Mohamed Ruwaid Rafiuddin
email: m.rafiuddin@hud.ac.uk

Shu Guo

Giovanni Donato

Andrew P Grosvenor

Nicolas Dacheux

Robert J Cava

Adel Mesbah
email: adel.mesbah@irecelyon.univ-lyon1.fr

Structural and Magnetic Properties of Churchite-type REPO 4 •2H 2 O Materials

published or not. The documents may come

Introduction

Rare-earth phosphates belong to a unique class of materials with rich structural chemistry and owing to this, these materials have wide-ranging applications in fields such as photonics, catalysis, radioactive waste management, and biomedical imaging. [1][START_REF] Onoda | Formation and catalytic characterization of various rare earth phosphates[END_REF][START_REF] Dacheux | Monazite as a promising long-term radioactive waste matrix: Benefits of high-structural flexibility and chemical durability[END_REF][4] These materials, depending on the synthetic conditions, can crystallize in the following structure-types: Monazite C2/c). [START_REF] Ni | Crystal chemistry of the monazite and xenotime structures[END_REF][START_REF] Mesbah | Monoclinic form of the rhabdophane compounds: REEPO4•0.667H2O[END_REF][START_REF] Subramani | Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO4•2H2O (RE = Gd to Lu) Materials[END_REF] Studies discussing the synthetic and structural aspects of rare-earth phosphates have been reported in the literature. [START_REF] Achary | Recent progress on synthesis and structural aspects of rare-earth phosphates[END_REF][START_REF] Rafiuddin | A Structural Investigation of Hydrous and Anhydrous Rare-Earth Phosphates[END_REF][START_REF] Clavier | Crystal chemistry of the monazite structure[END_REF][START_REF] Rafiuddin | Synthesis and structural investigation of churchitetype REPO4•2H2O (RE = Y, Gd, Dy) nanocrystals[END_REF] Similarly, reports on the optical properties, radiation-resistance, and chemical durability of rareearth phosphates can also be found in the literature. [START_REF] Guan | Facile Preparation Method for Rare Earth Phosphate Hollow Spheres and Their Photoluminescence Properties[END_REF][START_REF] Zhang | Synthesis of Tributylphosphate Capped Luminescent Rare Earth Phosphate Nanocrystals in an Ionic Liquid Microemulsion[END_REF][START_REF] Rafiuddin | An in-situ electron microscopy study of dual ion-beam irradiated xenotime-type ErPO4[END_REF][START_REF] Lenz | The Quantification of Radiation Damage in Orthophosphates Using Confocal μ-Luminescence Spectroscopy of Nd3+[END_REF][START_REF] Rafiuddin | Probing the effect of radiation damage on the structure of rare-earth phosphates[END_REF][START_REF] Rafiuddin | An investigation of the chemical durability of hydrous and anhydrous rare-earth phosphates[END_REF][START_REF] Arinicheva | Influence of temperature on the dissolution kinetics of synthetic LaPO4-monazite in acidic media between 50 and 130 °C[END_REF][START_REF] Gausse | Dissolution kinetics of monazite LnPO4 (Ln = La to Gd): A multiparametric study[END_REF] The presence of rare-earth (RE) ions can give rise to interesting magnetic properties, thereby enabling the use of rare-earth phosphates as a contrast agent in magnetic resonance imaging (MRI) applications. [START_REF] Liu | Are Rare-Earth Nanoparticles Suitable for In Vivo Applications?[END_REF][START_REF] Dumont | DNA Surface Modified Gadolinium Phosphate Nanoparticles as MRI Contrast Agents[END_REF] However, the magnetic properties of rare-earth phosphates have not been reported in detail, and to the best of our knowledge, only few reports have been published.

In one of the earliest reports, Saji et al. determined the magnetic properties of rare-earth phosphates and vanadates, RXO 4 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb; X = P and V) adopting the monazite and xenotime structures. [START_REF] Saji | Magnetic Properties of RXO4 System[END_REF] It was shown in that study that the RE 3+ cations predominantly gave rise to the magnetic moments observed in this material and that the temperature dependent magnetizations of these materials obeyed the Curie-Weiss law above 200 K. [START_REF] Saji | Magnetic Properties of RXO4 System[END_REF]. In a study by Cook et al., the magnetic properties of xenotime-type HoPO 4 were determined, and it was reported that HoPO 4 undergoes a transition to an antiferromagnetic state at a Neel temperature of 1.39 K. [START_REF] Cooke | Magnetic interactions in holmium phosphate, HoPO 4[END_REF]. In a recent study by Kumar et al., the magnetic properties of monazite-type Nd 3+ doped GdPO 4 were determined, and it was shown that the material exhibits paramagnetic behaviour and was attributed to the presence of paramagnetic Gd 3+ ions. [START_REF] Kumar | Spectroscopic and magnetic properties of neodymium doped in GdPO4 sub-micron-stars prepared by solvothermal method[END_REF].

