- Email: [email protected]
Cation exchange rates and mobility in aluminum-doped lithium orthosilicate: High-resolution lithium-6 NMR results
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IONICS Solid State Ionics 78 (1995) IA-L8
Cation exchange rates and mobility in aluminum-doped lithium orthosilicate: high-resolution lithium-6 NMR results J.F. Stebbins aT*, Z. Xu a, D. Vollath b aDepartment of Geological and Enuironmental Sciences, Stanford University, Stanfor CA 94305-2115, USA b Forschungszentrum Karlsruhe GmbH, Institiit fir Materialforschung III, Postfach 3640, D-76021 Karlsruhe, Germany Received 28 February 1995; accepted for publication 8 March 1995
Abstract Magic-angle spinning NhJR spectra on 6Li in Li,SiO,, Li,~,Al,,Si,,O,, and Li,,,Al,,SiO, resolve multiple lithium cation sites, and, with increasing temperature, show averaging due to exchange of Lif among sites. These exchange rates predict measured ionic conductivity accurately. Doping with aluminum increases Li mobility significantly. Keywords:
Lithium diffusion; NMR-MAS; Silicate
1. Introduction For several decades, NMR has been applied to the study of solid ionic conductors [l-S]. Studies have
been especially common for 7Li because of the often great mobility of Li+, its importance to high conductivity in a wide variety of materials, and the ease of observation of this nuclide. Most of this work has involved measurement of spin-lattice relaxation times T,, or of the overall NMR peak width. Although relaxation can reveal a great deal about cation mobility over wide ranges of temperature, interpretations are often theoretically complex. Particularly at low temperatures, local, within-site cation motions can dominate relaxation but be of little importance for through-going diffusion and conductivity. 7Li NMR peak shapes are, in general, dominated by internuclear dinolar coupling and/or by quadrupolar
* Corresponding author: Tel. (415)723-1140; Fax: (415)7252199; E-mail: [email protected].
effects. Both of these can be at least partially averaged by local motion, again making extraction of data relevant to conduction complex. In contrast to 7Li, the much smaller quadrupolar moment of 6Li (7.4% natural abundance), and its weak homonuclear dipolar coupling, allow high-reso. lution MAS (magic-angle spiting) NMR spectra to be collected in which the peak shape and position are dominated by chemical shift interactions, at least in samples with low contents of paramagnetic centers. Even though the total range of chemical shifts for this nuclide is small [9,10], structurally distinct sites may produce resolvable peaks. This allows the possibility of directly observing the exchange of cations from one site to another, because such exchange causes motional averaging of spectra in a relatively simple, predictable fashion. Lithium orthosilicate (Li,SiO,) and its substituted derivatives have received considerable attention as possible fast ion conductors and as Li-rich ceramics for use in nuclear fusion reactor blankets. Several
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00122-O
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high conductivities may require an intergranular conductive phase such as LiOH [ll], although such a phase may inhibit tritium release. Li motion in these materials has been studied by 7Li NMR [ll]. However, it is clear that a complementary approach could be useful to help answer a fundamental question in these and related systems, that is critical to the design of useful bulk materials: What is the inherent conductivity of the major phase itself, independent of processing conditions and impurities that form connections among grains?
sets of conductivity data on the pure phase are in reasonably good agreement [6,11-171. Two types of Al substitution have been studied that seem to enhance Li mobility: one that introduces excess Li as the other that introinterstitials (Li 4+ x AIXSi,_,O,), duces excess Li vacancies (Li,_,,A1,SiO,) [13]. Increases of several orders of magnitude in conductivity [13,15], as well as increased rates of tritium release after neutron irradiation, have been reported in these phases [18,19]. However, recent work on sol-gel synthesized materials has suggested that such
A
I”“1~‘~~1”‘~l~‘~~l’~~~I’~~~~~~‘1~”~1”1”~111~’~~11’~‘~~“111’1~”’~~ -60
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PPm Fig. 1. zgSi MAS-NMR spectra for lithium orthosilicate samples. (a) Melt-grown Li,SiO.,, ambient temperature. Spectrum for sample synthesized by chemical precipitation process is similar, with slightly lower resolution; (b) as in (a), except data acquired in situ at 400°C; (c) Li,,Al,,lSi,~gO,, ambient temperature; (d) Li,,,Al,.,SiO,.
