Peer Schmidt [a] Walter ; Schnelle

Carola J Müller

Ulrich Schwarz

Thomas Doert
email: thomas.doert@chemie.tu-dresden.de

Dr Rüdiger Kniep

High-Pressure Synthesis, Crystal Structure and Properties of GdS2 with Thermodynamic Investigations in the Phase Diagram Gd -S

Keywords: Polysulfides, gadolinium, high-pressure synthesis, crystal structure, phase diagram Polysulfides, gadolinium, high-pressure synthesis, crystal structure, phase diagram

Gadolinium disulfide has been prepared by high-pressure synthesis at 8 GPa and 1173 K. It crystallizes in the monoclinic space group P12 1 /a1 (No. 14) with lattice parameters a = 7.879(1) Å; b = 3.936(1) Å, c = 7.926(1) Å and β = 90.08(1)°. The crystal structure is a twofold superstructure of the aristotype ZrSSi and consists of puckered cationic [GdS] + double slabs which are sandwiched by planar sulfur sheets containing - 2 2 S dumbbells.

The thermal decomposition of GdS 2 proceeds via the sulfurdeficient polysulfides GdS 1.9 , GdS 1.85 and GdS 1.77 and eventually results in the sesquisulfide Gd 2 S 3 . GdS 2 is a paramagnetic semiconductor which orders antiferromagnetically at T N = 7.7(1) K. A metamagnetic transition is observed in the magnetically ordered state.

Introduction

Polychalcogenides of rare-earth elements are known since 1908 when Biltz synthesized CeS 2 [1]. The crystal structures of the polysulfides LnX 2-δ (Ln = Y, La, Ce -Lu; X = S, Se; 0 ≤ δ ≤ ≈ 0.2) of the trivalent rare-earth metals contain puckered double slabs [Ln 3+ X 2-] which are sandwiched by planar chalcogen layers [X δ-]. The same atomic pattern is also found in the ZrSSi type which thus can be considered as the aristotype for these rare-earth metal polychalcogenides. The differences in the chalcogen content of theses compounds severely affect the electronic structure and the arrangement of the atoms in the planar layers. The number of chalcogen defects and their ordering pattern determine which kind of superstructure is formed. In recent years considerable efforts have been made to understand the phase relations between polychalcogenides of different composition and to determine their closely related superstructures (cf. [2 -7]).

Two crystalline modifications for stoichiometric compounds LnS 2 and LnSe 2 have been characterized. The monoclinic α-modification (also called CeSe 2 type, P12 1 /a1, Z = 4) is adopted by the disulfides and diselenides from lanthanum to neodymium. [2,6,8] The polytypic orthorhombic β-modification (space group Pnma, Z = 8) is known for the disulfides from lanthanum to neodymium [6,9]. Chalcogen-poorer compounds of the formula type LnX 1.9 crystallize in the CeSe 1.9 type (space group P4 2 /n, Z = 20). This structural pattern is formed for polysulfides LnS 1.9 with Ln = La -Nd, Sm and Gd and for polyselenides LnSe 1.9 with Ln = La -Nd and Sm [3,10]. For yttrium and the smaller lanthanide metals (terbium -lutetium) only compounds with higher chalcogen deficiency like LnX 1.875 or LnX 1.85 (Ln = Y, Gd -Er, X = S, Se) have been synthesized [4,5,7,11].

Thermoanalytical and tensimetric studies reveal that polysulfides and polyselenides LnX 2-δ generally decompose at elevated temperatures by a stepwise release of molecular chalcogen X 2 until the sesquichalcogenide Ln 2 X 3 is finally reached [4,12]. Polychalcogenides of the smaller rare-earth metals have a higher decomposition pressure which is the reason that all attempts to synthesize stoichiometric disulfides or diselenides of those metals failed up to now. The increasing decomposition tendency with decreasing lattice parameters of the polychalcogenides is mainly attributed to growing anti-bonding interactions between the chalcogenide anions in the planar layers.

High-pressure synthesis is one tool to directly counteract the decomposition of rare-earth metal dichalcogenides LnX 2.0 to chalcogen defect polychalcogenides LnX 2-δ . Earlier, Webb and Hall have prepared compounds LnX 2-δ by highpressure synthesis in a pressure range from 1 to 7 GPa and temperatures up to 1700 K [13]. They reported a pseudotetragonal and a pseudocubic modification of the lanthanide disulfides LnS 2 (Ln = Gd -Lu) and a pseudotetragonal modification of the lanthanide diselenides LnSe 2 (Ln = Er -Lu). However, structure determinations on the basis of single crystal data and chemical analyses of the reaction products are missing. Later, Yanagisawa et al. succeeded to grow single crystals of β-CeS 2 under highpressure conditions [9]. They also observed evidence for a phase transition from the monoclinic to the orthorhombic modification at p > 5.5 GPa for the compounds PrS 2 and NdS 2 . Raman spectroscopic investigations up to 16 GPa at 293 K could not validate these results for the lanthanide disulfides with Ln = La -Nd [14]. Recently, Schleid et al. synthesized the orthorhombic modification of the disulfides LnS 2 of La, Ce and Pr at ambient pressure [6].

In this work we present the first results of high-pressure studies on rare-earth metal polysulfides. In this contribution we focus on the synthesis of GdS 2 , its crystal structure and some physical properties as well as on thermodynamic investigations in the binary system gadolinium -sulfur.

Results and Discussion

Synthesis

In order to find suitable conditions for formation and crystal growth of GdS 2 several reaction routes were tested at high-pressure conditions:

Gd + 2 S → GdS 2 , (1)
GdS 1.9 + 0.1 S → GdS 2 , (2)
Gd 2 S 3 + S → 2 GdS 2 (3)
and

2 GdCl 3 + 3 K 2 S 2 → 2 GdS 2 + 6 KCl + 2 S. (4)
A slight excess of sulfur has been used, namely starting ratios Gd : S of 1 : 2.2, for all reaction routes to avoid the formation of chalcogen deficient polysulfides GdS 2-δ with δ > 0. Besides the reaction route (i. e. the starting material) temperature, pressure and reaction time were varied in the experiments. The target compound GdS 2 is obtained at 8 GPa, 1173 K and 3 h annealing time from any of the selected reactions, see equations ( 1) -( 4). The excess of sulfur could not be detected in the powder X-ray diffraction patterns of the reaction products; it was most probably obtained as an amorphous phase. At a pressure of 3 GPa and a similar induction heating and annealing program only GdS 1.9 was obtained. At p = 15 GPa and a temperature of 1173 K GdS 2 was yielded as a by-product to Gd 2 S 3 . Interestingly GdS 2 does also form at ambient temperature if a pressure of 8 GPa is applied for 50 h.

The optimization of the temperature program was done with mixtures of gadolinium sesquisulfide and sulfur at 8 GPa. Different reaction temperatures between 1373 K and 1823 K (induction heating period, usually held for 15 min) followed by different annealing steps (1173 K -1573 K, for 3 h -43 h) were tested. The sample from which the crystal for the X-ray structure determination was taken was heated to 1373 K in 15 min, kept at this temperature for another 15 min and subsequently cooled to 1173 K during 2 h. This temperature was then kept for 15 h. For reaction path (4) the alkali-metal halide can be removed by leaching with a mixture of water and ethanol but the excess sulfur still remains in the product. Adding the flux material in ratios GdS 2 : CsCl of 1 : 6 and 6 : 1 or changing the element ratio of Gd : S to 1 : 3 or even 1 : 6 did not improve the crystal quality of the samples.

In short, according to the powder X-ray diffraction diagrams of the reaction products pure polycrystalline α-GdS 2 could be obtained from routes (1), ( 2) and (3) at p ≥ 8 GPa and 973 K ≤ T ≤ 1573 K. During our investigations, we found no evidence for polysulfides with higher sulfur content than GdS 2 or for a phase transition of monoclinic α-GdS 2 to the orthorhombic β-modification.

The reflections in the X-ray diffraction patterns of crystalline powders synthesized in different batches exhibit strongly varying full widths at half maximum and signal/background ratios. This result is taken as an indication for marked differences in the size of the crystalline domains of various batches. Powder patterns indicating poor quality were obtained from products prepared according to the route of Webb and Hall (1) starting from the elements [13]. The solid state metathesis reaction (4) results in diffraction patterns of better quality but due to the small content of target material and the large amount of by-products this method was not considered for further experiments. The quality of samples synthesized from gadolinium sesquisulfide (2) or gadolinium polysulfide GdS 1.9 ( 3) is similar. Since the synthesis of GdS 1.9 is more difficult and time-consuming than that of Gd 2 S 3 , all subsequent high-pressure experiments were performed starting from mixtures of gadolinium sesquisulfide and sulfur.

The domain size of the reaction products could be increased by annealing the samples in the temperature range of the sulfur melt at 8 GPa. This effect is attributed to an enhanced diffusion in the solid phase at high annealing temperatures which accelerates the growth of the grains. The effect that during the cooling phase bigger seed crystals grow at the expense of smaller ones is the well-known Ostwald ripening. When the temperature exceeds about 1700 K gadolinium disulfide decomposes to gadolinium polysulfide GdS 1.9 , gadolinium sesquisulfide Gd 2 S 3 and sulfur.

Chemical Analysis

The carrier gas hot extraction method did not evidence oxygen, nitrogen or hydrogen in the analyzed sample. The EDXS analysis on different parts of a sample resulted in a composition Gd : S of 1 : 1.97(2) confirmed in good agreement with the assumed composition GdS 2 .

Crystal Structure

The results of the crystal structure analysis reveal that As has been discussed, the atomic pattern consists of puckered cationic [GdS] + double slabs of Gd 3+ and S 2-ions (S1 in tables 2 and3 -1.00 / 0.95 twin matrices (0 -0.5 0, 2 0 0, 0 0 1) (-1 0 0, 0 -1 0, 0 0 1) (0 0.5 0, -2 0 0, 0 0 1) twin volume ratios 0.076(4), 0.076(2), 0.638(2), 0.210(2)

Insert Figure 1 here As stated above, the crystal structure of monoclinic GdS 2 (CeS 2 type) can be derived from the tetragonal ZrSSi type structure. The symmetry reduction is required due to the formation of disulfide anions in the planar chalcogen layers. The Bärnighausen tree for the three-step symmetry reduction to maximum subgroups each has been developed for isotypic PrSe 2 [2]. ZrSSi crystallizes in space group P4/nmm (No. 129). In a first translationengleich step of index two the symmetry is reduced to the orthorhombic subgroup Pmmn (No. 59). In this t2 step the fourfold rotation along [001] is lost. The next symmetry reduction results in monoclinic space group P2 1 /m (No. 11). In the course of this t2 descent the twofold rotation along [001] is also destroyed. By doubling the a axis, a klassengleich symmetry reduction of index two, the actual space group P12 1 /a1 (No. 14) is reached. During both t2 steps twinning is likely to occur and accordingly the investigated crystal turned out to be a fourfold twin. The twinning matrices resemble the loss of fourfold (and twofold) rotation axes along [001], see table 1. Pseudomerohedral twinning of α-LnX 2 structures has frequently been found [2, 6, 8d-f]. Due to the monoclinic angle β being close to 90° and the particular relation of the lattice parameters a ≈ c ≈ 2 b combined with reflection conditions which are inconsistent with space group symmetry, the diffraction pattern of a (multiply) twinned α-LnX 2 crystal could be mistaken for pseudocubic. This may be the explanation for Webb and Hall's interpretation of the diffraction images [13]. The cell parameter 7.882(3) Å of their pseusocubic phase agrees well with the cell parameter a of our investigated crystal.

