Suppression of strain coupling in perovskiteLa0.6Sr0.1TiO3by cation disorder

Abstract

An octahedral tilting transition has been investigated in two samples of ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.1}\mathrm{Ti}{\mathrm{O}}_{3}$ by powder neutron diffraction. One (slowly cooled) sample had a degree of cation/vacancy ordering on the $A$ sites and was tetragonal, $P4∕mmm$, at high temperatures. The second (quenched) sample had disordered cations and cubic, $Pm\overline{3}m$, symmetry at high temperatures. On cooling, both underwent the same $R$-point tilting transition at $\ensuremath{\sim}560--570\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, to give symmetry changes $P4∕mmm\ensuremath{\leftrightarrow}Cmmm$ and $Pm\overline{3}m\ensuremath{\leftrightarrow}I4∕mcm$, respectively. From the evolution of the tilt angles, the transition appears to be close to tricritical in both cases. As expected from a Landau expansion in the two order parameters, the transition temperature was found to be only weakly dependent on the degree of cation order. In contrast with the expectation of standard patterns of strain to order parameter coupling, however, the tilted (tetragonal) form of the disordered sample remained metrically cubic, implying that coupling of the tetragonal shear strain with the octahedral tilting was suppressed. It is proposed that cation disordering causes the development of local strain heterogeneities in crystals of ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.1}\mathrm{Ti}{\mathrm{O}}_{3}$ which prevent a coherent, long-range shear strain from developing. Manipulation of the degree of cation order in perovskites with carefully selected compositions might therefore provide a means by which the magnitudes of ferroelastic strains could be engineered according to the requirement of particular applications.