2 research outputs found
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.Comment: 20 pages, 5 figures, 1 tabl
Structural Phase Transitions in Perovskite BaCeO<sub>3</sub> 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 R3Ì…c phase is predominantly stabilized over a broad temperature
field, masking all subsequent phases up until the cubic Pm3Ì…m. 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