Quasi-Isostructural Solid-Solution Formation of Cu4TiSe4 with Cu3TMQ4 (TM: V, Nb, Ta; Q: S, Se): Tuning “Stuffing” Concentration for Ultralow Thermal Conductivity

Abstract In this study, we adopted a quasi-isostructural solid-solution strategy involving selenotitanate Cu4TiSe4 and sulvanite-type Cu3TMQ4 (TM: V, Nb, Ta; Q: S, Se). At room temperature, both systems crystallize in the cubic P4̅3m space group, where Cu4TiSe4 additionally contains positionally disordered Cu+ cations occupying two Wyckoff sites (1b and 4e) inside the sulvanite framework. The crystal structures of the solid-solution phases were determined by X-ray diffraction studies. Substituting Ti4+ for TM5+ in Cu3TMQ4 allows additional Cu+ to dissolve, maintaining the overall charge neutrality. The additional “interstitial” Cu+ ions occupy positionally disordered sites (1b and 4e) without affecting the three-dimensional (3D) sulvanite frameworks, with their occupancy controlled by the Ti4+/TM5+ ratio. Moreover, an ordering tendency of the interstitial Cu+ ions is observed within the cubic unit cell of the S-substituted solid solutions. This solid-solution formation leads to a substantial reduction in effective thermal conductivity relative to the parent sulvanite-type compounds, reaching the ultralow regime, while the optical band gaps vary systematically between 0.9 and 2.62 eV. The combination of this low thermal conductivity and tunable optical bandgap makes these materials highly attractive for diverse applications.