Ferroelectricity in Perovskites with s0 A-Site Cations: Toward Near-Room-Temperature Multiferroics

Abstract

materials is based on perovskites, ABO3, with two different cations A and B (for example A 2+ and B 4+ or A 3+ and B 3+ ), in which corner-sharing BO6 octahedra form the three-dimen- sional lattice with every B8 cube containing one A cation. An ideal cubic perovskite, in which the B-OB bonds are linear with A located at the center of each B8 cube (thus forming a AO12 polyhedron), is expected when the tolerance factor t = (rO + rA)/ ffiffiffi 2 p (r O+rB) is unity (rA and rB are the ionic radii of the A and B cations, respectively, with rO as that of the O 2� anion). In most cases, the A cations are small, so that t < 1 and the AO bonds are too long to maintain the ideal cubic structure. Consequently, the A cation moves away from the center of the B8 cube, which is accompanied by the bending of the B-O-B bonds and the rotation of the BO6 octahedra to form a lower-coordinate AOn polyhedron (n < 12) with short AO bonds. The distortion of the ideal cubic perovskite toward a non-centrosymmetric FE structure requires another local instability apart from t < 1, namely, the second-order Jahn-Teller (SOJT) instability (4-6) of the A-site and/or B-site