Noncentrosymmetric Mixed Chalcogenide Semiconductors La3Ga1.67(S1–xSex)7

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Abstract
The mixed chalcogenides La3Ga1.67(S1-xSex)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 La3Ga1.67Se7 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 La3Ga1.67S7 to 2.0 eV for La3Ga1.67Se7.
Funding Information
  • Government of Alberta
  • Canada Foundation for Innovation
  • Canada Research Chairs (CRC-2020-00352)
  • University of Alberta
  • Canada First Research Excellence Fund (T12-P01)
  • Natural Sciences and Engineering Research Council of Canada (RGPIN-2018-04294, RGPIN-2021-02540)
  • Alberta Innovates
  • Alliance de recherche num?rique du Canada

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