Ultrasonic Attenuation and Mechanism for the 250°K Antiferrodistortive Transition in BaMnF4

Abstract

Critical attenuation measurements are reported for two anomalous acoustic modes near the 250°K transition of BaMn${\mathrm{F}}_4$. A mechanism for the transition involving rotations of the Mn${\mathrm{F}}_6$ octahedra about the orthorhombic $b$ axis is proposed, with a pair of phonons at the Brillouin zone $S$ points $\pi (0, \frac{1}{b}, \pm \frac{1}{c})$ being the soft modes. This mechanism, which suggests strong two-dimensional correlations, appears consistent with previous work on BaMn${\mathrm{F}}_4$ as well as with the ultrasonic data.