Thermodynamic Studies

The thermal behaviour of solid GdS 2 is characterized by the incongruent thermal decomposition towards solid compounds richer in gadolinium (GdS 1.9 (s), GdS 1.85 (s), etc.) and gaseous sulfur (mainly S 2 (g)). In order to understand the decomposition mechanism and to evaluate the concrete decomposition conditions (temperature and pressure) several methods of investigation have been applied: By thermodynamic modeling of the heterogeneous equilibria the existence ranges (lg(p/bar) vs. T -1 ) of all gadolinium sulfides GdS 2-δ were estimated (Figure 2). Based on this information the phase barogram of the system Gd/S has been established. The decomposition reactions were experimentally verified by total pressure measurement (Figure 3) and thermal analysis in combination with mass spectrometry (TG/MS).

Including the known phases of gadolinium polysulfides GdS 2-δ , the decomposition of GdS 2 proceeds step-wise ( 5

S

of the gadolinium sulfides were taken from the literature for GdS and GdS 1.5 (see table 4; [17]). Unknown data for the polysulfides GdS 2-δ were estimated in comparison with the data of the praseodymium sulfides [12a]. Applying Kirchhoff's laws and Hess' law for the thermodynamic data of the phases in reactions ( 5

-8) yielded ) GdS ( x 0 R H ∆ and ) GdS ( x 0 R S ∆
for the respective polysulfides. Finally the phase barogram of the system Gd/S was developed by calculating the decomposition functions lg(p(S 2 )/bar) = f(T -1 ) (9) of gadolinium sulfides: R 303 . 2

) GdS ( 1 R 303 . 2 ) GdS ( ) )/bar S ( lg( x 0 R x 0 R 2 ⋅ ∆ + ⋅ ⋅ ∆ - = S T H p (9),
with R being the universal gas constant. The lines depicted in the barogram (Figure 2) mark the coexistence of adjacent phases, the areas in between the equilibrium lines correspondingly characterize the existence ranges of the individual phases. Following this way, the conditions for the synthesis of single phases GdS x can be deduced: The composition GdS 2 is thermodynamically stable at a given temperature with a pressure above the solid line. At T = 570 K the decomposition pressure p(S 2 ) is lower than 10 -3 bar (100 Pa) and the compound is stable at ambient conditions. A rise in temperature causes an increased decomposition pressure: p ≈ 1 bar (100 kPa) at T = 870 K and p ≈ 10 3 bar (0.1 GPa) at T = 1270 K. Thus, at higher reaction temperatures pressure has to be applied to counterbalance the decomposition pressure in order to obtain GdS 2 under equilibrium conditions.

Table 4: Thermodynamic data for of phases in the system Gd-S.

Insert Figure 2 here

The phase barogram of GdS 2 has been experimentally verified using total pressure measurements with a Bourdon manometer (see experimental section). The decomposition reaction starts at temperatures above 570 K with a pressure of 10 -3 bar (100 Pa), and follows the course of the equilibrium line (Figure 3). With an average sample mass of about 75 mg, the decomposition of GdS 2 is completed at about 670 K and a drift towards the decomposition line of GdS 1.9 is observed The course of measured decomposition pressures of GdS 2 and GdS 1.9 confirms the equilibrium conditions calculated in the phase barogram. As a conclusion, GdS 2 is stable at temperatures below 670 K under its own decomposition pressure, while the phase is unstable at temperatures above 670 K without compensation of the sulfur pressure.

Insert Figure 3 here Finally, the decomposition of GdS 2 was studied at ambient pressure using thermogravimetry in an argon atmosphere. In accordance with the phase barogram, the thermal degradation of GdS 2 starts at T ≈ 470 K and proceeds via four distinct steps, namely GdS 1.9 , GdS 1.85 , and GdS 1.77 , towards Gd 2 S 3 . The constant final mass is reached at T ≈ 1020 K; the solid product was identified by powder Xray diffraction as a mixture of A-type Gd 2 S 3 [18a] and Ctype Gd 2 S 3 [18b]. The gaseous decomposition products were analyzed by mass spectrometry: S 2 (m/z = 64) was identified as the main species. At lower temperatures the species S 8 (g), S 6 (g) and S 4 (g) were detected in minor amounts as well. Nevertheless the general decomposition mechanism of GdS 2 and GdS 2-δ according the equilibrium reactions (5 -8) is confirmed.

The release of sulfur has only consequences for the planar chalcogen layers of the polysulfide. In GdS 2 the planar sulfide layers consists of - S anions, single S 2-anions and vacancies can be found in the layers [3]. The structure of GdS 1.85 is not yet known, it might either adopt the modulated PrSe 1.85 type [7] or a defect variant of the Gd 8 Se 15 structure, a 24-fold superstructure of the ZrSSi type [5]. The higher chalcogen deficiency is reflected in a higher amount of vacancies and single X 2-anions in the respective chalcogen layers in both structure patterns. The formation of chalcogen deficient polychalcogenides by depletion of sulfur comes to an end with GdS 1.77 before the ordinary sulfide Gd 2 S 3 is formed. No meaningful structure model has been established for LnX 1.77 compounds yet although indications for the existence of phases of this approximate composition were found during several tensimetric studies [4,12].

Physical Properties

The temperature dependence of the resistivity ρ(T) could be followed down to 63 K where the resistance reached the upper limit of our experimental setup. The decrease in direction of increasing temperatures (Figure 4) indicates that GdS 2 is semiconducting. A lower estimate of 0.36 eV for the fundamental gap at room temperature may be obtained from the temperature dependence of ρ(T). The high resistivity of approximately 0.13 Ωm at 300 K indicates that the polycrystalline sample material has only a small concentration of point defects. This finding and the semiconducting properties of GdS 2 indicate a valence compound in which the electrons of the anionic partial structure are located in bonds or free electron pairs as reflected by the description [Gd 3+ S 2-] 2 [ - C p / J•mol - Insert Figure 4 here

The magnetic susceptibility χ = M/H of GdS 2 is strongly temperature dependant. The plot of the reciprocal susceptibility as a function of temperature (Figure 4) follows the Curie-Weiss law except for the lowest temperatures. From a fit for T > 20 K (µ 0 H = 0.1 T data) an effective magnetic moment µ eff of 7.92 µ B and a paramagnetic Weiss temperature θ P = -7.9 K is calculated, the latter indicating antiferromagnetic interactions. The Gd 3+ ion is, thanks to its half-filled 4f shell, a magnetic species without orbital moment (L = 0) and S = 7/2. The highly degenerate 8 S 7/2 ground state is therefore not split by crystal fields and the value of the effective moment of the free ion of 7.94 µ B agrees excellently with our observation. A cusp in χ(T) (see upper inset of Figure 5) observed at T N = 7.7( 1) K (in µ 0 H = 0.01 T) marks the long-range antiferromagnetic ordering of the Gd 3+ magnetic moments. An isothermal magnetization curve M(H) well below T N (1.8 K) shows a broad metamagnetic transition between fields of roughly 1 and 2 T. This transition is more clearly seen in a plot of M(H)/H (see lower inset of Fig. 5). The transition has no hysteresis in field and indicates a discontinuity in the gradual turning of the magnetic moment toward fully ferromagnetic ordering. Full alignment of the gadolinium spins (7 µ B ) is not achieved in our maximum field M(7 T) = 5.7 µ B .

Insert Figure 5 here

Conclusions

Gadolinium disulfide was obtained by high-pressure synthesis using different starting materials. GdS 2 adopts the monoclinic CeSe 2 type, a twofold superstructure of the ZrSSi type. GdS 2 is a paramagnetic semiconductor containing Gd 3+ and exhibits antiferromagnetic order at T N = 7.7(1) K. The formation of this disulfide is achieved by counterbalancing its decomposition pressure of, e.g., p ≈ 10 3 bar (0.1 GPa) at T = 1270 K. A synthesis of GdS 2 at ambient pressure thus fails either because of insufficient reactivity at lower temperatures (say below ≈ 850 K) or because the decomposition pressure is exceeded at elevated temperatures. It can be assumed that the syntheses of further, yet unknown rare-earth metal polysulfides under high-pressure conditions will be possible. The exploration of appropriate reaction conditions, especially for crystal growth, remains, however, still a challenge.

Experimental Section

Preparation and sample handling was performed under argon atmosphere in glove boxes (MBraun, H 2 O < 1 ppm, O 2 < 1 ppm). Gd 2 S 3 was synthesized from a mixture of gadolinium (99.9 %, Treibacher) and sulfur (99.95 %, Alfa Aesar) in a molar ratio of 2 : 3 (10). The elements were filled in silica ampoules with glassy carbon crucibles which were sealed under dynamic vacuum (p < 10 -4 mbar). The ampoules were placed in a muffle furnace and heated to 1273 K in 24 h. After annealing at this temperature for 72 h the ampoules were cooled to 298 K in 24 h.

2 Gd + 3 / 2 S 2 → Gd 2 S 3 (10) The sesquisulfide was obtained as a mixture of the A-type (space group Pnma, [18a]) and C-type (defect Th 3 O 4 type, d I 3 4 , [18b]) under these conditions.

For gadolinium polysulfide GdS 1.9 a solid state metathesis reaction was used [3], see equation ( 11). GdCl 3 (99.9 %, Strem) and Na 2 S 2 were mixed and filled in silica ampoules with glassy carbon crucibles. After sealing under dynamic vacuum the ampoules were annealed 9 days at 1023 K and afterwards cooled to room temperature with a rate of 3 K/h. 2 GdCl 3 + 3 Na 2 S 2 → 2 GdS 1.9 + 6 NaCl + 2.2 S (11)

The excessive sulfur was eliminated by sublimation (623 K → 298 K) in a horizontal tube furnace. The sodium chloride which is formed during the reaction acts as flux medium; it was removed with a mixture of water and ethanol (1 : 1).

High-pressure syntheses were performed with a 1000 t hydraulic press (Fa. Voggenreiter) at 8(0.8) GPa and temperatures between 293(15) K and 1373(69) K. For force redistribution and pressure generation a Walker-type module was used with MgO (doped with 5% Cr 2 O 3 ) octahedra of edge length 18 mm [19]. Starting materials were filled under argon into crucibles machined of hexagonal boron nitride. Stepped graphite tubes were used for heating [20]. No reaction between starting material or products and crucible material has been observed.

X-Ray diffraction experiments on powder samples were performed in transmission arrangements on a Huber Image Plate system G670 with Co Kα 1 radiation. The program package WinXPow was applied for the interpretation of the powder diffractograms [START_REF] Winxpow | Powder diffraction software[END_REF]. Single crystal experiments were performed on a Bruker Kappa CCD diffractometer with Mo Kα radiation using the software package SAINT [22]. Structure images have been realized with DIAMOND [START_REF]DIAMOND 3, Crystal and molecular structure visualization[END_REF].

Chemical analysis for hydrogen, nitrogen and oxygen was performed with the carrier gas hot extraction method on a LECO TCH 600. For EDXS analysis the sample were embedded with Technovit 550, ground with silicon carbide paper and polished with diamond suspensions. The EDXS analysis was performed with a Philips XL30 with LaB 6 cathode and a Si(Li) detector.

Thermogravimetric measurements were carried out with corundum crucibles in an analyzer STA 409 CD from NETZSCH skimmer-coupled to a mass spectrometer (Quadstar 422, Pfeiffer). Measurements were performed in the temperature range from T = 298 K to 1470 K with a heating rate of 10 K•min -1 in argon atmosphere. Buoyancy was corrected by measuring the empty crucible before the experiment. The particular decomposition temperature was determined at the onset of the corresponding weight loss.

