Distortion modes and related ferroic properties of the stuffed tridymite-type compoundsSrAl2O4andBaAl2O4

Abstract

The structural and associated ferroic properties of the stuffed tridymite-type compounds ${\text{BaAl}}_{2}{\text{O}}_{4}$ and ${\text{SrAl}}_{2}{\text{O}}_{4}$ have been investigated by means of ab initio calculations. Structures and energy landscapes have been analyzed in terms of symmetry-adapted distortion modes. Despite their rather different room-temperature symmetries, a triply-degenerate unstable antiferrodistortive rigid unit mode (RUM) of the ${\text{Al}}_{2}{\text{O}}_{4}$ tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave $(1\mathbf{q})$ distortion mode competes with a hexagonal configuration resulting from three superposed such waves $(3\mathbf{q})$. The very small energy difference between these two configurations in the case of ${\text{BaAl}}_{2}{\text{O}}_{4}$ would explain the recent electron microscope observation of orthorhombic symmetry on the nanoscale. A second unstable mode of polar character (also a RUM for the tetrahedral framework) is also present. While this second instability is too weak to condense in ${\text{BaAl}}_{2}{\text{O}}_{4}$, in the case of ${\text{SrAl}}_{2}{\text{O}}_{4}$ it is fundamental to make the $1\mathbf{q}$ configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in ${\text{SrAl}}_{2}{\text{O}}_{4}$ to monoclinic. In contrast with previous literature, ${\text{SrAl}}_{2}{\text{O}}_{4}$ is a proper ferroelectric and a pseudoproper ferroelastic, with an uncommon bilinear coupling between its spontaneous polarization and shear monoclinic strain. One expects direct switching of spontaneous polarization through a shear stress, and conversely switching of the ferroelastic spontaneous shear strain through an electric field. This property must be related to the elasticoluminescent and electroluminiscent properties reported in Eu-doped ${\text{SrAl}}_{2}{\text{O}}_{4}$. The calculated energy maps have been systematically compared with variations in the so-called global instability index, introduced within the empirical bond valence model. The similarity of the variation in both quantities (for the most unstable distortion modes) reported in other systems is also observed here.