CRYSTALLOGRAPHY OF THE Sb-Te-Ni SYSTEM

F. Laufek^1,3, M. Drábek¹, R. Skála², I. Císařová³

¹Czech Geological Survey, Geologická 6, Praha 5, 152 00, Czech Republic
² Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, Praha 6, 165 02, Czech Republic

³ Faculty of Science, Charles University, Hlavova 8, , Praha 2, 128 43, Czech Republic

The new unnamed nickel antimonide telluride Ni₂SbTe₂ was found as 6 μm grain by Vavřín and Frýda [1] at the Kunratice Cu-Ni deposit (North Bohemia). This ternary phase is in a close association with melonite (NiTe₂); this assembly is included in pyrrhotite (Fe_1-xS). In order to determine the crystal structure of this phase (from the synthetic analogue) and to further explore the Ni-Sb-Te phase diagram, the crystallography of this ternary system was investigated. The phases were prepared using the silica glass tube method. Our experiments were performed in the Experimental laboratory of Czech Geological Survey. High-purity elements – tellurium (99.999 %), antimony (99.99 %) and nickel (99.995 %) were used as starting materials. Before being used for syntheses, nickel was heated for 1 hour in a stream of H₂ at 800 ˚C. Carefully weighted samples were loaded into the high purity silica tubes and tightly fitting silica rods were placed on the top of the reagents in order to reduce the vapour volume during heating. The silica tubes with the charge were sealed under vacuum and then heated in the horizontal furnaces in which the temperature was controlled electronically. The maximum temperature variation did not exceed 4 ˚C. The samples were heated at 400˚C or at 800˚C for three weeks. The experiments were terminated either by quenching in a cold bath or by slow controlled cooling to room temperature.

The crystal structure of the new phase Ni₂SbTe₂ prepared at 800˚C (experiment terminated by quenching), determined from X-ray single diffraction data, is hexagonal, NiAs type, with lattice parameters: a = 3.9108(2), c = 5.2489(3) Å, space group P6₃/mmc (no. 194). The antimony and tellurium occupy the crystallographic position 2c; the position 2a is occupied by nickel atoms (Fig. 1)

The crystal structure of Ni₂SbTe₂ prepared at 400 ˚C (experiment terminated by slow controlled cooling to room temperature within interval 22 hours), originally described by [2], refined from single X-ray diffraction data, is hexagonal with lattice parameters a = 3.9110(2), c = 15.696(1) Å, space group P6₃/mmc (no. 194). The antimony and tellurium atoms occupy different crystallographic positions, antimony 2c and tellurium 4f. (Fig. 2). The crystal structure can be described as “hybrid” of NiSb and NiTe₂ with an elongated c-axis. In NiSb structure, the close packed anion layers are all Sb with ABAB stacking and all octahedral holes are filled with Ni. If every third plane of Ni atoms is removed from NiSb and Sb replaced with Te in two out of three layers, we can obtain the Ni₂SbTe₂ structure (low temperature modification). The atom stacking sequence is CABCAC (A – Ni, B – Sb, C – Te). The interlayer Te – Te distance is 3.543 Å. It is also to be noted that this interatomic distance is larger than Te₂ pair-cointaining phases (2.763 Å in HfTe₂ and 2.793 Å in ZrTe₂ ) or in elementary Te (2.84 Å) [3], but shorter than the van der Waals distance 4.12 Å (default van der Waals radius for Te is taken from [4]). The weak interlayer Te bonding results in a layered structure which corresponds to plate-like morphology of crystals.

The situation in the case of crystal structure of Ni₂SbTe₂ prepared at 400˚C (experiment terminated by quenching) is more complicated.The X-ray powder diffraction pattern corresponds to the high temperature phase, nevertheless diffraction profiles of 201 and 110 lines are asymetrical. This assymetry disappears in the powder pattern of Ni₂SbTe₂ prepared at 800 ˚C (experiment terminated by quenching, Fig. 3). On the Selected Area Electron Diffraction (SAED) pattern of reciprocal planes h0l it is possible to observe weak reflections near 1/3 and 2/3 of the distance between the sharp strong diffractions. These weak reflections are systematically shifted from 1/3 to the left and from 2/3 to the right, i.e. closer to the sharp strong diffraction.

In fact, there is a large number of compounds that exhibit Sb – Te bonding in which Sb is formally cationic. Examples of such compounds include K₃SbTe₃, TlSbTe₂, SnSb₂Te₄, BaSbTe₃ and NaSbTe₂ [2]. In many compounds the Sb and Te atoms are mixed on the same crystallographic site and thus are not new structure types. These include e.g. Ni₂SbTe, Mn₂SbTe, Co₂SbTe, FeSbTe, Pd₂SbTe and Cr₂SbTe [5]. Virtually, there are very few compounds which have Sb and Te atoms on distinct crystallographic positions. Examples of such compounds are Ni_7-δSbTe₂, Cu_9.1Sb₃Te and the new compound Ni₂SbTe₂. Our research showed that the crystal structure of this phase depends on the temperature during the formation.

The phase Ni₂SbTe₂ forms a solid solution with end members having a composition of 42,1 % Ni, 13,0 % Sb, 44,9 % Te and 43,0 % Ni, 28,4 % Sb, 28,6 % Te (at.%) at 400ºC. The most characteristic feature is a small change of the nickel content as well as significant differences of the antimony and tellurium content

This work was supported by a Grant Agency of Charles University (project number 43-203391).

Fig. 1. Crystal structure of Ni₂SbTe₂ prepared at 800˚C (experiment terminated by quenching)

Fig. 2. Crystal structure of Ni₂SbTe₂ prepared at 400˚C (experiment terminated by slowly cooling to room temperature)

Fig. 3. Diffraction profile of 110 line of Ni₂SbTe₂ prepared at 400˚C (above) and at 800˚C, CoKα₁

1. Vavřín, I. & Frýda, J. Věštník ČGÚ, 73 (1998) 177-180.

2. Reynolds T.K., Kelley R.F. & DiSalvo F.J. J. Alloy Comp., 366 (2004) 136 -144.

3. Bensch, W., Heid. W., Muhler, M., Jobic, S., Brec, R. & Rouxel, J. J. Solid State Chem. 121 (1996) 87 - 94.

4. Bondi, A. J.Phys.Chem. 68 (1964) 441-451.

5. Inorganic Crystal Structure Database (ICSD), 2003