Structural Phase Transitions in Perovskite BaCeO₃ with Data Mining and First-Principles Theoretical Calculations

Several neutron diffraction, Raman spectroscopy, and thermoanalytical experiments conducted over decades have revealed that 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 experiments are concerned, in the temperature range of 4.2 to 1273 K, four structures are crystallographically well characterized with three transitions, orthorhombic Pnma → orthorhombic Imma [563 K] → rhombohedral R3̅c [673 K] → cubic Pm3̅m [1173 K], which lately have been reciprocally realized in the studies of polarized Raman spectroscopy. In contrast, thermoanalytical methods such as dilatometry showed multiple singularities corresponding to at least three more structural phase transitions at around 830, 900, and 1030 K, in addition to those recorded by neutron studies. In account of these conflicting experimental findings, we computed a free-energy phase diagram for BaCeO3 polymorphs 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, and it has been found that a number of these polymorphs are thermodynamically competing with Pnma as the temperature rises. In particular, it has been predicted that the orthorhombic Cmcm and tetragonal P4/mbm phases surpass Pnma at 666 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 show that the rhombohedral R3̅c phase is predominantly stabilized over a broad temperature field, masking all subsequent phases up until the cubic Pm3̅m. Consequently, the novel phases predicted to occur in thermoanalytical studies are only fleetingly metastable. The calculated phonons additionally demonstrate that the high-temperature phases are not quenchable down to room temperature. The theoretical results presented reconcile the apparent inconsistencies observed thus far in the experiments