Elastic and anelastic relaxations associated with the incommensurate structure ofPr0.48Ca0.52MnO3

Abstract

The elastic and anelastic properties of a polycrystalline sample of ${\text{Pr}}_{0.48}{\text{Ca}}_{0.52}{\text{MnO}}_{3}$ have been investigated by resonant ultrasound spectroscopy, as a function of temperature (10--1130 K) and magnetic field strength (0--15 T). Marked softening of the shear modulus as the $Pnma\ensuremath{\leftrightarrow}\text{incommensurate}$ phase transition at $\ensuremath{\sim}235\text{ }\text{K}$ in zero field is approached from either side is consistent with pseudoproper ferroelastic character, driven by an order parameter with ${\ensuremath{\Gamma}}_{3}^{+}$ symmetry associated with Jahn-Teller ordering. This is accompanied by an increase in attenuation just below the transition point. The attenuation remains relatively high down to $\ensuremath{\sim}80\text{ }\text{K}$, where there is a distinct Debye peak. It is attributed to coupling of shear strain with the ${\ensuremath{\Gamma}}_{3}^{+}$ order parameter which, in turn, controls the repeat distance of the incommensurate structure. Kinetic data extracted from the Debye peak suggest that the rate-controlling process could be related to migration of polarons. Elastic softening and stiffening as a function of magnetic field at constant temperatures between 177 and $\ensuremath{\sim}225\text{ }\text{K}$ closely resembles the behavior as a function of temperature at 0, 5, and 10 T and is consistent with thermodynamically continuous behavior for the phase transition in both cases. This overall pattern can be rationalized in terms of linear/quadratic coupling between the ${\ensuremath{\Gamma}}_{3}^{+}$ order parameter and an order parameter with ${\ensuremath{\Sigma}}_{1}$ or ${\ensuremath{\Sigma}}_{2}$ symmetry. It is also consistent with a dominant role for spontaneous strains in determining the strength of coupling, evolution of the incommensurate microstructure, and equilibrium evolution of the Jahn-Teller ordered structure through multicomponent order-parameter space.