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High-pressure Raman scattering measurements have been carried out in ZnGa 2 Se 4 for both tetragonal defect chalcopyrite and defect stannite structures. Experimental results have been compared with theoretical lattice dynamics ab initio calculations and confirm that both phases exhibit different Raman-active phonons with slightly different pressure dependence. A pressure-induced phase transition to a Raman-inactive phase occurs for both phases; however, the sample with defect chalcopyrite structure requires slightly higher pressures than the sample with defect stannite structure to fully transform into the Raman-inactive phase. On downstroke, the Raman-inactive phase transforms into a phase that could be attributed to a disordered zincblende structure for both original phases; however, the sample with original defect chalcopyrite structure compressed just above 20 GPa, where the transformation to the Raman-inactive phase is not completed, returns on downstroke mainly to its original structure but shows a new peak that does not correspond to the defect chalcopyrite phase. The pressure dependence of the Raman spectra with this new peak and those of the disordered zincblende phase is also reported and discussed. V C 2013 AIP Publishing LLC.

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.

Order/disorder phenomena in Zn0.5Mn0.5Ga2Se4 related to changes in the cation occupancies are studied by Raman scattering. The cation distribution can be modified by submitting the sample to a controlled heating and cooling cycle. Three different stages of order/disorder type were observed. On heating above an onset temperature of ≈250 °C, the partially disordered as‐grown material (I ‐42m space group) evolves toward an ordered configuration with I ‐4 space group. At a temperature close to 500 °C the samples undergoes an order/disorder phase transition to a partially disordered structure that maintains the ordered stoichiometric vacancies. Slow cooling across the phase transition allows the cations to equilibrate and results in an almost fully ordered cation distribution at RT. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

It is known that optically anisotropic media can change the polarization of light propagating inside them. As regards Raman spectroscopy, this affects the light intensity measured in different geometrical configurations and results in an apparent unfulfillment of selection rules. We present an experimental and theoretical study of such effects in the defect chalcopyrite semiconductor MnGa 2 Se 4 . Optical anisotropy is taken into account by including in the calculation of Raman intensities the phase difference appearing between ordinary and extraordinary waves as they propagate, in uniaxial media, with different velocities. Birefringence can be obtained from Raman measurements provided that the distance run by the light is known.

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I space groups respectively, presenting either partial cation disorder ( m I 2 4 ) or total cation order ( 4 I ). In this work, a relation between the shape of a mode at ν≅180 cm -1 and the degree of cation order has been found. This band can be decomposed in a narrow and a broad component, related with the long range order and local non periodicity respectively. Cation ordering has been seen to increase after a heating treatment up to 600 ºC and further cooling to RT.

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