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dry Li,CO,, Al,O, and SiO,, decarbonating below the solidus, then melting and crystallizing. A chemical precipitation process, also described previously [l&19] was used for another Al-free sample as well as Li,,Al,,,SiO,. All samples were dried at 8501000°C immediately prior to collection of NMR s ectra. Samples were characterized by powder XRD, 29 Si and “‘A1 MAS-NMR. Minor proportions of other lithium silicates were detected in some samples by XRD, but are of too low abundance to significantly affect the NMR spectra described here. NMR data were collected with a modified Varian VXR-400s spectrometer at 58.8, 79.4 and 104.3 MHz for 6Li, 29Si and “‘Al respectively, with external 1 M aqueous LiCl, tetramethyl silane, and 1 M acidified Al(NO,), as frequency references. MAS
In recent work we reported that the 6Li MASNMR spectrum of Li,SiO, contains well-resolved peaks for multiple sites Li, and that these can be correlated with the Li+ coordination number [9,10]. Here we describe the effects of temperature on these spectra and those of Al-doped materials, and show that the cation exchange frequencies deduced from observed chemical exchange are closely related to the ionic conductivity within the ceramic grains.
2. Experimental Several pure Li,SiO, samples, as well as Li,,Al,,,Si,,,O,, were synthesized as previously described [lo] by mixing stoichiometric proportions of
I~‘~~I’~“I~i~‘l~‘~~I’~~~i~i~~l’~~’l”~~I~”’I~~”I~~“I’~“I~~~~l’i~~l”“~“r 130
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Fig. 2. 27M MAS-NMR
spectra for (a) Li,,,Al,,,SiO,;
(b) Li,,,Al 0.1Si 0.90 4, both acquired at ambient temperature.
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spectra for 6Li and 29Si were obtained with a Varian probe, at spinning rates of about 5 kHz. No weight gains during NMR experiments were detected, indicating insignificant water absorption. Radio frequency pulse tip angles and delay times were chosen to ensure that there was no effect of differential relaxation among signals for differing sites. Temperatures were calibrated using the relationship between the 207Pb peak position and temperature for Pb(NO,), [20]. Some 6Li and 29Si MAS spectra were collected up to 400°C with a high temperature MAS probe from Doty Scientific, Inc. “‘A1 MAS spectra were
State Ionics 78 (1955) Ll -L8
collected in a Doty Scientific, Inc. probe at spinning rates of about 10 kHz, with RF pulses with a tip angle of 15” for the central, l/2 to - l/2 transition.
3. Results 29Si MAS NMR spectra are shown in Fig. 1. All samples of the pure Li phase had similar spectra, with some variation in resolution. The presence of five 29Si peaks, two with doubled area, is consistent with the crystal structure [9,10], which has seven
Fig. 3. 6Li MAS-NMR spectra of Li,SiO,, (synthesized by chemical precipitation process) acquired assignments in the lowermost spectrum show the number of coordinating oxygen atoms [lo].
