Sulfur-sulfur bonding in the amorphous sulfides WS3, WS5, and Re2S7 from sulfur K-edge EXAFS studies

The measurement of extended X-ray absorption fine structure (EXAFS) using e nergy-selective fluorescence detection in the soft X-ray region has enabled sulfur K-edge EXAFS studies of the amorphous sulfides WS3, WS5 and Re2S7, to be carried out for the first time. Quantitative information about the lo cal environment of sulfur in these highly structurally disordered sulfides has been obtained and is compared with that obtained by previous EXAFS stud ies at the metal L-III edges. The average oxidation state of sulfur and hen ce the metals in these amorphous sulfides is derived from S-S co-ordination numbers. Results from the crystalline model compound [NH4](2)[W-3(mu(3)-S) (mu-S)(3)(S-4)(3)(NH3)(3)] are also presented to give an indication of the reliability of the results. In all the amorphous compounds studied S-S bond s with bond lengths close to 2.0 Angstrom are found. In WS3 each sulfur has on average 0.5 sulfur near neighbours and close to 2 tungsten near neighbo urs at 2.43 Angstrom. In WS5 each sulfur has on average 1 sulfur near neigh bour and approximate to 1.2 tungsten near neighbours at 2.46 Angstrom, cons istent with the material being formulated as W-V(S-2(2-))(2.5). Amorphous R e2S7 also contains sulfur with an average sulfur co-ordination number of 1 and the compound is thus formulated as Re-2(3.5)(S-2(2-))(3.5). The S-Re bo nd distance is found to be 2.32 Angstrom. All the S-M distances found are i n excellent agreement with those found from previous metal L-III-edge EXAFS studies. Possible structural models for the amorphous materials are discus sed.