Calorimetric study of high-pressure phase transitions among the CdGeO₃ polymorphs (pyroxenoid, garnet, ilmenite, and perovskite structures)

At high pressures, CdGeO₃ pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positive P-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positive dP/dT, while a negative dP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO₃ phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO₃ comounds including CdGeO₃, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8-12 fold coordination. The thermochemistry of the CdGeO₃ polymorphs shows several similarities to that of the CaGeO₃ system.