Phase transition and high-pressure elastic behavior of copper halides

The phase-transition pressures of copper halides (CuCl, CuBr, and CuI) have been computed by employing the minimization technique of Gibbs free-energy expressed as a function of pressure and charge transfer (or three-body interactions) caused by the overlap of electron shells of the adjacent ions. We have also investigated the equation of state, which shows an appreciable volume collapse at the phase-transition pressures. The calculated values for the phase-transition pressures and the associated volume collapses are generally in excellent agreement with their measured data. The pressure variations of the elastic constants (C₁₁, C₁₂, and C₄₄) and their combinations have been found to vary such that the values of C₁₁, C₁₂, BT [=(1/3(C₁₁+2C₁₂)], and CL [=1/2(C₁₁+C₁₂+2C₄₄)] increase with pressure, while those of the shear moduli C₄₄ and CS [=1/2(C₁₁-C₁₂)] decrease with it for all the copper halides. This feature is consistent with that observed in CuCl.