Phys Chem Minerals (1994) 21:373 386 PmlIICS/ me-HEMISTRY NMIIIERAIS 9 Springer-Verlag 1994 Diffusion and the Dynamics of Displacive Phase in Cryolite (Na3AIF6) and Chiolite (NasAl3F14): Multi-Nuclear NMR Studies Transitions Dane R. Spearing, Jonathan F. Stebbins, Ian Farnan* Department of Geological and EnvironmentalSciences, StanfordUniversity,Stanford, CA 94305, USA Received March 21, 1994/Revised, accepted July 21, 1994 Abstract. Cryolite is a mixed-cation perovskite (Na2(NaA1)F6) which undergoes a monoclinic to or- thorhombic displacive phase transition at ~ 550~ C. Chi- olite (NasA13F14) is associated with cryolite in natural deposits, and consists of sheets of corner sharing [A1F6] octahedra interlayered with edge-sharing [NaF6] octahe- dra. 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 mo- tions associated with the phase transition in cryolite, and Na diffusion in cryolite and chiolite. 27A1, 23Na, and 19F static NMR spectra and Tl's in cryolite suggest that os- cillatory motions of the [A1F6] octahedra among four mi- cro-twin and anti-phase domains in e-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 tempera- ture is reached. 27A1and 23Na Tl'S show the same behav- ior with increasing temperature, indicating the same re- laxation mechanisms are responsible for both. The first order nature of the cryolite transition is apparent as a jump in the 23Na and 27A1Tl's. Above the transition tem- perature, the TI'S decrease slightly indicating that the motions responsible for the drop in T 1 are still present above the transition, further supporting the dynamic na- ture 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. 27A1 and 23Na Tl's show similar behavior as in cry- olite, but there is no discontinuity due to a phase transi- tion. 19F TI's are constant from room temperature to 150~ C indicating no oscillatory motion of the [A1F6] oc- tahedra in chiolite. * Present address: Centrede Recherchessur la Physiquedes Hautes Temperatures, C.N.R.S, 1D, Avenue de la Recherche Scientifique, F-45071 Orleans Cedex 2, France Correspondence to: J.F. Stebbins Introduction Understanding the physical properties of compounds that exhibit the perovskite structure is becoming increas- ingly important in both the geological and materials sci- ences. Magnesium silicate (MgSiO3) perovskite is consid- ered to be the dominant phase within the Earth's lower mantle, and for the Earth as a whole (Williams et al. 1989). The perovskite structure is also adopted by a number of electroceramics (Amin 1989; Newnham 1989) and the new high-temperature superconductors, such as YBa2Cu307 (Wiley and Poeppelmeier 1989). The general perovskite structure (ABX3) consists of corner linked oc- tahedra of X anions (usually oxygen or fluorine) with the B cations at the center of the octahedra and the A cations in the interstices between the octahedra. Typi- cally, the A cation is divalent (e.g.- Ca, Mg, Sr, Ba, Fe 2§ etc.) and the B cation is tetravalent (Ti, Si, Nb, Zr, etc.). In the ideal cubic structure, all of the octahedra are translationally equivalent and aligned along their individual symmetry axes (i.e. - no tilting or distortion), as exhibited by SrTiO3. However, more often the struc- ture is modified by cation displacement, tilting, or distor- tion of the octahedra (Glazer 1972; Glazer 1975) result- ing in a lowering of the overall symmetry from cubic. This situation lends itself ideally to displacive phase tran- sitions: a slightly misaligned anion framework that can be "realigned" to higher symmetry with increasing tem- perature or pressure either through static displacements or motional averaging. Understanding how these displa- cive phase transitions occur, and the nature of the atomic displacements, is important in being able to predict the properties of such materials at various P- T conditions. Nuclear magnetic resonance (NMR) is a useful technique with which to study these types of phase transitions, and the motions associated with them, due to its sensitiv- ity to motions at two different time scales: the Hz-kHz range, from the line shape; and the MHz range, from the relaxation times. These are the time scales at which motions associated with phase transitions occur, such as ionic diffusion and soft mode fluctuations. Moreover, these are at time scales which are much shorter than