In the current study, the Churchite materials (REPO 4 •2H 2 O; RE = Gd -Lu) were synthesized using a low temperature precipitation route and the structures were analyzed using powder XRD and X-ray absorption near-edge spectroscopy (XANES). The temperature dependent magnetic susceptibilities of REPO 4 •2H 2 O (RE = Gd, Tb, Dy, Ho, Er) materials were measured between 300 and 1.8 K and the effective magnetic moments and the Curie-Weiss temperatures were determined. To the best of our knowledge, the local structure and magnetic properties of Churchite-type rare-earth phosphates have not been previously reported and this study aims to fill this literature gap.

Experimental

Synthesis

Churchite-type REPO 4 •2H 2 O materials were synthesized using a previously reported synthetic procedure. [START_REF] Subramani | Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO4•2H2O (RE = Gd to Lu) Materials[END_REF] The following precursors were used: GdCl 3 •6H 2 O (Sigma-Aldrich; 99%), TbCl 3 •6H 2 O (Sigma-Aldrich; 99.9%), DyCl 3 •6H 2 O (Sigma-Aldrich; 99.99%), HoCl 3 (Sigma-Aldrich; 99.9%), ErCl 3 •nH 2 O (Sigma-Aldrich; 99.9%), TmCl 3 (Sigma-Aldrich; 99.9%), YbCl 3 (Sigma-Aldrich; 99.9%), LuCl 3 (Sigma-Aldrich; 99.9%), H 3 PO 4 (85% Normapur), and HCl (37% Carbo Erba). The rare-earth chloride salts were dissolved in 1M HCl and the concentration of the final solution was determined using ICP-AES. The concentration of the RE ions varied between 0.5M and 1M. The rare-earth chloride solution was added to a 5M H 3 PO 4 solution (with an excess of 3 mol%) and stirred for a few hours before placing the solution mixture in a fridge (4ºC) for 24 months. Crystalline powders settled at the bottom of the container and the powders were recovered by centrifugation and washed twice with water and once with ethanol. The powders were then dried in air at room-temperature. To monitor the temperature-induced changes in the structure of REPO 4 .2H 2 O materials, powders of LuPO 4 •2H 2 O were heated to 300ºC and 1000ºC for 2 hours in air (Heating rate of 100ºC/h) followed by cooling to room temperature with a rate of 150ºC/h.

Powder X-ray Diffraction

Powder XRD measurements were carried out using a Bruker D8-Advance diffractometer equipped with a Cu K α1,2 radiation (λ = 1.5418 Å). The powder XRD patterns of the assynthesized REPO 4 •2H 2 O and annealed LuPO 4 samples were collected from 5º to 100º using step size of 0.0167º and a total time of 3 hours per sample. Rietveld refinements of the assynthesized REPO 4 •2H 2 O materials were determined using a Profex (version 4.3.3) software. [START_REF] Doebelin | Profex}: a graphical user interface for the Rietveld refinement program {\it BGMN}[END_REF] During the refinement, the lattice constants, isotropic thermal displacement parameters of RE and P ions were allowed to vary. The atomic coordinates and site occupancy factors were fixed during the refinement.

