Ion hopping in crystalline and glassy spodumeneLiAlSi2O6:

Fei Qi

¹

,

C. Rier

²

,

Roland Böhmer

³

et al.

Abstract

Nuclear magnetic resonance spectroscopy was used to study polycrystalline ␤-spodumene ͑␤-LiAlSi 2 O 6 ͒ as well as glassy specimens with the same chemical composition.7 Li spin-lattice relaxation measurements were carried out in a broad temperature range and for several Larmor frequencies. In addition to a pronounced rate maximum at high temperatures, stemming from the long-range Li motion in these aluminosilicates, we found a weak maximum in the crystalline modification near 120 K. The latter result confirms the existence of a local double-well structure in which the Li ions reside. The ionic motion was also monitored by solid-and stimulated-echo spectra as well as by the decay of the Jeener-Broekaert echo. Under conditions which are discussed in detail, the latter is a direct measure of the hopping correlation function. For the glass this function was found to decay faster and more stretched than that of the crystal at a given temperature. Furthermore, the relevant barriers against the high-temperature long-range Li motion are larger in the crystal as compared to the glass.

Atomic-scale measurement of ultraslow Li motions in glassyLiAlSi2O6by two-timeL6Wilkening1, Kuhn2, Heitjans3 2008Phys. Rev. BSelf Cite594500View full textAdd to dashboardCite

“…Thus, the corresponding SLR −1 rate decreases accordingly. 31 A larger difference is found when both, SLR −1 and SAE −1 ͑t p = 100 s͒ rates are compared with those values which were obtained from dc conductivity data ͑see Fig. 4͒.…”

“…frequencies 0 / 2 ranging from 10 to 77 MHz. 31,58 Plotting log͑1 / T 1diff ͒ vs ͑1 / T͒ revealed a diffusion induced rate peak for each frequency 0 / 2 . The maximum condition 0 SLR Ϸ 0.62 ͑according to BPP theory 59 ͒ allowed the estimation of SLR −1 at the temperature T M where the rate maximum appears.…”

“…It shows a value which is reasonable for phonon frequencies typically ranging between 10 12 and 10 14 s −1 . Analogous 7 Li SAE NMR measurements, which were performed recently, 31 yielded a somewhat smaller value ͓0.58͑3͒ eV͔. Interestingly, an activation energy E a,SAE of 0.69 eV agrees with that which was deduced by Roth and Böhm 53 as well as with that obtained by our group from impedance spectroscopy measurements 49,50 on the same sample ͓0.66͑2͒ eV, 400 K ՅT Յ 575 K, see That way, an activation energy was obtained from the high-temperature slope of the resultant log 10 ͑1 / T͒ peak.…”

“…3 are shown together with the Li jump rates SLR −1 which were determined via 7 Li SLR NMR measurements. 31,48,49,58 The latter were obtained by recording diffusion induced SLR rates over a broad temperature range and for different radio 6 Li spin-alignment echo NMR at two different preparation times t p ͑b : 100 s, : 300 s͒ plotted against inverse temperature. For comparison, the rates deduced from the maxima of the diffusion induced 7 Li SLR NMR rate peaks ͑ࡗ͒ ͑see Refs.…”

“…Advantageous over already well-established Li NMR methods [15][16][17][18][19] is the use of 7 Li stimulated echo, or more precisely, spin-alignment echo ͑SAE͒ NMR which has so far been applied mainly to crystalline Li conductors. 17,[20][21][22][23][24][25][26][27][28][29][30][31][32] Similar to modern twodimensional 6 Li and 7 Li exchange NMR spectroscopy, 33-38 7 Li SAE NMR is sensitive to extremely slow cation exchange processes and gives access to diffusion parameters from a microscopic point of view. Thus, it is possible to record single-particle motional correlation functions, 39 from which the atomic jump rate can be directly obtained, i.e., without the help of a diffusion model.…”