Order–disorder processes in adamantine ternary ordered‐vacancy compounds

Abstract

Adamantine ternary ordered-vacancy compounds (OVCs) of the AB 2 X 4 family derive from the zincblende structure of binary AX compounds and share many properties with the chalcopyrite-type ternary ABX 2 compounds. AB 2 X 4 mainly crystallize in the defect chalcopyrite (or thiogallate) structure but also in other vacancy-ordered phases, like the defect stannite (DS, or defect famatinite) and the pseudocubic (PC) structures, where vacancies occupy a fixed Wyckoff site in an ordered and stoichiometric fashion. Order-disorder phase transitions have been studied in several adamantine OVCs at high temperatures and/or pressures, where OVCs undergo a phase transition to a disordered zincblende (DZ) structure on increasing temperature and to a disordered rocksalt (DR) structure on increasing pressure. It has been suggested that these order-disorder transitions should undergo via intermediate phases of partial disorder between the fully ordered phases and the fully disordered phases. In this work we study the different possible intermediate phases of partial disorder and discuss the possibility to find them with the help of vibrational spectroscopies, like Raman scattering or infrared measurements.

High-pressure Raman scattering study of defect chalcopyrite and defect stannite ZnGa2Se4

“…Figure 1(e) shows the proposed DCA phase for ZnGa 2 Se 4 (model 6 in Ref. 34). Note that Ga atoms at 2a and vacancies at 2b sites of the DS phase get mixed to result in a structure with higher symmetry than the I-42m with cation and vacancy disorder.…”

“…The properties of this ordered DS phase are detailed in Refs. 15,33,34, and 41 and the justification about why calculations for the DS phase were performed for this ordered DS phase was already discussed in Ref. 28.…”

“…Note that Ga atoms at 2a and vacancies at 2b sites of the DS phase get mixed to result in a structure with higher symmetry than the I-42m with cation and vacancy disorder. This DCA phase has only 6 Raman-active modes; 34 i.e., much less than the DC phase. Therefore, we tentatively propose that the additional Raman mode observed in zone 2 of DC-ZnGa 2 Se 4 could be an indication that the DCA phase is coexisting with the DC phase.…”

“…[29][30][31][32][33] To this respect, in a recent work, we studied the different possible intermediate phases of partial disorder between the initial DC or DS phases and the DZ structure and discuss the possibility to find them by means of in situ vibrational spectroscopy. 34 In order to shed light on the pressure dependence of the Raman modes in DC-and DS-ZnGa 2 Se 4 and study how pressure affects order-disorder transitions in both phases, we report here HP Raman scattering measurements at room temperature (RT) for both DC and DS structures. Note that the DC sample is completely ordered but not the DS sample which has a cationic disorder [see Figs.…”

Structural and Vibrational Properties of CdAl2S4 under High Pressure: Experimental and Theoretical Approach

“…It must be mentioned that the discrepancies in the assignment of the symmetry of highfrequency modes in OVCs are very common because of the observation of quasi-modes instead of pure modes in uniaxial crystals. 17,33,[53][54][55] For CdAl 2 S 4 , the high quality of our Raman spectra and the similarity of the experimental and theoretical pressure dependence of the modes allows a rather clear assignment of the mode symmetry on the light of our theoretical calculations and previous Raman studies. A comparison with previous studies, in particular, with the results of Burlakov et al 11 , show that our main difference is that in our case we were able to distinguish experimentally between the B 4 and A 3 modes but not between E 5 and B 5 LO modes.…”