Total pressure measurements were carried out in a Bourdon manometer of silica glass [START_REF] Oppermann | [END_REF]. Measuring points have been collected every 20 K in the temperature range from T = 520 K to 820 K.

Estimation of standard data:

The standard enthalpies of formation of phases GdS 2-δ have been estimated based on the values of analogous praseodymium sulfides. As for all unknown standard entropies of formation and functions of heat capacity presented in this work, approximations of 0 been performed according to the Neumann-Kopp rule as the stoichiometric sum of the particular values of Gd 2 S 3 and S.

Physical properties of GdS 2 were measured on a polycrystalline piece of the same batch from which the crystal for structure determination and material for TG/MS analysis was taken. The electrical resistance was measured by the four-point method using direct current in the temperature range 4 -320 K. The magnetization was determined in magnetic fields µ 0 H between 0.01 T and 7 T and temperatures between 1.8 K and 400 K in a SQUID magnetometer (MPMS XL-7, Quantum Design).

Figure captions:

GdS 2

 2 adopts the CeS 2 (or α-LnS 2 ) type. It crystallizes in the monoclinic space group P12 1 /a1 (No. 14). The cell parameters are a = 7.879(1) Å, b = 3.936(1) Å, c = 7.926(1) Å and β = 90.08(2)°. The results of the crystal structure refinement and selected interatomic distances are listed in tables 1-3. 1

  ). These double slabs, which can more precisely be described with the Niggli formula [ coordination polyhedron, a capped slightly distorted quadratic antiprism.

2 2 S

 22 ] with completely filled valence shells of the sulfur ions.

Figure 1 :

 1 Figure 1: Crystal structure of GdS 2 , central projection of the unit cell along [010] (left) and dumbbell pattern of disulfide anions (right); ellipsoids at a 99.9% probability level.

Figure 2 :

 2 Figure 2: Phase barogram of the system Gd -S calculated according to equation (9) using the standard data of the polysulfide phases (table4).

Figure 3 :

 3 Figure 3: Measurement of the decomposition pressure of GdS 2 and GdS 1.85 [17] in comparison to calculation of the decomposition equilibria.

Figure 4 :

 4 Figure 4: Electrical resisitivity ρ(T) of polycrystalline GdS 2 .

Figure 5 :Figure 1 :Figure 2 :Figure 3 :Figure 5 :

 51235 Figure 5: Reciprocal magnetic susceptibility 1/χ of GdS 2 as function of temperature T in two different external fields µ 0 H (0.1 T and 1 T). Upper inset: magnetic susceptibility χ(T) for µ 0 H = 0.01 T at low temperatures. Lower inset: magnetic susceptibility χ as function of external field µ 0 H for T = 1.8 K. See text for discussion.

Table 1 :

 1 Crystallographic data and refinement parameters of GdS 2 .

	chemical composition	GdS 2
	M r , g/mol; F(000)	221.37; 384
	temperature, K	296(2)
	diffractometer type	Kappa CCD (Bruker-AXS)
	wavelength, Å	0.71073 (Mo Kα, graphite
		monochromator)
	crystal system	monoclinic
	space group (No.); Z	P12 1 /a1 (14); 4
	cell parameters	a = 7.879(1) Å, b = 3.936(1) Å
	(single crystal data)	c = 7.926(1) Å, β = 90.08(1)°
	cell volume, Å 3	245.8(1)
	density, g/cm 3	5.98
	absorption coefficient, mm -1	28.3
	absorption correction	multi-scan (SADABS, [15])
	T min / T max	0.2355 / 0.4650
	crystal size, mm 3	0.036 × 0.021 × 0.004
	measurement range	2.57 < Θ < 35.02
		-12 ≤ h, l ≤ 12
		-6 ≤ k ≤ 6
	N(meas); N(unique); N(obs)	6223; 1505; 1396
	R int	0.025
	refinement method	full-matrix least squares on F 2 ,
		JANA2006 [16]
	restrictions / parameters	0 / 31
	goodness-of-fit on F 2	1.02
	R 1 ;ωR 2 (reflections I > 3σ(I))	0.018; 0.040
	R 1 ;ωR 2 (all reflections)	0.021; 0.041
	∆ρ min / ∆ρ max , eÅ -3

Table 2 :

 2 Fractional coordinates and isotropic displacement parameters of GdS 2 .

	Atom	x	y	z	U eq , Å 2
	Gd	0.12938(3) 0.78670(7) 0.72427(2) 0.00744(4)
	S1	0.1251(1)	0.7609(2)	0.3674(1)	0.0078(2)
	S2	0.1097(1)	0.3399(2) -0.0031(1)	0.0088(2)

Table 3 :

 3 GdS 2 . Selected interatomic distances(d in Å).

	Gd -S1 #4	2.778(1) Gd -S2 #5	2.791(1)
	Gd -S1 #2	2.785(1) Gd -S2 #3	2.948(1)
	Gd -S1	2.831(1) Gd -S2 #2	3.024(1)
	Gd -S1 #1	2.925(1) Gd -S2 #5	3.072(1)
	Gd -S1 #3	3.031(1)
	S2 -S2 #8	2.140(1) S2 -S2 #9	2.960(1)
	S2 -S2 #7	2.960(1) S2 -S2 #10 3.186(2)

Symmetry codes

: #1 -x+1/2,y-1/2,-z+1; #2 -x+1/2,y+1/2,-z+1; #3 -x,-y+1,-z+1; #4 -x,-

y+2

,-z+1; #5 x,y,z+1; #6 x,y+1,z+1; #7 -x+1/2,y+1/2,-z; #8 -x,-y+1,-z; #9 -x+1/2,y-1/2,-z; #10 -x,-y,-z

Further details concerning the structure analysis can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, quoting the depository number CSD-421335.

Acknowledgments

The authors thank Susann Leipe for supporting high-pressure synthesis, Dr. Stefan Hoffmann and Susann Scharsach for the TG/MS measurements, Gudrun Kadner for the measurement of vapor pressure, and Dr. Marcus Schmidt as well as Prof. Dr. Juri Grin for helpful discussions. The competence groups Analytics and Structure at the MPI CPfS are gratefully acknowledged as well as financial support by the Deutsche Forschungsgemeinschaft. ____________


Sabine Zitzer

Dr Thomas Schleid
email: schleid@iac.uni-stuttgart.de

Professor Rüdiger Kniep Geburtstag

Na 2 Lu 3 I 3 [TeO 3 ]

Keywords: Sodium, Lutetium, Iodides, Oxotellurates(IV), Crystal Structure Sodium, Lutetium, Iodides, Oxotellurates(IV), Crystal Structure

spreading out parallel to (100) as well. Thus, the crystal structure contains two crystallographically distinct I -anions, of which (I1) -is coordinated nearly linear (∢(Na-I1-Na) = 179.6°) by two Na + cations, whereas (I2) -has contact to three of them displaying a distance of 114 pm from the triangular (Na + ) 3 plane. The crystal structure of

Einleitung

Halogenid-Derivate von Lanthanoid(III)-Oxotelluraten(IV) werden schon seit längerer Zeit erforscht, stellen aber dennoch eher eine Seltenheit dar. Zu jenen Verbindungen, die ausschließlich anionisch derivatisiert sind, zählen etwa HoClTeO 3 [1], NdXTeO 3 (X = Cl und Br) [2] und MClTe 2 O 5 (M = Nd und Gd) [3], HoClTe 2 O 5 [4] sowie die sehr halogenidarmen Vertreter Ho 11 ClTe 16 O 48 [5], Y 11 ClTe 16 O 48 [6], Y 6 Br 4 Te 11 O 29 [7] und

Strukturbeschreibung und Diskussion

Die Titelverbindung Na 2 Lu 3 I 3 [TeO 3 ] 4 kristallisiert monoklin in der Raumgruppe P2/c (Nr.

Abstract.

  Colourless platelets of Na 2 Lu 3 I 3 [TeO 3 ] 4 were obtained within five days at 775 °C by the reaction of Lu 2 O 3 and TeO 2 in a 3 : 8 molar ratio with NaI added in excess as both fluxing agent and reactant in evacuated silica ampoules. It crystallizes in the monoclinic space group P2/c with the lattice parameters a = 921.69(5), b = 552.71(3), c = 1664.37(9) pm, β = 90.218(4)° and Z = 2. The crystal structure of Na 2 Lu 3 I 3 [TeO 3 ] 4 exhibits two crystallographically different Lu 3+ cations, both coordinated by eight O 2-anions as square antiprisms. These polyhedra are interconnected via four common edges to build up ] layers (e = edge-linking) parallel to (100). Furthermore, the crystal structure includes a crystallographically unique Na + cation surrounded by four O 2-and four I -anions also in the shape of a square antiprism. These polyhedra connect via common (I2) -•••(I2) - strands that are further linked by (I1) - vertices to result in the formation of ] layers (v = vertex-linking)

Na 2

 2 Lu 3 I 3 [TeO 3 ] 4 is completed by two crystallographically independent Te 4+ cations that show stereochemically active non-bonding electron pairs ("lone pairs") and are located above and below the ] layers forming isolated ψ 1 -tetrahedral [TeO 3 ] 2-anions (d(Te-O) = 188 -190 pm) with all oxygen atoms.