at temperatures
shown in “C. Peak
J.F. Stebbins et al. /Solid
State Ionics 78 (199.5) Ll-L8
broadening could be due in part to increased quadrupolar coupling constants at the Al sites. As illustrated by Fig. 3, the 6Li MAS spectra for Li,SiO, has resolved or partially resolved peaks due to Li in distorted three-fold, four-fold, five-fold, and six-fold coordination [9,10]. This assignment is consistent with the known structure [21,22]. At low temperatures, the 6Li MAS spectra for the Al-substituted phases are similar to that of the undoped material (Figs. 4 and 5). In all samples, as temperature increases, the 6Li spectra become narrower as the peaks for separate sites collapse into a single averaged peak. This motional averaging or “chemical exchange” occurs when the rate of exchange of Li among the sites becomes similar to the frequency separation of the peaks. The observed averaging requires full exchange of Li in a site with one coordination number into a site with a different coordination number, and cannot be caused by local motion within sites. The temperatures of peak coalescence 1231, are approxi-
distinct Si sites [21]. The spectra for the Al-containing samples are less well resolved, as expected if disorder is introduced by the substitution. Particularly for the sample in which Al substitutes primarily into Li sites (Li,,Al,,SiO,), an extra peak appears at lower frequency (- 68.4 ppm), as expected for a Si site with a first cation neighbor of Al instead of all Li. “‘Al MAS spectra for the Al-substituted samples are shown in Fig. 2, and both contain multiple overlapping peaks. Peak positions for Li4,1A10,1S&O, (87 and 75 ppm) are relatively deshielded (high frequency), as expected if they occupy Q” sites with no connecting SiO, or AlO, tetrahedra. The major “‘A1 peak for Li,,Al,,SiO, is spread over a wider frequency range and is centered at a relatively shielded position (63 ppm>, consistent with Al substitution into primarily four-coordinated Li sites with Si neighbors, and possibly to a minor extent in five-coordinated sites. The greater peak width suggests greater disorder, but the shift and
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PPm Fig. 4. 6Li MAS-NMR
spectra of Li,~,Al,,,SiO,,
acquired at temperatures
shown in “C.
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J.F. Stebbins et al./Solid
mately 90, 33 and 44°C for Li,SiO,, Li,,Al,,,SiO, and Li,,A&,,SiO,,O,, respectively, with uncertainties of about f 10”. At this point, the mean exchange frequency is about 2.2 times the maximum frequency separation of the peaks, or about 300 &- 100 Hz [23]. A more quantitative measurement of the exchange rate from one-dimensional NMR spectra is likely to be difficult because of the large number of distinct sites in the structure and the complexity of the required simulation. However, recent two-dimensional exchange spectroscopy on Li,SiO, indicates that separate (and different) exchange rates among different sites, and corresponding activation energies, can be measured [9]. These are consistent with the one-dimensional data described here. “Si MAS spectra for pure Li,SiO, showed no motional averaging even at 400°C indicating that Si sites retain their structural identities in the presence of very rapid Li+ motion, although at higher temper-
State Ionics 78 (1995) Ll-L8
atures static (non-MAS) ing of the “Si chemical
spectra showed an averagshift anisotropy [24].
4. Discussion The site exchange that causes motional averaging in the 6Li MAS-NMR spectra involves most if not all of the Li sites, and requires Li+ hopping into a vacant site, followed by hopping into a second, previously occupied site. Site exchange thus should be closely related to electrical conductivity, if the latter is dominated by the rate of Li+ diffusion within the individual crystallites of the material. The presence of a minor grain boundary phase will not influence the 6Li MAS-NMR spectra significantly. The relationship between estimated NMR exchange frequencies and conductivity can be assessed by extrapolating published data on similar samples to
J.F. Stebbins et al./Solid
lower temperature, and applying walk model [7], with
a simple
random-
