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

Abstract

Abstract.Cryolite is a mixed-cation perovskite (Na2(NaA1)F6) which undergoes a monoclinic to orthorhombic displacive phase transition at ~ 550 ~ C. Chiolite (NasA13F14) is associated with cryolite in natural deposits, and consists of sheets of corner sharing [A1F6] 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. 27A1, 23Na, and 19F static NMR spectra and Tl's in cryolite suggest that oscillatory motions of the [A1F6] octahedra among four micro-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 temperature is reached. 27A1 and 23Na Tl'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 27A1 Tl's. Above the transition temperature, 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 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. 27A1 and 23Na Tl's show similar behavior as in cryolite, but there is no discontinuity due to a phase transition. 19F TI's are constant from room temperature to 150 ~ C indicating no oscillatory motion of the [A1F6] octahedra in chiolite.

Fast ionic mobility in cryolite studied by quasielastic neutron scattering

“…Results from NMR spectroscopy indicate an increased ionic mobility already above T = 100 • C, well within the stability range of the monoclinic phase [6,7]. The observed changes in the chemical shift and the merging of the two peaks from the differently coordinated Na ions are interpreted as rapid cation exchange between the Na V I and Na V III sites.…”

“…Diffusion rates may be obtained from NMR data [2,6,7], but the sensitivity of NMR is usually limited to jump frequencies up to the kHz range. For faster ionic mobility, experimental data are rare due to experimental difficulties of performing in situ experiments at high T where fluoride samples often become very reactive.…”

“…At T = 600 • C, which is above the phase transition to the cubic phase, the two sides are indistinguishable on the time scale of NMR with a minimum exchange frequency of 13 kHz. It was proposed [6,7] that the Na diffusion is governed by a vacancy mechanism and the respective diffusion rate is controlled by oscillations of the [AlF 6 ] octahedra. Molecular dynamics simulations suggest that the [AlF 6 ] octahedra rotate above T = 300 • C [13].…”

Sodium diffusion in cryolite at elevated temperatures studied by quasielastic neutron scattering

“…At low temperatures, far below the first transition, NMR studies confirmed Na + exchange between the six and eight fold coordinated site in the kHz frequency range [10,11]. At T = 880 K, which is in the stability field of the cubic phase, the two sites are indistinguishable on the time scale of NMR.…”

Cristalización de criolita a partir de un fundido: Rasgos macroscópicos y estructurales

“…Posee una estructura de tipo perowskita, de gran actualidad tanto desde el punto de vista geológico como para la ciencia de los materiales. Además exhibe una conducta muy especial, con transiciones de fase y movilizaciones ató-micas asociadas a ellas, que son responsables de sus peculiares propiedades mecánicas y eléctricas (2). Por encima de los 450ºC se comporta como un flujo plástico que podría ser la explicación de las brechas del yacimiento de Ivigtut (Groenlandia), donde criolita pura constituye el cemento de los clastos de la roca (Ivigtut ha sido el depósito industrial mas grande del mundo, en la actualidad prácticamente agotado) (3).…”