Chemical mixing and hard mode spectroscopy in ferroelastic lead phosphate arsenate: localsymmetry splitting and multiscaling behaviour

Abstract

The phase transition in ferroelasticPb3(PO4)2–Pb3(AsO4)2 mixed crystals shows typical multiscaling behaviour with two relevant length andtimescales. One length scale is macroscopic and shows uniform, weakly first-order phasetransitions between a rhombohedral paraphase () and a monoclinic ferroelastic phase (C 2/c). The second length scale is on the level of tetrahedral complexes which display monoclinicdistortions at temperatures well above the macroscopic transition point. For instance, inPb3(P0.43As0.57O4)2 theAsO4 polyhedra show staticdeformation up to ∼60 K abovethe deformation of the PO4 tetrahedra. The two timescales are either short compared withthe time of observation, namely the dynamic reorientation of thePO4 tetrahedral distortion, or very long. The long timescale refers then to the (quasi)static distortion of theAsO4 tetrahedra whichpersists at T>Tc. These distortions appear to be uncorrelated or only weakly correlated and their randomfield leads to an order/disorder aspect of the phase transition which remains, on a phonontimescale, essentially displacive in character.