Liza Basyrova

Vasilisa Bukina

Stanislav Balabanov

Alexander Belyaev

Viktor Drobotenko

Olga Dymshits

Irina Alekseeva

Marina Tsenter

Svetlana Zapalova

Alexander Khubetsov

Alexander Zhilin

Anna Volokitina

Vladimir Vitkin

Xavier Mateos

Josep Maria Serres

Patrice Camy

Pavel Loiko
email: pavel.loiko@ensicaen.fr

Synthesis, structure and spectroscopy of Fe2+:MgAl2O4 transparent ceramics and glass-ceramics

Keywords: transparent ceramics, glass-ceramics, spinel, iron ions, microstructure, spectroscopy

Introduction

Divalent iron ions (Fe 2+ ) embedded in II-IV group materials such as ZnS and ZnSe are known for their intense and broadband absorption and emission in the mid-infrared originating from the 5 E ↔ 5 T2 ( 5 D) transitions of Fe 2+ ions in tetrahedral (Td) crystal field [START_REF] Mirov | Progress in Cr 2+ and Fe 2+ doped mid-IR laser materials[END_REF]. For example, for Fe 2+ :ZnSe, the maximum absorption is at 3.1 μm and the emission band is centered at 4.35 μm [START_REF] Voronov | A continuous-wave Fe 2+ :ZnSe laser[END_REF][START_REF] Demirbas | Synthesis and characterization of diffusion-doped Cr 2+ :ZnSe and Fe 2+ :ZnSe[END_REF][START_REF] Voronov | Passive Fe 2+ :ZnSe single-crystal Q switch for 3-μm lasers[END_REF]. It makes Fe 2+ -doped chalcogenides suitable for laser gain media [START_REF] Voronov | A continuous-wave Fe 2+ :ZnSe laser[END_REF][START_REF] Firsov | High-energy room-temperature Fe 2+ :ZnS laser[END_REF][START_REF] Doroshenko | Tunable mid-infrared laser properties of Cr 2+ :ZnMgSe and Fe 2+ :ZnSe crystals[END_REF] operating in this spectral range with multiple applications in gas sensing, molecular spectroscopy, range finding and medicine. Due to high ground-state absorption (GSA) cross sections for Fe 2+ ions in Td sites (σGSA = 0.97×10 -18 cm 2 for Fe 2+ :ZnSe) and the lack of excited-state absorption [START_REF] Mirov | Progress in Cr 2+ and Fe 2+ doped mid-IR laser materials[END_REF], Fe 2+ -doped materials are also interesting for saturable absorbers (SAs) of mid-infrared lasers [START_REF] Voronov | Passive Fe 2+ :ZnSe single-crystal Q switch for 3-μm lasers[END_REF][START_REF] Denker | Short pulse formation in a 2.3 µm Tm-doped tellurite glass laser by an Fe 2+ :ZnS saturable absorber[END_REF][START_REF] Cankaya | Absorption saturation analysis of Cr 2+ :ZnSe and Fe 2+ :ZnSe[END_REF].

Because of relatively low hardness and laser-induced damage threshold (LIDT), high temperature coefficient of the refractive index of chalcogenides, and the lack of efficient pump lasers at a wavelength of 3.1 μm, it seems reasonable to find other hosts for Fe 2+ ions doping. One of the possible host candidates is cubic magnesium aluminium spinel, MgAl2O4 [START_REF] Ganesh | A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications[END_REF]. This broad energy gap material features good thermal and mechanical properties (e.g., high Mohs hardness of 8, high thermal conductivity of 24.7 Wm -1 K -1 for an undoped crystal [START_REF] Schulz | Thermophysical properties of sapphire, AlN and MgAl2O4 down to 70 K[END_REF] and relatively low thermal expansion coefficient of about 9×10 -6 K -1 between 20 and 1200 °C [START_REF] Kaprálik | Thermal expansion of spinels MgCr2O4, MgAl2O4 and MgFe2O4[END_REF]) together with a broad (for oxide crystals) transparency range of 0.2-5.5 μm. MgAl2O4 also exhibits high LIDT well exceeding that of chalcogenides. Spinel demonstrates widely variable occupation of two (tetrahedral, Td, and octahedral, Oh) sites by various transition metal ions and distribution of the cations between these sites. MgAl2O4 is known for doping with such transition metal ions as Co 2+ , Ni 2+ and Cr 3+ [START_REF] Kuleshov | Absorption and luminescence of tetrahedral Co 2+ ion in MgAl2O4[END_REF][START_REF] Kuleshov | Spectroscopy and excited-state absorption of Ni 2+ -doped MgAl2O4[END_REF][START_REF] Stręk | Optical properties of Cr 3+ in MgAl2O4 spinel[END_REF]. When doped with Co 2+ ions entering predominantly in the Td sites, it is recognized as a SA for eye-safe lasers operating at ~1.5 μm [START_REF] Karlsson | Diode-pumped Er-Yb:glass laser passively Q switched by use of Co 2+ :MgAl2O4 as a saturable absorber[END_REF][START_REF] Goldstein | Development of saturable absorbers for laser passive Q-switching near 1.5 µm based on transparent ceramic Co 2+ :MgAl2O4[END_REF].

Iron-containing spinels are common in both the crust and the Earth's mantle, in meteorites, and in lunar rocks. There is a continuous solid solution between MgAl2O4, spinel, endmember and FeAl2O4, hercynite, end-member, which also has a normal spinel structure. It is not surprising that there exist structural and optical studies of iron-doped MgAl2O4 minerals. Andreozzi et al. studied the distribution of Fe 2+ ions over the Td and Oh sites in MgAl2O4 -FeAl2O4 solid solutions and revealed the tetrahedral preference of Fe 2+ with respect to Mg 2+ [START_REF] Andreozzi | Intersite distribution of Fe 2+ and Mg in the spinel (sensu stricto)-hercynite series by single-crystal X-ray diffraction[END_REF]. Gaffney described the spectra of Fe 2+ ions in tetrahedral sites in spinel [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF], based on an earlier work of Slack [START_REF] Slack | FeAl2O4-MgAl2O4: growth and some thermal, optical, and magnetic properties of mixed single crystals[END_REF][START_REF] Slack | Optical absorption of tetrahedral Fe 2+ (3d 6 ) in cubic ZnS, CdTe, and MgAl2O4[END_REF]. Taran et al. described optical absorption of natural (Fe 3+ ,Fe 2+ )bearing spinels [START_REF] Taran | Electronic absorption spectroscopy of natural (Fe 2+ , Fe 3+ )-bearing spinels of spinel ss-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures[END_REF]; a study of iron-bearing aluminate spinel under lunar-like redox conditions was performed by Jackson et al. [START_REF] Jackson | Visible-infrared spectral properties of ironbearing aluminate spinel under lunar-like redox conditions[END_REF]. Halenius et al. proposed a model of Fe 2+ -Fe 3+ ion pairs in spinels [START_REF] Hålenius | Influence of cation distribution on the optical absorption spectra of Fe 3+ -bearing spinel ss-hercynite crystals: evidence for electron transitions in VI Fe 2+ -VI Fe 3+ clusters[END_REF].

The melting point of MgAl2O4 is rather high (2135 °C). The single crystals are typically grown by the Czochralski method [START_REF] Cockayne | The vertical pulling of MgAl2O4 single crystals[END_REF] using iridium crucibles which is challenging because the melting point of iridium is only slightly higher than that of spinel [START_REF] Volk | Anisotropy of nonlinear absorption in Co 2+ :MgAl2O4 crystal[END_REF]. We found only one paper on Fe 2+ -doped spinel single crystal [START_REF] Sackuvich | Spectroscopic characterization of Ti 3+ :AgGaS2 and Fe 2+ :MgAl2O4 crystals for mid-IR laser applications[END_REF] and one paper on growth of FeAl2O4, hercynite, single crystal [START_REF] Slack | FeAl2O4-MgAl2O4: growth and some thermal, optical, and magnetic properties of mixed single crystals[END_REF]. According to Sackuvich et al. [START_REF] Sackuvich | Spectroscopic characterization of Ti 3+ :AgGaS2 and Fe 2+ :MgAl2O4 crystals for mid-IR laser applications[END_REF], Fe 2+ ions in MgAl2O4 crys-tals feature broad absorption band with a maximum near 2 µm and a luminescence signal between 3 and 6 µm.

Thus, it is reasonable to propose other transparent MgAl2O4-based materials for doping with transition metal ions, namely, transparent polycrystalline ceramics [START_REF] Goldstein | Development of saturable absorbers for laser passive Q-switching near 1.5 µm based on transparent ceramic Co 2+ :MgAl2O4[END_REF] and transparent nanophase glass-ceramics (GCs) [START_REF] Reisfeld | Spectroscopy of Cr(III) in transparent glass ceramics containing spinel and gahnite[END_REF][START_REF] Malyarevich | Optical absorption and luminescence study of cobalt-doped magnesium aluminosilicate glass ceramics[END_REF][START_REF] Loiko | Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1[END_REF].

Nowadays, transparent ceramic technology is becoming competitive to traditional single crystal growth methods. It has several advantages [START_REF] Goldstein | Transparent ceramics at 50: progress made and further prospects[END_REF], such as (i) lower synthesis temperature, (ii) size scalable production, (iii) easier doping with active ions, (iv) the development of compositions which are unstable / hardly reproducible in the single-crystalline form. At the same time, optical ceramics provide spectroscopic and thermal properties similar to those of single crystals. Optical quality of ceramics can be greatly improved by proper adjusting the synthesis procedure. Recently, multiple studies were dedicated to the development of transparent spinel ceramics, such as MgAl2O4 and ZnAl2O4 (gahnite) both undoped and Co 2+ doped [START_REF] Goldstein | Development of saturable absorbers for laser passive Q-switching near 1.5 µm based on transparent ceramic Co 2+ :MgAl2O4[END_REF][START_REF] Belyaev | A new approach to producing transparent ZnAl2O4 ceramics[END_REF][START_REF] Wei | Fabrication and spectral properties of hot-pressed Co:MgAl2O4 transparent ceramics for saturable absorber[END_REF][START_REF] Loiko | Synthesis, characterization and absorption saturation of Co:ZnAl2O4 (gahnite) transparent ceramic and glass-ceramics: A comparative study[END_REF][START_REF] Balabanov | Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders[END_REF][START_REF] Su | A simple way to prepare Co:MgAl2O4 transparent ceramics for saturable absorber[END_REF]. For the consolidation of optical-quality spinel ceramics, either hot pressing with a fluoride-based sintering additive [START_REF] Balabanov | Densification peculiarities of transparent MgAl2O4 ceramics -Effect of LiF sintering additive[END_REF] or free sintering with a subsequent hot isostatic pressing (HIP) treatment is used [START_REF] Han | Preparation of high-quality transparent Alrich spinel ceramics by reactive sintering[END_REF]. Less frequent methods are spark plasma sintering (SPS) [START_REF] Alekseev | Transparent ceramics prepared from ultrapure magnesium aluminate spinel nanopowders by spark plasma sintering[END_REF] and microwave sintering [START_REF] Yu | Ultra-rapid microwave sintering of pure and Y2O3doped MgAl2O4[END_REF]. These methods allow to achieve high compaction rates and to preserve the sub-μm structure of ceramics which is responsible for their high micro-hardness. However, optical transmittance of such ceramics is usually lower due to incomplete pore removal during rapid compaction. Many methods in the preparation of spinel powders, such as sol-gel, solid-phase synthesis, pyrolysis of metal salts, etc., have been developed [START_REF] Du Merac | Fifty years of research and development coming to fruition; Unraveling the complex interactions during processing of transparent magnesium aluminate (MgAl2O4) spinel[END_REF]. The advantages of the sol-gel method of obtaining MgAl2O4 powders with the use of volatile metal alkoxides are high chemical purity of powders and a stable ratio of magnesium-to-aluminum ions of 1:2 [START_REF] Balabanov | Effect of magnesium aluminum isopropoxide hydrolysis conditions on the properties of magnesium aluminate spinel powders[END_REF]. High chemical purity of ceramics is an important requirement for their use in laser, terahertz and microwave technologies, especially for highpower applications [START_REF] Sanghera | Transparent ceramics for high-energy laser systems[END_REF][START_REF] Egorov | Terahertz dielectric properties of polycrystalline MgAl2O4 spinel obtained by microwave sintering and hot pressing[END_REF].