r = ce2d2/6 k,Ta . Here, T is the mean time between jumps at temperature T, c is the concentration of exchanging ions, d is the mean jump distance between sites (0.25 mn), k, is Boltzmann constant, (+ the conductivity, and e the charge of the electron. For pure Li,Si04 [25], we obtain a predicted hopping frequency at the 6Li NMR coalescence temperature (90°C) of between 170 and 360 Hz (activation energy ranging from 0.75 to 0.85 eV), in remarkably good agreement with that estimated from the spectrum (300 + 100 Hz). We conclude that for this material, the bulk conductivity of the polygranular ceramic is close to that within the individual grains. The 6Li spectral coalescence temperatures for the Al-doped phases are lower than for the pure phase, indicating significantly higher Li+ mobility in the former. Assuming that the mean exchange rate at the coalescence temperature of each phase is the same, and using a typical activation energy of 0.75 eV [ll], we can estimate the relative exchange rates at a constant temperature. Compared to pure Li,Si04, the Li inter-site mobility deduced from the NMR Li exchange rate is about 100 times higher in Li,,A&,,SiO,, in excellent agreement with the difference in dc bulk conductivity (measured by impedance spectroscopy) previously reported for these same samples [17]. The Li mobility in Li4,,Al,,Si,~,0, is about 30 times higher than in the undoped phase, at least qualitatively similar to effects on conductivity in similar materials [13]. The much lower conductivities reported for some sol-gel Li, + x Al,Si,_,O, synthesized ceramics [ll] are therefore probably the result not of intrinsically lower conductivity within the grains of the ceramic, but of poorer intergranular connections. This is consistent with the author’s conclusion that an intergranular LiOH phase was responsible for the much higher conductivity of their less homogeneous Al-doped materials. Percolation of current through a connected intergranular network of a high-conductive phase is apparently not required. The observation that addition of Al to lithium orthosilicate increases the rate of tritium release after neutron irradiation may thus be at least in part related to intrinsically higher Li
State Ionics 78 (1995) LI-L8
L7
mobility within the Al-doped ceramic grains [18,19]. This conclusion has been reached for other lithium ceramics, based on 7Li relaxation time measurements
[=I. The applicability of this approach to other materials requires, of course, that multiple cation sites are present and are observable in high-resolution NMR spectra. In less mobile systems, MAS-NMR at relatively high temperature may be required to observe exchange. Measurements up to 600°C are becoming feasible, as reported for 23Na in NaAlSiO, (nepheline) [27] and Na,AlF, (cryolite) [28]. Motional averaging of one-dimensional spectra can be observed only over limited temperature ranges, making such studies complements to, not replacements for, relaxation time measurements. However, two-dimensional exchange studies [9,29,30], can detect motions several orders of magnitude slower than those that affect 1-D spectra. These are beginning to show great promise for widening the temperature range and hopping frequencies accessible to direct spectral determinations of exchange rates.
References 111D. Brinkman, Prog. NMR Spcct. 24 (1992) 527. 121D. Brinkman, Mag. Res. Rev. 14 (1989) 101. [31 C. Berthier, in: Fast Ion Transport in Solids, eds. P. Vashishta, J.N. Mundy and G.K Shenoy (North-Holland, New York, 1979) p. 171. 141 J.L. Bjorkstam, J. Listerud and M. Villa, Solid State Ionics 18/19 (1986) 117. [51 M.S. Whittingham and B.G. Silbemagel, in: Solid Electrolytes, eds. P. Hagenmuller and W. Van Go01 (Academic Press, New York, 1978) p. 93. [d R.A. Huggins, Electrochim. Acta 22 (1977) 773. t71 I. Svare, F. Borsa, D.R. Torgeson and SW. Martin, Phys. Rev. B 48 (1993) 9336. k31M. Griine, W. MiilIer-Wannuth, P. Hebel and B. Krebs, Solid State Ionics 66 (1993) 165. 191 Z. Xu and J.F. Stebbins, Eos, Trans. Am. Geophys. Union 75 (1994) 705. [lOI Z. Xu and J.F. Stebbins, Solid State Mag. Reson. (19951, in press. 1111 M. Smaihi, D. Petit, J.P. Korb and J.P. Boilot, J. Solid State Chem. 94 (1991) 260. WI M. Smaihi, D. Petit, F. GourbiIleau, F. Chaput and J.P. Boilot, Sol. State Ionics 48 (1991) 213. 1131 K. Jackowska and A.R. West, J. Mat. Sci. 18 (1983) 2380. r141 I.M. Hodge, M.D. Ingram and A.R. West, J. Am. Ceram. Sot. 59 (1976) 360.
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[15] R.D. Shannon, B.E. Taylor, A.D. English and T. Berzins, Electrochim. Acta 77 (1977) 783. [16] AR. West, .I. Appl. Electochem. 3 (1973) 327. [17] K Noda, Y. Ishii, H. Matsui, D. Vollath and H. Watanabe, in: Fusion Technology 1990, eds. B.E. Keen, M. Hugnet and R. Hemsworth (Elsevier, New York, 1990) p. 939. [18] D. Vollath and H. Wedemeyer, Adv. Ceram. 27 (1990) 3. [19] D. Vollath, H. Wedemeyer, H. Zimmerman and H. Werle, J. Nucl. Mater. 174 (19901 86. [20] A. Bielecki and D.P. Burum, 36th Rocky Mount., Conf. An. Chem., Abs. (1994) 305. [21] B.H.W.S. de.long, D. Ellerbroek and A.L. Spek, Acta Cryst. B (1995) in press. [22] D. Tranquil, R.D. Shannon, H.Y. Chen, S. Iijima and W.H. Baur, Acta Cryst. B35 (1979) 2479. [23] J.K.M. Sanders and B.K. Hunter, Modem NMR spectroscopy (Oxford University Press, Oxford, 1987).
