Computer modeling and Brillouin scattering studies of anharmonicity and high-temperature disorder in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">LaF</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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.

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