Structural phase transitions in perovskite BaCeO3 with data mining and first-principles theoretical calculations
Several experiments conducted over decades have revealed that the perovskite-structured BaCeO3 goes through a series of temperature-induced structural phase transitions. However, it has been frequently observed that the number of phases and the sequence in which they appear as a function of temperature differ between experiments. Insofar as neutron diffraction and Raman spectroscopy experiments are concern, four structures are well characterized with three transitions: Pnma to Imma [563 K] to R-3c [673 K] to Pm-3m [1173 K]. In contrast, thermoanalytical methods showed multiple singularities corresponding to at-least three more structural transitions at around 830 K, 900 K, and 1030 K. In account of these conflicting experimental findings, we computed free energy phase diagram for BaCeO3 employing crystal structure data mining in conjunction with first principles electronic structure and phonon lattice dynamics. A total of 34 polymorphs have been predicted, the most stable of which follows the Glazer classification of the perovskite tilt system. It has been predicted that the Cmcm and P4/mbm phases surpass Pnma at 666 K and 1210 K, respectively. At any temperature, two alternate tetragonal phases (P42/nmc and I4/mcm) are also found to be 20 to 30 meV less favored than the Pnma. While the calculated stability order of the predicted polymorphs is in acceptable agreement with the results of neutron diffraction, the transitions observed in thermoanalytical studies could be ascribed to the development of four novel phases (Cmcm, P4/mbm, P42/nmc, and I4/mcm) at intermediate temperatures. However, we analyze that the R-3c phase predominantly stabilized over a broad temperature field, masking all subsequent phases up until the cubic Pm-3m. Consequently, the novel phases predicted to occur in thermoanalytical studies are only fleetingly metastable.