State Ionics 78 (1995) Ll-L8 [24] I. Faman and J.F. Stebbins, J. Am. Chem. Sot. 112 (1990) 32. [25] M. Smaihi, D. Petit, J.P. Boilot, F.M. Botter, J. Mougin and M.J. Boncoeur, in: Fusion Technology 1990, eds. B.E. Keen, M. Hugnet and R. Hemsworth (Elsevier, New York, 1990) p. 817. [26] H. Ohno, S. Konishi, T. Nagasaki, T. Kurasawa, H. Katsuta and H. Watanabe, J. Nucl. Mater. 133/134 (1985) 181. [27] J.F. Stebbins, I. Farnan, E.H. Williams and J. Roux, Phys. Chem. Miner. 16 (1989) 763. [28] D.R. Spearing, J.F. Stebbins and I. Faman, Phys. Chem. Miner. 21 (1994) 373. [29] I. Faman and J.F. Stebbins, J. Non-Cryst. Solids 124 (1990) 207. [30] I. Faman and J.F. Stebbins, Science 265 (1994) 1206.
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IONICS Solid State Ionics 78 (1995) IA-L8
Cation exchange rates and mobility in aluminum-doped lithium orthosilicate: high-resolution lithium-6 NMR results J.F. Stebbins aT*, Z. Xu a, D. Vollath b aDepartment of Geological and Enuironmental Sciences, Stanford University, Stanfor CA 94305-2115, USA b Forschungszentrum Karlsruhe GmbH, Institiit fir Materialforschung III, Postfach 3640, D-76021 Karlsruhe, Germany Received 28 February 1995; accepted for publication 8 March 1995
Abstract Magic-angle spinning NhJR spectra on 6Li in Li,SiO,, Li,~,Al,,Si,,O,, and Li,,,Al,,SiO, resolve multiple lithium cation sites, and, with increasing temperature, show averaging due to exchange of Lif among sites. These exchange rates predict measured ionic conductivity accurately. Doping with aluminum increases Li mobility significantly. Keywords:
Lithium diffusion; NMR-MAS; Silicate
1. Introduction For several decades, NMR has been applied to the study of solid ionic conductors [l-S]. Studies have
been especially common for 7Li because of the often great mobility of Li+, its importance to high conductivity in a wide variety of materials, and the ease of observation of this nuclide. Most of this work has involved measurement of spin-lattice relaxation times T,, or of the overall NMR peak width. Although relaxation can reveal a great deal about cation mobility over wide ranges of temperature, interpretations are often theoretically complex. Particularly at low temperatures, local, within-site cation motions can dominate relaxation but be of little importance for through-going diffusion and conductivity. 7Li NMR peak shapes are, in general, dominated by internuclear dinolar coupling and/or by quadrupolar
* Corresponding author: Tel. (415)723-1140; Fax: (415)7252199; E-mail: [email protected].
effects. Both of these can be at least partially averaged by local motion, again making extraction of data relevant to conduction complex. In contrast to 7Li, the much smaller quadrupolar moment of 6Li (7.4% natural abundance), and its weak homonuclear dipolar coupling, allow high-reso. lution MAS (magic-angle spiting) NMR spectra to be collected in which the peak shape and position are dominated by chemical shift interactions, at least in samples with low contents of paramagnetic centers. Even though the total range of chemical shifts for this nuclide is small [9,10], structurally distinct sites may produce resolvable peaks. This allows the possibility of directly observing the exchange of cations from one site to another, because such exchange causes motional averaging of spectra in a relatively simple, predictable fashion. Lithium orthosilicate (Li,SiO,) and its substituted derivatives have received considerable attention as possible fast ion conductors and as Li-rich ceramics for use in nuclear fusion reactor blankets. Several
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00122-O
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State Ionics 78 (1995) Ll-L8
high conductivities may require an intergranular conductive phase such as LiOH [ll], although such a phase may inhibit tritium release. Li motion in these materials has been studied by 7Li NMR [ll]. However, it is clear that a complementary approach could be useful to help answer a fundamental question in these and related systems, that is critical to the design of useful bulk materials: What is the inherent conductivity of the major phase itself, independent of processing conditions and impurities that form connections among grains?
