Domain boundary pinning and elastic softening in KMnF3 and KMn1-xCaxF3

Anelastic softening related to the movement of twin boundaries is observed in improper ferroelastic KMnF3 and KMn(1-x)CaxF(3). Wall movement in KMnF3 shows a frequency dependence which is described in terms of an extended Debye relaxation with an extension exponent of 0.54. This exponent indicates a fairly narrow distribution of activation energies near 0.43 eV. Wall movements in Ca-doped samples are best described in terms of Vogel-Fulcher (VF) relaxations with a VF energy of 0.23 eV. The activation energies are related to interaction between F vacancies or interstitials and the moving domain walls; Ca doping appears to increase the tendency to form glass-like states. No domain freezing occurs at temperatures above the subsequent phase transition I4/ mcm-Pnma; the Pnma phase does not show any domain movement and anelastic behaviour. Elastic precursor softening is observed above the transition temperature between the cubic and the tetragonal phase. The softening can be described empirically using a power law: [(T-T-o)/T-o](-K) with values of the exponent K around 0.5.