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 amorph ous chromium sulfide, CrS3, the amorphous chromium selenide, CrSe3, an d the crystalline vanadium sulfide model compound, VS4, have been carr ied out at both the metal and chalcogen K edges. Structural models bas ed on the distances and co ordination numbers obtained from the analys is of these data are presented for CrS3 and CrSe3. In CrS3, chromium i s surrounded by an average of six sulfurs at 2.35 Angstrom. The sulfur -edge data can be fitted by two shells containing two chromium neighbo urs at 2.34 Angstrom and one sulfur neighbour at 2.03 Angstrom. The re sults 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, Del ta E, of the chromium K edge of CrS3 (Delta E = 6.37 eV) relative to c hromium metal, is close to that of Cr2S3 (Delta E = 6.88 eV), supporti ng the assignment of the oxidation state of chromium as III. In CrSe3, chromium is surrounded by an average of six seleniums at a distance o f 2.50 Angstrom. Each selenium has an average of two chromium neighbou rs 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 shi ft of CrSe3 (Delta E = 5.79 eV) is large for a chromium selenide and p rovides 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 Cr Se3, supporting the assignment of different oxidation states to chromi um in the two materials.