sets of conductivity data on the pure phase are in reasonably good agreement [6,11-171. Two types of Al substitution have been studied that seem to enhance Li mobility: one that introduces excess Li as the other that introinterstitials (Li 4+ x AIXSi,_,O,), duces excess Li vacancies (Li,_,,A1,SiO,) [13]. Increases of several orders of magnitude in conductivity [13,15], as well as increased rates of tritium release after neutron irradiation, have been reported in these phases [18,19]. However, recent work on sol-gel synthesized materials has suggested that such
A
I”“1~‘~~1”‘~l~‘~~l’~~~I’~~~~~~‘1~”~1”1”~111~’~~11’~‘~~“111’1~”’~~ -60
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PPm Fig. 1. zgSi MAS-NMR spectra for lithium orthosilicate samples. (a) Melt-grown Li,SiO.,, ambient temperature. Spectrum for sample synthesized by chemical precipitation process is similar, with slightly lower resolution; (b) as in (a), except data acquired in situ at 400°C; (c) Li,,Al,,lSi,~gO,, ambient temperature; (d) Li,,,Al,.,SiO,.
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State Zonics 78 (1995) Ll-L8
dry Li,CO,, Al,O, and SiO,, decarbonating below the solidus, then melting and crystallizing. A chemical precipitation process, also described previously [l&19] was used for another Al-free sample as well as Li,,Al,,,SiO,. All samples were dried at 8501000°C immediately prior to collection of NMR s ectra. Samples were characterized by powder XRD, 29 Si and “‘A1 MAS-NMR. Minor proportions of other lithium silicates were detected in some samples by XRD, but are of too low abundance to significantly affect the NMR spectra described here. NMR data were collected with a modified Varian VXR-400s spectrometer at 58.8, 79.4 and 104.3 MHz for 6Li, 29Si and “‘Al respectively, with external 1 M aqueous LiCl, tetramethyl silane, and 1 M acidified Al(NO,), as frequency references. MAS
In recent work we reported that the 6Li MASNMR spectrum of Li,SiO, contains well-resolved peaks for multiple sites Li, and that these can be correlated with the Li+ coordination number [9,10]. Here we describe the effects of temperature on these spectra and those of Al-doped materials, and show that the cation exchange frequencies deduced from observed chemical exchange are closely related to the ionic conductivity within the ceramic grains.
2. Experimental Several pure Li,SiO, samples, as well as Li,,Al,,,Si,,,O,, were synthesized as previously described [lo] by mixing stoichiometric proportions of
I~‘~~I’~“I~i~‘l~‘~~I’~~~i~i~~l’~~’l”~~I~”’I~~”I~~“I’~“I~~~~l’i~~l”“~“r 130
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Fig. 2. 27M MAS-NMR
spectra for (a) Li,,,Al,,,SiO,;
(b) Li,,,Al 0.1Si 0.90 4, both acquired at ambient temperature.
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J.F. Stebbins et al. /Solid
spectra for 6Li and 29Si were obtained with a Varian probe, at spinning rates of about 5 kHz. No weight gains during NMR experiments were detected, indicating insignificant water absorption. Radio frequency pulse tip angles and delay times were chosen to ensure that there was no effect of differential relaxation among signals for differing sites. Temperatures were calibrated using the relationship between the 207Pb peak position and temperature for Pb(NO,), [20]. Some 6Li and 29Si MAS spectra were collected up to 400°C with a high temperature MAS probe from Doty Scientific, Inc. “‘A1 MAS spectra were
State Ionics 78 (1955) Ll -L8
collected in a Doty Scientific, Inc. probe at spinning rates of about 10 kHz, with RF pulses with a tip angle of 15” for the central, l/2 to - l/2 transition.
