Incommensurate and inhomogeneous phases in improper ferroelastic MgSiF6·6H2O: Mn2 studied by EPR

Abstract

The results of EPR study of inhomogeneous and incommensurate phases in improper ferroelastic crystals MgSiF6·6H2O:Mn2+ are presented. On the basis of analysis of temperature and angular dependences of experimental parameters and numerical calculations the conclusion has been drawn that at Ti1=370±0.3 K considered crystals undergo transition to structurally incommensurate phase and the order parameter of this transition may be angle of Mg[H2O]62+ octahedra rotation around crystal C3 axis. From Ti1 to Ti2=343±0.3 K the incommensurate phase evolve according to classical complete scenario: origin of incommensurate structure with plane-wave modulation at Ti1, appearance of structural phase soliton lattice with temperature decrease, and, finally, abrupt decrease in soliton density at Ti2 down to values ≈0.1. The assumption was expressed that at Ti2 the crystals investigated undergo phase transition from incommensurate phase to another inhomogeneous phase, in which studied crystals consist of two types of regions characterized by homogeneous and inhomogeneous structural organization. The performed calculation of spectral density functions has shown that the structure of former regions almost does not change with temperature variation whereas that of latter ones considerably modifies when the temperature tends to the improper ferroelastic phase transition temperature TC=296.5±0.3 K.