Pseudoproper ferroelastic softening behaviour and dynamic microstructure of monoclinic PrAlO₃ perovskite

Elastic and anelastic properties of single crystal PrAlO₃ have been investigated by resonant ultrasound spectroscopy at 0.2–1.2 MHz. Non-linear (pseudoproper) softening with falling temperature in the <a name="ILM0001"> </a> stability field below ∼690 K is related to the elastic stability limit <a name="ILM0005"> </a>. Before this instability point is reached, the first order <a name="ILM0002"> </a> <i>Imma</i> transition intervenes and is marked by a steep minimum in shear elastic constants. Further pseudoproper ferroelastic softening then occurs, consistent with <a name="ILM0003"> </a> at ∼151 K for the second-order <i>Imma</i> ↔ <i>C</i>2/<i>m</i> transition. Superattenuation of acoustic resonances within the stability field of the <i>C</i>2/<i>m</i> structure implies that the monoclinic twin walls are highly mobile, apart from in a narrow temperature interval near 100 K where the structure becomes metrically tetragonal. PrAlO₃ appears to represent a general class of Jahn–Teller phase transitions in perovskites for which the predominant order parameter transforms according to <a name="ILM0004"> </a> symmetry and which displays an unusual anelastic dissipation