Disorder-Mediated Structural Transformation in the Cu4TiSe4–xSx (0 ≤ x ≤ 4) System and Its Effects on the Thermal Transport Property

Abstract

Copper-transition-metal chalcogenides can offer low-cost and environmentally benign solutions to trap heat and heat to electric energy conversion. In this report, we present the synthesis and characterization of Cu–Ti-based mixed chalcogenides, Cu4TiSe4–xSx (x = 0–4). At room temperature, Cu4TiSe4 adopts a sulvanite-type cubic structure (P4̅3m), whereas Cu4TiS4 crystallizes in a body-centered tetragonal space group (I4̅2m), where the lattice parameter is doubled along the c-direction w.r.t. the sulvanite. The structure of the S-analogue is completely ordered, while the Se-analogue hosts positional disorder distributed over two Cu-sites (1a and 4e Wyckoff sites). A systematic investigation of a series of compositions of Cu4TiSe4–xSx (0 ≤ x ≤ 4) indicates that S insertion in the Cu4TiSe4–xSx boosts the disordered 4e site to coalesce into the 1a site. Up to x ≈ 2.6, Cu4TiSe4–xSx forms the cubic phase similar to Cu4TiSe4, whereas, for x ≥ 3.5, the pure tetragonal phase related to Cu4TiS4 appears. Herein, the cubic-to-tetragonal phase transformation is rationalized by theoretical calculations. Thermal conductivity measurements show a significant increase in the lattice thermal conductivity (κL) values from the cubic (0.3–0.47 Wm–1 K–1) to the tetragonal (above 0.7 Wm–1 K–1) phases. Phonon band structure and phonon density of state calculations suggest that both Cu and Se atoms are responsible for the anharmonic scattering of the acoustic phonons in the Se-rich cubic phase, whereas Cu atoms primarily contribute to this scattering process in the S-rich tetragonal phase.