XANES

P L 2,3 -edge XANES

The P L 2,3 -edge XANES spectra were collected from as-synthesized (ErPO 4 •2H 2 O,

HoPO 4 •2H 2 O, LuPO 4 •2H 2 O
) and annealed LuPO 4 (300ºC and 1000ºC) materials using the Variable Line Spacing -Plane Grating Monochromator (VLS-PGM; 11ID-2) beamline at the Canadian Light Source, CLS (Saskatoon, Canada). [START_REF] Hu | VLS-PGM beamline at the Canadian light source[END_REF] The P L 2,3 -edge spectra were collected using a high-energy grating monochromator and the resolution of the spectra at the P L 3 -edge is 0.01 eV. Fine powders of ErPO 4 •2H 2 O and HoPO 4 •2H 2 O were placed on carbon tape, and the spectra was collected under vacuum in a total fluorescence yield mode using a step size of 0.05 eV. The calibration of the P L 2,3 -edge spectra were done using a spectrum collected from Red P and choosing the maximum in the first derivative curve to 130 eV. All the XANES spectra discussed in this study were analyzed using the Athena software program. [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF]

Lu L 1 -edge XANES

Lu L 1 -edge XANES spectra were collected from as-synthesized LuPO 4 •2H 2 O and annealed LuPO 4 samples (300ºC and 1000ºC) using the Sector 20 -bending magnet (BM) beamline located at the Advanced Photon Source (APS; Argonne National Laboratory). [START_REF] Heald | XAFS and micro-XAFS at the PNC-CAT beamlines[END_REF] All the spectra were collected using a Si (111) double crystal monochromator in the transmission mode and the ionization chambers were filled with 100% N 2 . The spectral resolution of the Lu L 1 -edge spectra at the Lu L 1 -edge (10870 eV) is 1.5 eV. Powders of as-synthesized and annealed samples were placed on a Kapton tape and the spectra was collected using a step size of 0.15 eV through the Lu L 1 -edge. The Lu L 1 -edge spectra was calibrated using a Ga metal foil (Ga K-edge energy = 10367 eV) placed in between the transmission (I t ) and reference ionization chambers (I ref ).

Magnetic Measurements

Temperature-dependent magnetizations (M) were measured on powder samples of approximate weight 0.1 g between 1.8 and 300 K in an applied field (H) of 2000 Oe on a Quantum Design PPMS (Physical Property Measurement System) Dynacool. Susceptibility was defined as M/H. Measurements were made in the DC mode, using the vibrating sample magnetometer function.

The field-dependent magnetization of REPO 4 •2H 2 O (RE = Gd, Tb, Dy, Ho, Er) were collected at 2 K.

Results and Discussion

Crystal structure and Local structure of REPO 4 •2H 2 O materials

The PXRD patterns of the as-synthesized REPO 4 •2H 2 O (RE = Gd -Lu) materials indicate that they are phase-pure and crystallize in the monoclinic crystal system (Space group: C2/c) (Figure 1a). [START_REF] Ivashkevich | PREPARATION AND STRUCTURE OF THE YTTRIUM PHOSPHATE DIHYDRATE[END_REF] Rietveld refinement was performed on one of the members of the churchite series 1b). [START_REF] Ivashkevich | PREPARATION AND STRUCTURE OF THE YTTRIUM PHOSPHATE DIHYDRATE[END_REF] The crystallographic data of refined LuPO 4 •2H 2 O is presented in Table 1. It can be observed in Figure 1a that the diffraction peaks consistently shift to higher 2θ on moving from 2). The crystal structure of Churchite-type REPO 4 •2H 2 O is shown in Figure 2. [START_REF] Ivashkevich | PREPARATION AND STRUCTURE OF THE YTTRIUM PHOSPHATE DIHYDRATE[END_REF] In this structure, the RE 3+ ions are coordinated to eight oxygen atoms and the resulting REO 8 polyhedra adopts a distorted square antiprism geometry; in the REO 8 polyhedra, the six oxygen atoms are provided by the phosphate groups and two oxygen atoms are provided by the water molecules. The REO 8 polyhedra are connected to each other via PO 4 tetrahedra, resulting in the formation of 1D chains along the [100] direction. These 1D chains are in turn connected to each other via edge-sharing of the REO 8 polyhedra and this results in the formation of 2D layers. These 2D layers are stacked along the b-axis and the space between the 2D layers is occupied by water molecules.