Transparent nanophase GCs [START_REF] Beall | Nanophase glass-ceramics[END_REF] are obtained by secondary heat-treatments of initial glasses of specially developed compositions. The initial glasses are produced by the meltquenching technique, and they can be doped by active ions. As a result, the material contains nanosized crystals of one or several crystalline phases uniformly distributed in the residual glass. Fabrication of GCs is in many cases easier than that of transparent ceramics. GCs also benefit from higher LIDT [START_REF] Loiko | Synthesis, characterization and absorption saturation of Co:ZnAl2O4 (gahnite) transparent ceramic and glass-ceramics: A comparative study[END_REF]. However, it is important to control the distribution of active ions between the nanocrystalline phase (or phases) and the residual glass. Moreover, thermal conductivity of GCs is typically lower than that of optical ceramics. So far, multiple studies were dedicated to fabrication and characterization of spinel-based transparent GCs doped with Cr 3+ [START_REF] Reisfeld | Spectroscopy of Cr(III) in transparent glass ceramics containing spinel and gahnite[END_REF][START_REF] Molla | Microstructure, mechanical, thermal, EPR, and optical properties of MgAl2O4:Cr 3+ spinel glass-ceramic nanocomposites[END_REF], Co 2+ [START_REF] Malyarevich | Optical absorption and luminescence study of cobalt-doped magnesium aluminosilicate glass ceramics[END_REF][START_REF] Loiko | Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1[END_REF] and Ni 2+ ions [START_REF] Golubkov | The influence of nickel oxide additives on the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF][START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF][START_REF] Dugué | Structural transformations and spectroscopic properties of Ni-doped magnesium aluminosilicate glass-ceramics nucleated by a mixture of TiO2 and ZrO2 for broadband near-IR light emission[END_REF].

In this work, we aim to fabricate and study the microstructure, vibronic and optical properties of novel transparent ceramics and GCs based on Fe 2+ :MgAl2O4 spinel crystals. Previously, the authors from the Institute of Electrophysics UD RAS reported the development of transparent Fe 2+ :MgAl2O4 ceramics [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF][50][START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF]. The obtained ceramics [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF][50][START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF] contained secondary oxide phase of (MgO)0.91(FeO)0.09 causing a substantial decrease in transmittance in the visible range. In those ceramics Fe 3+ ions are located both in the primary spi-nel phase Fe:MgAl2O4 and in the secondary phase Fe:MgO, which results in a very low determined GSA cross sections of Fe 2+ ions in tetrahedral site symmetry [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF]. Recently, first GCs based on Fe 2+ :ZnAl2O4 [START_REF] Eremeev | Spectral properties of novel transparent glass-ceramics based on Fe 2+ :ZnAl2O4 nanocrystals[END_REF][START_REF] Eremeev | The influence of the Fe 2+ doping concentration on structure and spectroscopic properties of transparent glass-ceramics based on Fe 2+ :ZnAl2O4 nanocrystals[END_REF] nanocrystals in zinc aluminosilicate glass system were prepared and studied. Lin et al. reported on the development of transparent GCs based on Fe:(Ga2-xAlx)O3 nanocrystals with spinel structure in the sodium-magnesium-alumino-gallosilicate glass system and demonstrated the proof-of-the-concept of their SA performance [START_REF] Lin | In situ and tunable structuring of semiconductor-in-glass transparent composite[END_REF]. Though there exist studies on the iron-doped GCs of the magnesium aluminosilicate system, they aim to develop opaque GCs based of iron-doped cordierite [START_REF] Wang | Effects of Fe on crystallization and properties of a new high infrared radiance glassceramics[END_REF][START_REF] Wang | Crystallization and infrared radiation properties of iron ion doped cordierite glass-ceramics[END_REF][START_REF] Liu | Effect of Fe2O3 doping on structure, physical-mechanical properties and luminescence performance of magnesium-aluminumsilicon based glass-ceramics[END_REF]. The preliminary results on the development of Fe:MgAl2O4-based transparent glass-ceramics were first published in [START_REF] Bukina | Optical glass-ceramics based on nanosized crystals of magnesium aluminate spinel doped with iron ions[END_REF][START_REF] Bukina | Novel transparent glass-ceramics based on Fe 2+ :MgAl2O4 spi-nel nanocrystals[END_REF].

Synthesis of ceramics and glass-ceramics 2.1. Synthesis of ceramics

The magnesium aluminium spinel powder doped with Fe 2+ ions was synthesized by the sol-gel method. Metallic magnesium and aluminum (>99% purity) and isopropyl alcohol (99.9% purity) were used as starting materials. Mg and Al metals (31 and 69 wt%, respectively, which corresponds to a molar ratio of 1:2) were melted at 600 °С in an atmosphere of nitrogen. The alloy was crushed, mixed with 0.1 wt% of SnCl2 and 0.2 wt% of NH4Cl, which are the catalysts for the dissolution of the metals in alcohol, and heat-treated in an enclosed stainless steel vessel. MgAl2(OPr i )8 (Pr i stands for the isopropyl groups) was synthesized in a tube reactor at 80-150 °С by an addition of small portions of isopropyl alcohol to the granulated magnesium-aluminum alloy. After that, the final product was purified by vacuum distillation. Under synthesis conditions, other elements do not form volatile isopropylates. Thus, a single vacuum distillation of MgAl2(OPr i )8 results in its high chemical purity and an exact ratio of 1:2 of magnesium to aluminum metals. The hydrolysis of 50 vol% solution of MgAl2(OPr i )8 in isopropyl alcohol was conducted by using 1200 ml of isopropyl alcoholwater azeotropic mixture per 1 mol of alkoxide (hydrolysis with pure water leads to a strong agglomeration of hydroxides). The MgAl2(OPr i )8:H2O ratio was 1:8, which is an equimolar ratio required for complete hydrolysis. The azeotropic mixture contained iron (II) chloride (FeCl2) as a source of iron ions. The addition of LiF (the sintering aid) was carried out as follows. 72 ml of suspension prepared by interaction of LiNO3•3H2O and NH4F aqueous solutions was added to 1200 ml of Pr i OH-H2O azeotrope and further added into 1 mol of MgAl2(OPr i )8 under stirring. After hydrolysis, the suspensions were cooled down to room temperature (RT) in air. Obtained suspensions were dried at 150 °С under ~1 kPa pressure followed by calcination in air at 750 °C for 3 h. Resulted spinel powders contained 0.1 mol% Fe and about 1.5 wt% of LiF.

To obtain Fe 2+ :MgAl2O4 optical ceramics, the synthesized powders were pre-shaped, loaded into a graphite mold and heated from the RT up to 1600 °С with a heating rate of ∼7.5 °C/min and held for 6 h before cooling down. The load was applied at a temperature of 800 °C and then increased up to ~50 MPa in 30 min. The pressure was released before free cooling of the furnace. The transparent ceramic disks were polished to laser-grade quality on both surfaces using a diamond suspension. The disk had dimensions of Ø13 × 2.8 mm (diameter × thickness) and a weak grey coloration due to the iron doping, see Fig. 1.

For comparison, undoped MgAl2O4 ceramic was prepared using a similar approach. The ceramic disk (Ø13 × 2.0 mm) was transparent and colorless, Fig. 1.

Synthesis of glass-ceramics

The glass with a composition of 20 MgO, 20 Al2O3, 60 SiO2 (mol%) was nucleated by 10 mol% TiO2 [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF][START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF] and doped with 0.1 mol% FeO both added above 100% of the base glass composition. The raw materials were reagent grade. The batch for producing 400 g of glass was melted in a crucible made of quartz ceramics at 1550 °C for 3 h with stirring. The glass was poured onto a cold metal plate and annealed at 640 °C.

The transparent pale-yellow glass, Fig. 2, was subjected to double-stage secondary heat-treatments. The first stage was at 750 °C for 6 h and the temperature of the second stage ranged from 800 to 1100 °C with the same holding time of 6 h. As a result, transparent GCs were synthesized. Their color changed with the heat-treatment temperature from the pale yellow to the deep gray-brown (Fig. 2).

Experimental

Characterization of ceramics

The X-ray powder diffraction (XRD) patterns were measured with a Shimadzu XRD-6000 diffractometer, Cu Kα radiation with a Ni filter. The spinel lattice parameter a was determined by the Rietveld refinement. Morphology of the fracture surface of ceramics was characterized by scanning electron microscopy (SEM) using a MERLIN SEM microscope (Carl Zeiss). The SEM images were analyzed with the ImageJ software.

The RT Raman spectra were measured with the Renishaw inVia confocal Raman microscope using a Leica x50 objective (N.A. = 0.75). The excitation wavelength λexc was 514 nm (Ar + laser line).

The RT transmission spectra in the visible to near-IR and in the mid-IR were measured using a Shimadzu UV-3600 spectrophotometer and a FTIR spectrometer Bruker Tensor 27, respectively.

Characterization of glass-ceramics

The density of the initial glass and GCs was measured by the Archimedes method with toluene. The experimental error was about 0.0005 g/cm 3 .

The XRD patterns were recorded using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation and a Ni filter. The mean crystal size, DXRD, was estimated from broadening of the diffraction peaks according to the Scherrer's equation:

, cos ) 2 ( XRD θ θ λ ⋅ ∆ = K D (1)
where λ is the wavelength of the X-ray radiation (1.5406 Å), θ is the diffraction angle, Δ(2θ) is the width of the peak at half of its maximum and K is the constant assumed to be 1 [START_REF] Lipson | Interpretation of X-ray Powder Patterns[END_REF]. The error for the crystal size estimation is ~5%. The size of spinel crystals was determined using the diffraction peak with the Miller's indices (hkl) = (440) at 2θ ≈ 65.5°. The size of crystals of magnesium aluminotitanate (MAT) xMgTi2O5yAl2TiO5 solid solution (ss) with a pseudobrukite structure [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF][START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF] was determined for the (110) diffraction peak at 2θ ≈ 25.7°. The spinel lattice parameter a was determined using the same diffraction peak with the Miller's indices (hkl) = (440) at 2θ ≈ 65.5° according to the equation:

2 2 2 l k h d a hkl + + = (2)
For differential scanning calorimetry (DSC), bulk samples of about 20 mg in weight were used. A simultaneous thermal analyzer NETZSCH STA 449 F3 Jupiter with a dynamic flow atmosphere of Ar was employed. The temperature range was RT -1250 °C. The heating rate was 10 °C/min. The samples were the initial glass and the glass heat-treated at the nucleation stage of 750 °C for 6 h. To assign the exothermal DSC peaks to certain crystalline phases, bulk samples of about 80 mg in weight were heated in the DSC furnace with the same heating rate of 10 °C/min up to a temperature of the certain exothermal peak appearence, cooled down to RT and subjected to powder XRD analysis.

