Phase transitions in defect chalcopyrite compounds under hydrostatic pressure

Abstract

Tetrahedrally bonded AGa 2 X 4 (AϭCd, Zn; XϭS, Se͒ compounds crystallizing in defect chalcopyrite and defect famatinite structures have been studied by Raman spectroscopy under hydrostatic pressure. The pressure-induced changes in the Raman spectra of both ordered defect chalcopyrite and partially disordered defect famatinite structures were attributed to an order-disorder phase transition in the cation sublattice, which proved to occur in two stages, as predicted by Bernard and Zunger ͓Phys. Rev. B 37, 6835 ͑1988͔͒. The first stage of the transition was found to depend on the tetragonal distortion of the initial crystals. An irreversible disappearance of Raman signal at elevated pressures was observed and attributed to a phase transition from the adamantine structure to a high-symmetry rocksalt-type structure. The pressure of this transition in the compounds under consideration exhibits a dependence on bond ionicity and bond length. The structure of decompressed samples was explained on the basis of pressure dependence of potential barriers for the phase transition by using a configuration-coordinate model.

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

“…On the other hand, the B modes correspond to vibrations of cations and anions in the z direction while the E modes correspond to vibrations of cations and anions in the x-y plane. 11 As pressure increases, the Raman peaks of the highfrequency region shift to higher frequencies while most of the peaks of the low-frequency region show a negligible or even negative pressure coefficient. Above 18 GPa, all Raman peak intensities are hardly seen, thus indicating the onset of a phase transition that is completed at 20.2 GPa with the disappearance of all the Raman peaks of the DC phase.…”

“…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.…”

“…This mode is also known as the "breathing" mode because it is associated to the symmetric oscillation of the anions against the stoichiometric vacancy. 11 It must be noted that the non-polar A modes in OVCs correspond to oscillations of the anions around the vacancy so the frequencies of the A modes are barely affected by the cation masses and scale with the anion mass. On the other hand, the B modes correspond to vibrations of cations and anions in the z direction while the E modes correspond to vibrations of cations and anions in the x-y plane.…”

“…16 In summary, previous high-pressure Raman scattering studies in DC-CdGa 2 Se 4 were far from being conclusive because (i) there is some controversy regarding the assignment of the symmetries of the Raman-active modes, (ii) there is controversy regarding the presence of the two stages of disorder in this compound at high pressure; and (iii) there is controversy regarding the reversibility of the orderdisorder phase transition leading from the DC to the DR structure taking place near 20 GPa in CdGa 2 Se 4 . Furthermore, the mechanisms of the order-disorder phase transitions are far from being completely understood 11 these stages of disorder are difficult to be evidenced by x-ray diffraction measurements. Therefore, in this work, we report high-pressure optical absorption and Raman scattering measurements at room temperature in DC-CdGa 2 Se 4 during two upstrokes above 22 GPa and two downstrokes.…”

“…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)].…”