Extended X-ray absorption fine structure studies of the amorphous tungsten sulfides and selenides, WS5, WSe5 and WS3

Abstract

Extended X-ray absorption fine structure (EXAFS) and S K-edge absorption studies of the amorphous tungsten sulfide, WS₅, and EXAFS studies of the amorphous tungsten selenide, WSe₅, suggest these compounds can be formulated as W^V(S₂^2–) and W^V(Se₂^2–)_2.5. The observed metal–metal distances of ca. 2.75 Å are consistent with the formation of a metal–metal bond between these d¹ metal centres; the metal–metal bond would explain the observed diamagnetism of these compounds. The Se—Se bond length of 2.34 Å, 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 agreement 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 absorption spectrum of WS₃ suggests that this compound contains both S^–I and S^–II. W L_III-edge EXAFS studies of WS₃ show much greater disorder in 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 formulations, W^V(S^2–)₂(S₂^2–)_0.5 and W^IV(S^2–)(S₂^2–), 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 Å, which are found in the EXAFS studies. Possible structural models for WS₅ and WSe₅ are presented.