EVIDENCES FOR THE FORMATION OF CHROMIUM IN THE UNUSUAL OXIDATION-STATE CR(IV) .1. CHEMICAL-REACTIVITY, MICROHOMOGENEITY, AND CRYSTAL-STRUCTURES OF THE NONSTOICHIOMETRIC CHANNEL COMPOUNDS TLXCR5SE8 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1)

The topotactic redox reaction between TlCr5Se8 and bromine in acetonit rile leads to the formation of metastable samples with 0 less than or equal to x<1 of TlxCr5Se8. Preparation at elevated temperatures yields for the first time a ternary transition metal chalcogenide crystalliz ing in the TlV5S8 type structure with a Tl content x<0.33. As expected the topotactic redox reaction does not follow simple kinetics but is rather explained on the basis of the superposition of at least three d ifferent fundamental steps, EDAX investigations conducted on selected single crystals reveal that dependent on the deintercalation temperatu re, Tl is laterally inhomogeneous distributed along the needle axis wh ich coincides with the crystallographic b axis. A pronounced maximum i s observed at the middle of the crystals. In stoichiometric TlCr5Se8 t he detailed analysis of the anisotropic displacement parameters of the Tl atoms reveal that the Tl atoms are displaced from the central posi tion by about 0.26 Angstrom. As a consequence the Tl atoms are coordin ated by seven Se atoms in an irregular coordination polyhedron, Since the positions x 0 z are related by a center of symmetry they cannot be occupied simultaneously, Hence it must be assumed that TlCr5Se8 has a domain structure with local symmetry Cm. In the Tl poorer phases the refinement of the Tl atoms at the central position leads to unusual hi gh U-22 components, The observed microinhomogenity as well as an enhan ced mobility and/or static disorder of the Tl atom within the channel may be responsible for this. The value for U-11 decreases with decreas ing Tl content whereas U-33 is not affected, This observation is indic ative for a different displacement of the Tl atoms from the central po sition, With respect to the possible reaction mechanisms, according to our structural in investigations, the oxidation of monovalent Tl to t rivalent Tl can be excluded, With decreasing Tl content the lattice pa rameters exhibit very anisotropic behavior, which is a direct conseque nce of the large changes of the interatomic Cr-Cr distances, The avera ge [Cr-Se] as well as the nonbonding Se-Se distances are only slightly affected, These results suggest an oxidation of the formally trivalen t Cr to the unusual tetravalent state Cr(IV), rather than the formatio n of valence band holes. (C) 1996 Academic Press, Inc.