Noncentrosymmetric Mixed Chalcogenide Semiconductors La₃Ga_1.67(S_1-Se)₇

The mixed chalcogenides La₃Ga_1.67(S_1-Se)₇ form a complete solid solution, with members at increments of x = 0.14 prepared as phase-pure samples. Based on their powder X-ray diffraction (XRD) patterns, the cell volume increases with greater Se substitution, but the cell parameters vary in a nonmonotonic way. Single-crystal XRD studies at room temperature indicated that they adopt noncentrosymmetric hexagonal structures (in space group P6₃) containing stacks of Ga-centered tetrahedra and stacks of octahedra with partially occupied Ga sites. Strong preferences of S vs Se atoms within three types of chalcogen sites lead to unusual structural changes in the intermediate members of the solid solution and can be understood in terms of the need to satisfy optimum bonding requirements. Structure determination of the selenide end-member La₃Ga_1.67Se₇ at low temperature (100 K) revealed a supercell (in space group P6₁) with a tripled c-axis characterized by distortions of the partially occupied Ga sites and the coordinating Se atoms, which help relieve bond strain. Two Ga sites in tetrahedral and roughly octahedral geometry were assigned by ⁷¹Ga solid-state nuclear magnetic resonance spectroscopy. The experimental optical band gaps vary from 2.6 eV for La₃Ga_1.67S₇ to 2.0 eV for La₃Ga_1.67Se₇.