Crystal structure at (T = 295 and 173 K) of[(NH₄)_0.63Li_0.37]₂TeBr₆

1 Introduction

Gillespie and Nyholm (1957), Gillespie (1970) developed the valence shell electron-pair repulsion theory, according to which the lone pair of electrons present in, for example, the hexachlorotellurate(IV) complex anion should be stereochemically active and distort the coordination polyhedron from being regular octahedral (Gillespie effect). However, several X-ray crystal structures of A₂[TeX₆] salts (A being an alkali metal and X an halogen) have been determined (Engel, 1935; Hazell, 1966; Aynsley and Hazell, 1963; Brown, 1964; Webster and Collins, 1973), and in all cases reported so far the coordination was found to be regular octahedral at room temperature.

For compounds A₂TeBr₆ (with A = K, NH₄, Rb, Cs) the high temperature phases show the anti-fluorite (K₂[PtCl₆]-type) structure with space group Fm  $\overline{3}$  m(Oh⁵). On lowering temperature, a second order phase transition can be observed, especially when the anions are comparatively large (Das et al., 1966).

Of particular interest is (NH₄)₂TeBr₆, which exhibits a Cubic–Tetragonal phase transition at low temperature. In order to examine the effect and the influence of cationic substitution over symmetry and physical properties, we have extended these investigations to[(NH₄)_xLi_1−x]₂TeBr₆.

A description of the crystal structure at room and low temperatures of[(NH₄)_0.63Li_0.37]₂TeBr₆ is reported in this paper.

2 Experimental

The mixed compound of the composition[(NH₄)_xLi_1−x]₂TeBr₆ was synthesized by adding the hot saturated solutions of LiBr and NH₄Br and TeO₂ in HBr (1:1:1 mole ratio). The precipitated powder was thoroughly dried and sealed under vacuum. All starting components were of a purity of 99.99 wt.%.

Slow cooling gave bright red octahedral single crystals of[(NH₄)_xLi_1−x]₂TeBr₆ which were filtered and stored for several days in a desiccators containing a small breaker of potassium hydroxide pellets in addition to the silica gel. As the crystals of the hexabromotellurate are very sensitive to moisture, they were protected by paraffin-oil.

The formula[(NH₄)_0.63Li_0.37]₂TeBr₆ was determined by refinement of the crystal structure at two different temperatures.

The differential scanning calorimetry measurements were performed between 123 and 523 K with a DCS METTLER TA4000 at heating speed of 10 K mn⁻¹.

Crystal data collection procedure and structure refinement, at room and low temperatures, is given in Table 1. Total reflections were collected with OXFORD KM4CCD diffractometer, corrections were made for Lorentz-polarisation effects and absorption.

The positions of the tellurium atoms were determined from a three dimensional Patterson synthesis. Bromine, nitrogen, lithium and hydrogen atoms were located by three-dimensional Fourier function. Structure solution and refinement were carried out using SHELX programs (Sheldrick, 1986, 1997). The non-hydrogen atoms were refined anisotropically. The H atoms were attributed isotropic thermal factors equal to those of the atoms on which they are linked. The atomic coordinates of the room and low temperatures are given in Table 2 while the anisotropic displacement parameters are presented in Table 3.

3 Results

3.1

3.1 Calorimetric study

The DSC curve of the[(NH₄)_0.63Li_0.37]₂TeBr₆ crystals for heating-up is shown in Fig. 1. We observe three endothermic peaks at 195, 395 and 498 K. The corresponding enthalpy changes are ΔH1 = 0.24 J/g, ΔH₂ = 6.24 J/g and ΔH₃ = 22.18 J/g, respectively. The third peak corresponds to the decomposition of the materials.

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Figure 1

The DSC curves of[(NH₄)_0.63Li_0.37]₂TeBr₆ for the heating-up runs.

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3.2

3.2 Structural properties

3.2.1

3.2.1 Room temperature (298)

The structure of this family of salts was first deduced by Wyckoff and Posnjak (1921). The Te atoms lie on the 4(a) sites of the Fm  $\overline{3}$  m(Oh⁵) space group, surrounded by an octahedron of halogen atoms in the 24(e) positions with coordinates (x, 0, 0) and $x\simeq 0.24$ . The average distance Te–Br is 2.6747(4) Å.

