Local structures of the amorphous chromium sulfide, CrS3, and selenide, CrSe3, from X-ray absorption studies

Extended X-ray absorption fine structure (EXAFS) studies of the amorphous chromium sulfide, CrS3, the amorphous chromium selenide, CrSe3, and the crystalline vanadium sulfide model compound, VS4, have been carried out at both the metal and chalcogen K edges. Structural models based on the distances and co ordination numbers obtained from the analysis of these data are presented for CrS3 and CrSe3. In CrS3, chromium is surrounded by an average of six sulfurs at 2.35 Angstrom. The sulfur-edge data can be fitted by two shells containing two chromium neighbours at 2.34 Angstrom and one sulfur neighbour at 2.03 Angstrom. The results of sulfur K-edge absorption spectroscopy of CrS3,Cr2S3, VS4 and alpha-S suggest that all the su!fur is found in one oxidation state, S-1, and we formulate CrS3 as Cr-III(S-2(-1))(1.5). The edge shift, Delta E, of the chromium K edge of CrS3 (Delta E = 6.37 eV) relative to chromium metal, is close to that of Cr2S3 (Delta E = 6.88 eV), supporting the assignment of the oxidation state of chromium as III. In CrSe3, chromium is surrounded by an average of six seleniums at a distance of 2.50 Angstrom. Each selenium has an average of two chromium neighbours at 2.45 Angstrom, and 0.67 of a selenium neighbour at 2.34 Angstrom. We formulate CrSe3 as Cr-IV(Se-2(-1))Se--II. The chromium K-edge shift of CrSe3 (Delta E = 5.79 eV) is large for a chromium selenide and provides further evidence for the assignment of the oxidation state of chromium in CrSe3 as IV. The magnetic moments per chromium measured at room temperature were 1.74(3) mu(B) for CrS3 and 2.62(3) mu(B) for CrSe3, supporting the assignment of different oxidation states to chromium in the two materials.