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 I4¯2m 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 I4¯ 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.

HgGa2Se4 under high pressure: An optical absorption study

“…These results indicate that dark linear defects are not reversible and the change of the bandgap energy above 12 GPa (once dark linear defects appear) is also not reversible. The decrease of the bandgap energy observed in DC‐HgGa 2 Se 4 above 12 GPa, that we have attributed to increasing disorder, is similar to that observed in chalcopyrites and in other ternary OVCs, like CdGa 2 Te 4 , CdGa 2 Se 4 , HgGa 2 S 4 , and ZnGa 2 Se 4 . We want to stress that the irreversibility of the direct bandgap energy and its decrease in value in the recovered samples of OVCs can be explained by the irreversible cation–cation and cation–vacancy order–disorder processes which may result in different recovered structures from that of the original phase.…”

“…High‐pressure studies of OVCs with A II B 2 III X 4 VI stoichiometry are receiving increasing attention in the last years . The vast majority of these works have been focused on the study of the structural and vibrational properties of A II B 2 III X 4 VI compounds by means of X‐ray diffraction (XRD) and Raman scattering (RS) measurements, respectively.…”

Vibrational study of HgGa2S4 under high pressure

“…In particular, the broadening of the most intense A mode and of the highfrequency modes suggests that the recovered sample corresponds to the DS phase rather than to the original DC phase. 35 Note that the broadening of many Raman modes in the spectrum of the DS phase with respect to the spectrum of the DC phase was also evidenced in CdGa 2 Se 4 , 23 I. Experimental (exp.)…”

“…A possible justification for the observation of polytype 4 of the DS phase instead of model 2, more common in Znbased compounds, 25,[35][36][37] is that Hg (A cation) is rather larger compared to Ga (B cation); therefore, the mix of Hg and Ga cations at the same cation plane can be difficult. This would avoid the formation of model 2 of the DS phase 23 and would lead to the more favorable mixing of Ga cations and vacancies at the same cation plane resulting in model 4 of the DS phase.…”

“…23 The case of HgGa 2 S 4 is the opposite to the case of ZnGa 2 S 4 and ZnGa 2 Se 4 , where model 2 is the assumed DS phase occurring at ambient conditions because it is easy for Zn and Ga atoms with similar sizes to get mixed. 25,[35][36][37] …”