Nd 5 O 4

 54 Cl 3 [TeO 3 ] 2[8]. Darüber hinaus kennt man bereits auch einige gleichzeitig kationisch und anionisch derivatisierte Oxotellurate(IV), bei denen zusätzlich zu den Halogenid-Anionen noch Übergangsmetall-Kationen in die Strukturen eingebaut werden, wie z. B. CuMX[TeO 3 ] 2 (M = Dy und Er mit X = Cl; M = Er mit X = Br), CuDy 2 Br 2 Te 5 O 13 und MnSm 2 Cl 2 Te 5 O 13 oder CuNd 4 Cl 3 [TeO 3 ] 5 [9]. Reine Alkalimetall-Derivate von Lanthanoid(III)-Oxotelluraten(IV) sind dagegen bislang unbekannt, allerdings wurden solche Verbindungen von Lanthanoid(III)-Oxotelluraten(VI) beschrieben, die zusätzlich zu einem Alkali-auch noch ein Erdalkalimetall-Kation in der Kristallstruktur aufweisen, z. B. LiEAPr[TeO 6 ] (EA = Ca, Sr, Ba), LiSrEu[TeO 6 ] sowie AMgLa[TeO 6 ] und ABaLa[TeO 6 ] [10, 11] (A = Li und Na). Die einzige Verbindung, die ausschließlich durch Alkalimetall-Kationen derivatisiert wird, ist das Kalium-Cer(IV)-Oxotellurat(VI) K 2 CeTe 3 O 12 [12]. Somit sind Kristallstrukturen, die Alkalimetall-Kationen, Iodid-Anionen oder gar deren Kombinationen enthalten, unter den Lanthanoid(III)-Oxotelluraten(IV) noch gänzlich unbeschrieben und die Titelverbindung Na 2 Lu 3 I 3 [TeO 3 ] 4 stellt den ersten Vertreter hierfür dar. Experimentelles Einkristalle von Na 2 Lu 3 I 3 [TeO 3 ] 4 sind durch die Umsetzung von Lu 2 O 3 (99,9 %, ChemPur, Karlsruhe) und TeO 2 (99,9995 %, Alfa Aesar, Karlsruhe) im molaren Verhältnis von 3 : 8 mit NaI (Suprapur, Merck, Darmstadt) im Überschuss als Flussmittel und Reaktand zu erhalten. Die Edukte werden dazu in evakuierten Kieselglasampullen für vier Tage im Muffelofen bei 775 °C getempert. Im Anschluss lässt man zur besseren Kristallisation innerhalb von 45 h langsam auf 500 °C abkühlen, um danach bei abgeschaltetem Ofen Raumtemperatur zu erreichen. Unter dem Mikroskop fallen farblose Einkristalle der Titelverbindung mit plättchenförmigem Habitus auf, die sehr feuchtigkeitsunbeständig sind, jedoch unter getrocknetem Paraffinöl problemlos selektierbar sind. Die Strukturbestimmung erfolgte anhand röntgendiffraktometrischer Einkristalldaten (κ-CCD Röntgendiffraktometer; Bruker-Nonius). Die Strukturlösung und -verfeinerung gelang mit Hilfe des Programmpakets SHELX-97 [13] mittels Direkter Methoden. Eine numerische Absorptionskorrektur wurde mit dem Programm X-SHAPE auf der Basis des Programms HABITUS [14] durchgeführt. Die Visualisierung der Kristallstruktur erfolgte mit dem Programm DIAMOND [15]. Weitere Einzelheiten zur Kristallstrukturanalyse können unter Angabe der Hinterlegungsnummer CSD-420167 beim Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (E-mail: crysdata@fiz-karlsruhe.de), erhalten werden. Die Tabellen 1 -4 enthalten weitere Einzelheiten zur Strukturlösung und -verfeinerung sowie die Atomlagen, die anisotropen Temperaturfaktoren, ausgewählte interatomare Abstände und Winkel sowie die Motive der gegenseitigen Zuordnung [17] der beteiligten Atome. auch mit Hilfe von röntgenographischen Pulverbeugungsmethoden untersucht (Pulverdiffraktometer STADI P, Fa. Stoe; Germanium-monochromatisierte Cu-K -Strahlung: λ = 154,1 pm), wobei als kristalline Nebenphasen nur noch NaI und β-TeO 2 [23] detektierbar waren.

Tabelle 1 . 2 Berechnete

 12 13) mit den Gitterparametern a = 921,69(5), b = 552,71(3), c = 1664,37(9) pm, β = 90,218(4)° und Z = 2. In der Kristallstruktur liegen zwei kristallographisch unterschiedliche Lu 3+ -Kationen vor, die beide achtfach von Sauerstoffatomen in Form von quadratischen Antiprismen umgeben sind (Abb. 1, oben). Die Abstände im [(Lu1)O 8 ] 13--Polyeder betragen d((Lu1) 3+ -O 2-) = 226 -248 pm (Tab. 4) mit ECoN(Lu1) = 7,69 (nach Hoppe: ECoN = Effective Coordination Number [17]), für die [(Lu2)O 8 ] 13--Polyeder ergeben sich Abstände von d((Lu2) 3+ -O 2-) = 228 -246 pm (Tab. 4) mit ECoN(Lu2) = 7,82. Im Vergleich zu dem binären Oxid Lu 2 O 3 im C-Typ, bei dem die Lu 3+ -O 2--Abstände zwischen 221 und 225 pm [18] liegen, überstreichen die Werte in Na 2 Lu 3 I 3 [TeO 3 ] 4 also einen etwas größeren Bereich. Dies lässt sich wohl darauf zurückführen, dass die Lu 3+ -Kationen in C-Lu 2 O 3 mit Bixbyit-Struktur von nur sechs Sauerstoffatomen koordiniert werden, die dementsprechend näher an die Zentralteilchen heranrücken können. Die Polyeder um (Lu1) 3+ und (Lu2) 3+ verknüpfen zunächst über zwei Kanten zu trimeren Grundbaueinheiten ] (t = terminal, k = kantenverknüpfend), die durch weitere Kantenverknüpfung ] sind nochmals so über gemeinsame Kanten kondensiert, dass jedes [LuO 8 ] 13--Polyeder über vier Kanten mit anderen verbunden ist und sich damit eine ] -Schicht parallel (100) ausbildet (Abb. 2). Darin wird ein jedes [(Lu1)O 8 ] 13- -Polyeder von vier [(Lu2)O 8 ] 13--Einheiten, umgekehrt aber die quadratischen Antiprismen [(Lu2)O 8 ] 13-von je zwei [(Lu1)O 8 ] 13--und [(Lu2)O 8 ] 13--Polyedern umgeben. Zudem weist die Kristallstruktur noch kristallographisch singuläre Na + -Kationen auf, die von je vier O 2-und I --Anionen in Form von quadratischen Antiprismen [NaO 4 I 4 ] 11-(Abb. 1, unten) in Abständen von d(Na + -O 2-) = 224 -290 pm und d(Na + -I -) = 329 -338 pm (Tab. 4) koordiniert sind. Bei der Betrachtung von Literaturwerten für das binäre Oxid Na 2 O [19] mit d(Na + -O 2-) = 241 pm (4×) und das binäre Iodid NaI [20] mit d(Na + -I -) = 324 pm (6×), aber auch das gleichzeitig Oxid-und Iodid-Anionen enthaltende Natrium-Oxidiodid Na 4 OI 2 [21], das Werte von d(Na + -O 2-) = 234 -248 pm sowie d(Na + -I -) = 324 -331 pm aufweist, fällt auf, dass zwar die Na + -I --Abstände in Na 2 Lu 3 I 3 [TeO 3 ] 4 im gleichen Bereich liegen wie die in der Literatur angegebenen, zwei Werte für die Na + -O 2--Abstände (277 und 290 pm, Tab. 4) jedoch recht weit darüber hinaus gehen. Dieser Sachverhalt spiegelt sich auch in dem kleinen Wert für ECoN(Na + ) von 6,31 wider, der durch die quadratischprismatische Gesamtkoordination eigentlich einen Zahlenwert von etwa acht annehmen sollte. Entsprechend geringe Individualbeiträge (δ-ECoN = 0,22 bzw. 0,10) werden daher von den weiter entfernten O 2--Anionen zum ECoN-Gesamtwert geleistet. Die [NaO 4 I 4 ] 11--Polyeder verknüpfen über jeweils zwei gemeinsame (I2) -•••(I2) -eckenverknüpfend; Abb. 3) parallel (100) ausbilden. Die so entstandenen natriumhaltigen Schichten kommen nun dergestalt ober-und unterhalb der ] zu liegen, dass die Na + -Kationen nun die Lücken innerhalb letzterer besetzen. Demnach müssen in der Kristallstruktur von Na 2 Lu 3 I 3 [TeO 3 ] 4 zwei kristallographisch unabhängige I --Anionen vorhanden sein. Im Falle von (I1) -koordinieren zwei Na + -Kationen nahezu linear (Tab. 4), um (I2) -bilden drei Na + -Kationen ein Dreieck, aus dessen Ebene (I2) -um etwa 114 pm ausgelenkt ist. Die I --Anionen kommen in der Kristallstruktur so zu liegen, dass sie in (100) etwa die gleichen Positionen besetzen wie die Lu 3+ -Kationen, zu diesen jedoch um 1 / 2 entlang [100] verschoben sind (Abb. 6) und damit keinen koordinativen Kontakt mit ihnen unterhalten können. Die beiden in der Kristallstruktur ebenfalls enthaltenen und kristallographisch unterschiedlichen Te 4+ -Kationen dekorieren, ebenso wie die Na + -Kationen, oberhalb und unterhalb der ] deren noch verfügbare Lücken. Die Lücken dieser Schichten werden somit vollständig durch Te 4+ -und Na + -Kationen im Verhältnis 2 : 1 besetzt. Längs [001] betrachtet (Abb. 2), folgen dabei auf eine Reihe mit vollständiger Te 4+ -Belegung zwei Reihen, die sowohl mit Te 4+als auch Na + -Kationen versehen sind. Die Te 4+ -Kationen bilden mit jeweils drei O 2--Anionen isolierte ψ 1 -tetraedrische [TeO 3 ] 2--Einheiten (d(Te 4+ -O 2-) = 188 -190 pm, Tab. 4) mit stereochemisch aktivem nicht-bindendem Elektronenpaar aus, welche die Kristallstruktur letztlich komplettieren, wobei die "lone pairs" stets in die -Schichten zeigen (Abb. 6). Vergleicht man die Literaturdaten der binären Telluroxide TeO 2 (α-TeO 2 : d(Te 4+ -O 2-) = 190 -209 pm [22], β-TeO 2 : d(Te 4+ -O 2-) = 188 -220 pm [23], CN(Te 4+ ) = 4) und TeO 3 (d(Te 6+ -O 2-) = 192 pm, 6×) [24], die allesamt kürzeste Te-O-Abstände von rund 190 pm aufweisen, so liegen die Werte für Na 2 Lu 3 I 3 [TeO 3 ] 4 ebenfalls gut in diesem Bereich, fallen aber geringfügig kleiner aus, weil kondensierte Netzwerke in den binären Oxiden isolierten ψ 1 -Tetraedern in Na 2 Lu 3 I 3 [TeO 3 ] 4 gegenübergestellt werden. Letztere reflektieren mit O-Te-O-Winkeln zwischen 87 und 103° (vgl. Tab. 4) einen höheren p-Charakter in den Te-O-Bindungen als man für ein ideal sp 3 -hydridisiertes Te 4+ -Zentralkation mit 109,5° erwarten würde und auch deren Auslenkungen von 102 -105 pm aus den zugehörigen (O 2-) 3 -Dreiecksebenen folgen diesem Trend. Dies wird besonders deutlich im Vergleich mit dem fast unverzerrten ψ 1 -Tetraeder [(I2)Na 3 ] 2+ (d(I2-Na) = 336 -338 pm; ∢(Na-I2-Na) = 108 -110°, Tab. 4). Mit kürzesten "Sekundärkontakten" von 304 pm (O2•••Te2) und 315 pm (O5•••Te1) bzw. 345 pm (I2•••Te1) und 347 pm (I2•••Te2) kann für beide [TeO 3 ] 2--Anionen von einem echten koordinativen Zuwachs nicht die Rede sein. Na 2 Lu 3 I 3 [TeO 3 ] 4 stellt nach HoCl[TeO 3 ] [1], Nd 5 O 4 Cl 3 [TeO 3 ] 2 [8], den kupferhaltigen Beispielen CuDyCl[TeO 3 ] 2 und CuErBr[TeO 3 ] 2 [9] sowie CuNd 4 Cl 3 [TeO 3 ] 5 [9] prinzipiell erst das sechste Lanthanoid(III)-Oxotellurat(IV)-Derivat überhaupt dar, das nachweislich vollständig isolierte [TeO 3 ] 2--Pyramiden enthält, nachdem sowohl in den ternären Verbindungen M 2 Te 3 O 9 (M = Nd, Dy, Tm -Lu) [25 -27], M 2 Te 4 O 11 (M = Y, La -Nd, Sm -Lu) [28 -34] und M 2 Te 5 O 13 (M = Sc, Y, Dy -Lu) [34 -37], als auch in allen anderen in der Einleitung aufgeführten Beispielen, Oligomere oder durch starke Sekundärkontakte verbundene Oxotellurat(IV)-Aggregate vorliegen. Nachdem uns mittlerweile die Synthese einer ganzen Serie von zur Titelverbindung isotypen Na 2 M 3 X 3 [TeO 3 ] 4 -Vertretern (X = Cl: M = Y, La, Sm, Dy, Tm; X = Br: M = La -Nd, Sm -Lu; X = I: M = Pr, Eu, Ho, Er, Yb, Lu) [38] mit Na 2 Lu 3 I 3 [TeO 3 ] 4 -Struktur gelungen ist, werden in Zukunft Untersuchungen zur Lumineszenz (z. B. an lanthanoid-dotiertem Na 2 Y 3 X 3 [TeO 3 ] 4 [39]) und zur Na + -Ionenmobilität (laut Tabelle 2 weist Na + neben den I --Anionen bei großer U 23 -Anisotropie die weitaus höchsten U ij -Werte aller unabhängigen Atome in Na 2 Lu 3 I 3 [TeO 3 ] 4 auf) im Fokus unseres Interesses stehen. Dank Der Deutschen Forschungsgemeinschaft (Bonn), dem Fonds der Chemischen Industrie (Frankfurt/Main) und dem Land Baden-Württemberg (Stuttgart) danken wir für die finanzielle Unterstützung sowie Herrn Dr. Ingo Hartenbach für die Sammlung der Röntgenbeugungsdaten. Na 2 Lu 3 I 3 [TeO 3 ] 4 : Kristallographische Daten und ihre Bestimmung. Dichte a) (D x in g/cm 3 ) 6,479 Molvolumen a) (V m in cm

Abbildung 1 . 4 . 2 .Abbildung 4 .Abbildung 5 .Abbildung 6 .

