EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDIES OF THE AMORPHOUS TUNGSTEN SULFIDES AND SELENIDES, WS5, WSE5, AND WS3

Extended X-ray absorption fine structure (EXAFS) and S K-edge absorpti on studies of the amorphous tungsten sulfide, WS5, and EXAFS studies o f the amorphous tungsten selenide, WSe5, suggest these compounds can b e formulated as W-V(S-2(2-)) and W-V(Se-2(2-))(2.5). The observed meta l-metal distances of ca. 2.75 Angstrom are consistent with the formati on of a metal-metal bond between these d(1) metal centres; the metal-m etal bond would explain the observed diamagnetism of these compounds. The Se-Se bond length of 2.34 Angstrom, determined from the Se K-edge EXAFS studies, is typical of a diselenide group. Comparison of the S K -edge absorption spectrum for WS5 with spectra from related compounds suggests that the sulfur is in the -1 oxidation state. This is in agre ement with the observed absorption in the infrared at 518 cm(-1) which we assign to an S-S stretch in a disulfide group. The S K-edge absorp tion spectrum of WS3 suggests that this compound contains both S--I an d S--II. W L(m)-edge EXAFS studies of WS3 show much greater disorder i n the W-S bonding than in WS5, which can be attributed to the presence of sulfur in two oxidation states. Models based on two possible formu lations, W-V(S2-)(2)(S-2(2-))(0.5) and W-IV(S2-)(S-2(2-)), give almost equally good fits to the experimental data. The observed diamagnetism of WS3 can be explained by the formation of W-W bonds of ca. 2.75 Ang strom, which are found in the EXAFS studies. Possible structural model s for WS5 and WSe5 are presented.