High-pressure single-crystal elasticity study of CO2 across phase I-III transition

Sound velocities and elastic moduli of solid single-crystal CO2 were measured at pressures up to 11.7(3) GPa by Brillouin spectroscopy. The aggregate adiabatic bulk modulus (KS), shear modulus (G), and their pressure derivatives for CO2 Phase I are KS0 = 3.4(6) GPa, G0 = 1.8(2) GPa, (dKS/dP)0 = 7.8(3), (dG/dP)0 = 2.5(1), (d2KS/dP2)0 = −0.23(3) GPa−1, and (d2G/dP2)0 = −0.10(1) GPa−1. A small increase of elastic properties was observed between 9.8(1) and 10.5(3) GPa, in agreement with the CO2 I-III transition pressure determined from previous x-ray diffraction experiments. Above the transition pressure PT, we observed a mixture dominated by CO2-I, with minor CO2-III. The CO2-I + III mixture shows slightly increased sound velocities compared to pure CO2-I. Elastic anisotropy calculated from the single-crystal elasticity tensor exhibits a decrease with pressure beginning at 7.9(1) GPa, which is lower than PT. Our results coincide with recent X-ray Raman observations, suggesting that a pressure-induced electronic transition is related to local structural and optical changes