Raman spectroscopy of CaIrO₃ postperovskite up to 30 GPa

We have measured Raman spectra of the postperovskite (PPv) phase in CaIrO₃ up to 30 GPa to constrain the Grüneisen parameter (γ). We identified a total of 4 strong modes between 200 and 650 cm^-1, which is in contrast with the Raman spectra of Mn₂O₃ and MgGeO₃-PPV where at least nine different modes have been detected. We found no sign of a phase transition in the Raman spectra of PPvCaIrO₃,which supports the stability of the PPv phase up to 30 GPa and room temperature in CaIrO₃. The spectroscopic Grüneisen parameter γ_sp.0 = 1.66-1.72, constrained from our Raman data, is in excellent agreement with the thermodynamic Grüneisen parameter, γ_th.0 = 1.75 ± 0.05, calculated from recent XRD measurements (Martin et al. 2007) on CaIrO₃-PPv synthesized at high pressure and temperature similar to our starting material. Our result suggests that γ_sp constrained by Raman measurements provides a reasonable estimate on the γ_th of the PPv phase in CaIrO₃.