X-ray powder diffraction as a tool for facing twins: the case of the monoclinic NbCoTe2 and TaCoTe2 phases

TaCoTe[sub 2] has been synthesized from the elements at 900[degrees]C in evacuated silica tubes; the crystals of TaCoTe[sub 2] are monoclinic, space group P2[sub 1]/c (No. 14), with a=8.524(6)[angstrom], b = 6.2649(4) [angstrom], c = 7.7945(5)[angstrom], [beta] = 116.789(4)[degrees], and Z = 4, and appear to be heavily twinned about the (100) plane. Therefore, powder diffraction data and Rietveld refinement were used to assess the correct stereochemistry of the atoms of this new compound. In addition, the already reported NbCoTe[sub 2] compound has been found to be, in the bulk, monoclinic [a = 8.1612(4) [angstrom], b = 6.2976(3) [angstrom], c = 7.8518(4) [angstrom], [beta] = 117.487(3)[degrees]], rather than orthorhombic, and isostructural to TaCoTe[sub 2]. The stereochemical interpretation of this change of crystal symmetry is given in terms of a significant asymmetrical bending of one tellurium atom toward one Nb (or Ta) atom, making these metals five-, rather than six-, coordinated.