Transformation processes inLaAlO3: Neutron diffraction, dielectric, thermal, optical, and Raman studies

Abstract

The behavior of the $Pm\overline{3}m\text{\ensuremath{-}}R\overline{3}c$ phase transition in $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ (${T}_{\mathrm{C}}=813\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from differential scanning calorimetry measurements) has been studied using temperature-dependent measurements of the crystal structure, dielectric relaxation, specific heat, birefringence, and the frequencies of the two soft modes (via Raman spectroscopy). While all these experiments show behavior near ${T}_{\mathrm{C}}$ consistent with a second-order Landau transition, there is extensive evidence for additional anomalous behavior below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Below this temperature, the two soft mode frequencies are not proportional to each other, the spontaneous strain is not proportional to the square of the $\mathrm{Al}{\mathrm{O}}_{6}$ rotation angle, and anomalies are seen in the birefringence. Twin domains, which are mobile above $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, are frozen below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These anomalies are consistent with biquadratic coupling between the primary order parameter of the transition ($\mathrm{Al}{\mathrm{O}}_{6}$ rotation) and a second process. From the dielectric results, which indicate a smooth but rapid increase in conductivity in the temperature range $500--800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we propose that this second process is hopping of intrinsic oxygen vacancies. These vacancies are essentially static below $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and dynamically disordered above $730\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The interaction between static vacancies and the displacive phase transition is unfavorable. A similar anomaly may be observed in other aluminate perovskites undergoing the same transition.