Evidence for the formation of valence band holes due to topotactical Tl removal in the ternary channel compound TlCr 3S 5: chemical reactivity and experimental electronic structure

Abstract

Tl-poor samples Tl 3 Cr 3S 5 (0.5 ≤ × ≤ 1) are prepared via a topolactical redox reaction between TlCr 3S 5 and Br 2 in CH 3CN. Unexpectedly, the reaction stops at a composition Tl 1Cr 3S 5 with x ≈ 0.50(5). Single crystals with a reduced Tl content are very brittle, indicating a high mechanical strain. X-ray photoemission investigations performed on TlCr 3S 5 and on Tl 0.5Cr 3S 5 demonstrate that the Cr 2p and Tl 4f core levels are identical for both samples. As a consequence, the possibility of an oxidation of Cr(III) or Tl(I) during the redox reaction is excluded. In contrast, significant changes are observed for the S 2p core level, suggesting a partial oxidation of the S 2− anions. Ultraviolet photoemission experiments conducted with He I and He II radiation reveal that TlCr 3S 5 and Tl 0.5Cr 3S 5 are semiconductors with a slightly smaller band gap in the Tl-poorer sample. The valence band spectra show pronounced differences between the two samples. The detailed analysis of both the core level spectra and the valence band spectra strongly suggests that the topotactic redox reaction leads to the formation of valence band holes, i.e. a partial oxidation of the sulfur anions. The results of all experiments indicate that the termination of the chemical redox reaction at a composition Tl 0.5Cr 3S 5 is caused by lattice strain and not due to the loss of the electrical conductivity of the host lattice.