The microstructure of the initial glass and GCs was studied by transmission electron microscopy (TEM) with the JEOL TEM-1011 microscope (100 kV acceleration voltage, 0.4 nm point resolution). The samples were finely powdered and dispersed in ethanol. The obtained solution was dropped on a TEM grid which was dried for ~30 min. The TEM images were analyzed with the ImageJ software.

The Raman spectra of the initial glass and GCs were recorded with the same confocal Raman microscope as that used for studying the ceramics. For the measurment in the frequency range of 100-1200 cm -1 , an edge filter was used. The Raman spectra in the frequency range of 5-350 cm -1 were measured with a notch-filter. The spatial resolution was about 2 cm -1 .

The absorption spectra of the initial glass and GCs were recorded on a Shimadzu UV-3600 spectrophotometer. Double-sided polished samples (1.0 mm thick) were used. The luminescence spectra were measured using the confocal Raman microscope with λexc = 488 nm (Ar + laser line).

Transparent ceramics 4.1. Structure

The phase purity and the structure of ceramics were confirmed by XRD, see Fig. 3(a). MgAl2O4 and Fe:MgAl2O4 are cubic (sp. gr. O 7 h -Fd3‾m, No. 227). The lattice constants of both spinels are similar, a = 8.083±0.003 Å (for the Fe:MgAl2O4 spinel) and 8.088±0.003 Å (for the undoped MgAl2O4 one). Quite similar lattice constants a = 8.086±0.002 Å and 8.087±0.002 Å and were reported in [50] for MgAl2O4 doped with 0.1 and 1wt% Fe3O4, respectively. The lattice parameter in Fe:MgAl2O4 ceramic is slightly larger than that for the initial powder (a = 8.063 Å [START_REF] Balabanov | Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders[END_REF]), which is most likely due to the presence of a sintering aid of lithium fluoride in the powder that can enter the spinel structure [START_REF] Mordekovitz | The effect of lithium doping on the sintering and grain growth of SPS-processed, non-stoichiometric magnesium aluminate spinel[END_REF]. During hot-pressing, the sintering aid evaporates and the fluorine and lithium ions are replaced by larger ions of oxygen and magnesium or aluminum, respectively.

No traces of any impurity phase were found. Figure 3(a) shows that the intensities of the peaks with the Miller' indices (422) and (533) in the XRD pattern of the Fe:MgAl2O4 ceramics increase as compared with the standard values (see ICSD card No. 82-2424 [START_REF] Sawada | An electron density residual study of magnesium aluminum oxide spinel[END_REF]) and with those of the MgAl2O4 ceramics. The changes in the peak intensity indicate the presence of a disordered spinel phase in the sample [START_REF] Simeone | Order-disorder phase transition induced by swift ions in MgAl2O4 and ZnAl2O4 spinels[END_REF][START_REF] Sickafus | Comment on 'order-disorder phase transition induced by swift ions in MgAl2O4 and ZnAl2O4 spinels[END_REF][START_REF] Barpanda | Chemically induced order disorder transition in magnesium aluminium spinel[END_REF] caused by the iron entering the spinel crystals.

Let us shortly discuss the structure of spinels. Their chemical formula is AB2O4. Each unit cell contains 32 anions (O 2-) in a face-centered cubic packing. In the oxygen lattice, the cations (A 2+ and B 3+ ) have 32 Oh and 64 Td interstitial sites per unit cell. For a normal spinel, IV [A] VI [B]2O4, 1/8 of the tetrahedral sites are occupied by the A 2+ cations and 1/2 of the octahedral ones are occupied by the B 3+ cations. Here the superscripts are the coordination numbers (C.N. = IV and VI for the Td and Oh sites, respectively). Typically, spinels demonstrate a certain degree of cation disorder. It is described by the so-called inversion parameter δ -a fraction of trivalent cations in the Td sites (or, equivalently, a fraction of A 2+ cations in the Oh sites). Thus, a general formula for spinels is IV [A1-δBδ] VI [B2-δAδ]O4 (δ = 0 and 1 correspond to a normal / inverse spinel, respectively) [START_REF] Ardit | Structural relaxation in tetrahedrally coordinated Co 2+ along the gahnite-Co-aluminate spinel solid solution[END_REF]. Note that as we mentioned before, there exists Fe-based normal spinel with δ ≈ 0 (FeAl2O4, hercynite) [START_REF] Harrison | The temperature dependence of the cation distribution in synthetic hercynite (FeAl2O4) from in-situ neutron structure refinements[END_REF].

The Fe 2+ ions are expected to replace the Mg 2+ ones in Td sites owing to the closeness of their ionic radii (RFe = 0.63 Å and RMg = 0.57 Å for IV-fold oxygen coordination [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]). The Fe 2+ doping can also induce an additional cation disorder leading to appearance of both Fe 2+ and Mg 2+ ions in Oh sites. Andreozzi et al. had shown that compared with Fe 2+ , Mg 2+ has some preference in substituting for octahedral Al 3+ , and that in the MgAl2O4 -FeAl2O4 isostructural series of solid solutions, the fraction of Fe 2+ ions in Oh sites increases with the Fe content while it is near zero for low iron doping (as in our case) [START_REF] Andreozzi | Intersite distribution of Fe 2+ and Mg in the spinel (sensu stricto)-hercynite series by single-crystal X-ray diffraction[END_REF].

The morphology of the fracture surface was characterized by scanning electron microscopy (SEM), Fig. 4(a). The grain boundaries are clean and there is no secondary phase at the boundaries. This is in agreement with the XRD finding of the absence of any secondary crystal phase (Fig. 3). Due to low-defect grain boundaries, the bonds between the grains have strength comparable to that in the grain volume so that the inter-and intragranular fracture mechanisms compete in ceramics. Usually, when using sintering aid, it tends to accumulate at the grain boundaries, which weakens the bonds between the grains compared to the grain volume. This leads to the intergranular destruction of such ceramics [START_REF] Balabanov | Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders[END_REF]. In our case, the relatively small initial LiF content of 1.5 wt% and the long exposure time at the hot pressing temperature made it possible to completely remove the sintering aid. Another important factor for the absence of impurities at the grain boundaries may be the initially high chemical purity of the powders, achieved by vacuum distillation of the precursor. The ceramic is also almost pore-free. Low porosity and the absence of secondary phases make it possible to achieve high optical quality of ceramics (see below). The average grain size is about 50 μm (as determined by analyzing more than 100 grains), see Fig. 4(b). The grain size was obtained by multiplying the measured lengths by a shape factor of 1.2.

Raman spectra

Vibronic properties of ceramics were studied by Raman spectroscopy, Fig. 5. According to [START_REF] Mordekovitz | The effect of lithium doping on the sintering and grain growth of SPS-processed, non-stoichiometric magnesium aluminate spinel[END_REF], the following vibrational modes are characteristic of the spinel structure: Г = A1g(R) + Eg(R)+ 3T2g(R) + 4T1u(IR)+ T1g+2T2u + 2A2u + 2Eu. Among these modes, the Raman-active ones are A1g, Eg, and T2g, while T1u is IR-active. The oriented MgAI2O4 spinel single crystal has the following vibrational frequencies: 311 cm -1 (T2g) -lattice, 410 cm -1 (Eg), 492 cm -1 (T2g) -both bending, 671 cm -1 (T2g) -asymmetric stretching, 772 cm -1 (A1g) -symmetric stretching [START_REF] O'horo | Lattice vibrations of MgAI2O4 spinel[END_REF].

The same vibrations at ~195, 251, 303, 405, 496, 667, 721 and 764 cm -1 are found in the Raman spectra of Fe-doped and undoped ceramics. The Raman bands at 303, 405, 496, 667 and 764 cm -1 are close in positions to the data for the MgAl2O4 spinel single crystal [START_REF] O'horo | Lattice vibrations of MgAI2O4 spinel[END_REF]. Although the Raman spectra of two ceramics are rather similar, the spectrum of the irondoped spinel is more complex: the band at 405 cm -1 is more broad, asymmetric and intense, the bands at 496 and 721 cm -1 are more pronounced, and there are also bands at 583 and above 800 cm -1 that are hardly seen in the spectrum of the undoped spinel. The appearance of the band at 721 cm -1 and a low-frequency shoulder on the intense band at 405 cm -1 are typical features of synthetic spinel [START_REF] Cynn | High-temperature Raman investigation of order-disorder behavior in the MgAl2O4 spinel[END_REF]. The band at 721 cm -1 is assigned to symmetric Al-O stretching vibration of AlO4 groups created by redistribution of some Al 3+ ions from octahedral to tetrahedral sites and evidences a certain disorder of the spinel structure, while the lowfrequency shoulder of the 405-cm -1 band appears to be the bending mode for Al ions in tetrahedral sites [START_REF] De Wijs | First-principles calculation of the phonon spectrum of MgAl2O4 spinel[END_REF][START_REF] Slotznick | In situ Raman spectroscopy measurements of MgAl2O4 spinel up to 1400 °C[END_REF]. Apparently, the bands at 205 and 251 cm -1 , in the similar manner as the band at 300 cm -1 , can be attributed to the external lattice vibrations in polycrystalline ceramics.

Raman spectroscopy was successfully used to study cation disorder in spinels. It has been suggested [START_REF] Slotznick | In situ Raman spectroscopy measurements of MgAl2O4 spinel up to 1400 °C[END_REF] that most of extra features in Raman spectra of spinels are related to cation disordering. We speculate that appearance of additional bands in the spectrum of the Fedoped spinel at 583 and at above 800 cm -1 can also stand for a certain degree of its disorder caused by Fe 2+ doping. Thus, we can conclude that the comparison of Raman spectra of Fedoped and undoped spinel proves the increase of the cation disorder in spinel with an addition of iron ions, which is in accordance with the XRD findings. It is also known [START_REF] Slotznick | In situ Raman spectroscopy measurements of MgAl2O4 spinel up to 1400 °C[END_REF] that the complex Raman spectrum of iron-doped spinel can be affected not only by the substitution Fe 2+ → Mg 2+ and the cation disorder, but also by the entrance of small amounts of Fe 3+ in the spinel structure. It is not surprising that the addition of 0.1% Fe does not affect the position of the Raman peaks. According to [START_REF] Ippolito | Raman study of MgAl2O4-FeAl2O4 and MgAl2O4-MgFe2O4 spinel solid solutions, Period. di Mineral[END_REF], while the substitution Fe 2+ → Mg 2+ proceeds along the MgAl2O4 -FeAl2O4 series, no change in the wavenumber of the Eg, T2g, and A1g modes is observed up to the Fe 2+ content of ~20%.

In agreement with the XRD data, no signals attributed to impurities or secondary phases are identified in the Raman spectra of both spinels.

Optical absorption

The in-line transmission spectrum of the polished Fe:MgAl2O4 ceramic disk (recalculated for a reference thickness of 1.0 mm) is presented in Fig. 6(a) and compared with that for the undoped MgAl2O4 ceramic. The transparency range for both samples is similar, 0.2-6.0 μm. MgAl2O4 possesses a large direct bandgap of Eg = 7.8 eV (the corresponding wavelength of the UV absorption edge λUV ~0.16 μm) [START_REF] Jiang | Ab initio many-body study of the electronic and optical properties of MgAl2O4 spinel[END_REF]. For the undoped ceramic, the in-line (smallsignal) transmission T at the wavelength of ~1 μm is 82.2% (compare with the theoretical value set by the Fresnel losses, T0 = 87.0% for a refractive index n = 1.704 [START_REF] Ganesh | A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications[END_REF]). For the Fedoped ceramics at the same wavelength of ~1 μm, T is reduced to a value of 74.4%.

