Perovskite twinned structures

In order to obtain reliable results, magnetostriction should be measured in epitaxial films, free of twins, which are typical for materials of perovskite structure. In the case of films it is not easy, because usually the films are deposited on perovskite substrates, thus presenting difficulties in eliminating twin structures. 

Abstract The twin structure of La0.95Sr0.05Ga0.9Mg0.1O2.925 perovskite-type crystals was... 

The twin structure in small LSGMO ciystals tends to form chevronlike wall configurations that allows for a stress-lfee co-existence of four different orientation states. This pattern of domain walls is expected to be characteristic also for other perovskite-type compounds with a sequence of ferroelastic phase transitions related to those of LSGMO. Examples are mixed conductivity perovskites, which are used as electrodes and interconnectors in SOFC batteries. 

Cao, W., Cross, L.E. Theory of tetragonal twin structures in ferroelectric perovskites with a first-order phase transition. Phys. Rev. B 44, 5-12 (1991)... 

The perovskite structure is, of course, of special significance in the electroceramics context since the ferroelectric perovskites are dominant in the ceramic capacitor, PTC thermistor and electromechanical transducer industries. The structure favours the existence of soft modes (low frequency phonons) as evidenced by its tendency to instability, for example the ferroelectric-paraelectric transition. Instability is evident in the case of the T23 compound which exhibits a tetragonal-orthorhombic transition in the region of 700 °C (the exact temperature depends on the oxygen content). Extensive twinning, very reminiscent of ferroelectric domain structures, is observed. 

The BOg octahedra can not only deform but may also tilt and rotate along their fourfold or twofold axes, giving rise to different superstmctures or modulated structures. Besides, there is a strong dependence of structural symmetry on temperature at lower temperatures, numerous modifications or structural distortions from the ideal perovskite stmcture exist [43—45], and all of these causes lower the symmetry of the structure from cubic to tetragonal, orthorhombic, rhombohedral, or monoclinic. A lowering in symmetry will introduce different orientation variants (twins) and translation variants (antiphase boundaries). Stmctures with a lower symmetry, derived from the cubic structure by tilting and/or deformation of the BOg octahedra, become stable such that one (or several) phase transformation(s) may take place. 

Miigge and others found that the only minerals that could easily be deformed under ambient conditions were the alkali halides and a few sulfides and carbonates. An exception to this was periclase (MgO), which deformed by 110 (110) dodecahedral glide in the same way as halite (NaCl). A more recently discovered exception is SrTiOs with the cubic perovskite structure, which can be deformed plastically at ambient and high temperatures but is brittle at intermediate temperatures (see Section 9.4.7). Other oxides and silicate minerals either cleaved or twinned when attempts were made to deform them at normal temperatures and pressures [1]. 

---