THE EFFECT OF PARTIAL SUBSTITUTION OF SULFUR BY PHOSPHORUS IN TLXV6S8- CRYSTAL-STRUCTURES AND ELECTRICAL-PROPERTIES OF TLXV6S8-YPY (Y=0.7 AND 1.1)

Tl0.85V6S7.3P0.7 and Tl0.77V6S6.9P1.1 crystallize in the hexagonal Nb3 Se4 type structure with Tl atoms confined within the hexagonal channel s. The Tl atom exhibits a high one-dimensional disorder as evidenced b y the high value for the anisotropic displacement parameter U-33. The P atoms in the two compounds are exclusively distributed on one of the two possible S sites and are in a trigonal prismatic environment. The vanadium to vanadium distance within the zig-zag chains parallel to t he crystallographic c-axis are larger than in the pure ternary sulfide and the separation between these chains is smaller. Resistivity measu rements performed between 300 K and 1.8 K exhibit a metallic behaviour for Tl0.85V6S7.3P0.7 with rho(300K) about 800(1) mu Omega cm. At abou t 2.3 K the resistivity drops by a factor of about 3 but does not reac h zero.