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

At room temperature, both systems crystallize in the cubic <i>P</i>4̅3<i>m</i> space group, where Cu₄TiSe₄ additionally contains positionally disordered Cu⁺ cations occupying two Wyckoff sites (1<i>b</i> and 4<i>e</i>) inside the sulvanite framework.

The crystal structures of the solid-solution phases were determined by X-ray diffraction studies. Substituting Ti⁴⁺ for <i>TM</i>⁵⁺ in Cu₃<i>TMQ</i>₄ allows additional Cu⁺ to dissolve, maintaining the overall charge neutrality. The additional “<i>interstitial</i>” Cu⁺ ions occupy positionally disordered sites (1<i>b</i> and 4<i>e</i>) without affecting the three-dimensional (3D) sulvanite frameworks, with their occupancy controlled by the Ti⁴⁺/<i>TM</i>⁵⁺ ratio.