Strain mechanism for order-parameter coupling through successive phase transitions inPrAlO3

Abstract

High-resolution powder neutron diffraction data have been collected at $\ensuremath{\sim}50$ temperatures between 4.2 and 300 K for perovskites ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{AlO}}_{3}$ with $x=0$, 0.05, 0.1, and 0.25. Structural and lattice parameter data extracted by Rietveld refinement have then been used to follow details of octahedral tilting and spontaneous strains associated with the sequence of phase transitions $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c\ensuremath{\leftrightarrow}Imma\ensuremath{\leftrightarrow}C2∕m$. These are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a $Pm\overline{3}m$ reference structure with two instabilities (${\mathrm{R}}_{4}^{+}$ and ${\ensuremath{\Gamma}}_{3}^{+}$ active). Data from the literature relating to the $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c$ transition are consistent with second-order character and an extrapolated transition temperature of $1864\ifmmode\pm\else\textpm\fi{}31\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $R\overline{3}c\ensuremath{\leftrightarrow}Imma$ transition is first order in character and can be understood as occurring because coupling takes place between tilting and electronic order-parameter components via a common tetragonal strain. Strains for the $Imma\ensuremath{\leftrightarrow}C2∕m$ transition conform closely to the Landau solution for a proper ferroelastic transition with second-order character and low-temperature saturation. The acoustic anomaly reported at 118 K appears to be associated with a metrically tetragonal structure which develops, as an accidental strain degeneracy, at a temperature between $\ensuremath{\sim}110$ and $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Differences in saturation temperatures for the order parameters associated with tilting and electronic ordering are probably responsible for small additional anomalies in the evolution of strains below $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.