In the spectrum of the Fe-doped ceramics, there is an intense absorption band in the near-mid-IR, spanning from ~1.2 to 3.7 μm with the local maxima at 1.90 and 2.82 μm. It is assigned to the absorption of Fe 2+ ions in Td sites in spinel. The electronic configuration of Fe 2+ ion is [Ar]3d 6 . Thus, the energy-level scheme for the Fe 2+ ion in the tetrahedral ligand field is equivalent to a 3d (10-n) = 3d 4 ion in the octahedral ligand field. The ground state of the free ion is the 5 D one and it is split in the tetrahedral ligand field into the 5 E and 5 T2 levels separated by an energy Δ = 10Dq. The spin-allowed transitions between these levels determine the intense absorption and emission bands of IV Fe 2+ ions. Indeed, an intense absorption band in the NIR range centered at ~5000 cm -1 has been widely attributed to the spin-allowed electronic d-d transition ( 5 E → 5 T2) in tetrahedrally coordinated Fe 2+ [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF][START_REF] Slack | FeAl2O4-MgAl2O4: growth and some thermal, optical, and magnetic properties of mixed single crystals[END_REF][START_REF] Slack | Optical absorption of tetrahedral Fe 2+ (3d 6 ) in cubic ZnS, CdTe, and MgAl2O4[END_REF][START_REF] Taran | Electronic absorption spectroscopy of natural (Fe 2+ , Fe 3+ )-bearing spinels of spinel ss-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures[END_REF][START_REF] Jackson | Visible-infrared spectral properties of ironbearing aluminate spinel under lunar-like redox conditions[END_REF][START_REF] Rossman | Spectroscopic standards for four-and fivefold-coordinated Fe 2+ in oxygen-based minerals[END_REF][START_REF] Taran | Electronic absorption spectra of Fe 2+ ions in oxygen-based rockforming minerals at temperatures between 297 and 600 K[END_REF][START_REF] Skogby | An FTIR study of tetrahedrally coordinated ferrous iron in the spinel-hercynite solid solution[END_REF][START_REF] Ippolito | Color mechanisms in spinel: cobalt and iron interplay for the blue color[END_REF]. The band is characterized by a distinct shoulder at ~3500 cm -1 , due to the dynamic Jahn-Teller effect for tetrahedral Fe 2+ ions in the spinel structure [START_REF] Taran | Electronic absorption spectroscopy of natural (Fe 2+ , Fe 3+ )-bearing spinels of spinel ss-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures[END_REF][START_REF] Skogby | An FTIR study of tetrahedrally coordinated ferrous iron in the spinel-hercynite solid solution[END_REF]. Our data are in agreement with Gaffney [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF] who reported on the following energies of this transition in absorption: 3600 and 4900 cm -1 (λ = 2.78 and 2.04 μm). For tetrahedrally coordinated Fe 2+ ions, there also exist higher-lying triplet states ( 3 H, 3 P, 3 F, 3 G), however, the transitions to these states are spinforbidden and thus very weak.

Apart from IV Fe 2+ ions, as pointed out above, there can exist VI Fe 2+ ones in the disordered spinel structure. In general, because the Oh site has a center of symmetry, the transition intensities for transition metal ions located in the ligand field of octahedral symmetry are by several orders of magnitude weaker than those in the field of tetrahedral symmetry. Absorption of VI Fe 2+ ions was observed before mainly in highly doped samples. Gaffney [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF] assigned transitions at 9500 and 10800 cm -1 (λ = 1.05 and 0.93 μm, respectively) to the spinallowed 5 T2 → 5 E ( 5 D) absorption transition in the octahedral (Oh) field. No absorption at these wavelengths is found in the ceramic sample under study. Dickson et al. predicted the molar extinction coefficient ε for this absorption band to be 0.5l l•mol -1 •cm -1 [START_REF] Dickson | Low-temperature optical absorption and Mössbauer spectra of staurolite and spinel[END_REF] (compare with the value of 59 l•mol -1 •cm -1 for the IV Fe 2+ absorption [START_REF] Hålenius | Influence of cation distribution on the optical absorption spectra of Fe 3+ -bearing spinel ss-hercynite crystals: evidence for electron transitions in VI Fe 2+ -VI Fe 3+ clusters[END_REF]. According to [START_REF] Andreozzi | Intersite distribution of Fe 2+ and Mg in the spinel (sensu stricto)-hercynite series by single-crystal X-ray diffraction[END_REF], in spinel, Fe 2+ shows a marked preference for tetrahedral coordination with respect to Mg 2+ , thus, the absence of absorption caused by the VI Fe 2+ ions is not surprising.

Another possibility for Fe ions to be accommodated in the spinel lattice is to do it in the form of Fe 3+ (in the Oh sites, by replacing Al 3+ ions). The corresponding ionic radii are RFe = 0.645 Å, high-spin, and RAl = 0.535 Å for VI-fold oxygen coordination [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. The presence of Fe 3+ ions in oxide crystals can be manifested by absorption transitions of both isolated Fe 3+ ions (d-d transition) and Fe 2+ -Fe 3+ ion pairs (intervalence charge transfer, IV CT, transition) [START_REF] Hålenius | Influence of cation distribution on the optical absorption spectra of Fe 3+ -bearing spinel ss-hercynite crystals: evidence for electron transitions in VI Fe 2+ -VI Fe 3+ clusters[END_REF]. The characteristic transitions are at 21700 and 21300 cm -1 (λ = 0.46 and 0.47 μm, respectively) -for the 6 A1g → 4 A1g + 4 Eg( 4 G) transitions for the isolated VI Fe 2+ ions and ion pairs, respectively [START_REF] Hålenius | Influence of cation distribution on the optical absorption spectra of Fe 3+ -bearing spinel ss-hercynite crystals: evidence for electron transitions in VI Fe 2+ -VI Fe 3+ clusters[END_REF]. None of these bands were observed in the studied ceramics. This finding is in accordance with [START_REF] Bruschini | Fe-Mg substitution in aluminate spinels: effects on elastic properties investigated by Brillouin scattering[END_REF], which states the iron atoms at low concentration are dilut-ed in the system and located far from each other. However, we cannot rule out that we noticed no trace of Fe 3+ ions in the Oh sites due to the low intensity of their absorption bands.

In the previous works [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF][50][START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF][START_REF] Devaraja | Luminescence properties of MgO:Fe 3+ nanopowders for WLEDs under NUV excitation prepared via propellant combustion route[END_REF][START_REF] Kutty | Cationic distribution and its influence on the luminescent properties of Fe 3+ -doped LiAl5O8 prepared by wet chemical methods[END_REF][START_REF] Sosman | Optical spectroscopy of MgGa2O4: Fe 3+[END_REF], the presence of Fe 3+ was detected by measuring the visible luminescence spectra of iron-doped samples revealing emissions centered at ~0.51, 0.62 and 0.72 μm and originated from the 4 G excited states. The emission spectrum for our ceramic sample is shown in Fig. 7. In fact, Fig. 7 demonstrates a typical red fluorescence spectrum of MgAl2O4:Cr 3+ [START_REF] Wood | Optical Spectrum of Cr 3+ ions in spinels[END_REF][START_REF] Brik | Luminescence of Cr 3+ ions in ZnAl2O4 and MgAl2O4 spinels: correlation between experimental spectroscopic studies and crystal field calculations[END_REF]. The red emission at 687 nm is assigned to the 2 Eg → 4 A2g spin-forbidden transition of Cr 3+ ions located at the Al 3+ sites [START_REF] Brik | Luminescence of Cr 3+ ions in ZnAl2O4 and MgAl2O4 spinels: correlation between experimental spectroscopic studies and crystal field calculations[END_REF]. A very week luminescence of Fe 3+ ions can probably reveal itself by the broad band of low intensity in the range of 540 -650 nm and partly be concealed in the luminescence spectrum of the Cr 3+ ion. The absorption spectrum of MgAl2O4:Cr 3+ consists of two broad bands in the visible spectral range, at ~390 nm (the 4 A2g → 4 T1g transition) and at ~540 nm (the 4 A2g → 4 T2g transition). We did not observe the characteristic absorption bands of Cr 3+ ion in the absorption spectrum of our Fe:MgAl2O4 ceramic sample (see Fig. 6). Therefore, the concentration of this uncontrolled impurity ion in the ceramic sample is extremely low. According to the study of Bubnova and Solomonov [START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF], the characteristic luminescence bands of Cr 3+ ions are not observed in spinels containing Fe 3+ ions due to the quenching originating from the nonradiative energy transfer between chromium excited-states and the lower-lying levels of iron ions. Consequently, the fact that the luminescence spectrum of our Fe:MgAl2O4 spinel ceramic presents the luminescence of Cr 3+ ions is an evidence of the absence or an extremely low content of Fe 3+ ions in the material.

In the UV part of the absorption spectrum of the Fe:MgAl2O4 ceramic, there is an absorption band centered at 0.26 μm. This band is too intense for the spin-forbidden IV Fe 2+ transitions. It can originate from the oxygen to Fe 2+ and probably to Fe 3+ charge transfer (oxide to metal charge transfer, OMCT). The corresponding absorption peaks are located at about 250 nm and 210 nm, respectively [START_REF] Sherman | Reassignment of the iron(III) absorption bands in the spectra of Mars[END_REF]. This suggestion is in accordance with Gaffney's finding [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF] that Fe 3+ impurities do not contribute directly to the visible spectra of most of these minerals, with the exception that Fe 3+ probably contributes to the intense ultraviolet absorption.

The ground-state absorption (GSA) cross section, σGSA, for the 5 E → 5 T2 ( 5 D) transition of Fe 2+ in MgAl2O4 was calculated assuming that all the iron ions in the ceramic are optically active and there is no contribution of VI Fe 2+ and Fe 3+ species. The results are shown in Fig. 6(b). Here, we use the nominal iron concentration NFe = 1.516×10 19 cm -3 (as calculated for the theoretical spinel density ρ = 3.578 g/cm 3 ). The maximum σGSA is calculated to be 0.28×10 -18 cm 2 at 1.90 μm.

For comparison, in Fig. 6(b), we also plotted the GSA cross section spectra for Fe 2+ ions in ZnS and ZnSe crystals (measured in this work using samples fabricated at G.G. Devyatykh Institute of Chemistry of High-Purity Substances, RAS). The absorption band in the Fe-doped spinel is blue-shifted by ~1 μm as compared to that for the Fe:ZnSe material while showing a very similar shape. The value of σGSA for the Fe:MgAl2O4 spinel is reduced by a factor of ~3 (compare with σGSA = 0.97×10 -18 cm 2 at 3.11 μm for the Fe:ZnSe sample). We believe that this difference mainly originates from the host matrix. In part, it may also originate from the presence of traces of VI Fe 2+ and Fe 3+ species that do not contribute to the absorption at 1.90 μm). Another possible mechanism responsible for decreasing the concentration of optically active iron ions in the ceramic compared to its initial concentration in the powder may be reduction of the Fe 2+ ions to the metallic state due to the presence of residual hydrocarbons and carbon monoxide from the graphite equipment of the hot press. However, one of the features of LiF as a sintering additive is effective removal of carbon-containing impurities from ceramics at the open porosity stage. This prevents intensive reduction of Fe 2+ ions. We believe that the concentration of optically active iron is close to its initial concentration in the powder.

