Raman scattering in CdGa₂Se₄ under pressure

Abstract

The Raman scattering spectra of CdGa 2 Se 4 under pressure were investigated at 300 K up to 20.9 GPa. TO as well as LO modes were observed. Two stages were observed in the pressure dependences of Raman bands. Such a behaviour was attributed to the order -disorder phase transition in the cation sublattice. A first-order phase transition under pressure was observed at 20.2 GPa. Using the Harrison -Keating model of the lattice dynamics modified for crystals with the tetragonal structure, the bulk modulus B and the mode-Grüneisen parameters Γ i were determined for the first time. It is shown that a better agreement between the experimental and calculated values of Γ i is observed if one takes into consideration the different behaviour with pressure for the bond-bending and the bond-stretching parameters, which determine the low-(lower than 140 cm -1 ) and high-(higher than 140 cm -1 ) frequency phonons, respectively.

High-pressure optical and vibrational properties of CdGa2Se4: Order-disorder processes in adamantine compounds

“…Thanks to our lattice dynamical calculations and the comparison of the experimental and calculated frequencies and pressure coefficients, we tentatively assign the modes observed at 244, 257, and 274 cm À1 to the E TO modes at 242, 255, and 272 cm À1 to the E LO 4 , E LO 5 , and B LO 4 modes, respectively. 12 Therefore, we think that in contrast with previous measurements, we have not observed the E LO 4 mode or this mode is coincident in frequency with one of the three modes of highest frequency. Finally, as regards to ab initio calculations, we have to mention that our calculated Raman frequencies, pressure coefficients, and ordering of the different modes are in rather good agreement with previous ab initio calculations of Fuentes-Cabrera.…”

“…These two studies provided two different symmetry assignments of the Raman-active modes of the DC phase that were not discussed in the latter study despite their use of a Keating-Harrison model. 12 However, both studies agreed on the existence of two stages of cation disorder prior to the phase transition to a) Author to whom correspondence should be addressed. Electronic mail: osgohi@fis.upv.es.…”

“…Therefore, we have used our ab initio calculations as a help to assign and discuss the symmetry of the measured Raman modes and their possible TO-LO splitting and to compare them with previous measurements and calculations. [11][12][13] The zero-pressure frequencies, pressure coefficients, and derivatives of the pressure coefficients for experimental and calculated Raman modes in DC-CdGa 2 Se 4 are summarized in Table I and compared with the experimental results of Ursaki et al 11 In general, our assignment of the Raman mode frequencies agrees with those of Ursaki et al 11 and also with those of Mitani et al 12 The main difference between ours and previous results is the assignment of the last three highfrequency modes. Thanks to our lattice dynamical calculations and the comparison of the experimental and calculated frequencies and pressure coefficients, we tentatively assign the modes observed at 244, 257, and 274 cm À1 to the E TO modes at 242, 255, and 272 cm À1 to the E LO 4 , E LO 5 , and B LO 4 modes, respectively.…”

“…In previous high-pressure Raman works, the disorder process at cation sites in OVCs with DC or DS structure was mainly suggested on the light of the extraordinary changes of the intensity and width of some Raman modes above certain pressures. 11,12,19,20,29 Indeed, cation disorder should affect drastically the intensities and widths of Raman modes because it is known that cation disorder in the DS structure results in broader Raman spectra with overlapped and less intense peaks than Raman spectra of the DC phase. 30 Therefore, to discuss the possible disorder processes occurring in DC-CdGa 2 Se 4 at cation sites, we have plotted the FWHM and the intensity of the most intense Raman modes in Figs.…”

“…CdGa 2 Se 4 is one of the most studied adamantine ternary compounds, and the behavior of DC-CdGa 2 Se 4 under pressure has been subject of recent studies. [11][12][13][14][15][16] In particular, two Raman scattering measurements under pressure 11,12 found a phase transition near 20 GPa to the disordered rock salt (DR) structure [see Fig. 1(c)].…”