EVIDENCE FOR THE FORMATION OF VALENCE-BAND HOLES DUE TO TOPOTACTICAL TL REMOVAL IN THE TERNARY CHANNEL COMPOUND TLCR3S5 - CHEMICAL-REACTIVITY AND EXPERIMENTAL ELECTRONIC-STRUCTURE

Tl-poor samples TlxCr3S5 (0.5 less than or equal to x less than or equ al to 1) are prepared via a topotactical redox reaction between TlCr3S 5 and Br-5 in CH3CN. Unexpectedly, the reaction stops at a composition TlxCr3S5 with x approximate to 0.50(5). Single crystals with a reduce d Tl content are very brittle, indicating a high mechanical strain. X- ray photoemission investigations performed on TlCr3S5 and on Tl0.5Cr3S 5 demonstrate that the Cr 2p and Tl 4f core levels are identical for b oth samples. As a consequence, the possibility of an oxidation of Cr(I II) or TI(I) during the redox reaction is excluded. In contrast, signi ficant changes are observed for the S 2p core level, suggesting a part ial oxidation of the S2- anions. Ultraviolet photoemission experiments conducted with HeI and Hell radiation reveal that TlCr3S5 and Tl0.05C r3S5 are semiconductors with a slightly smaller band gap in the Tl-poo rer sample. The valence band spectra show pronounced differences betwe en the two samples. The detailed analysis of both the core level spect ra and the valence band spectra strongly suggests that the topotactic redox reaction leads to the formation of valence band holes, i.e. a pa rtial oxidation of the sulfur anions. The results of all experiments i ndicate that the termination of the chemical redox reaction at a compo sition Tl0.5Cr3S5 is caused by lattice strain and not due to the loss of the electrical conductivity of the host lattice.