Low temperature thermoelastic and structural properties of LaGaO3 perovskite in the Pbnm phase

The thermoelastic and structural properties of LaGaO3 perovskite have been studied using high resolution neutron diffractometry at 158 temperatures between 11 K and 548 K. Data collected in 2 K intervals between 410 K and 430 K show no evidence for an intermediate phase between the low temperature phase in space group Pbnm   and the high temperature phase in space group F3¯2/n. From a simultaneous fit of the unit cell volume and the isochoric heat capacity, the phonon density of states in the Pbnm phase is shown to be approximated by a two-term Debye model, with characteristic temperatures 294(1) K and 831(1) K. Vibrational Debye temperatures, determined from fitting the temperature variation of the atomic displacement parameters, show the cations to be more associated with the lower characteristic temperature, whilst that for the anions, is closer to the higher characteristic temperature. Structural parameters are presented as the amplitudes of the seven symmetry-adapted basis-vectors of the aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is outlined. The phase transition in both temperature and pressure arises when a non-bonded La–O distance approaches the La coordination sphere.

The weight fraction of the rhombohedral phase of LaGaO3 from the onset of the phase transition at 408 K to its completion at 430 K.Figure optionsDownload as PowerPoint slideHighlights
► Thermoelastic properties of LaGaO3 analyzed as a two-term Debye model.
► Crystal structure and bonding analyzed in terms of symmetry-adapted basis-vectors.
► Consistency between calorimetric and crystallographic measurements.