The Li or N atoms occupy the 8(c) (¼, ¼, ¼) sites, and the H atoms (by implication) the 32(f) (x, x, x) sites. Diffraction experiments on the isomorphous (NH₄)₂SiF₆ by Schlemper et al. (1966) have provided information on the orientation and the thermal motion of the ammonium group, and similar models of ordering have been used in this structural refinement of[(NH₄)_0.63Li_0.37]₂TeBr₆ (see Fig. 2).

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Figure 2

Room temperature cubic unit cell of[(NH₄)_0.63Li_0.37]₂TeBr₆ having the K₂PtCl₆ type structure.

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The ammonium tetrahedron may be placed in two similar positions with its axis along[1 1 1], but with either the base or the apex towards the origin. The H atoms occupy the 32(f) positions with $x\simeq 0.19$ and $x\simeq 0.31$ , respectively.

In the room temperature data refinement, the R factors corresponding to the two previous sites are 5.63% and 6.14%, respectively. It was hard to confirm one of the tendencies, as the R factors values are very close. The H atom was later placed in the position selected from three-dimensional Fourier function and this gave x_H = 0.3007, y_H = 0.1993, z_H = 0.1993 and R = 4.64%. The H atoms of the ammonium group are orientated with its apex towards the corners, none occupied by Te atoms of the one-eighth cell. The orientation of the ammonium group in[(NH₄)_0.63Li_0.37]₂TeBr₆ is thus the opposite of that observed at room temperature in (NH₄)₂SiF₆.

The ${\text{NH}}_4^{+}$ tetrahedra, like Li atoms, reside in the tetrahedral site of the F.C.C cell. This presence of both Li⁺ and ${\text{NH}}_4^{+}$ cations induce the coexistence of two types of bonds:

• –

  Ionic bonding between cationic entities Li⁺ and[TeBr₆]²⁻ anionic complexes.

• –

  H bonding contacts N–H⋯Br providing a linkage between cationic entities ${\text{NH}}_4^{+}$ and[TeBr₆]²⁻ anionic complexes.

Li/N atoms are 12-fold coordinated by Br atom neighbors. The average distance Li–Br is 3.790(2) Å.

3.2.2

3.2.2 Low temperature 173 K

At low-temperature (173 K),[(NH₄)_0.63Li_0.37]₂TeBr₆ crystallizes in the tetragonal space group (P4/mnc (No. 128)). The tellurium atoms are located on the 4-fold axis along[0 0 1] direction. Each Te atom is surrounded by six Br atoms forming a slightly distorted octahedral structure with the distances 2.6806(2) and 2.6881(3) Å. The small differences of Te–Br bond lengths in the low temperature structure (Fig.3) are not significant considering the standard deviations and are typical for a tetragonal refinement of positional parameters. So the point symmetry of the ${\text{TeBr}}_6^{2-}$ ion is still m3m. There appears to be no stereochemical active lone pair of electrons at Te(IV) in this hexahalogeno complex ion.

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Figure 3

Environment of tellurium atom at 173 K.

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The decreasing of the temperature gives the following results:

1. The thermal motion of Te, Br and Li is reduced. Thus the tellurium atom at low temperature tends to have more ordered and stable octahedral coordination.

2. An anti-ferrorotative displacement of the ${\text{TeBr}}_6^{2-}$ octahedral (tilt angle 6 °).

3. The coordination of Li by Br changes from a 12-fold (Fm  $\overline{3}$  m) to a 4 + 4 + 4 (P4/mnc) one, while the (mean) distances Li–Br decrease from 298 to 175 K.

Fig. 4 shows the tetragonal structure in a projection along the 4-fold axis.

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Figure 4

Projection of the tetragonal structure along the 4-fold axis at low temperature.

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4 Conclusion

The new mixed compound[(NH₄)_0.63Li_0.37]₂TeBr₆ crystallises in the cubic system with the space group Fm  $\overline{3}$  m at room temperature (295 K) and in the tetragonal system P4/mnc at 173 K. It appears from this study that no distortion of the type predicted for[TeBr₆]²⁻ by Gillespie and Nyholm has been found at room and low temperatures in the mixed compound. There appears to be no stereochemical active lone pair of electrons at Te(IV) in this hexahalogeno complex ion.

This study shows that the title compound undergoes a phase transition at about 195 K as determined by DSC. This phase transition is characterised by an anti-ferrorotative displacement of the ${\text{TeBr}}_6^{2-}$ octahedral (tilt angle 6 °).

Further experiments should enable us to study the effect of this substitution on phase transition temperatures.