Ferroelasticity in Palmierite-Type(1 − X)pb3(po4)2 −Xpb3(aso4)2

Lead phosphate-arsenate Pb3(P1-x As x O4)2 undergoes an improper ferroelastic phase transition from a rhombohedral paraphase [Formula: see text] to a monoclinic ferrophase [Formula: see text] leading to distinct twin boundary patterns. On cooling compounds with x larger than 0.8 undergo further transitions to monoclinic low-temperature phases, whereas the composition with x = 0.8 shows order-parameter coupling phenomena. The transformation [Formula: see text]-[Formula: see text] was described on the basis of a three-state Potts model and the existence of precursors of monoclinic clusters in the rhombohedral paraphase. The system is one of the best studied improper ferroelastics. Due to its two-mode phonon behaviour the solid solution exhibits multistep temperature- as well as pressure-driven structural transformations with different length and time scales. Relevant investigations and findings of this palmierite-type material have been made by Prof E K H Salje. Some of the most prominent results from x-ray diffraction, optical microscopy and Raman scattering are reviewed, and the potential implications for domain-wall structures and engineering are discussed.