Brillouin scattering studies of ${\mathrm{LaF}}_3$ at high temperatures have yielded the complete set of acoustic-mode frequencies associated with the elastic constants. These show a linear decrease with temperature up to about 1150 K, this being associated with lattice expansion. Above 1150 K, the elastic constants show a marked reduction with the exception of $C_{44}$, which remains nearly constant. This behavior, which results from a diffuse superionic transition to a disordered state, has been computationally simulated by energy-minimization techniques. The nature of the dominant high-temperature disorder has been examined with the use of a supercell approach to permit appropriate defect concentrations and types. The calculations favor anion Frenkel disorder in an estimated concentration of 2 mole % at 1400 K.

• Received 29 August 1989

DOI:https://doi.org/10.1103/PhysRevB.41.3815