 142456 Abbildung 1. Anionische Koordinationsumgebung der beiden kristallographisch unabhängigen Lu 3+ -Kationen (oben) und des kristallographisch singulären Na + -Kations

  Atomkoordinaten und Koeffizienten der anisotropen Temperaturfaktoren a) (U ij /pm 2 ) für Na 2 Lu 3 I 3 [TeO 3 ]. U ij , definiert im anisotropen Temperaturfaktor der Form: exp [-2π 2 (a* 2 h 2 U 11 + b* 2 k 2 U 22 + c* 2 l 2 U 33 +2 b* c* k l U 23 + 2 a* c* h l U 13 + 2 a* b* h k U 12 ].

							ZAAC	Page 12 of 23
	Tabelle 2. Tabelle 2.	(Fortsetzung).
	Atom		U 11		U 22	U 33	U 23	U 13	U 12
	Atom O1	Wyckoff-Lage 131(26)	71(25)	x/a 148(22)	y/b 6(18)	-7(19)	z/c 20(19)
	Na O2	4g 167(28)	121(27)	0,2976(4) 80(20)	0,2585(6) 26(18)	35(18)	0,0865(2) -20(21)
	Lu1 O3		2e 68(23)		87(26)	0 121(20)	0,24530(7) 37(17) -15(17)	1 / 4 6(17)
	Lu2 O4	4g 143(26)	101(24)	0,00087(3) 77(20)	0,25421(5) 16(18)	5(17)	0,91538(1) -33(20)
	Te1 O5	4g 153(26)		92(26)	0,24849(5) 93(20)	0,20685(8) -43(18) -53(18)	0,73314(3) 60(20)
	Te2 O6	4g 130(26)	3 /mol) 71(27)	255,30 0,24449(5) 129(21)	0,28751(8) 11(18) -19(18)	0,43439(3) -5(19)
	Diffraktometer, Wellenlänge I1 2f	κ-CCD Diffraktometer (Bruker-Nonius), 1 / 2 0,26046(13) 1 / 4
							Mo-Kα: λ = 71,07 pm
	Messbereich (±h max , ±k max , ±l max ) I2 4g	12, 7, 22 0,49377(5)	0,24403(9)	0,91628(2)
	2θ max (in grd) O1			4g			56,35 0,1347(5)	0,4548(9)	0,6888(3)
	F(000) Absorptionskoeffizient µ (in mm -1 ) O2 4g	1396 29,64 0,1280(5)	0,0775(9)	0,8142(3)
	Datenkorrektur O3		4g			Untergrund, Polarisations-und Lorentz-0,1585(5) 0,0097(9) 0,1593(3)
	O4			4g			Faktoren; numerische Absorptionskorrektur: Programm X-SHAPE [14] 0,1283(5) 0,0439(9) 0,4790(3)
	Gemessene Reflexe O5	4g			18345 0,1198(5)	0,4142(9)	0,3547(3)
	Symmetrieunabhängige Reflexe O6 4g	2092 0,1542(5)	0,4932(9)	0,0069(3)
	R int / R σ						0,089 / 0,046
	Zahl der Reflexe mit |F o | ≥ 4σ(F o ) Atom U 11 U 22	1735 U 33	U 23	U 13	U 12
	Strukturbestimmung und -verfeinerung Programmpaket SHELX-97 [13] Na 211(19) 232(19) 260(17) 160(13) -6(13)	6(13)
	Streufaktoren Lu1		90(2)		33(2)	International Tables, Vol. C [16] 53(2) 0 -19(2)	0
	R 1 / R 1 mit |F o | ≥ 4σ(F o )			0,030 / 0,041
	wR 2 / Goodness of Fit (GooF) Lu2 102(2) 49(2)	0,074 / 1,026 63(2)	2(1)	-10(1)	4(1)
	Extinktion (g) Te1	71(2)		56(2)	0,00088(6) 70(2)	6(2)	-15(1)	-6(2)
	Restelektronendichte, max. / min. (ρ in e -10 -6 pm -3 ) Te2 73(2) 61(2)	2,44(5) / -3,06(5) 72(2) 10(2)	-13(1)	7(2)
	a) Einkristalldaten. I1 249(5)		125(4)	199(4)	0	-46(3)	0
	I2		193(4)		167(4)	142(3)	-21(2)	-5(3)	10(2)
						Wiley-VCH

a)

Tabelle 3.

Motive der gegenseitigen Zuordnung [START_REF] Hoppe | [END_REF]


Dr Hubert ; Huppertz
email: hubert.huppertz@uibk.ac.at

Almut Haberer

Reinhard Kaindl

Jürgen Konzett

Robert Glaum

Professor Rüdiger Kniep

Synthesis of Er 5 (BO 3 ) 2 F 9 and Properties of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb)

Keywords: Erbium fluoride oxide, High-pressure, Crystal structure Erbium fluoride oxide, High-pressure, Crystal structure

Er 5 (BO 3 ) 2 F 9 was synthesized under conditions of 3 GPa and 800 °C in a Walker-type multianvil apparatus. The crystal structure was determined on the basis of single-crystal X-ray diffraction data, collected at room temperature. Er 5 (BO 3 ) 2 F 9 is isotypic to the recently synthesized Yb 5 (BO 3 ) 2 F 9 and crystallizes in C2/c with the lattice parameters a = 2031.2(4) pm, b = 609.5(2) pm, c = 824.6(2) pm, and β = 100.29(3)°. The physical properties of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) including high temperature behaviour and single crystal IR-/ Raman spectroscopy were investigated.

Introduction

In the past years, the field of rare-earth fluorido-and fluoride borates could be greatly extended by the application of high-pressure / high-temperature techniques, leading to the new compounds Yb 5 (BO 3 ) 2 F 9 [1], Pr 4 B 3 O 10 F [2], and Gd 4 B 4 O 11 F 2 [3].

Before we started our high-pressure / high-temperature research, rare-earth fluorido-and fluoride borates were only represented by the compounds RE 3 (BO 3 ) 2 F 3 (RE = Sm, Eu, Gd) [4] and Gd 2 (BO 3 )F 3 [5], synthesized by heating a stoichiometric mixture of RE 2 O 3 , B 2 O 3 , and REF 3 under ambient pressure conditions. There is a close relationship between these two structure-types and the recently discovered high-pressure ytterbium fluoride borate Yb 5 (BO 3 ) 2 F 9 [1]. All three compounds can be described via alternating layers of the formal compositions "REBO 3 " and "REF 3 " in the bc-plane. It should be emphasized that the crystal structures of the actual compounds REBO 3 and REF 3 can not be compared with the structures of the layers, so that "REBO 3 " and "REF 3 " only stand for the formal composition of the layers. We are now trying to substitute the Yb 3+ ions of Yb 5 (BO 3 ) 2 F 9 with other rare-earth cations to yield isotypic RE 5 (BO 3 ) 2 F 9 phases, different structures, or new compositions dependent of the size of the rare-earth ion. E.g., during our studies of high-pressure phases of the rareearth borates, the compounds RE 4 B 6 O 15 (RE = Dy, Ho) [6][7][8] were intensively investigated. Nevertheless, it was not possible to obtain isotypical compounds with RE = Er, Tm, Yb, or Lu. For these rare-earth elements, high-pressure / high-temperature experiments always yielded the compounds RE 3 B 5 O 12 (RE = Er-Lu) [9].

In analogy to these investigations of the rare-earth borates, we are trying to define the formation region for the composition RE 5 (BO 3 ) 2 F 9 . While Yb 5 (BO 3 ) 2 F 9 was synthesized at 7.5 GPa [1], an isotypic erbium fluoride borate Er 5 (BO 3 ) 2 F 9 could be obtained now at a pressure of 3 GPa. This indicates that there is a large pressure region for the formation of these high-pressure phases and the application of similar synthetic conditions to neighbouring rare-earth elements could thus lead to additional isotypic compounds. Since it was possible to obtain Er 5 (BO 3 ) 2 F 9 , the existence of an isotypic thulium compound is more than likely. Future studies will show, if the synthesis with larger rare-earth cations is possible as well and at which point we observe the formation of another structure type as in the borate case. However, experiments on Yb 5 (BO 3 ) 2 F 9 at higher pressures (10 GPa) resulted already in the formation of an apatite-structured compound, wherein all boron atoms showed a fourfold coordination [10].

In this paper, we present the synthesis and structural details of Er 5 (BO 3 ) 2 F 9 in comparison to the isotypic phase Yb 5 (BO 3 ) 2 F 9 . Furthermore, several physical properties of the compounds RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) are reported.

Experimental Section

To synthesize the compound Er 5 (BO 3 ) 2 F 9 , high-pressure / hightemperature conditions of 3 GPa and 800 °C were applied, starting from Er 2 O 3 (Strem Chemicals, 99.9%), B 2 O 3 (Strem Chemicals, 99.9+%), and ErF 3 (Strem Chemicals, 99.+%) at equal molar ratios. The starting materials were ground up and filled into a boron nitride crucible (Henze BNP GmbH, HeBoSint S100, Kempten, Germany). The compounds were compressed and heated via a multianvil assembly, based on a Walker-type module and a 1000 ton press (both devices from the company Voggenreiter, Mainleus, Germany). Eight tungsten carbide cubes (TSM-20, Ceratizit, Reutte, Austria) compressed the boron nitride crucible, which was positioned inside the centre of an 18/11 assembly. A detailed description of the assembly can be found in reference [11]. The 18/11 assembly was compressed up to 3 GPa in 1.25 h and heated to 800 °C (cylindrical graphite furnace) in the following 10 min, kept there for 20 min, and cooled down to 600 °C in 20 min at constant pressure. After natural cooling down to room temperature by switching off the heating, a decompression period of 3.75 hours was required. Then the recovered octahedral pressure medium (MgO, Ceramic Substrates & Components Ltd., Newport, Isle of Wight, UK) was broken apart and the sample carefully separated from the surrounding graphite and boron nitride. Pink air-and water-resistant crystals of Er 5 (BO 3 ) 2 F 9 were obtained.

