Anion disorder in LnF3 (Ln = La,Ce, Pr) crystals

The mechanism of lattice “melting” in tysonite-structure superionic trifluorides is analyzed in relation to the structure of LaF₃ crystals. The ion displacements within the unit cell are shown to correlate with the dynamics of the Raman scattering parameters of the crystals. The frequency and intensity of the lines at 370, 392 (LaF₃), 376, and 404 cm⁻¹ (PrF₃) show anomalous behavior in three temperature ranges: 160–180, 240–280, and 420–460 K. The anomalies in the two higher temperature ranges are due to lattice disordering in the superionic transition region. The scattering in the range 160–180 K is tentatively attributed to fluoride ion displacements in split sites with a few potential wells. It is shown based on a split-site model that the thermally activated formation of interstices comparable in size to the mobile fluoride ion is an inherent feature of the LnF₃ structure.