3. Results 29Si MAS NMR spectra are shown in Fig. 1. All samples of the pure Li phase had similar spectra, with some variation in resolution. The presence of five 29Si peaks, two with doubled area, is consistent with the crystal structure [9,10], which has seven
Fig. 3. 6Li MAS-NMR spectra of Li,SiO,, (synthesized by chemical precipitation process) acquired assignments in the lowermost spectrum show the number of coordinating oxygen atoms [lo].
at temperatures
shown in “C. Peak
J.F. Stebbins et al. /Solid
State Ionics 78 (199.5) Ll-L8
broadening could be due in part to increased quadrupolar coupling constants at the Al sites. As illustrated by Fig. 3, the 6Li MAS spectra for Li,SiO, has resolved or partially resolved peaks due to Li in distorted three-fold, four-fold, five-fold, and six-fold coordination [9,10]. This assignment is consistent with the known structure [21,22]. At low temperatures, the 6Li MAS spectra for the Al-substituted phases are similar to that of the undoped material (Figs. 4 and 5). In all samples, as temperature increases, the 6Li spectra become narrower as the peaks for separate sites collapse into a single averaged peak. This motional averaging or “chemical exchange” occurs when the rate of exchange of Li among the sites becomes similar to the frequency separation of the peaks. The observed averaging requires full exchange of Li in a site with one coordination number into a site with a different coordination number, and cannot be caused by local motion within sites. The temperatures of peak coalescence 1231, are approxi-
distinct Si sites [21]. The spectra for the Al-containing samples are less well resolved, as expected if disorder is introduced by the substitution. Particularly for the sample in which Al substitutes primarily into Li sites (Li,,Al,,SiO,), an extra peak appears at lower frequency (- 68.4 ppm), as expected for a Si site with a first cation neighbor of Al instead of all Li. “‘Al MAS spectra for the Al-substituted samples are shown in Fig. 2, and both contain multiple overlapping peaks. Peak positions for Li4,1A10,1S&O, (87 and 75 ppm) are relatively deshielded (high frequency), as expected if they occupy Q” sites with no connecting SiO, or AlO, tetrahedra. The major “‘A1 peak for Li,,Al,,SiO, is spread over a wider frequency range and is centered at a relatively shielded position (63 ppm>, consistent with Al substitution into primarily four-coordinated Li sites with Si neighbors, and possibly to a minor extent in five-coordinated sites. The greater peak width suggests greater disorder, but the shift and
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PPm Fig. 4. 6Li MAS-NMR
spectra of Li,~,Al,,,SiO,,
acquired at temperatures
shown in “C.
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J.F. Stebbins et al./Solid
mately 90, 33 and 44°C for Li,SiO,, Li,,Al,,,SiO, and Li,,A&,,SiO,,O,, respectively, with uncertainties of about f 10”. At this point, the mean exchange frequency is about 2.2 times the maximum frequency separation of the peaks, or about 300 &- 100 Hz [23]. A more quantitative measurement of the exchange rate from one-dimensional NMR spectra is likely to be difficult because of the large number of distinct sites in the structure and the complexity of the required simulation. However, recent two-dimensional exchange spectroscopy on Li,SiO, indicates that separate (and different) exchange rates among different sites, and corresponding activation energies, can be measured [9]. These are consistent with the one-dimensional data described here. “Si MAS spectra for pure Li,SiO, showed no motional averaging even at 400°C indicating that Si sites retain their structural identities in the presence of very rapid Li+ motion, although at higher temper-
State Ionics 78 (1995) Ll-L8
atures static (non-MAS) ing of the “Si chemical
spectra showed an averagshift anisotropy [24].