(LuPO 4 •2H 2 O) using the churchite-type YPO 4 •2H 2 O
GdPO 4 •2H 2 O to LuPO 4 •2H 2 O
The local structure of P in the Churchite materials was determined by collecting the P L 2,3 -edge XANES spectra (Figure 3). In those spectra, X-ray induced electronic transitions occur from the P 2p state into the P 3s and P 3d conduction states. In Figure 3, however, the various spectral features are attributed to an electronic transition from the P 2p state into overlapping P 3s and P 3d conduction states. [START_REF] Rafiuddin | A Structural Investigation of Hydrous and Anhydrous Rare-Earth Phosphates[END_REF][START_REF] Rafiuddin | X-ray Spectroscopic Study of the Electronic Structure of Monazite-and Xenotime-Type Rare-Earth Phosphates[END_REF] The spectral line shape of the materials investigated in this study are similar, an indication that all the REPO 4 •2H 2 O (RE = Ho, Er, Tm, Lu) materials adopt the Churchite structure (Figure 3). Similarly, the local structure of RE in the Churchite materials were determined in this study via collection of Lu L 1 -edge spectra from Churchite-type 4). The Lu L 1 -edge spectra consists of a weak pre-edge shoulder (2s  6p-5d) and a strong main-edge (2s  6p) peak (Figure 4). [START_REF] Asakura | Local Structure and La L1 and L3-Edge XANES Spectra of Lanthanum Complex Oxides[END_REF][START_REF] Asakura | Local Structure of Pr, Nd, and Sm Complex Oxides and Their X-ray Absorption Near Edge Structure Spectra[END_REF][START_REF] Asakura | Local Structure and L1-and L3-Edge X-ray Absorption Near Edge Structures of Middle Lanthanoid Elements (Eu, Gd, Tb, and Dy) in Their Complex Oxides[END_REF] The pre-edge shoulder is possible due to the distorted square antiprism geometry of the LuO 8 polyhedra which therefore allows for the hybridization of the 6p and 5d states. It can be observed from Figures 3 and4 that the P L 2,3and Lu L 1 -edge spectra are structurally rich and display many features in the near-edge region thereby making them useful probes to monitor the changes in the local structure of materials with increasing temperature.

LuPO 4 •2H 2 O (Figure

Effect of temperature on the long-range and local structure

In a previous study, it was shown using thermogravimetry (TGA) that Churchite materials lose structural water between 200ºC -300ºC and that the powder XRD patterns of the post TGA samples (that were heated to 1000ºC) revealed the structural transformation to the xenotime structure. [START_REF] Subramani | Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO4•2H2O (RE = Gd to Lu) Materials[END_REF] However, it is unknown whether the structural transformation is direct or whether it proceeds via an anhydrous Churchite structure intermediate. To address this question, one of the members of the churchite series (LuPO 4 •2H 2 O) was annealed at both 300ºC and 1000ºC in air.

The ex-situ ambient temperature powder XRD patterns of the annealed materials are shown in Figure 5. Upon heating to 300ºC, the data show that the Churchite structure is not stable and transforms directly to the xenotime (I4 1 /amd) structure; an anhydrous Churchite structure intermediate is not observed. The crystal structure of xenotime-type LuPO 4 is shown in Figure 6.

It is interesting to note that the rhabdophane-type material SmPO 4 •0.667H 2 O, which also contains structural water, loses water in two steps, each step being associated with a structural transformation. [START_REF] Mesbah | In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO4.0.667H2O to the hexagonal LnPO4 (Ln = Nd, Sm, Gd, Eu and Dy)[END_REF] In the first (at ~80ºC) and the second (at ~213ºC) steps of water loss for that material, rhabdophane (SmPO 4 •0.667H 2 O) undergoes a structural transformation to hemihydrate (SmPO 4 •0.5H 2 O) and anhydrous (SmPO 4 ) phases, respectively. [START_REF] Mesbah | In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO4.0.667H2O to the hexagonal LnPO4 (Ln = Nd, Sm, Gd, Eu and Dy)[END_REF] It should be noted that the anhydrous rhabdophane structure is stable up to ~500ºC and beyond that temperature undergoes a structural transformation to the monazite structure. [START_REF] Mesbah | In pursuit of the rhabdophane crystal structure: from the hydrated monoclinic LnPO4.0.667H2O to the hexagonal LnPO4 (Ln = Nd, Sm, Gd, Eu and Dy)[END_REF] In the current study, after heating the Lu-Churchite at 300ºC, the diffraction peaks are broad and is due to the small size of xenotime crystallites (Figure 5). At 1000ºC, the diffraction peaks sharpen due to an increase in the crystallite size (Figure 5). The lattice constants of the as-synthesized LuPO 4 •2H 2 O and annealed LuPO 4 materials are provided in Table 3.