It is worth mentioning that the described reduction mechanism can be partly responsible for the appearance of optically inactive iron in [START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF], as supported by much lower σGSA of only (1.66±0.14)×10 -20 cm 2 reported in that work. Its indication is the dark coloration of samples in [START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF]. Another reason for such a low σGSA is the presence of VI Fe 2+ in this ceramics, as manifested by intense absorption band at ~1 μm [START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF].

Transparent glass-ceramics

Structure

The density variation for the initial glass with the heat-treatment temperature is shown in Fig. 8. It demonstrates a complex behavior typical for density variation of spinel based GCs [START_REF] Loiko | Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1[END_REF]. The density of the initial glass is 2.6120 g/cm 3 . After the heat-treatment at 750 °C, the density slightly increases to 2.6151 g/cm 3 , and after two-stage heat-treatments in the temperature range of 800-900 °C it rapidly increases up to 2.7147 g/cm 3 . The GCs prepared at the second stage in the temperature range of 950-1000 °C have lower densities (the GC prepared at 1000 °C has a density of 2.6809 g/cm 3 ). The density of GC prepared at 1050 °C is higher, namely 2.7148 g/cm 3 .

The DSC curves for the initial glass and the glass heat-treated at 750 °C for 6 h are drastically different, Fig. 9. Though the glass transition temperature, Tg, is about 740 °C for both glasses, in the temperature range of 800-1000 °C they behave differently. For the initial glass, there is an intense narrow peak with the crystallization onset temperature (Ton) of 880 °C and the maximum crystallization temperature of 909 °C. This peak demonstrates an asymmetric wing from the low-temperature side. For the glass heat-treated at 750 °C for 6 h, there are three rather broad exothermal peaks with the crystallization onset temperatures of 795, 934 and 1085 °C and maximum crystallization temperatures of about 859, 973 and 1160 °C, respectively, Fig. 10. It was challenging to determine the origin of these peaks for both DSC curves. That is why, after the crystallization onset temperatures were determined, the samples of the initial glass and the glass preliminary heat-treated at 750 °C for 6 h, both about 80 mg in weight, were heated in the DSC instrument up to the crystallization onset temperatures.

According to the XRD data, Fig. 10(a), in the DSC curve of the initial glass, the first exothermal peak is associated with crystallization of the magnesium aluminotitanate (MAT) solid solution (ss) (probably, its crystallization corresponds to the low-temperature wing of the exothermal peak) and of the magnesium aluminosilicate with a quartz-like structure. The sample heat-treated up to the temperature of the second exothermic peak (1160 °C) had a rich phase composition of the MAT ss, magnesium aluminosilicate with a quartz-like structure, sapphirine, rutile, traces of indialite, a high-temperature modification of cordierite, and traces of mullite.

The XRD patterns of GCs prepared from the glass heat-treated at 750 °C for 6 h showed that the first crystallization peak is due to formation of spinel and traces of MAT ss. The sample heated up to 970 °C contained crystals of both MAT ss and spinel, while after heating up to 1150 °C, the sample contained the MAT ss and sapphirine crystals, as shown in Fig. 10(b).

The XRD patterns of the initial glass and GCs obtained by secondary heat treatments in the temperature range from 750 to 1050 °C are shown in Fig. 11. The initial glass and the glass heat-treated at the nucleation stage (at 750 °C for 6 h) are X-ray amorphous. The color change (Fig. 2) and density increase (Fig. 8) after the heat treatment at 750 °C suggest amorphous phase separation of the initial glass. This assumption will be confirmed below using TEM, Raman and optical spectroscopy data.

The XRD patterns show that after two-stage heat-treatments with the second stage in the temperature range of 800-1000 °C, two crystalline phases, MAT ss and spinel, crystallize and grow. With increasing the heat-treatment temperature, intensities of the peaks on the XRD patterns increase, and the intensity of the amorphous halo decreases. It means that the crystallinity fraction in GCs increases with the heat-treatment temperature. When the temperature rises from 800 to 1000 °C, the spinel mean crystal size grows from 37 to 74 Å, while the size of MAT crystals increases from 64 to 206 Å (see Table 1). The spinel unit cell parameter a increases with the heat-treatment temperature from 8.002 Å (at 800 °C) to 8.079 Å (at 1000 °C). As it was mentioned before, there exists an infinite solid solution in the MgAl2O4-FeAl2O4 system with the unit cell parameter a variation from 8.0855 to 8.1646 Å [START_REF] Andreozzi | Intersite distribution of Fe 2+ and Mg in the spinel (sensu stricto)-hercynite series by single-crystal X-ray diffraction[END_REF]. Therefore, one would expect larger spinel unit cell parameter in GCs. The low value of the parameter a implies that the composition of the spinel under study is enriched in alumina, which agrees with our previous observations [START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF]. An increase of the lattice constant of spinel under study with the heat-treatment temperature could be explained mainly by the decrease of the excess of aluminum in the spinel composition and additionally by an increase of the order in the spinel structure [START_REF] Loiko | Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1[END_REF].

As soon as the crystals of spinel and MAT ss are formed at 800 °C, there is a shift of the amorphous halo position to smaller angles, which manifests a change in the composition of the residual glass. With increasing the heat-treatment temperature, the position of the amorphous halo becomes close to that of the vitreous silica, as the residual glass composition becomes more and more silica enriched.

In the temperature range of 1050-1100 °C, sapphirine, the solid solution with a composition lying between 2MgO•2Al2O3•1SiO2 and 7MgO•9Al2O3•3SiO2 [START_REF] Higgins | Sapphirine I. Crystal chemical contributions[END_REF] is also formed. After the two-stage heat treatment with the second hold at 1050 °C, the GC is transparent, while after the heat treatment at 1100 °C, it is translucent. It is the first time to our knowledge that transparent GC based on sapphirine are prepared (see Fig. 2 and Fig. 11, 1050 °C).

A typical TEM image of the initial glass, Fig. 12(a), shows amorphous phase separation in accordance with our previous studies [START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF]. Note that the XRD pattern of the sample indi-cates the lack of any crystalline phase. The phase separation develops after the nucleation stage of the heat treatment at 750 °C, Fig. 12(b). The phase separated regions (mean size: DTEM ~3.3 nm) are still X-ray amorphous. In the TEM image of the GC sample obtained at 950 °C, nanocrystals (DTEM ~6.4 nm) with a rectangular and elongated shape are observed. They are most probably the spinel nanocrystals. Indeed, the corresponding DXRD = 7.4 nm (spinel) and 20.6 nm (MAT ss), see Table 1.

The complex character of the density variation with the heat-treatment temperature, when the density first increases and then slightly decreases, cf. Fig. 8, is related to a complex structure of the GCs described above and, in particular, to different densities of the spinel and MAT solid solutions and the amorphous phase with composition close to the vitreous silica, whose ratio changes with temperature. In addition, the composition of solid solutions also change in a complex way. The density increase is observed for the GC prepared at 1050 °C and containing sapphirine nanocrystals and increased fraction of MAT ss, Fig. 10. It could not be associated with the crystallization of sapphirine as the densities of spinel (~3.58 g/cm 3 [START_REF] Ganesh | A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications[END_REF]) and sapphirine (3.40-3.58 g/cm 3 , depending on its composition [START_REF]Handbook of Mineralogy[END_REF]) are similar. Thus, this effect is probably connected with increased fraction of the MAT ss containing Fe ions (see below). Note that the armalcolite density is ~4.94 g/cm 3 [START_REF]Handbook of Mineralogy[END_REF].

Raman spectra

The Raman spectra of the initial and heat-treated glasses demonstrate the structural evolution of the material, see Fig. 13(a). The spectrum of the initial glass contains two broad bands, one with a maximum at 478 cm -1 , and another of a complex shape with several maxima at 802, 920 and 1025 cm -1 . The bands at 478, 802 and 1025 cm -1 are attributed to vibrations of tetrahedra of the aluminosilicate network. The very intense band at 920 cm -1 is attributed to vibrations of the [TiO4] tetrahedra incorporated into the aluminosilicate network [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF].

After the heat-treatment of the glass at 750 °C for 6 h (the nucleation stage), its Raman spectrum changes. The broad band at 478 cm -1 shifts to 466 cm -1 and the band intensities in the high-frequency region are redistributed: the band at 920 cm -1 shifts to 910 cm -1 and attenuates, and the band at 800 cm -1 shifts to 814 cm -1 and enhances. These changes are caused by development of the phase separation of the glass: the magnesium aluminotitanate amorphous phase (with a characteristic band at 810 cm -1 ) and the phase enriched in magnesia and alumina are formed in the residual silica-rich phase (the manifestation of the latter phase is the shift of the band characteristic for the glass network from 478 cm -1 to 466 cm -1 ). Thus, the socalled three-phase immiscibility [START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF] is developed. The high intensity of the 814 cm -1 band can be explained by the superposition of vibrations of the [SiO4] tetrahedra and those of the Ti-O bonds in [TiO]5 and in [TiO]6 polyhedrons in the amorphous magnesium aluminotitanate phase [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF].

The Raman spectra of GCs prepared by two-stage heat treatments are different from those previously discussed. In the Raman spectrum of GC prepared at 750 °C (the first stage) and at 800 °C (the second one), there is a broad band at 450 cm -1 (note that for pure SiO2, it is located at 440 cm -1 ), an intense band of 810 cm -1 with an inflection at 915 cm -1 , and a number of weak bands at 148, 285 and ~690 cm -1 . These changes indicate further phase separation of the glass and beginning of the MAT ss crystallization. The bands at 148, 285, 603 and ~690 cm -1 and the intense band at 810 cm -1 which is superimposed on the band of the amorphous magnesium aluminotitanate phase, can be attributed to the spectrum of the MAT ss with a rather high alumina content [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF]. After further increasing the temperature of the second stage of the heat treatment to 850 °C, the bands at 219, 272, 385, ~487, ~690 and 802 cm -1 appear in the Raman spectrum that can be attributed to the MAT ss crystals. Note that the weakest bands in the frequency range of 334-496 cm -1 are superimposed on the band at ~450 cm -1 originating from the residual glass phase, and it is difficult to determine their position more precisely. With a successive increase in the heat-treatment temperature of the glass from 900 to 1000 °C, almost all bands of the MAT ss are observed in the Raman spectrum. They are located at 167, 209, 261, ~375, ~487, 668, 789 and ~900 cm -1 (a weak wing of the intense band near 800 cm -1 ), and their positions slightly vary reflecting the variation of the ratio of magnesium and aluminum in the MAT ss at the particular temperature. After the heattreatment with the second stage at 1050 °C, the bands are shifted to higher frequencies of 170, 215, 266, ~383, ~487, 681, and 797 cm -1 , which indicates an additional enrichment of the solid solution with alumina (note that the weak band at 456 cm -1 does not belong to the spectrum of the MAT ss).

