Nuclear Magnetic Resonance Studies of Pure and Samarium-Doped CaF2 Crystals

Abstract

The spin—lattice relaxation time T1 of 19F was measured over a temperature range of 77° to 1300°K in natural and synthetic CaF2 crystals and in CaF2 crystals doped with 0.05% Sm, 0.16% Sm, and 1.20% Sm. Most of the data was taken with the magnetic field along the [100] and [111] directions using the magnetic recovery method. For annealed samples T1 at room temperature and below is found to be inversely proportional to the concentration of samarium. Measurements show that in the high-temperature regions T1 is primarily governed by translational diffusion. Activation energies in the extrinsic and intrinsic regions were found to be 0.36 and 2.3 eV, respectively, for the pure sample and 0.53 and 2.4 eV for the doped samples. These are in good agreement with the activation energies obtained from conductivity measurements. The knee separating the extrinsic and intrinsic regions occurred at higher temperatures as the amount of dopant was increased. The T1-vs−1/T curves had a minimum at 1120°K for all the samples. Below 400°K T1 was found to be directly proportional to temperature for the doped samples. The low-temperature data indicate that the correlation time of samarium at room temperature is of the order of 10−14 sec.

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

“…The narrow temperature range over which we were able to observe the 19F TI's shows a behavior similar to other superionic conductors, such as CaF2 (Lysiak and Mahendroo 1966). 19F T 1 measurements in Sin-doped CaF2 crystals show that in the high temperature regions, the Ta is primarily governed by the translational diffusion of the F, while in the lower temperature region, the log(T1)vs inverse temperature slope is much shallower, and the T1 is governed by the concentration of paramagnetic Sm 2+ (Lysiak and Mahendroo 1966).…”

“…19F T 1 measurements in Sin-doped CaF2 crystals show that in the high temperature regions, the Ta is primarily governed by the translational diffusion of the F, while in the lower temperature region, the log(T1)vs inverse temperature slope is much shallower, and the T1 is governed by the concentration of paramagnetic Sm 2+ (Lysiak and Mahendroo 1966). We observe a near-linear decrease in T~ with increasing temperature which is most likely controlled by spin-spin coupling to paramagnetics.…”

Direct Observation of Defect Dynamics in Nanocrystalline CaF2: Results from 19F MAS NMR Spectroscopy

“…2010, 1, 1126-1129 pubs.acs.org/JPCL -108.5 ppm, the 19 F MAS NMR spectra of the nanocrystalline samples are characterized by an additional peak centered around -131. 6 ppm that constitutes about 0.55% of the total 19 F NMR signal for the 25 nm crystallites and decreases to ∼0.27% as the crystallite size increases to 50 nm ( Figure 1). …”

“…[1][2][3][4] In the high-temperature regime (T g 600 °C) the electrical conductivity of the solid electrolyte CaF 2 is known to be "intrinsic" and thus controlled by thermally activated hopping of Fas well as by the formation of fluorine vacancies/interstitial Fions (i.e., defects) in the lattice. [5][6][7] On the other hand, at lower temperatures, the conductivity is considered to be "extrinsic", being controlled exclusively by the motion of the defects. Such a change in the mechanism of ionic transport results in a corresponding change in the activation energy of ionic conductivity of CaF 2 from a value of ∼2.0 eV in the "intrinsic" regime to ∼0.9 eV in the "extrinsic" regime.…”

“…Such a change in the mechanism of ionic transport results in a corresponding change in the activation energy of ionic conductivity of CaF 2 from a value of ∼2.0 eV in the "intrinsic" regime to ∼0.9 eV in the "extrinsic" regime. 5,6 However, whether the conductivity is controlled by the motion of the fluorine vacancies or the interstitials, i.e., vacancy versus interstitialcy mechanism, remains unknown. A recent study has shown that the ionic conductivity of nanocrystalline CaF 2 can be orders of magnitude higher than its microcrystalline counterpart.…”