The changes in the local structure of Churchite materials was determined as a function of temperature by collecting the P L 2,3 -edge and Lu L 1 -edge XANES spectra of as-synthesized LuPO 4 •2H 2 O and annealed LuPO 4 (300ºC, 1000ºC) materials (Figures 7 and8). In the P L 2,3edge spectra, a distinct change in the spectral line shape and a shift in the near-edge is observed for the material annealed at 300ºC and 1000ºC (Figure 7). This observation indicates that the local structure around the P atom in the material that has been heated to 300ºC and 1000ºC is 4). As a result, the P L 2,3 -edge spectra of churchite-type LuPO 4 •2H 2 O has a less pronounced fine structure at the near edge (Figure 7). Upon heating the LuPO 4 •2H 2 O material to 300ºC and 1000ºC, the churchite structure transforms to the xenotime structure. This structural transformation is reflected in the P L 2,3 -edge spectra in the form of a more pronounced fine structure at the near-edge region and is a result of the symmetrical nature of the PO 4 tetrahedra in the xenotime structure (Figure 7).

In the Lu L 1 -edge spectra of the materials annealed at 300ºC and 1000ºC, the near-edge region becomes narrower and a slight increase in the intensity of the main-edge peak is observed in comparison to as-synthesized LuPO 4 •2H 2 O (Figure 8). The change in the Lu L 1 -edge spectral lineshape as a function of temperature can also be explained by comparing the Lu-O bond distances in the churchite and xenotime structures (See Table 4). The LuO 8). [START_REF] Subramani | Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO4•2H2O (RE = Gd to Lu) Materials[END_REF][START_REF] Boatner | Synthesis, Structure, and Properties of Monazite, Pretulite, and Xenotime[END_REF]

Magnetic Properties of REPO 4 •2H 2 O materials

To reveal the magnetic properties of REPO 4 •2H 2 O (RE = Gd, Tb, Dy, Ho, and Er), Curie-Weiss fits were applied to the inverse susceptibilities, based on the formula χ-χ 0 = C/(T -) where χ refers to the measured susceptibility, χ 0 is the temperature independent contribution to the susceptibility (negligible in these cases), C is the Curie Constant and  is the Curie-Weiss temperature. µ eff is used for calculating the effective magnetic moments.

As shown in Figure 9, long range magnetic ordering is absent down to 1. 5 and Figure 10). The relationship between polycrystalline Curie-Weiss temperatures  of REPO 4 •2H 2 O and de Gennes factor dG (dG = (g J -1) 2 J(J+1)) is depicted in Figure 10 (right), where g refers to Lande splitting factor and J is the total angular momentum.

The isothermal magnetization M(H) curves at 2 K for REPO 4 •2H 2 O are shown in Figure 11 hysteresis loops were absent for these materials and nonlinear M versus H behavior were observed in the field-dependent magnetization of all members at 2 K. For the Gd-and Tbsamples, the M(H) curves saturates at ∼6.76 μ B (Gd) and ∼4.93 μ B (Tb), respectively for the applied field of μ 0 H = 9 T. Nonlinear responses were observed for other members; however, saturations were not found up to μ 0 H = 9 T in the M(H) curves for Ln = Dy, Ho, and Er.

To facilitate the comparison, the normalization of the temperature dependent susceptibilities of Ho material suggests that it may be heading for a transition where lower magnetic moment is observed, just below the temperature range of our measurements, and thus it may be suitable for future study. Due to the relatively localized 4f electrons of the rare earth elements, the magnetic interactions are weak and further experiments (ultralow temperature specific heat, ultralow temperature AC susceptibilities, and neutron scattering for example) would be needed to reveal the magnetic ground states of all these materials.

REPO 4 •2H 2 O (RE = Gd,

Conclusion

Materials adopting the Churchite-type structure (REPO

(REPO 4 ;

