Pressure dependence of the Raman modes and pressure-induced phase changes in CuGaS2 and AgGaS2

Carlone, C. ; Olego, D. ; Jayaraman, A. ; Cardona, M. (1980) Pressure dependence of the Raman modes and pressure-induced phase changes in CuGaS2 and AgGaS2 Physical Review B, 22 (8). pp. 3877-3885. ISSN 0163-1829

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Official URL: http://prb.aps.org/abstract/PRB/v22/i8/p3877_1

Related URL: http://dx.doi.org/10.1103/PhysRevB.22.3877

Abstract

The pressure dependence of the Raman peaks of CuGaS2 and AgGaS2 (chalcopyrite structure) has been studied up to 20 GPa in a gasketed diamond anvil cell. Nonlinear pressure dependence of several phonon frequencies have been noted, including a softening of the lowest Γ5 phonon, prior to a phase transition, which takes place at 16.5±0.5 GPa in CuGaS2 and 4.2±0.5 GPa in AgGaS2. In the latter material, discontinuous changes of the Raman frequencies indicate another phase transition at 11.6±0.5 GPa. The transition in CuGaS2 is believed to be to the disordered rocksalt structure, and in AgGaS2, the high-pressure transition is to the rocksalt, or to the closely related α-NaFeO2 structure. In AgGaS2, the softening of the lowest Γ5 phonons can be followed across the 4.2-GPa phase transition, an observation which to our knowledge has been made for the first time in a tetrahedral semiconductor. It is concluded that the first phase transition takes place in all these materials whenever the lowest Γ5 mode (or its equivalent) is lowered to 0.7 times its zero-pressure value. The effect of pressure on the phonon frequencies is discussed in terms of Born's transverse dynamical charges and their pressure dependence.

Item Type:Article
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