Disorder-Mediated Structural Transformation in the Cu₄TiSe_4-xS_x (0 ≤ x ≤ 4) System and Its Effects on the Thermal Transport Property

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, Cu₄TiSe_4-xS_x (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 Cu₄TiSe_4-xS_x (0 ≤ x ≤ 4) indicates that S insertion in the Cu₄TiSe_4-xS_x boosts the disordered 4e site to coalesce into the 1a site. Up to x ≈ 2.6, Cu₄TiSe_4-xS_x 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 (κ_L) 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.