DIFFUSION AND THE DYNAMICS OF DISPLACIVE PHASE-TRANSITIONS IN CRYOLITE (NA3ALF6) AND CHIOLITE (NA5AL3F14) - MULTINUCLEAR NMR-STUDIES

Cryolite is a mixed-cation perovskite (Na-2(NaAl)F-6) which undergoes a monoclinic to orthorhombic displacive phase transition at similar to 550 degrees C. Chiolite (Na5Al3F14) is associated with cryolite in na tural deposits, and consists of sheets of corner sharing [AlF6] octahe dra interlayered with edge-sharing [NaF6] octahedra. Multi-nuclear NMR line shape and relaxation time (T-1) studies were performed on cryoli te and chiolite in order to gain a better understanding of the atomic motions associated with the phase transition in cryolite, and Na diffu sion in cryolite and chiolite. Al-27, Na-23, and F-19 static NMR spect ra and T-1's in cryolite suggest that oscillatory motions of the [AlF6 ] octahedra among four micro-twin and anti-phase domains in a-cryolite begin at least 150 degrees C below the transition temperature and per sist above it. Variable temperature Na-23 MAS NMR further indicates di ffusional exchange at a rate of at least 13 kHz between the Na sites b y the time the transition temperature is reached. Al-27 and Na-23 T-1' s show the same behavior with increasing temperature, indicating the s ame relaxation mechanisms are responsible for both. The first order na ture of the cryolite transition is apparent as a jump in the Na-23 and Al-27 T-1's Above the transition temperature, the T-1's decrease slig htly indicating that the motions responsible for the drop in T-1 are s till present above the transition, further supporting the dynamic natu re of the high temperature phase of cryolite. Chiolite Na-23 static sp ectra decrease in linewidth with increasing temperature, indicating in creased Na diffusion, which is interpreted as occurring within the [Na F6] sheets in the chiolite structure, but not between the two differen t Na sites. Al-27 and Na-23 T-1's show similar behavior as in cryolite , but there is no discontinuity due to a phase transition. F-19 T-1's are constant from room temperature to 150 degrees C indicating no osci llatory motion of the [AlF6] octahedra in chiolite.