Crystal structure of the mixed-metal trisulfide BaCu1/3Ta2/3S3

In the structure of BaCu1/3Ta2/3S3, the Cu and Ta atoms are occupationally disordered on the same site in a ratio of 1/3:2/3.

The mixed-metal title compound, BaCu 1/3 Ta 2/3 S 3 [barium copper(II) tantalum(V) trisulfide], was prepared through solid-state reactions. The crystal structure adopts the BaTaS 3 structure type and consists of face-sharing [MS 6 ] (M = Ta,Cu) octahedra (point-group symmetry 3m.) that are condensed into infinite chains along [001]. Adjacent chains are linked through the barium cations (site symmetry 6m2), which exhibit a coordination number of twelve. The M site is occupied by 2/3 of Ta V and 1/3 of Cu II , whereby the average M-S distances are slightly longer than those of ordered BaTaS 3 . The classical charge balance of the title compound can be represented by [Ba 2+ ] [(Ta/Cu) 4+ ] [S 2À ] 3 .

Chemical context
Barium vanadium trisulfide, BaVS 3 (Takano et al., 1977), with which BaTaS 3 (Gardner et al., 1969) crystallizes isotypically in space group P6 3 /mmc, has a chain structure. The observed conductivity was attributed to the formation of conduction bands via vanadiumÁ Á Ávanadium d-orbital overlap. It shows three phase transitions and exhibits a number of intriguing physical properties (Nakamura et al., 1994). While both BaVS 3 and BaTaS 3 are composed of the same type of linear chains, BaTaS 3 shows metallic conductivity and a Curie-Weiss behaviour of the magnetic susceptibility (Gardner et al., 1969). To explore the physical properties of BaTaS 3 and related Face-sharing of MS 6 (M = Cu, Ta) octahedra in the structure of BaCu 1/3 -Ta 2/3 S 3 . Displacement ellipsoids are drawn at the 50% probability level. compounds, we have introduced copper and studied mixedmetal phases Ba(Ta/Cu)S 3 . Here we report on the synthesis and structural characterization of the mixed-metal trisulfide with composition BaCu 1/3 Ta 2/3 S 3 .
The M site is occupationally disordered and contains 1/3 Cu and 2/3 Ta. It is surrounded by six S atoms with an M-S bond length of 2.475 (4) Å , which is slightly longer than that of ordered BaTaS 3 (2.461 Å ; Gardner et al., 1969). This trend is in agreement with the different ionic radii of Ta (0.64 Å for Ta V with coordination number of six) and Cu II (0.73 Å ) using the data provided by Shannon (1976).

Synthesis and crystallization
The title compound was prepared using solid-state reactions between the elements Cu, Ta, S and BaS. Ta powder (99.999%, Alfa Aesar Puratronic), Cu powder (99.999%, Alfa Aesar Puratronic), S powder (99.999%, Alfa Aesar Puratronic), and BaS powder (99.999%, Alfa Aesar Puratronic) were mixed in a fused-silica tube in an Ta:Cu:S:BaS molar ratio of 0.67:0.33:2:1. The tube was evacuated to 0.1 Pa, sealed and heated gradually (60 K h À1 ) to 973 K, where it was kept for 2 d. The tube was then cooled to 673 K at a rate of 3 K h À1 and then quenched to room temperature. The crystals are stable in air and alcohol.
Scanning electron microscopy (SEM) images of selected crystals were taken on a Hitachi S-4800 microscope equipped with an electron microprobe analyzer for a semiquantitative elemental analysis in the energy dispersive X-ray spectroscopy (EDX) mode. The presence of both copper and tantalum was confirmed (Fig. 3). The crystal structure of BaCu 1/3 Ta 2/3 S 3 , viewed down [001].

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The refinement of the model with occupational disorder on the M site resulted in a significant decrease of the reliability factors in comparison with a fully occupied Ta site (R1 = 0.73, wR = 0.197). No evidence, e.g. in the form of superstructure reflections, was found for an ordering of this site and thus a statistically disordered model was considered. In the final model, atoms of the disordered site were restrained to have the same displacement parameters, with a fixed Cu:Ta ratio of 1/3:2/3 required for charge neutrality and in good agreement with the EDX measurement. The remaining maximum and minimum electron densities are located 1.06 Å from the (Cu,Ta)1 site and 1.96 Å from the S1, respectively. (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2007); software used to prepare material for publication: publCIF (Westrip, 2010).

Barium copper(II) tantalum(V) trisulfide
Crystal data BaCu 0.33 Ta 0.67 S 3 M r = 375.14 Hexagonal, P6 3 /mmc a = 6.8350 (6)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.