 4 RE = La-Gd; Space group = P2 1 /n), Xenotime (RE'PO 4 ; RE = Tb-Lu & Y; Space group = I4 1 /amd), Rhabdophane (REPO 4 •H 2 O; RE = La-Dy; Space group -C2), and Churchite (REPO 4 •2H 2 O; RE = Gd-Lu & Y; Space group -

  and is a result of the decrease in the ionic radii of RE 3+ ions. Consistent with the shift in the diffraction peaks, the lattice constants of REPO 4 .2H 2 O also decrease on moving from GdPO 4 •2H 2 O to LuPO 4 •2H 2 O thereby indicating the contraction of the unit cell (Table

  different in comparison to the as-synthesized Churchite-type LuPO 4 •2H 2 O material and is attributed to the structure transformation from the Churchite to xenotime structure. The change in lineshape can be explained by comparing the P -O bond distances of the PO 4 tetrahedra in churchite and xenotime structures. Churchite-type LuPO 4 •2H 2 O has a slightly distorted PO 4 tetrahedra (P -O bond length = 1.506Å -1.516Å) in comparison to the PO 4 tetrahedra of xenotime-type LuPO 4 (P -O bond length = 1.534Å; See Table

  8 polyhedra in churchite (Lu -O = 2.258Å -2.446Å) is more distorted in comparison to the LuO 8 polyhedra in the xenotime (Lu -O = 2.257Å and 2.344Å). Thus, the changes in the local symmetry of LuO 8 polyhedra is reflected in the form of changes in the intensity of the Lu L 1 main-edge peak of churchite-type LuPO 4 •2H 2 O and xenotime-type LuPO 4 . Upon heating LuPO 4 •2H 2 O to 300ºC and 1000ºC, the intensity of the main-edge peak in the Lu L 1 -edge spectra increases and indicates a structure transformation from churchite (LuO 8 -square antiprism geometry) to xenotime (LuO 8dodecahedron geometry) structure (Figure

  8 K, according to the DC magnetization data for all tested members in the REPO 4 •2H 2 O family. For GdPO 4 •2H 2 O, a nice Curie-Weiss fitting was applied in the high temperature range (150-280 K), which gives a small negative Weiss temperature  = -0.1 K and effective magnetic moment μ eff = 7.83 μ B /Gd for GdPO 4 •2H 2 O (Figure 9a). The value of effective moment is in a good agreement with the expected value of the free ion for Gd (7.94 μ B /Gd). The small Weiss temperature implies weak magnetic interaction for GdPO 4 •2H 2 O. Similar Curie-Weiss fittings were also performed for the other members (Figure 9b -9e) in the high-temperature regime. The Weiss temperature  and effective magnetic moment μ eff for the studied materials are summarized in Table 5. The Weiss temperatures for TbPO 4 •2H 2 O, DyPO 4 •2H 2 O, HoPO 4 •2H 2 O, ErPO 4 •2H 2 O are -7.4 K, -8.5 K, -9.5 K, and -10.1 K, respectively. These values are relatively large compared with  = -0.1 K for GdPO 4 .2H 2 O. The effective moments for TbPO 4 •2H 2 O, DyPO 4 •2H 2 O, HoPO 4 •2H 2 O, ErPO 4 •2H 2 O are close to the expected values of the free ion for RE (Table

  Tb, Dy, Ho, and Er) are depicted in Figure12. This figure supports the nicely paramagnetic behavior of this family of materials over a wide temperature range. The divergences for some of the materials at low temperatures (T/ < 2) (Figure12, inset) are likely attributed to crystal electric field (CEF) effects. The anomalous behavior displayed by the

Figure 1 Figure 2 -

 12 Figure 1a) Normalized PXRD patterns of as-synthesized Churchite-type REPO 4 •2H 2 O (RE =

Figure 3 -

 3 Figure 3 -Normalized P L 2,3 -edge XANES spectra of as-synthesized churchite-type

Figure 4 -

 4 Figure 4 -Lu L 1 -edge XANES spectra of as-synthesized churchite type LuPO 4 •2H 2 O material.

Figure 5 -

 5 Figure 5 -PXRD patterns of as-synthesized LuPO 4 •2H 2 O and heated (300ºC, 1000ºC) LuPO 4

Figure 6 -

 6 Figure 6 -Crystal structure of Xenotime-type LuPO 4 •2H 2 O. The crystal structure was generated

Figure 7 -

 7 Figure 7 -P L 2,3 -edge XANES spectra of as-synthesized LuPO 4 •2H 2 O and heated (300ºC,

Figure 8 -

 8 Figure 8 -Lu L 1 -edge XANES spectra of as-synthesized LuPO 4 •2H 2 O and heated (300ºC,

Figure 9 -

 9 Figure 9 -Temperature-dependence of the magnetic susceptibilities (blue) and inverse magnetic

Figure 10 .

