SYNTHESIS, CRYSTAL AND ELECTRONIC-STRUCTURE OF THE TERNARY LAYERED COMPOUNDS NBA(1 2)TE(2) (A=SI, GE)/

The crystal and electronic structures of the materials NbA(1/2)Te(2) ( A = Si, Ge) are investigated with a view to a further understanding of the short Te...Te interslab contacts found in the MA(1/2)Te(2) (M = N b, Ta; A = Si, Ge; 1/3 less than or equal to x less than or equal to 1 /2) family of materials. The average cell of the two analogous phases can be described in the C2/c space group (NbSi1/2Te2: a = 14.216(2)Ang strom, b/2 = 3.9440(8)Angstrom, c = 6.3362(13)Angstrom, beta = 97.97(2 )degrees; NbGe1/2Te2: a = 14.216(2) Angstrom, b/2 = 3.9615(5) Angstrom , c = 6.448(2) Angstrom, beta = 97.86(2)degrees). However, a proper de scription of the ordering of the cations and hence of the Te-Te distan ces and of the electronic structure requires a refinement of the cryst al structure in supercell (doubling of b). Such refinements are carrie d out for the silicon phase without additional reflections and for the germanium phase using satellite reflections. The results are used to fully redetermine the electronic band structure of the NbSi1/2Te2 mate rial. It is shown by an analysis of the Te-Te overlap populations that the relationship between the shortest Te...Te interslab contact dista nce and the value of x in the MA(x)Te(2) family is not due to electron donation as originally anticipated.