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 [image omitted] stability field below 690 K is related to the elastic stability limit [image omitted]. Before this instability point is reached, the first order [image omitted] Imma transition intervenes and is marked by a steep minimum in shear elastic constants. Further pseudoproper ferroelastic softening then occurs, consistent with [image omitted] at 151 K for the second-order Imma C2/m transition. Superattenuation of acoustic resonances within the stability field of the C2/m 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 [image omitted] symmetry and which displays an unusual anelastic dissipation.