 10 Figure 10. Changes of effective magnetic moments, free R 3+ ion moment, and Weiss temperatures () as a function of de Gennes factor: (g J -1) 2 J(J+1). data for REPO 4 •2H 2 O (R =

Figure 11 .

 11 Figure 11. Field-dependent magnetization for REPO 4 •2H 2 O (RE = Gd, Tb, Dy, Ho, and Er) at 2

Figure 12 .

 12 Figure 12. Normalization and comparison of the temperature dependent susceptibilities of

Figure 6 29

 6 Figure 6

Figure 9 Figure 10 33

 910 Figure 7

Figure 11

 11 Figure 11

  structural model proposed by Ivashkevich et al and the results indicate a good match with the experimental and calculated PXRD patterns (Figure

Table 2 :

 2 4 •2H 2 O; Space group: C2/c) were synthesized using a low-temperature precipitation method. The materials were found to be stable over a narrow temperature range (T < 300ºC). At temperatures 300ºC and above, they transform into the xenotime structure. Distinct changes in the local structure of P and RE (RE = Lu) ions were observed in the P L 2,3 -edge and Lu L 1 -edge spectra of as-synthesized LuPO 4 •2H 2 O and annealed LuPO 4 materials. The magnetic properties of the as-synthesized REPO 4 •2H 2 O materials were determined via Field-dependent Magnetization, and Temperature-dependent Susceptibility measurements. None of the members of the Churchite series investigated in this study were found to display long-range magnetic ordering down to 1.8 K. The effective magnetic moments (µ eff ) of all the REPO 4 •2H 2 O materials were found to be similar to those of free RE (RE = Gd, Tb, Dy, Ho, Er) ions. Field dependent magnetization measurements at 2 K revealed the absence of hysteresis loop and the existence of a non-linear M vs H relationship in these materials. This study reported, for the first time, the variations in the local structure of Churchite-type REPO 4 •2H 2 O materials as a function of temperature in addition to determining the magnetic properties of the REPO 4 •2H 2 O materials. Lattice constants of the Churchite phases LnPO 4 •2H 2 O

	Tables
	Table	1

Table 3 :

 3 Lattice constants of as-synthesized LuPO 4 •2H 2 O and annealed LuPO 4 materials

	Compounds	a	b	c	β	Volume (Å) Structure
	As-synthesized	6.0888 (1)	14.8592 (3)	5.53047 (8)	115.801 (1)	450.487 (4)	Churchite
	300ºC	6.797 (2)		5.966 (2)		275.6 (3)	Xenotime
	1000ºC	6.7944 (2)		5.9575 (3)		275.0 (2)	Xenotime

Table 5 .

 5 Sample information, De Gennes factor, dG = (g J -1) 2 J(J+1), temperature independent contribution (χ 0 ), Effective moments (μ eff ), and Weiss temperatures () determined by fitting of the Curie-Weiss law to the magnetic susceptibility data. µ fi = free ion magnetic moment.

	RE	Mass(mg)	dG	HighT fit (K)	 	μ eff (μ B )	 (K)	μ fi (μ B )
	Gd	16.8	15.75	150-280	-5.13×10 -4	7.83	-0.1	7.94
	Tb	16.7	10.50	150-280	-4.77×10 -5	9.69	-7.4	9.72
	Dy	23.7	7.08	150-280	-2.08×10 -5	10.52	-8.5	10.63
	Ho	18.5	4.50	150-280	-2.59×10 -4	10.58	-9.7	10.60
	Er	21.3	2.55	150-280	-2.43×10 -4	9.56	-10.1	9.58

Acknowledgements

The authors would like to thank the French National Research Agency (ANR JCJC-X-MAS; Project # ANR-17-CE06-0004) for the financial support. The P L 2,3 -edge XANES spectra was collected at the VLS-PGM beamline (1ID-2), Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan. The Lu L 1 -edge XANES spectra was collected at the 20 BM beamline (CLS@APS Sector 20, APS). Sector 20 facilities at the Advanced Photon Source, and research at these facilities, are supported by the U.S. Department of Energy -Basic Energy Sciences, the Canadian Light Source and its funding partners, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne


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Andrei Petrov

Jeanne Allinne

Iryna Pirozhkova

Dalila Laoudj-Chenivesse

Marc Lipinski

Yegor Vassetzky

A nuclear matrix

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