Crystal Structure Analysis

The sample was characterized by powder X-ray diffraction, which was performed in transmission geometry on a flat sample of the reaction product, using a STOE STADI P powder diffractometer with MoK α1 radiation (Ge monochromator, λ = 71.073 pm). The powder pattern showed reflections of Er 5 (BO 3 ) 2 F 9 and of µ-ErBO 3 [12] as a by-product of the synthesis (Figure 1). The experimental powder pattern tallied well with the theoretical pattern of Er 5 (BO 3 ) 2 F 9 , simulated from single-crystal data. Indexing the reflections of the erbium fluoride borate, we got the parameters a = 2033.0(3) pm, b = 610.6(3) pm, and c = 825.5(4) pm, with β = 100.29(6)° and a volume of 1008.2(9) Å 3 . This confirmed the lattice parameters, obtained from single-crystal X-ray diffraction (Table 1). Intensity data of a single-crystal of Er 5 (BO 3 ) 2 F 9 were collected at room temperature by use of a Kappa CCD diffractometer (Bruker AXS / Nonius, Karlsruhe), equipped with a Miracol Fiber Optics Collimator and a Nonius FR590 generator (graphitemonochromatized MoK α1 radiation, λ = 71.073 pm). An absorption correction, based on multi-scans [13], was applied to the data set.

All relevant details of the data collection and evaluation are listed in Table 1. The structure solution and the parameter refinement (full-matrix least-squares against F 2 ) were successfully performed, using the SHELX-97 software suite [START_REF] Sheldrick | SHELXS-97 and SHELXL-97, Program suite for the solution and refinement of crystal structures[END_REF][START_REF] Sheldrick | [END_REF] with anisotropic atomic displacement parameters for all atoms. According to the systematic extinctions, the orthorhombic space groups C2/c and Cc were derived. The structure solution in C2/c (no. 15) succeeded. The final difference Fourier syntheses did not reveal any significant residual peaks in all refinements. The positional parameters of the refinements, anisotropic displacement parameters, interatomic distances, and interatomic angles are listed in the Tables 2345. Further information of the crystal structure is available from the Fachinformationszentrum Karlsruhe (crysdata@fiz-karlsruhe.de), D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No. CSD-421469. Table 2. Atomic coordinates and isotropic equivalent displacement parameters (U eq / Å 2 ) for Er 5 (BO 3 ) 2 F 9 (space group: C2/c). Wyckoff sites are 4e for Er3 and F3 and 8f for all other atoms. U eq is defined as one-third of the trace of the orthogonalized U ij tensor.

Results and Discussion

Crystal Structure of Er 5 (BO 3 ) 2 F 9

The structure of Er 5 (BO 3 ) 2 F 9 consists of isolated BO 3groups, erbium cations, and fluoride anions (Figure 2). The arrangement can be described via alternating layers of the formal compositions "ErBO 3 " and "ErF 3 ", spreading into the bc-plane. For a more detailed description of the structure, including a comparison to the related structures of RE 3 (BO 3 ) 2 F 3 (RE = Sm, Eu, Gd) [5] and Gd 2 (BO 3 )F 3 [6], the reader is referred to the description of the isotypic compound Yb 5 (BO 3 ) 2 F 9 (ref. [1]). In this paper, we briefly compare the main features of the compounds. Table 3. Interatomic distances (pm) in Er 5 (BO 3 ) 2 F 9 , calculated with the single-crystal lattice parameters. 5) pm). This can be ascribed to the slightly larger size of Er 3+ (ionic radius = 120 pm for a ninefold coordination [16]), compared to Yb 3+ (ionic radius = 118 pm for coordination number nine [16]). Since the size difference is marginally, no greater deviances of the bond lengths and angles are observed. In both compounds, each of the three crystallographically independent rare-earth cations exhibits a coordination number of nine. In Er Inside the BO 3 -groups, the B-O distances of Er 5 (BO 3 ) 2 F 9 vary between 136.4(7) and 146.1(8) pm (Table 3). The B-O bond lengths tally with those in Yb 5 (BO 3 ) 2 F 9 (between 138.7 (10) and 148.0(12) pm). The unusually large B-O2 bond of 148 pm, observed in Yb 5 (BO 3 ) 2 F 9 , is slightly shorter in the erbium compound (146.1(8) pm), but still exeeds typical bond lengths in BO 3 -groups, which are usually in a range around 137 pm, e.g. in borates with a calcite structure (AlBO 3 (137.96(4) pm) [17], β-YbBO 3 (137.8(4) pm) [18], and FeBO 3 (137.9(2) pm) [19]). Table 4. Interatomic angles (deg) in Er 5 (BO 3 ) 2 F 9 , calculated with the single-crystal lattice parameters.

In analogy to Yb 5 (BO 3 ) 2 F 9 , we have calculated the charge distribution of the atoms in Er 5 (BO 3 ) 2 F 9 via bond valence sums (ΣV) with VaList (Bond Valence Calculation and Listing) [START_REF] Wills | VaList Version 3.0.13[END_REF] and with the CHARDI concept (ΣQ) [START_REF] Brown | [END_REF][22][START_REF] Hoppe | Less-Common Met[END_REF], confirming the formal valence states in the fluoride borate. Table 5 displays the results of the formal ionic charges of the atoms, which are in agreement within the limits of both concepts. In Yb 5 (BO 3 ) 2 F 9 , the bond valence sums for the boron atom were extraordinarily low (ΣV = 2.59), due to the large B-O2 bond present in the structure. Since the boron bond lengths are shorter in Er 5 (BO 3 ) 2 F 9 , we now obtain a better value of ΣV = 2.73 for B1. Furthermore, we have calculated the MAPLE value (Madelung Part of Lattice Energy according to Hoppe [START_REF] Hoppe | [END_REF][25][START_REF]MAPLE (version 4), program for the calculation of MAPLE values[END_REF]) of Er 5 (BO 3 ) 2 F 9 to compare it with the sum of the MAPLE values for the underlying binary components, namely the high-pressure modification of Er 2 O 3 [START_REF] Hoekstra | [END_REF], the ambient-pressure modification of B 2 O 3 (B 2 O 3 -I) [28], and ErF 3 [29]. We obtained a value of 53879 kJ/mol in comparison to 53240 kJ/mol (deviation: 1.2 %), starting from the binary components [1 × Er 2 O 3 (15283 kJ/mol) + 1 × B 2 O 3 -I (20626 kJ/mol) + 3 × ErF 3 (17331 kJ/mol)]. Table 5. Charge distribution in Er 5 (BO 3 ) 2 F 9 , calculated with VaList (ΣV) [START_REF] Wills | VaList Version 3.0.13[END_REF] and the CHARDI concept (ΣQ) [START_REF] Brown | [END_REF][22][START_REF] Hoppe | Less-Common Met[END_REF].

Elemental analyses

Since it is nearly impossible to distinguish between fluoride ions and hydroxyl groups by means of electron density or bond lengths, we performed elemental analyses on our samples of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) to assure the atom assignments in both structures. EDX measurements were performed on a JEOL JSM-6500F scanning electron microscope, equipped with a field emission gun at an acceleration voltage of 16 kV. Samples were prepared by placing single crystals of both phases on adhesive conductive pads and subsequently coating them with a thin conductive carbon film. Each EDX spectrum (Oxford Instruments) was recorded with the analyzed area limited on one single crystal to avoid the influence of possibly contaminating phases. The crystals of Er 5 (BO 3 ) 2 F 9 showed average atomic compositions (theoretical values in brackets) of Er 63.1 (74.3) wt%; B 8.9 (1.9) wt% ; O 9.3 (8.5) wt% ; F 18.7 (15.2) wt%. For Yb 5 (BO 3 ) 2 F 9 , average compositions (wt%) of Yb 70.7 (75.0) wt% ; B 4.6 (1.9) wt% ; O 8.4 (8.3) wt% ; F 16.3 (14.8) wt% were obtained. Due to the light weight of boron, measurements have to be taken with caution, but still, these results confirm the presence of all elements and the composition, obtained from the single crystal structure determination.

Furthermore, the composition of Yb 5 (BO 3 ) 2 F 9 was confirmed by chemical analysis with a JEOL JXA-8100 electron microprobe. The sample was prepared by embedding bulk material in epoxy resin and polishing with diamond polishing paste. Analytical conditions were a 15 kV acceleration voltage and a 20 nA beam current with measurement times of 40 sec on the peaks and 20 sec on the backgrounds of the O-K α , F-K α , and Yb-L α X-ray lines. SiO 2 , Al 2 SiO 4 F 2 , and Yb 5 P 5 O 14 were used as standards for O, F, and Yb, respectively, and the raw counts were corrected with the ZAF correction procedure. Boron could not be determined quantitatively with the electron microprobe due to its low concentration. An average of 3 analyses on three different crystals yielded (theoretical values in brackets): Yb 74.4±0.2 (75.0) wt%; O 8.6±0.4 (8.3) wt%; F 14.5±0.6 (14.8) wt%.

Physical Properties of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb)

UV/vis spectroscopy of Yb 5 (BO 3 ) 2 F 9

The electronic absorption spectrum of Yb 5 (BO 3 ) 2 F 9 was measured at 293 K in the nir/vis/UV (5800 -28000 cm -1 , step-width ∆λ(UV/vis) = 1 nm, ∆λ(nir) = 2 nm), using a strongly modified CARY 17 microcrystal spectrophotometer (Spectra Services, ANU Canberra, Australia). Details of the spectrometer have already been described in literature [30,31]. A polycrystalline agglomerate with dimensions 0.2 × 0.2 × 0.05 mm 3 was selected for the investigation. The noisy spectrum (Fig. 3) results from the small size and rather modest optical quality of the agglomerate. The reference intensity was measured with a pinhole instead of the crystal mounted on an aperture. The electronic absorption spectrum of a polycrystalline agglomerate of Yb 5 (BO 3 ) 2 F 9 shows the transition 2 F 7/2 → 2 F 5/2 , expected for Yb 3+ at ν% ≈ 10400 cm -1 [START_REF] Henderson | Optical Spectroscopy of Solids[END_REF]. Due to weak ligand-field interaction, the signal is split absorbance units (a. u.)

ν 3 ν 3 ν 1 ν 1 ν 2 ν 2 ν 4 ν 4
into a sharp peak at ν% = 10236 cm -1 and a broader shoulder at ν% = 10660 cm -1 . Figure 3. Electronic absorption spectrum of a polycrystalline agglomerate of Yb 5 (BO 3 ) 2 F 9 .