We speculate that iron ions Fe 2+ can enter the crystals of MAT ss. There exist a mineral armalcolite, which has a general chemical formula of (Mg 2+ ,Fe 2+ )Ti2O5. Together with MAT, it belongs to the pseudobrookite group. The Raman spectrum of armalcolite [START_REF] Kawasaki | Possible armalcolite pseudomorph-bearing garnet-sillimanite gneiss from Skallevikshalsen, Lützow-Holm Complex, East Antarctica: Implications for ultrahigh-temperature metamorphism[END_REF] is similar to that of the MAT ss. Thus, one can expect that entering the Fe 2+ ions into the structure of MAT ss should not disturb the structure and will not cause a serious change of the Raman spectrum.

After the two-stage heat treatments with a second stage at 950-1000 °C, a weak and then more distinct maximum appears at 409 cm -1 , which corresponds to the most intense band of spinel [START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. Note that, apparently, due to the large difference in the scattering cross sections for crystals of MAT ss and spinel, in the Raman spectra of the heat-treated glass the bands of MAT ss are always the most intense, and spinel is hard to be observed.

After increasing the heat-treatment temperature to 1050 °C, according to XRD data, Fig. 11, sapphirine crystals are detected in addition to spinel and MAT ss. We tried to find any signs of sapphirine in this Raman spectrum. In the RRUFF database [93], there are Raman spectra of several sapphirine minerals with peak positions at about 220, 410, 495, 565, 685, 750, 825, 915 and 985 cm -1 . Two of these bands are the most intense, i.e., at 565 and 685 cm -1 , of which the second is about 1.6 times more intense than the first. Figure 13(a) shows that the position of the most intense Raman band of sapphirine at 685 cm -1 nearly coincides with the position of the band of MAT ss at 681 cm -1 . A very weak trace of the second most intense sapphirine band is found at 562 cm -1 . The trace of the band at ~ 409 cm -1 is the superposition of spinel and sapphirine bands. The sapphirine band of intermediate intensity at 915 cm -1 is superimposed on the high-frequency tail of the intense band of the MAT ss at 795 cm -1 . Thus, there is no significant impact of sapphirine crystallization on the Raman spectrum of GC since intensities of all the bands do not change noticeably when comparing the spectrum of the GC prepared at 1000 °C with that of the GC prepared at 1050 °C.

Low-frequency Raman spectra

The Raman spectra of the initial glass and the glass subjected to heat-treatment at the nucleation stage (at 750 °C for 6 h) did not show any low-frequency band. This band appears after two-stage heat-treatments and its position shifts to lower frequencies with increasing the heat-treatment temperature, Fig. 13(b).

There is a correlation between the position of the low-frequency Raman band and the size of inhomogeneous regions [START_REF] Tamura | Lattice vibrations and specific heat of a small particle[END_REF][START_REF] Petrov | Raman scattering by acoustic phonons of TiO2 submicrocrystals in glasses[END_REF]

: , 2 s 02 s 02 c R l ⋅ = π υ ξ ν (3)
where ν s 02 is the frequency of the spheroidal vibration mode corresponding to the lowfrequency peak, which is mostly active in Raman scattering, ξ s 02 is the phenomenological coefficient depending on the relation between the transversal, υt, and longitudinal, υl, velocities of sound in the inhomogeneous regions and on the particle interface, R is the radius of the inhomogeneous region (assuming its nearly-spherical shape) and c is the speed of light. The mean size (DRaman) of inhomogeneous regions is [START_REF] Champagnon | Size determination of semiconductor nanocrystallites by low frequency inelastic scattering (LOFIS)[END_REF]:

. 8 . 0 s 02 Raman ν υ c D l ≈ (4)
Our earlier study of phase separation in heat-treated magnesium aluminosilicate glasses nucleated by TiO2 using small-angle X-ray scattering [START_REF] Golubkov | On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system[END_REF][START_REF] Chuvaeva | Low-frequency Raman scattering of magnesium aluminosilicate glasses and glassceramics[END_REF] showed that a bidispersed system of inhomogeneities is formed at the initial stages of phase separation. The sizes of the smaller regions were found to be consistent with the sizes obtained from the position of the low-frequency Raman band under an assumption that at the beginning of the process, these are amorphous silicate inhomogeneities enriched in magnesium and aluminum, in which spinel crystallizes with increasing the heat-treatment temperature. Large-size inhomogeneities, at first also of amorphous nature, form magnesium-aluminum-titanate phase, from which the MAT ss crystallizes with increasing the heat-treatment temperature.

The sound velocity for spinel crystals υl = 10×10 5 cms -1 [START_REF] Mason | Physical Acoustics, Applications to Quantum and Solid State Physics[END_REF]. When calculating the sizes of inhomogeneous regions DRaman listed in Table 1, we have chosen a slightly lower sound velocity, υl = 8×10 5 cms -1 , since a certain amount of SiO2 is always present in magnesium aluminate regions of inhomogeneity. For SiO2, υl is less than 6×10 5 cms -1 [START_REF] Mason | Physical Acoustics, Applications to Quantum and Solid State Physics[END_REF]. The results presented in Table 1 allowed us to conclude that these regions of inhomogeneity are not completely crystallized, since the diameter of spinel crystals according to the XRD and TEM data is approximately half of that calculated from the position of the low-frequency Raman band.

Optical absorption

The absorption spectra of the initial glass and GCs are presented in Fig. 14(a-c). For the initial glass, the UV absorption edge is observed at λUV = 0.34 μm. The spectrum contains a broad band in the range of 400-600 nm, a maximum of which is superimposed with the absorption edge, a weak and broad band at 0.75-1.5 μm with a peak at ~1.05 μm and a weak shoulder extending until 2.5 μm, and a broad asymmetric band in the in the range of 2.7 -3.1 μm. The spectrum of the glass heat-treated at 750 °C for 6 h (the nucleation stage) is different from the spectrum of the initial glass mainly in the UV and visible ranges: the absorption edge is red-shifted by about 7 nm, and a broad band in the visible intensifies and becomes broader, it now spans until 700 nm.

The spectrum of the GC prepared by the two-stage heat-treatment with the second stage at 800 °C demonstrates a similar evolution: the absorption edge is further red-shifted by about 6 nm, and a broad band in the visible spectral range intensifies and becomes even broader now spanning until 900 nm.

With increasing the temperature of the heat-treatment at the second stage (850-1000 °C), the spectra of the GCs change in a different way. The slope of the UV absorption edge becomes smoother, while its position is slightly changing in the range of λUV = 0.34-0.35 μm. There is also a great raise of absorption in the visible with a maximum at ~0.55 μm and in the near IR, and an increase in intensity of the broad absorption band at 0.75-1.5 μm, Fig. 14(b). A broad band at 1.4-2.5 μm with a maximum at ~1.88 μm appears and grow with increasing the heat-treatment temperature. The mid-IR absorption is structured with two components at 2.73 and 2.92 μm, Fig. 14(c).

Finally, for the GC obtained at 1050 °C, the absorption edge experiences a notable redshift to λUV = 0.37 μm. The absorption at ~1.88 μm decreases. Simultaneously, the absorption in the visible and at ~1.1 μm raises.

The interpretation of the absorption spectra is complicated because in the initial glass and in GCs, there are two polyvalent transition metal ions that can be responsible for light absorption, i.e., titanium and iron ones, and both ions participate in phase transformations caused by heat-treatments, which alternates their surrounding.

In general, titanium ions are found in glasses and minerals under two oxidation states of Ti 3+ (3d 1 ) and Ti 4+ (3d 0 ). The absorption band due to the 2 T2g → Eg transition of the Ti 3+ ion in Oh site symmetry is located at about 500 -625 nm depending on the ligand field strength and distortions of its symmetry [START_REF] Mao | Crystal-field effects of trivalent titanium in fassaite from the Pueblo de Allende meteorite[END_REF]100], while the band due to the Eg → 2 T2g transition of Ti 3+ in Td site symmetry is located at about 1000 nm [100,101]. The O-Ti 3+ OMCT band is predicted in the UV spectral range at about 240 nm [102]. Ti 4+ ions do not exhibit any d-d transition while they are responsible for the O-Ti 4+ OMCT band which is located in the UV spectral range at about 300 nm [102], as well as participate in homonuclear IV CT Ti 4+ /Ti 3+ and heteronuclear Fe 2+ + Ti 4+ → Fe 3+ + Ti 3+ transitions that give rise to intense absorption bands in the visible spectral range. In different minerals, IV CT Ti 4+ /Ti 3+ transition gives rise to a band at about 480 nm [103], 660 -670 nm [104].

Iron ions in glasses exist under two oxidation states, ferrous, Fe 2+ (3d 6 ) and ferric, Fe 3+ (3d 5 ). The analysis of the absorption spectra caused by iron ions in glasses is difficult because all transitions of Fe 3+ and Fe 2+ ions, except of those giving rise to bands in the region of 1 and 2 µm, are spin-forbidden and therefore have weak intensities; the corresponsing bands are superimposed on very intense (especially in case of Fe 3+ ions) IV CT (at least 10 times more intense) and OMCT (100 to 1000 more intense) bands. Moreover, Fe 3+ and Fe 2+ ions often coexist, they can be fourfold-, fivefold-and sixfold-coordinated, and similar forbidden bands fall into the same spectral range and overlap [101,105]. For the assignment of absorption bands in the spectra of glasses, a comparison with absorption spectra of minerals with wellestablished structure is employed [105], additional methods of analysis are used, i.e., highresolution X-ray absorption near-edge structure (XANES) spectra [106] or Mössbauer spectra [100,107].

In the spectra of ferric ions in silicate glasses, the OMCT Fe 2+ -O band is located at about 235 nm, while weak spin-forbidden bands can be found around 21500 and 23400 cm -1 (at 465 and 430 nm, respectively) [105] and are assigned to the Fe 2+ spin-forbidden transitions 5 T2(D) → 3 T1(H) and 5 T2(D) → 3 T2(H), respectively. The broad bands at about 1000 and 2000 nm are attributed to the spin-allowed electronic d-d transitions 5 T2 → 5 E and 5 E → 5 T2 for octahedrally and tetrahedrally coordinated Fe 2+ ions, respectively [100,101].

In the spectra of ferric ions in silicate glasses, the OMCT Fe 3+ -O band is located at about 270 nm [105] and it spans until the visible spectral range. It often even overlaps with peaks located at about 370, 380, 420, 440, and 480 nm and assigned to d-d transitions of [4] Fe 3+ , [5] Fe 3+ and [6] Fe 3+ ions [105].

Fe 2+ -Fe 3+ and Fe 2+ -Ti 4+ coordination clusters may coexist in the same structure, and give rise to homonuclear Fe 2+ → Fe 3+ and heteronuclear Fe 2+ → Ti 4+ intervalence transitions in the visible region, thereby complicating the assignment of absorption bands [104]. The Fe 2+ -Ti 4+ IV CT transition is believed to contribute to the UV-visible spectra of synthetic Fe-Ti silicate glasses [108]. In [100], it was found at about 450 nm contributing to the absorption edge of the glass. IV CT Fe 2+ →Fe 3+ transitions are observed in the range of 540 -660 nm, as well as at around 730-770 nm [104].

The spectral changes observed upon the heat-treatments of the initial glass under study confirm that the titanium and iron ions are involved in phase transformations. The absorption edge of the initial glass is caused by the O-Ti 4+ , O-Fe 2+ , and O-Fe 3+ OMCT overlapping spectral bands. According to Mysen [109], in magnesium aluminosilicate glasses, iron exists in the forms of ferrous (Fe 2+ ) and ferric (Fe 3+ ) ions with 4-, 5-, and 6-fold coordination states. The absorption band in the range of 400-600 nm is probably due to spin-forbidden d-d absorption bands of ferrous and ferric ions [105], while the weak and broad absorption band at 0.75-1.5 μm with a peak at ~1.05 μm can be associated with Fe 2+ ions in Oh sites in the glass network. A weak shoulder extending until 2.5 μm can be connected with a minor fraction of Fe 2+ ions in Td sites. A broad asymmetric band in the in the range of 2.7 -3.1 μm is caused by very intense absorption bands due to O-H vibrations of hydroxyl groups in glass.

