Diffusion and the dynamics of displacive phase transitions in cryolite (Na3AlF6) and chiolite (Na5Al3F14): Multi-nuclear NMR studies

Cryolite is a mixed-cation perovskite (Na₂(NaAl)F₆) which undergoes a monoclinic to orthorhombic displacive phase transition at ～550° C. Chiolite (Na₅Al₃F₁₄) is associated with cryolite in natural deposits, and consists of sheets of corner sharing AlF₆] octahedra interlayered with edge-sharing NaF₆] octahedra. Multi-nuclear NMR line shape and relaxation time (T₁) studies were performed on cryolite and chiolite in order to gain a better understanding of the atomic motions associated with the phase transition in cryolite, and Na diffusion in cryolite and chiolite. ²⁷Al, ²³Na, and ¹⁹F static NMR spectra and T₁'s in cryolite suggest that oscillatory motions of the AlF₆] octahedra among four micro-twin and anti-phase domains in α-cryolite begin at least 150° C below the transition temperature and persist above it. Variable temperature ²³Na MAS NMR further indicates diffusional exchange at a rate of at least 13 kHz between the Na sites by the time the transition temperature is reached. ²⁷Al and ²³Na T₁'s show the same behavior with increasing temperature, indicating the same relaxation mechanisms are responsible for both. The first order nature of the cryolite transition is apparent as a jump in the ²³Na and ²⁷Al T₁'s. Above the transition temperature, the T₁'s decrease slightly indicating that the motions responsible for the drop in T₁, are still present above the transition, further supporting the dynamic nature of the high temperature phase of cryolite. Chiolite ²³Na static spectra decrease in linewidth with increasing temperature, indicating increased Na diffusion, which is interpreted as occurring within the NaF₆] sheets in the chiolite structure, but not between the two different Na sites. ²⁷Al and ²³Na T₁'s show similar behavior as in cryolite, but there is no discontinuity due to a phase transition. ¹⁹F T₁'s are constant from room temperature to 150° C indicating no oscillatory motion of the AlF₆] octahedra in chiolite.