IR spectroscopy

FTIR absorbance spectra of single crystals of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) were measured in transmitted, polarized light on a BaF 2 plate using a BRUKER VERTEX 70 spectrometer, attached to a HYPERION 3000 microscope, a MIR light source, and a LN-MCT detector. Spectral resolution was ~ 4 cm -1 . Minimum-maximum normalization was done by the OPUS 6.5 software (BRUKER). In Figure 4, the spectra of both compounds are displayed. The absorptions of Yb 5 (BO 3 ) 2 F 9 (Fig. 4, bottom) correspond to the absorptions measured on the bulk material (Ref. [1]). In the IR-spectrum of the isotypic erbium compound (Fig. 4, top), the absorption bands are slightly shifted to higher wavelengths. The absorption patterns of the IR spectra are typical for borates exhibiting triangular BO 3 -groups [1,[START_REF] Laperches | [END_REF]34]. Below 600 cm -1 for Yb 5 (BO 3 ) 2 F 9 and below 650 cm -1 for Er 5 (BO 3 ) 2 F 9 , bands are evoked by the in plane bending (ν 4 ) of the BO 3 -groups. The strong and usually sharp absorptions derived from the out of plane bending (ν 2 ) of the trigonal ion occur in the range of 600 -780 cm -1 for Yb 5 (BO 3 ) 2 F 9 and 650 -850 cm -1 for Er 5 (BO 3 ) 2 F 9 . According to the crystalline environment, the absorption bands between 800 -950 cm -1 for Yb 5 (BO 3 ) 2 F 9 and 900 -1000 cm -1 for Er 5 (BO 3 ) 2 F 9 can be classified as symmetric stretching vibrations (ν 1 ). Asymmetric stretching vibrations (ν 3 ) of the BO 3 -groups are assigned in the areas of 1150 -1400 cm -1 for Yb 5 (BO 3 ) 2 F 9 and of 1200 -1500 cm -1 for Er 5 (BO 3 ) 2 F 9 . Interestingly, two bands between 3400 and 3600 cm -1 were detected in the single crystal spectra of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) (Figure 5), which were not present in the corresponding spectrum of dried bulk Yb 5 (BO 3 ) 2 F 9 [1]. Bands in this range usually indicate OH-groups in watercontaining borates. The substitution of fluoride with hydroxyl groups is a problem commonly known from fluoride borates [3,4]. The single crystals, which were examined via IR spectroscopy, have been exposed to air and humidity and were not dried under high vacuum like the bulk material of 1 000 1 500 wavenumber (cm -1 ) Yb 5 (BO 3 ) 2 F 9 . Thus, the exchange of fluoride with OHgroups occurs due to hygroscopic alteration. In apatites, the exchange of hydroxyl and fluoride ions was previously observed and studied [35,36].

Raman spectroscopy

Raman spectra of single crystals of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) were measured without polarizers in the range 100 -1600 cm -1 , using a HORIBA LABRAM HR-800 confocal micro-spectrometer and a 785 nm diode laser. Laser focus and power on the sample surface was ~1 µm and ~10 mW, respectively. Depth resolution was ~4 µm, spectral resolution ~6 cm -1 . Wavenumber accuracy of ~3 cm -1 was achieved by regularly adjusting the zero-order position of the grating and checked by a Neon calibration lamp. Third order polynomial background subtraction, normalization and band fitting by Gauss-Lorentz functions were done by the LABSPEC 5 software (HORIBA). Some glue contamination cannot be excluded. The Raman spectra of the isotypic compounds RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) are quite similar with the most intense bands < 500 cm -1 and the narrow, "isolated" band at ~940 cm -1 (Figure 6). Above 1000 cm -1 , one broad band for Yb 5 (BO 3 ) 2 F 9 and several broader bands for Er 5 (BO 3 ) 2 F 9 are observed. Bands around 900 cm -1 in borates are usually assigned to stretching modes of BO 4 -tetrahedra whereas BO 3 -groups are expected > 1100 cm -1 [4, 37 -42]. However, in hydrated monoborates very intense bands at 882 and 501 cm -1 were observed and assigned to symmetric stretching vibrations of the isolated BO 3 -groups [39]. The unusually large variation of B-O distances inside the BO 3 -groups (see above) might account for the comparatively low wavenumber and the large wavenumber range of these bands. Bands < 500 cm -1 can be assigned to bending and stretching vibrations of the isolated BO 3 -groups, RE-O, RE-F, F-O bonds, and lattice vibrations.

Thermal behaviour of Yb 5 (BO 3 ) 2 F 9

In situ X-ray powder diffraction experiments were done on a STOE STADI P powder diffractometer [MoK α1 radiation (λ = 71.073 pm)] with a computer controlled STOE furnace: The sample was enclosed in a silica capillary and heated from room temperature to 500 °C in 100 °C steps, and from 500 °C to 1100 °C in 50 °C steps. The heating rate was set to 40 °C/min. Afterwards, the sample was cooled down to 500 °C in 50 °C steps, and from 500 °C to room temperature in 100 °C steps (cooling rate: 50 °C/min). After each heating step, a diffraction pattern was recorded over the angular range 3° ≤ 2θ ≤ 40°. The temperature-dependent X-ray powder diffraction experiment on Yb 5 (BO 3 ) 2 F 9 indicated a decomposition and subsequent phase changes of the decomposition products (Figure 7). Successive heating of Yb 5 (BO 3 ) 2 F 9 in the range of 650-850 °C resulted in a decomposition into a mixture of π-YbBO 3 [43] and another still unidentified phase. Further heating led to a transformation of π-YbBO 3 into the high-temperature polymorph µ-YbBO 3 [43] in the range of 900-1100 °C. Additionally, reflections of YbF 3 and a formation of the silicate Yb 2 Si 2 O 7 [44] were observed. The latter was formed by a reaction with the silica capillary, which is known to happen occasionally. Subsequent cooling showed a complete retransformation into the low-temperature polymorph π-YbBO 3 below 500 °C. The formation and transformation of π-YbBO 3 into the high-temperature polymorph µ-YbBO 3 and back have been often observed in temperature-dependent X-ray powder diffraction experiments of ytterbium borates, e.g. with Yb 3 B 5 O 12 [9].

Conclusions

The successful syntheses of Yb 5 (BO 3 ) 2 F 9 (7.5 GPa) and the isotypic erbium fluoride borate Er 5 (BO 3 ) 2 F 9 at a pressure of 3 GPa suggest a large formation region for these high pressure phases. The application of similar synthetic conditions to neighbouring rare-earth elements could lead to additional isotypic compounds and will be studied in the future. However, in the case of RE 3+ = Yb 3+ , experiments at higher pressures (10 GPa) resulted in the formation of an apatite-structured compound, wherein all boron atoms showed a fourfold coordination [10].

Figure 1 .

 1 Figure 1. Experimental powder pattern of Er 5 (BO 3 ) 2 F 9 in comparison to the theoretical powder pattern beneath, based on single-crystal diffraction data. Reflections of µ-ErBO 3 are marked with lines.

  data Single-crystal diffractometer Bruker AXS / Nonius Kappa CCD Radiation MoK α1 (λ = 71.073 pm) a (pm) 2031.2(4) b (pm) 609.5(2) c (pm) 824int = 0.0382) Reflections with I > 2σ(I) 2315 (R σ = 0.0389) Data / parameters 2700 / 102 Absorption correction Multi-scan [13] Goodness-of-fit (F 2 ) 1.062 Final R indices (I > 2σ(I)) R1 = 0.0289 wR2 = 0.0530 R indices (all data) R1 = 0.0384 wR2 = 0.0558 Largest differ. peak / deepest hole (e/Å -3 ) 2.71 / -2.55

Figure 2 .

 2 Figure 2. Crystal structure of Er 5 (BO 3 ) 2 F 9 , showing isolated BO 3groups.

5 (

 5 BO 3 ) 2 F 9 , we find Er-O/F distances in the range of 231.2(3) -288.8(4) pm, which tally well with the values obtained for the Yb-O/F distances in Yb 5 (BO 3 ) 2 F 9 (227.4(5) to 294.2(5) pm). Focussing on the coordination spheres of the fluoride ions, we find a coordination number of three for all fluoride ions in the two isotypic compounds. In Yb 5 (BO 3 ) 2 F 9 , the F-Yb distances are slightly shorter than the F-Er bond lengths in Er 5 (BO 3 ) 2 F 9 , which agrees with the slight size difference of the cations.

Figure 4 .

 4 Figure 4. FTIR absorbance spectra of single crystals of Er 5 (BO 3 ) 2 F 9 (top) and Yb 5 (BO 3 ) 2 F 9 (bottom) in the range 550-2000 cm -1 .

Figure 5 .

 5 Figure 5. FTIR absorbance spectra on single crystals of Er 5 (BO 3 ) 2 F 9 (top) and Yb 5 (BO 3 ) 2 F 9 (bottom) in the range 3000-4000 cm -1 .

Figure 6 .

 6 Figure 6. Raman-spectra on single crystals of Yb 5 (BO 3 ) 2 F 9 (top) and Er 5 (BO 3 ) 2 F 9 (bottom) in the range 100-1600 cm -1 .

Figure 7 .

 7 Figure 7. Temperature-dependent X-ray powder patterns, following the decomposition reaction of Yb 5 (BO 3 ) 2 F 9 .

Table 1 .

 1 Crystal data and structure refinement of Er 5 (BO 3 ) 2 F 9

	Empirical Formula	Er 5 (BO 3 ) 2 F 9
	Molar mass (g⋅mol -1 )	1124.92
	Crystal system	monoclinic
	Space group	C2/c (No. 15)
	Lattice parameters from powder data
	Powder diffractometer	STOE STADI P
	Radiation	MoK

α1 (λ = 71.

Acknowledgments

We thank Dr. Gunter Heymann for collecting the single-crystal data. Special thanks go to Christian Minke (LMU Munich) for the EDX analyses. ____________


Bernhard Neumüller

Dr Kurt Dehnicke
email: dehnicke@chemie.uni-marburg.de

Keywords: Beryllium, N-Methylimidazole complexes, X-ray diffraction Beryllium, N-Methylimidazole complexes, X-ray diffraction

Me-Im) 4 ]I 2 und [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 N-Methylimidazole Complexes of Beryllium: [Be(Me-Im) 4 ]I 2 and [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3

in the presence of moisture air. According to X-ray diffraction studies, two different crystal individuals (2a and 2b) result, depending on the starting materials BeCl 2 and (Ph 4 P) 2 [Be 2 Cl 6 ], respectively [2a: Space group P2 1 /n, Z = 4; 2b: Space group P 1, Z = 2]. As a side-product from the reaction of N-methylimidazole with (Ph 4 P) 2 [Be 2 Cl 6 ] single crystals of (Ph 4 P)Cl•CH 2 Cl 2 (3) were identified crystallographically (P2 1 /n, Z = 4) which are isotypical with the corresponding known bromide (Ph 4 P)Br•CH 2 Cl 2 .

___________________________________________________________________________ 1 Einleitung

In vorangegangenen Arbeiten haben wir über Synthesen und Kristallstrukturen von Polyiodiden des Berylliums mit homoleptisch solvatisierten Dikationen [Be(L) 4 ] 2+ mit L = Pyridin [1], Tetramethylharnstoff [2] und Diisopropylcyanamid [3] O [16] und dem Chloridoderivat (Ph 4 P) 2 [Be 3 (µ-OH) 3 (H 2 O) 6 ]Cl 5 , das auch durch DFT-Rechnungen charakterisiert wurde [17].

(µ-OH) 3 (C 5 H 4 N-CO 2 ) 3 ]•H 2 O [13], von den Pyrazolylboraten [Be 3 (µ-OH) 3 {HB(pz) 3 } 3 ] [14] und [Be 3 (µ-OH) 3 {HB(3,5-Me 2 pz) 3 } 3 ] [15] sowie von dem Kaliummalonat K 3 [Be 3 (µ-OH) 3 (O 2 CCH 2 CO 2 ) 3 ]•6H 2
Das in wäßrigen Lösungen stabile [Be 3 (µ-OH) 3 (H 2 O) 6 ] 3+ -Ion wurde auch kernresonanzspektroskopisch untersucht [18].

