Octahedral Tilting in Perovskites. II. Structure Stabilizing Forces

Abstract

The 23 Glazer tilt systems describing octahedral tilting in perovskites have been investigated. The various tilt systems have been compared in terms of their A-cation coordination and it is shown that those tilt systems in which all the A-cation sites remain crystallographically equivalent are strongly favored, when all the A sites are occupied by the same ion. Calculations based on both ionic and covalent models have been performed to compare the seven equivalent A-site tilt systems. Both methods predict that when the tilt angles become large, the orthorhombic a+b-b -tilt system will result in the lowest energy structure. This tilt system gives the lowest energy structure because it maximizes the number of short A---O interactions. The rhombohedral a-a-a-tilt system gives a structure with a slightly lower Madelung energy, but increased ion-ion repulsions destabilize this structure as the tilt angles increase. Consequently, it is stabilized by highly charged A cations and small to moderate tilt angles. The ideal cubic a°a°a° tilt system is only observed when stabilized by oversized A cations and/or M---O 7r-bonding. Tilt systems with nonequivalent A-site environments are observed when at least two A cations with different sizes and/or bonding preferences are present. In these compounds the ratio of large-to-small cations dictates the most stable tilt system.

Correlating interfacial octahedral rotations with magnetism in (LaMnO3+δ)N/(SrTiO3)N superlattices

“…For the N ¼ 16 superlattice and the LaMnO 3 þ d film, the Mn valence is close to þ 3, and correspondingly, a distorted orthorhombic (DO) phase emerges with in-phase rotation around one of the in-plane axis [15][16][17]31,32 , resulting in obvious two-half-integer peaks. The observed onehalf-integer peaks cannot be produced by OOR, and are probably caused by La displacements 31 , which are well known to occur in bulk orthorhombic phases 14 .…”

“…Details of the sample growth, the measurements and theoretical calculation are described in Methods section. At room temperature, bulk LaMnO 3 þ d exhibits either orthorhombic structure with a À a À c þ type octahedral rotation (if Mn valence is close to 3 þ ), or rhombohedra structure with a À a À a À rotation (if Mn valence is higher) [14][15][16][17] . SrTiO 3 in bulk exhibits a cubic structure above 105 K with no octahedral rotations.…”

Designing a robustly metallic noncenstrosymmetric ruthenate oxide with large thermopower anisotropy

“…The Ru-O-Ru bond angles are 157.1°and 145.6°(perpendicular to the CaO/SrO layers) and 150.4°(within the layers). The stabilization of this rotation pattern is understood on the grounds that the p* band, which is nearly full because of the t 4 2g configuration, is shifted to lower energy by the electronegative Ru 4 þ cation 28 .…”

Strain-driven spin-state transition and superexchange interaction in LaCoO3:Abinitiostudy

“…1 (a). It is described with space group c R3 with a distorted corner-sharing CoO 6 octahedral network (a -a -a -in the Glazer tilt system [42,43]). We compare the calculated structural parameters for bulk LCO with experiment in Table 1.…”