The variation of the absorption spectrum after the heat-treatment at the nucleation stage is connected with the development of the three-phase immiscibility. Ferrous ions probably enter magnesium aluminate and magnesium aluminotitanate amorphous regions resulting in Fe 2+ + Ti 4+ → Fe 3+ + Ti 3+ IVCT transitions.

The red shift of the absorption edge and a great raise of absorption in the visible spectral range after secondary heat treatments is consistent with crystallization of spinel and MAT ss and possible entering of iron ions into these phases. Absorption spectra of iron-doped spinels are well-documented and discussed [START_REF] Gaffney | Spectra of Tetrahedral Fe 2+ in MgAl2O4[END_REF][START_REF] Slack | Optical absorption of tetrahedral Fe 2+ (3d 6 ) in cubic ZnS, CdTe, and MgAl2O4[END_REF][START_REF] Taran | Electronic absorption spectroscopy of natural (Fe 2+ , Fe 3+ )-bearing spinels of spinel ss-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures[END_REF][START_REF] Jackson | Visible-infrared spectral properties of ironbearing aluminate spinel under lunar-like redox conditions[END_REF][START_REF] Hålenius | Influence of cation distribution on the optical absorption spectra of Fe 3+ -bearing spinel ss-hercynite crystals: evidence for electron transitions in VI Fe 2+ -VI Fe 3+ clusters[END_REF][START_REF] Rossman | Spectroscopic standards for four-and fivefold-coordinated Fe 2+ in oxygen-based minerals[END_REF][START_REF] Taran | Electronic absorption spectra of Fe 2+ ions in oxygen-based rockforming minerals at temperatures between 297 and 600 K[END_REF][START_REF] Skogby | An FTIR study of tetrahedrally coordinated ferrous iron in the spinel-hercynite solid solution[END_REF][START_REF] Ippolito | Color mechanisms in spinel: cobalt and iron interplay for the blue color[END_REF][START_REF] Dickson | Low-temperature optical absorption and Mössbauer spectra of staurolite and spinel[END_REF]110]. The broad absorption bands in the visible part of spectra of Fe-doped spinels are usually attributed to different spinforbidden d-d transitions of Fe 2+ and Fe 3+ ions in the spinel ligand field [START_REF] Taran | Electronic absorption spectroscopy of natural (Fe 2+ , Fe 3+ )-bearing spinels of spinel ss-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures[END_REF]110], while the additional weak band at ~15000 cm -1 (~670 nm) can be assigned to the Fe 2+ -Fe 3+ exchange interaction [110]. In the spectra of GCs under study we can undoubtedly see the signs of Fe 2+ and Fe 3+ ions in the spinel structure, as well as IV CT transitions associated with Ti 3+ /Ti 4+ , Fe 2+ /Fe 3+ and Fe 2+ + Ti 4+ → Fe 3+ + Ti 3+ . It should be noted that broad d-d absorption bands of Ti 3+ ions in synthetic magnesium aluminate spinel are localized in the spectral region of 510 and 760 nm [111]. We cannot exclude their appearance in absorption spectra of GCs under study. An appearance and growth of the bands in the IR spectral range is due to the entry of Fe 2+ ions into spinel nanocrystals in both the tetrahedral (predominantly) and octahedral positions. The mid-IR absorption with two components at 2.73 and 2.92 μm is developed on the basement of the broad asymmetric unstructured band of OH -groups in the residual glass. It is interesting to note that the position of the short wavelength component of this band experiences a gradual blue-shift from 2.8 to 2.73 μm with increasing the temperature of the heattreatment, Fig. 14(c). We connect the structuring of the absorption bands due to O-H vibrations with spinel crystallization because it is observed only in the spectra of samples containing spinel nanocrystals. Note that in synthetic spinel, the structuring of the band associated with OH groups was observed and assigned to OH groups entering the spinel structure [112,113]. However, the positions of these bands at 2985 nm (3350 cm -1 ) and 2818 nm (3548 cm -1 ) are different from those observed in our GCs while the reason for this discrepancy still remains unclear.

For the GC obtained at 1050 °C, a notable red-shift of the UV absorption edge reflects an increase in light scattering by relatively large sapphirine crystals. The absorption at ~1.88 μm associated with Fe 2+ ions in Td sites in spinel decreases. Simultaneously, the absorption in the visible and at ~1.1 μm raises. This correlates with the decrease in the amount of precipitated spinel and appearance of sapphirine. In sapphirine, Mg 2+ ions are in sixfold coordination while Al 3+ ions are in four-and sixfold coordinated sites [114]. Sapphirine accommodates iron ions and exhibits the substitutions Mg → Fe 2+ (predominantly) and Al → Fe 3+ [113] with Fe 3+ ions assigned to tetrahedral positions [115]. Thus, upon sapphirine formation from spinel and residual highly siliceous glass, iron ions are present as the VI Fe 2+ and IV Fe 3+ species, which is reflected in absorption spectra.

Due to the complex distribution of iron ions over several phases, as well as the presence of iron species with different coordination and valence states, it is very difficult to make a reliable estimation of the fraction of iron ions located in spinel nanocrystals. The determination of the ground-state absorption cross-section for Fe 2+ ions in MgAl2O4 (e.g., in an absorption saturation experiment) may facilitate such estimates.

Luminescence

The luminescence spectrum of GCs (Fig. 15) strikingly resembles that of ceramic (Fig. 7) that was assigned to impurity Cr 3+ ions in Oh sites of spinel. This means that the evolution of the luminescence properties on the initial glass with the heat-treatment should be connected with impurity Cr 3+ ions located in different phases. Let us discuss the spectralluminescence properties of Cr 3+ ions in the initial glass and in GCs. In oxide glasses, Cr 3+ ions occupy a variety of sites with different crystal field strengths due to site variability and compositional disorder. Cr 3+ is generally recognized as being in Oh symmetry with certain distortions [116]. According to [117], for trivalent chromium in octahedral coordination, the 4 T2g → 4 A2g transition is typical in the low crystal field materials with a substantial participation of the site-to-site disorder, i.e., in glasses. The Cr 3+ luminescence in silicate glasses in low-crystal field is at ~800 nm [116,118,119]. The luminescence spectrum of the initial glass in the visible contains a weak broad band with a maximum at about 750 nm (Fig. 15). After the heat-treatment at 750 °C, the intensity of the luminescence band increases. A disproportional increase in intensity of the short-wavelength wing changes the shape of the luminescence band, while the peak position at about 750 nm does not change. After two-stage heattreatments and with increasing the temperature at the second stage (800-1050 °C), the luminescence spectra of the GCs further change. Sharp peaks with maxima at 667, 676, 688, 698, 709, and 717 nm are formed on the basement of the broad unstructured band. Their intensities sequentially increase with the heat-treatment temperature in the range of 800-1000 °C while the intensity of the broad unstructured band has a non-monotonic behavior. The maximum intensity of the latter band is observed in GC prepared by the heat-treatment at the second stage of 850 and 900 °C, then it slightly decreases and remains unchanged for GCs prepared at 950-1050 °C (Fig. 16). The intensities of the peaks with maxima at 667, 676, 688, 698, 709, and 717 nm slightly decrease in the spectrum of the GC prepared at 1050 °C.

The luminescence spectra of GCs prepared by heat-treatments at the second stage at 850 -1050 °C can be considered as a superposition of spectral features of the Cr 3+ :MgAl2O4 luminescence and the band underlying these features and the longer wavelength band attributed to high-and low-crystal field Cr 3+ sites, respectively, in the amorphous residual phase [116,119].

Discussion

In this section, we aimed to describe our results by comparing the structure of ceramic and GC and the behavior of iron and impurity chromium ions in these two hosts, wherever it is possible. Previously, we used this approach in the study of Co:ZnAl2O4 transparent ceramic and GC [START_REF] Loiko | Synthesis, characterization and absorption saturation of Co:ZnAl2O4 (gahnite) transparent ceramic and glass-ceramics: A comparative study[END_REF]. Such a comparison clearly demonstrates the characteristic features of both materials. The XRD pattern of ceramics presents a single-phase spinel material. The XRD pattern of the Fe:MgAl2O4-based GC reveals a multiphase material exhibiting the strongly broadened diffraction peaks of spinel located at the same positions as for the ceramic, as well as broad peaks of low intensity due to precipitation of MAT ss nanosized crystals and a massive halo due to a high content of the residual highly siliceous glass, Fig. 16(a). A relatively low spinel crystalline fraction in GC becomes evident.

A comparison of Raman spectra of ceramic and GC helped us to find characteristic vibrations of spinel nanocrystals in the complex Raman spectrum of GC containing the features of the MAT ss and those of the residual glass. Previously, we were certain that spinel vibrations are too weak in intensity to be noticed in the spectra of GCs at the background of in-tense vibrations of MAT ss [START_REF] Dymshits | Raman spectroscopic study of phase transformations in titaniumcontaining magnesium aluminosilicate glasses[END_REF]. The most intense spinel band located at 409 cm -1 can be easily found because in this spectral range there are no modes of the MAT ss. Moreover, we managed to find the other bands typical for spinel nanocrystals only by comparison with the spectrum of ceramics. Now we can notice the peaks at 200, 251, 306, 409, 496, 668, 724 and 764 cm -1 in the spectrum of GC and attribute them to vibrations connected with spinel crystals, Fig. 16(b).

The comparison of absorption spectra of ceramic and GC, Fig. 16(c), clearly indicates the presence of iron ions in ceramics only in one oxidation state, Fe 2+ , and predominantly in the Td sites as opposed to multiple valence states and different coordinations in GCs. This difference is the result of different phase compositions of the materials. The ceramic is a single-phase spinel while the GCs are multiphase materials containing spinel nanocrystals. Indeed, the glass under study demonstrates the three-phase immiscibility, i.e., formation of the magnesium aluminotitanate amorphous phase and an amorphous phase enriched in magnesia and alumina. These phases are located in the residual silica-rich glass. In fact, Fe ions are distributed between these three phases according to a partition coefficient. During further crystallization within amorphous regions enriched in magnesia and alumina and magnesium aluminotitanate amorphous regions, iron ions become a component of the spinel and MAT ss crystals, as well as remain in the residual glass. In the GCs, iron ions are located in spinel nanocrystals as IV Fe 2+ and VI Fe 2+ , IV Fe 3+ and VI Fe 3+ species, in the MAT ss probably as IV Fe 2+ species and predominantly in the form of Fe 3+ species in the residual glass enriched in silica. The spinels in the ceramic and GCs are very different due to their different structures (i.e., different degree of inversion determined by composition and fabrication conditions), compositions (in GCs as opposed to ceramics, there are solid solutions of spinel enriched in alumina and containing titanium ions) and sizes (nm-sized in GCs vs. μm-sized in ceramic).