Ergebnisse

[Be (N-Methylimidazol) [Be(Tetramethylharnstoff) 4 ](I 3 ) 2 [2] und [Be(Diisopropylcyanamid) 4 ](I 3 ) 2 [3]. Blaßgelbe

  berichtet, die sich sehr leicht bereits bei Raumtemperatur aus Berylliumpulver, überschüssigem Iod und den Solvatmolekülen L herstellen lassen. Hierbei wirken die Solventien L als Lösungsmittel und Reaktanden zugleich. Wir beschreiben hier den auf analoge Weise zugänglichen N-Methylimidazol-Komplex [Be(Me-Im) 4 ]I 2 , der überraschenderweise auch mit überschüssigem Iod kein Polyiodid bildet. Homoleptische Imidazolkomplexe zweiwertiger Metallionen sind nur von Kupfer und Zink beschrieben. Neben dem Kupfer(II)-Imidazolkomplex [Cu II (Im) 5 ](PhOPO 3 )•4H 2 O [4] kennt man auch den Kupfer(I)-N-Methylimidazolkomplex [Cu I (Me-Im) 4 ]ClO 4 mit 4-Symmetrie [5], dessen Kation [Cu(Me-Im) 4 ] + isoelektronisch ist mit dem Dikation des Zinkkomplexes [Zn(Me-Im) 4 ](ClO 4 ) 2 [6], der seinerseits isotyp mit dem in dieser Arbeit beschriebenen Berylliumkomplex [Be(Me-Im) 4 ]I 2 (1) in der Raumgruppe I 42d kristallisiert. Darüber hinaus sind auch die Strukturen der Tetraimidazolkomplexe des Zinks [Zn(Im) 4 ](X) 2 mit den Gegenionen X = (BF 4 ) -[7], X = (ClO 4 ) -[8] und X = 2-Hydroxy-3,5-dinitrobenzoat [9] bekannt sowie auch die durch Variation der Gegenionen zugänglichen Hexa-Imidazolkomplexe [Zn(Im) 6 ]Cl 2 •4H 2 O [10] und [Zn(Im) 6 ](Ph 3 C-CO 2 ) 2 [11]. Wir ergänzen unseren Bericht mit erfolglos verlaufenen Reaktionen von N-Methylimidazol mit BeCl 2 und (Ph 4 P) 2 [Be 2 Cl 6 ], die im Gegensatz zu anderen N-Donormolekülen nicht zu kristallinen Komplexen der Typen [BeCl 2 (L) 2 ] bzw. [BeCl 3 (L)] - [12] führen, wohl aber bei Zutritt von Luftfeuchtigkeit zwei Kristallvarianten von Tris(µhydroxo)hexamethylimidazoltriberyllium-Trichlorid, [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 , bilden. Kristallstrukturanalysen von Komplexen mit dem [Be 3 (µ-OH) 3 ] 3+ -Fragment liegen vor von dem Pyridin-2-carboxylat [Be 3

Einkristalle von Verbindung 1 , 3 )

 13 die für eine Kristallstrukturanalyse geeignet waren (siehe unten), erhielten wir nach Überschichten der Reaktionslösung mit Pentan und anschließendem Schockgefrieren mittels flüssigen Stickstoffs. Im IR-Spektrum von 1 werden die meisten Frequenzen des Spektrums von N-Methylimidazol durch die Koordination am Berylliumion nur unwesentlich beeinflußt. Eine Ausnahme macht die starke Absorption bei 819 cm -1 , die im Spektrum von 1 eine Aufspaltung in das Triplett 800, 827, 847 cm -1 erfährt. Zudem tritt im Spektrum von 1 eine neue sehr starke Bande bei 718 cm -1 auf, deren Ursache wir als eine Folge der durch die Be-N-Bindung veränderten Ringkonjugation ansehen. Darüber hinaus beobachten wir im Spektrum von 1 noch zwei weitere Absorptionsbanden, die im Spektrum von N-Methylimidazol nicht auftreten, nämlich eine sehr starke Bande bei 993 cm -1 und eine Bande mittlerer Intensität bei 929 cm -1 , die wir den beiden für I 4-Symmetrie zu erwartenden Be-N-Valenzschwingungen [19] der Rassen E 1 und B 2 zuordnen. Diese Banden sind noch kürzerwellig als die Be-N-Valenzschwingungen im Kation von [Be(Pyridin) 4 ] 2 (I 3 ) 2 (I 4 ) (∼ 900 cm -1 , vst, br [1]), was aber gut zu den Be-N-Bindungslängen paßt. Diese sind in der Struktur der Verbindung 1 nochmals drei pm kürzer als die ohnehin schon sehr kurzen Be-N-Bindungen im Pyridinkomplex mit im Mittel 173 pm [1]. Zum vollständigen IR-Spektrum siehe Experimentellen Teil. [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 (2) und (Ph 4 P)Cl•CH 2 Cl 2 (Versuche in der Absicht, definierte Donor-Akzeptorkomplexe der Typen [BeCl 2 (L) 2 ] und [BeCl 3 (L)] -mit L = N-Methylimidazol in Dichlormethan aus BeCl 2 bzw. (Ph 4 P) 2 [Be 2 Cl 6 ] herzustellen, waren trotz Variation der Versuchsbedingungen erfolglos. Dies überrascht, da viele N-Donormoleküle L komplikationslos Molekülkomplexe [BeCl 2 (L) 2 ] [20, 12] und anionische Varianten [BeCl 3 (L)] -[12] bilden.

3 Schema 1 2 [ 23 ]

 31223 Abbildung 3 sind die beiden Struktureinheiten zum Vergleich wiedergegeben.

  unter Bildung farbloser Einkristalle, die zur Kristallstrukturanalyse geeignet sind. Die Verbindungen 2a und 2b unterscheiden sich nicht nur durch die verschiedenen Anteile der in die Kristallgitter eingelagerten Solvensmoleküle, sondern auch durch die Orientierung der Donormoleküle Me-Im zueinander (siehe unten). Bei Anwendung von (Ph 4 P) 2 [Be 2 Cl 6 ] läßt sich durch Auslesen eine zweite Kristallsorte isolieren, bei der es sich um (Ph 4 P)Cl•CH 2 Cl 2 (3) handelt, was wir kristallographisch ermittelt haben. Mögliche weitere Produkte als die in Schema 1 genannten haben wir nicht identifizieren können.

	Me-Im
	BeCl 2	[Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 •1.25CH 2 Cl 2 •0.63H 2 O
	CH 2 Cl 2 , H 2 O
		2a
		[Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 •3CH 2 Cl 2
	Me-Im
	(Ph 4 P) 2 [Be 2 Cl 6 ]	2b
	CH 2 Cl 2 , H 2 O

Lösungen von BeCl 2 oder (Ph 4 P) 2 [Be 2 Cl 6 ] [21] in Dichlormethan/N-Methylimidazol-Gemischen reagieren aber bei dosierter Zufuhr von feuchter Luft in langsamer Reaktion gemäß dem Schema 1 (Ph 4 P)Cl•CH 2 Cl 2

Tabelle 1

 1 Kristalldaten und Angaben zu den Strukturanalysen von [Be(Me-Im) 4 ]I 2 (1), [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 •1.25CH 2 Cl 2 •0.63H 2 O (2a), [Be 3 (µ-OH) 3 (Me-Im) 6 ]Cl 3 •3CH 2 Cl 2 (2b) und (Ph 4 P)Cl•CH 2 Cl 2 (3) Summenformel C 16 H 24 BeI 2 N 8 C 25.25 H 42.75 Be 3 Cl 5.5 N 12 O 3.63 C 27 H 45 Be 3 Cl 9 N 12 O 3 C 25 H 22 Cl 3 P -16 ≤ h ≤ 16, -27 ≤ k ≤ 27, -16 ≤ l ≤ 16 -16 ≤ h ≤ 16, -16 ≤ k ≤ 16, -16 ≤ l ≤ 17 -12 ≤ h ≤ 12, -20 ≤ k ≤ 20,

						ZAAC				Page 14 of 20
	Raumgruppe Flack-Parameter	I 42d -0.08(5)			P2 1 /n -			P 1 -		P2 1 /n -
	Absorptionskorrektur R 1	numerisch 0.0238			numerisch 0.0479			numerisch 0.0588		numerisch 0.0366
	µ/cm -1 wR 2 (alle Daten)	28.53 0.0391			4.35 0.11			6.01 0.1519		4.83 0.0772
	Meßtemperatur/K max. Restelektronendichte	100(2) 0.409	1		193(2) 0.546	2a		193(2) 0.573	2b	193(2) 0.207	3
	Instrument 2θ max /° /10 -6 e•pm -3	IPDS 2T (Stoe) 51.76		IPDS I (Stoe) 52.44		IPDS I (Stoe) 51.80	IPDS I (Stoe) 52.16
	Strahlung hkl-Werte	Mo-K α -21 ≤ h ≤ 16, -19 ≤ k ≤ 21, a) w =	Mo-K α a) w =			Mo-K α a) w =		Mo-K α a) w =
		-8 ≤ l ≤ 7 1/[σ 2 (F o 2 )+(0.0141•P) 2 ;	1/[σ 2 (F o 2 )+(0.0343•P) 2 ;	1/[σ 2 (F o 2 )+(0.0691•P) 2 ;	-16 ≤ l ≤ 16 1/[σ 2 (F o 2 )+(0.0356•P) 2 ;
	Formelgewicht/g•mol -1 gemessene Reflexe	591.24 3208 P = [max(F o 2 , 0)+2•F c	2 ]/3	794.47 39749 P = [max(F o 2 , 0)+2•F c	2 ]/3	931.83 20514 P = [max(F o 2 , 0)+2•F c	2 ]/3	459.75 18450 P = [max(F o 2 , 0)+2•F c	2 ]/3
	Kristallabmessungen/mm 0.05 × 0.05 × 0.03 unabhängige Reflexe 1006		0.34 × 0.33 × 0.15 7925		0.33 × 0.27 × 0.15 8219	0.38 × 0.28 × 0.25 4295
	a/pm R int	1784.9(1) 0.0392			1358.6(1) 0.1646			1356.3(2) 0.1026		1027.3(1) 0.0691
	b/pm Reflexe mit F o >4σ(F o )	1784.9(1) 905			2258.3(2) 2708			1369.0(2) 3527		1668.1(2) 2488
	c/pm Parameter	696.2(1) 64			1358.6(1) 457			1402.1(2) 503		1335.9(1) 271
	α/° Strukturlösung a)	90 direkte Methoden		90 direkte Methoden		72.99(2) direkte Methoden	90 direkte Methoden
	β/°	90 (SIR-92 [27])		103.11(1) (SIR-92 [27])		74.37(1) (SIR-92 [27])	96.71(1) (SHELXS-97 [28])
	γ/° Verfeinerung gegen F 2	90 SHELXL-97 [28]		90 SHELXL-97 [28]		67.07(1) SHELXL-97 [28]	90 SHELXL-97 [28]
	Zellvolumen/pm 3 H-Atome	2218.0(4)•10 6 berechnete Positionen mit	4059.7(6)•10 6 berechnete Positionen mit	2256.8(6)•10 6 berechnete Positionen mit	2273.6(4)•10 6 berechnete Positionen mit
		gemeinsamem Auslen-	gemeinsamem Auslen-	gemeinsamem Auslen-	gemeinsamem Auslen-
	Z	4 kungsparameter		4 kungsparameter; H1 -H3	2 kungsparameter; H1 -H3	4 kungsparameter; freie
	d rönt /g•cm -3	1.771			1.300 wurden frei verfeinert	1.371 wurden frei verfeinert	1.343 Verfeinerung für H251 und
	Kristallsystem	tetragonal			monoklin			triklin		monoklin H252
						Wiley-VCH

Page 15 of 20 Wiley-VCH ZAAC