Thermally activated cation ordering in ZnGa2Se4single crystals studied by Raman scattering, optical absorption, andab initiocalculations

Abstract

Order–disorder phase transitions induced by thermal annealing have been studied in the ordered-vacancy compound ZnGa2Se4 by means of Raman scattering and optical absorption measurements. The partially disordered as-grown sample with tetragonal defect stannite (DS) structure and space group has been subjected to controlled heating and cooling cycles. In situ Raman scattering measurements carried out during the whole annealing cycle show that annealing the sample to 400°C results in a cation ordering in the sample, leading to the crystallization of the ordered tetragonal defect chalcopyrite (DC) structure with space group. On decreasing temperature the ordered cation scheme of the DC phase can be retained at ambient conditions. The symmetry of the Raman-active modes in both DS and DC phases is discussed and the similarities and differences between the Raman spectra of the two phases emphasized. The ordered structure of annealed samples is confirmed by optical absorption measurements and ab initio calculations, that show that the direct bandgap of DC-ZnGa2Se4 is larger than that of DS-ZnGa2Se4.