The luminescence spectra of both materials are very similar, Fig. 16(d). They are formed by luminescence of Cr 3+ ions substituting for Al 3+ ions in spinel crystals. The only difference in the spectra is the presence of a broad unstructured band with two maxima at about 720 and 750 nm observed in the spectrum of GCs. It is connected to a multiphase nature of GCs: this broad unstructured luminescence band is typical for Cr 3+ ions in glass and reflects the presence of the residual glass in GCs. The maximum at about 720 nm on this broad band can be connected with the luminescence of Fe 3+ ions in Oh symmetry in the Al 3+ position in spinel nanocrystals in GCs [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF][50][START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF].

We are aware that spinel ceramic often contains an admixture of secondary phases, i.e., kyanite (Al2O3•SiO2) [START_REF] Bubnova | Luminescence analysis of ceramic magnesium aluminum spinel Fe 2+ :MgAl2O4 synthesized from nanosized powders via syntering in air and vacuum[END_REF], (MgO)0.91(FeO)0.09 or periclase (MgO) [START_REF] Osipov | Fabrication and characterization of IR-transparent Fe 2+ doped MgAl2O4 ceramics[END_REF]50]. These phases are manifested by luminescence of Fe 3+ ions. The absence of Fe 3+ ion luminescence in spinel ceramics proves that it is a single-phase material that does not contain Fe 3+ ions.

Let us also comment on the synthesis method of our ceramics. In contrast to [START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF] where the Fe 2+ :MgAl2O4 powders were obtained by laser ablation followed by free vacuum sintering, in our work the powders were synthesized by sol-gel method, and their consolidation was carried out by hot pressing in the presence of LiF sintering additive. This approach made it possible to obtain higher optical quality ceramics, which is especially noticeable in the visible and near-IR wavelength ranges. In addition, the main part of the iron remained in the +2 oxi-dation state in Td sites. Thus, at close initial concentration of iron in spinel powders as compared to [START_REF] Dugué | In situ evolution of Ni environment in magnesium aluminosilicate glasses and glass-ceramics-Influence of ZrO2 and TiO2 nucleating agents[END_REF], the absorption of Fe 2+ ions in our ceramics is several times greater, see Fig. 6(a).

Conclusions

We fabricated and studied the structure, phase transformations, optical and spectroscopic properties of the Fe:MgAl2O4 (spinel) transparent ceramic and Fe:MgAl2O4-based transparent nanophase glass-ceramics. The main difference of two materials is their phase composition which determines the valence state and coordination of the iron ions leading to different spectroscopic properties. The 0.1 mol% Fe:MgAl2O4 ceramic obtained by hot-pressing at 1600 °C / 50 MPa of powders obtained by the sol-gel method and using LiF as a sintering aid is a single-phase material being isostructural to undoped spinel (sp. gr. Fd3‾m, the lattice constant a = 8.083±0.003 Å). Its microstructure is determined by relatively large grains (mean size: 50 μm) with clean grain boundaries and the lack of any secondary phases at their boundaries. The iron ions are replacing the Mg 2+ ones in tetrahedral (Td) sites. The optical absorption and visible luminescence studies indicated that the content of the VI Fe 2+ and Fe 3+ species, if present, is very low. The absorption of the Fe:MgAl2O4 ceramic is determined by a broad and intense band related to the 5 E → 5 T2 ( 5 D) transition of Fe 2+ ions in Td sites and corresponding to a ground-state absorption cross section of 0.28×10 -18 cm 2 at the peak wavelength of 1.90 μm. This absorption band is similar in shape to that observed in Fe 2+ -doped zinc chalcogenides (ZnS, ZnSe) while is blue-shifted. This is for the first time that transparent Fe:MgAl2O4-based glass-ceramics were obtained. They were fabricated by secondary two-stage heat-treatment of the initial magnesium aluminosilicate glass nucleated by 10 mol% TiO2 and doped with 0.1 mol% FeO. A complex picture of phase transformations in the initial glass and further in the GCs was determined. For the second stage of the heat-treatment at 800 -1000 °C for 6 h (the first, nucleation, stage, was at 750 °C for 6 h), nanosized spinel crystals (mean size: 3.7 -7.4 nm, lattice constant: a = 8.002 -8.079 Å, both increasing with the heat-tretment temperature) precipitated. The transparent GCs were multi-phase materials containing, together with the spinel nanophase magnesium aluminotitanate solid solution (MAT ss) nanocrystals (mean size: 6.4-20.6 nm), as well as enriched in silica residual glass phase. The iron ions in these GCs were present as IV Fe 2+ , VI Fe 2+ and Fe 3+ species in the spinel structure, as well as Fe 2+ and Fe 3+ species in magnesium aluminotitanate solid solutions and in the residual glass. The presence of Ti 3+ and Ti 4+ ions (from the crystallization catalyst) determined the absorption of GCs in the UVvisible spectral range. Absorption is partly caused by interplay of iron and titanium ions in different valence states, coordination sites, and locations while the Cr 3+ ions in spinel and in the residual glass (an uncontrolled impurity) determined the spectrum of visible luminescence.

In the present paper, we focused on revealing the nature of the optically active iron centers in spinel transparent ceramic and GCs. This is of key importance for potential applications of such materials, e.g., as laser gain media and saturable absorbers of mid-infrared lasers emitting at the wavelength of 2-3 μm. For applications, particularly the Fe 2+ ions in Td sites are preferable. Besides, one cannot neglect the difference in the thermo-mechanical properties of the host materials and especially their resistance to the laser-induced damage which is much higher for GCs. Further work on Fe:MgAl2O4 based transparent materials should focus on revealing their mid-infrared emission properties (the spectrum and the lifetime), as well as the saturable absorption properties. The latter may serve as an independent tool to confirm the ground-state absorption cross sections for IV Fe 2+ species in spinel. *DXRD -size determined from the XRD studies using the Scherrer formula, Eq. ( 1); DRaman -size determined from small-frequency Raman spectra, Eq. ( 4); DTEM -size determined from TEM images.

List of figure captions

Figure 1 .

 1 Figure 1. Photograph of the laser-grade-polished undoped MgAl2O4 (left) and 0.1 mol% Fe:MgAl2O4 (right) transparent ceramic disks.

Figure 2 .

 2 Figure 2. Photographs of the initial glass and transparent GCs obtained by various secondary heat-treatments, (temperature, °C / duration, h).

Figure 3 .

 3 Figure 3. XRD patterns of an undoped MgAl2O4 ceramic and a 0.1 mol% Fe:MgAl2O4 ceramic; numbers denote the Miller's indices (hkl). The patterns are shifted for the convenience of observation. Red peaks -theoretical pattern for MgAl2O4 (ICSD card No. 82-2424).

Figure 4 .

 4 Figure 4. (a) SEM image of a fracture surface of a 0.1 mol% Fe:MgAl2O4 ceramic; (b) a typical grain size distribution.

Figure 5 .

 5 Figure 5. Unpolarized Raman spectra of an undoped MgAl2O4 ceramic and a 0.1% Fe:MgAl2O4 ceramic, λexc = 514 nm, numbers denote positions of the Raman peaks in cm -1 .

Figure 6 .

 6 Figure 6. (a) In-line transmission spectra of laser-grade-polished undoped MgAl2O4 and 0.1 mol% Fe:MgAl2O4 ceramic disks (t = 1.0 mm), green curve indicates the theoretical Fresnel losses; (b) the absorption cross sections, σabs, for the 5 E → 5 T2( 5 D) transition of Fe 2+ ions in Td sites in MgAl2O4 ceramic, ZnSe and ZnS materials.

Figure 7 .

 7 Figure 7. Luminescence spectrum of the 0.1 mol% Fe:MgAl2O4 ceramic. The excitation wavelength is 488 nm. Numbers denote the positions of the emission peaks in nm.

Figure 8 .

 8 Figure 8. Variation of density with the heat-treatment temperature for the initial glass and glass-ceramics (for the samples heat-treated at 800 °C and above, the first stage is at 750 °C). The line serves as a guide for the eye. The error bars match the size of symbols.

Figure 9 .

 9 Figure 9. DSC curves for the initial glass and the glass heat-treated at 750 °C for 6 h. Tg is the glass transition temperature, Ton is the crystallization onset temperature. The curves are shifted for the convenience of observation.

Figure 10 .

 10 Figure 10. XRD patterns (a) of the initial glass heat-treated in the DSC furnace up to 915 and 1160 °C; (b) of the sample preheated at 750 °C for 6 h and heat-

Figure 11 .

 11 Figure 11. XRD patterns of 0.1% Fe-doped the initial glass and transparent GCs. Labels 800 -1050 °C indicate the heat-treatment temperature at the second stage (holding time: 6 h). The nucleation stage is at 750 °C for 6 h. o -spinel, x -magnesium aluminotitanate (MAT) solid solution (ss), v -sapphirine. The patterns are shifted for the convenience of observation.

Figure 12 .

 12 Figure 12. TEM images of 0.1 wt.% Fe-doped (a) initial glass, (b) glass heattreated at 750 °C for 6 h, and (c) glass heat-treated at 750 °C for 6 h and at 950 °C for 6 h. Insets in (b),(c) -size distributions of the nanocrystals. Note the different magnification.

Figure 13 .

 13 Figure 13. Unpolarized Raman spectra of the initial glass and transparent glassceramics: (a) the overview of the frequency range of 100-1300 cm -1 , (b) a close look at the frequency range of 5-350 cm -1 with a notch filter. Labels 800 -1050 °C indicate the heat-treatment temperature at the second stage. λexc = 514 nm, numbers denote the position of the Raman peaks in cm -1 . The curves are shifted for the convenience of observation.

Figure 14 .

 14 Figure 14. Absorption spectra of the initial glass and transparent glass-ceramics in different spectral ranges: (a) overview spectrum, (b) a close look on the 0.3-2.4 μm range, (c) a close look on the OH --group absorption at 2.6 -3.2 μm. Labels 800 -1050 °C indicate the heat-treatment temperature at the second stage. The first stage of the heat-treatment is always at 750 °C for 6 h. Numbers in (c) indicate the peak positions in nm.

Figure 15 .

 15 Figure 15. Spectra of visible luminescence from the initial glass and transparent glass-ceramics. Labels 800 -1050 °C indicate the heat-treatment temperature at the second stage. λexc = 488 nm.

Figure 16 .

 16 Figure 16. Comparison of a 0.1 mol% Fe:MgAl2O4 ceramic and Fe:MgAl2O4 based glass-ceramics prepared by the heat-treatment with the second stage at 1000 o C for 6 h: (a) XRD patterns, (b) Raman spectra, λexc = 514 nm, numbers denote the Raman frequencies in cm -1 , (c) absorption spectra (for ceramics, the scattering losses are subtracted), and (d) spectra of visible luminescence, λexc = 488 nm, numbers denote the positions of the emission peaks in nm.

Table 1 .

 1 Mean size of nanocrystals D in transparent GCs estimated by different methods* and the unit-cell parameter of spinel nanocrystals a.

	Heat-treatment	XRD			Raman	TEM
	regime	Spinel		MAT
		а, Å	DXRD, Å DXRD, Å DRaman, Å DTEM, Å
	750 °C/6 h + 800 °C/6 h	8.002 37	64	98	-
	750 °C/6 h + 850 °C/6 h	8.020 41	103	115	-
	750 °C/6 h + 900 °C/6 h	8.074 60	127	115	-
	750 °C/6 h + 950 °C/6 h	8.079 74	146	124	64
	750 °C/6 h + 1000 °C/6 h 8.079 74	206	137	-

Acknowledgements

This work was partly supported by the RFBR (Grant 19-03-00855). The work at ICh-HPS was performed within the state assignment No. 0095-2019-005.