Phase transitions among the CaGeO3 polymorphs (wollastonite, garnet, and perovskite structures): Studies by high‐pressure synthesis, high‐temperature calorimetry, and vibrational spectroscopy and calculation

Phase boundaries for the reactions CaGeO3 (wollastonite → garnet) and CaGeO3 (garnet → perovskite) were determined at high pressure and temperature. The enthalpies of these transitions were measured by high temperature calorimetry. These studies indicate a positive dP/dT for the wollastonitegarnet transition and a negative dP/dT for the garnet‐perovskite transition. These PT slopes are further supported by calculations of lattice vibrational entropies based on Kieffer's model and the infrared and Raman spectra of the three polymorphs.