Dynamic elastic response of KMn1-xCaxF3: Elastic softening and domain freezing

Results of continuous-wave resonance (MHz-region) and ultralow-frequency (Hz-region) dynamic mechanical analyzer measurements of pure KMnF3 and mixed crystals KMn1-xCaxF3 (x < 0.017) are presented in a broad temperature range including the phase-transition regions. The elastic anomalies depend strongly on the measurement frequency: The MHz-elastic constants can be well fitted by Landau theory including the difference between isothermal and adiabatic behavior. Contrary, at low frequencies (0.1-50 Hz) an elastic softening (superelasticity) due to domain-wall motion is found quite similar to that observed recently in isostructural SrTiO3 [A. V. Kityk , Phys. Rev. B 61, 946 (2000)]. However, unlike to SrTiO3, for the mixed system KMn1-xCaxF3 we have measured freezing of the domain-wall movement around 100 K in a certain range of concentration of Ca2+ ions (x=0.003). Calculating the dynamic elastic response to the ac-stress field by taking into account the motion of randomly pinned ferroelastic domain walls in KMn1-xCaxF3 yields excellent agreement between theory and experiment.