THE CRYSTAL-CHEMISTRY AND ROLE OF METAL-METAL BONDING IN THE MONOCHALCOGENIDES TIS, VS, TISE, AND VSE AND THEIR SOLID-SOLUTIONS

The influence of metal-metal bonding on the physical properties of the titanium and vanadium monosulfides, monoselenides, and their solid so lutions is explored. Compounds were synthesized at 800 degrees C and c haracterized by room-temperature X-ray diffraction, chemical analysis, density, differential thermal analysis (DTA), magnetic susceptibility , and electrical resistance. All compounds formed one of three closely related crystal structures: hexagonal B8(1), orthorhombic B31, or a r hombohedral 9R polytype. In all these structures, partially occupied o rbitals of 3d parentage form narrow bands as a result of metal-metal i nteractions; all compounds were metallic and Pauli paramagnetic except nominal VSe, which exhibits an antiferromagnetic Neel temperature T-N similar or equal to 240 K and becomes semiconductive below 50 K. Evid ence of anion vacancies was found for all samples; compounds formed fr om larger selenium atoms also incorporate interstitial metals into tri gonal bipyramidal sites, stabilizing the hexagonal B8(1) structure and avoiding distortions to the ideal (''unstuffed'') B31 or 9R structure s. (C) 1995 Academic Press, Inc.