Distortion modes and related ferroic properties of the stuffed tridymite-type compoundsSrAl2O4andBaAl2

Abstract

The structural and associated ferroic properties of the stuffed tridymite-type compounds BaAl 2 O 4 and SrAl 2 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 ͑RUM͒ of the Al 2 O 4 tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave ͑1q͒ distortion mode competes with a hexagonal configuration resulting from three superposed such waves ͑3q͒. The very small energy difference between these two configurations in the case of BaAl 2 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 framework͒ is also present. While this second instability is too weak to condense in BaAl 2 O 4 , in the case of SrAl 2 O 4 it is fundamental to make the 1q configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in SrAl 2 O 4 to monoclinic. In contrast with previous literature, SrAl 2 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 SrAl 2 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 modes͒ reported in other systems is also observed here.

Mode crystallography of distorted structures

“…In Perez-Mato et al (2009) the P2 1 experimental structure of SrAl 2 O 4 was compared with an ab initio relaxed structure and the coincidence of both models is also excellent. In Perez-Mato et al (2009) the P2 1 experimental structure of SrAl 2 O 4 was compared with an ab initio relaxed structure and the coincidence of both models is also excellent.…”

A Fluctuating State in the Framework Compounds (Ba,Sr)Al2O4

“…These RUMs sometimes act as a soft mode that induces a structural phase transition, for example, the structural phase transitions in quartz4, tridymite5, and nepheline6. For the framework compound BaAl 2 O 4 , it has also been argued that a RUM is the dominant structural instability11.…”

Dipolar-glass-like relaxor ferroelectric behaviour in the 0.5BaTiO3-0.5Bi(Mg1/2Ti1/2)O3 electroceramic

“…13,14,23 Frozen in, largely transverse correlated, [111] p (subscript p for parent perovskite substructure) chain dipoles of precisely this type are known to be responsible for the rhombohedral ferroelectric phases of the Pb(Zr 1Àx Ti x )O 3 (PZT) system 24 and BiFeO 3 . 25 In order to gain insight into the crystal chemistry underlying the existence of these 1-D h111i nano-scale, dipolar chains as well as for the wide spread in relaxation frequencies associated with the observed dipolar relaxation, bond valence sum calculations [26][27][28][29] assuming an ideal undistorted, a ¼ 4.0129 Å , Pm 3m perovskite average structure were carried out. The calculated apparent valences (AVs) of the cations without any off-centre displacements are as follows: AV(Ba 2þ ) ¼ 2.589 valence units (vu), AV(Bi 3þ ) ¼ 1.757 vu, AV(Mg 2þ ) ¼ 2.230 vu, and AV(Ti 4þ ) ¼ 3.520 vu.…”