Distortion modes and related ferroic properties of the stuffed tridymite-type compounds SrAl2O4 and BaAl2O4

The structural and associated ferroic properties of the stuffed tridymite-type compounds BaAl2O4 and SrAl2O4 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 Al2O4 tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave (1q) distortion mode competes with a hexagonal configuration resulting from three superposed such waves (3q). The very small energy difference between these two configurations in the case of BaAl2O4 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 BaAl2O4, in the case of SrAl2O4 it is fundamental to make the 1q configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in SrAl2O4 to monoclinic. In contrast with previous literature, SrAl2O4 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 SrAl2O4. 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.