Evidences for the Formation of Chromium in the Unusual Oxidation State Cr(IV): I. Chemical Reactivity, Microhomogeneity, and Crystal Structures of the Nonstoichiometric Channel Compounds TlxCr5Se8(0 ≤x≤ 1)

Abstract

The topotactic redox reaction between TlCr5Se8and bromine in acetonitrile leads to the formation of metastable samples with 0 ≤x< 1 of TlxCr5Se8. Preparation at elevated temperatures yields for the first time a ternary transition metal chalcogenide crystallizing in the TlV5S8type structure with a Tl contentx< 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 different fundamental steps. EDAX investigations conducted on selected single crystals reveal that dependent on the deintercalation temperature, Tl is laterally inhomogeneous distributed along the needle axis which coincides with the crystallographicbaxis. A pronounced maximum is observed at the middle of the crystals. In stoichiometric TlCr5Se8the detailed analysis of the anisotropic displacement parameters of the Tl atoms reveal that the Tl atoms are displaced from the central position by about 0.26 Å. As a consequence the Tl atoms are coordinated by seven Se atoms in an irregular coordination polyhedron. Since the positionsx0zare related by a center of symmetry they cannot be occupied simultaneously. Hence it must be assumed that TlCr5Se8has a domain structure with local symmetryCm. In the Tl poorer phases the refinement of the Tl atoms at the central position leads to unusual highU22components. The observed microinhomogenity as well as an enhanced mobility and/or static disorder of the Tl atom within the channel may be responsible for this. The value forU11decreases with decreasing Tl content whereasU33is not affected. This observation is indicative for a different displacement of the Tl atoms from the central position. With respect to the possible reaction mechanisms, according to our structural investigations, the oxidation of monovalent Tl to trivalent Tl can be excluded. With decreasing Tl content the lattice parameters exhibit very anisotropic behavior, which is a direct consequence of the large changes of the interatomic Cr–Cr distances. The average 〈Cr–Se〉 as well as the nonbonding Se–Se distances are only slightly affected. These results suggest an oxidation of the formally trivalent Cr to the unusual tetravalent state Cr(IV), rather than the formation of valence band holes.