4. Discussion The site exchange that causes motional averaging in the 6Li MAS-NMR spectra involves most if not all of the Li sites, and requires Li+ hopping into a vacant site, followed by hopping into a second, previously occupied site. Site exchange thus should be closely related to electrical conductivity, if the latter is dominated by the rate of Li+ diffusion within the individual crystallites of the material. The presence of a minor grain boundary phase will not influence the 6Li MAS-NMR spectra significantly. The relationship between estimated NMR exchange frequencies and conductivity can be assessed by extrapolating published data on similar samples to
J.F. Stebbins et al./Solid
lower temperature, and applying walk model [7], with
a simple
random-
r = ce2d2/6 k,Ta . Here, T is the mean time between jumps at temperature T, c is the concentration of exchanging ions, d is the mean jump distance between sites (0.25 mn), k, is Boltzmann constant, (+ the conductivity, and e the charge of the electron. For pure Li,Si04 [25], we obtain a predicted hopping frequency at the 6Li NMR coalescence temperature (90°C) of between 170 and 360 Hz (activation energy ranging from 0.75 to 0.85 eV), in remarkably good agreement with that estimated from the spectrum (300 + 100 Hz). We conclude that for this material, the bulk conductivity of the polygranular ceramic is close to that within the individual grains. The 6Li spectral coalescence temperatures for the Al-doped phases are lower than for the pure phase, indicating significantly higher Li+ mobility in the former. Assuming that the mean exchange rate at the coalescence temperature of each phase is the same, and using a typical activation energy of 0.75 eV [ll], we can estimate the relative exchange rates at a constant temperature. Compared to pure Li,Si04, the Li inter-site mobility deduced from the NMR Li exchange rate is about 100 times higher in Li,,A&,,SiO,, in excellent agreement with the difference in dc bulk conductivity (measured by impedance spectroscopy) previously reported for these same samples [17]. The Li mobility in Li4,,Al,,Si,~,0, is about 30 times higher than in the undoped phase, at least qualitatively similar to effects on conductivity in similar materials [13]. The much lower conductivities reported for some sol-gel Li, + x Al,Si,_,O, synthesized ceramics [ll] are therefore probably the result not of intrinsically lower conductivity within the grains of the ceramic, but of poorer intergranular connections. This is consistent with the author’s conclusion that an intergranular LiOH phase was responsible for the much higher conductivity of their less homogeneous Al-doped materials. Percolation of current through a connected intergranular network of a high-conductive phase is apparently not required. The observation that addition of Al to lithium orthosilicate increases the rate of tritium release after neutron irradiation may thus be at least in part related to intrinsically higher Li
State Ionics 78 (1995) LI-L8
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mobility within the Al-doped ceramic grains [18,19]. This conclusion has been reached for other lithium ceramics, based on 7Li relaxation time measurements
[=I. The applicability of this approach to other materials requires, of course, that multiple cation sites are present and are observable in high-resolution NMR spectra. In less mobile systems, MAS-NMR at relatively high temperature may be required to observe exchange. Measurements up to 600°C are becoming feasible, as reported for 23Na in NaAlSiO, (nepheline) [27] and Na,AlF, (cryolite) [28]. Motional averaging of one-dimensional spectra can be observed only over limited temperature ranges, making such studies complements to, not replacements for, relaxation time measurements. However, two-dimensional exchange studies [9,29,30], can detect motions several orders of magnitude slower than those that affect 1-D spectra. These are beginning to show great promise for widening the temperature range and hopping frequencies accessible to direct spectral determinations of exchange rates.
References 111D. Brinkman, Prog. NMR Spcct. 24 (1992) 527. 121D. Brinkman, Mag. Res. Rev. 14 (1989) 101. [31 C. Berthier, in: Fast Ion Transport in Solids, eds. P. Vashishta, J.N. Mundy and G.K Shenoy (North-Holland, New York, 1979) p. 171. 141 J.L. Bjorkstam, J. Listerud and M. Villa, Solid State Ionics 18/19 (1986) 117. [51 M.S. Whittingham and B.G. Silbemagel, in: Solid Electrolytes, eds. P. Hagenmuller and W. Van Go01 (Academic Press, New York, 1978) p. 93. [d R.A. Huggins, Electrochim. Acta 22 (1977) 773. t71 I. Svare, F. Borsa, D.R. Torgeson and SW. Martin, Phys. Rev. B 48 (1993) 9336. k31M. Griine, W. MiilIer-Wannuth, P. Hebel and B. Krebs, Solid State Ionics 66 (1993) 165. 191 Z. Xu and J.F. Stebbins, Eos, Trans. Am. Geophys. Union 75 (1994) 705. [lOI Z. Xu and J.F. Stebbins, Solid State Mag. Reson. (19951, in press. 1111 M. Smaihi, D. Petit, J.P. Korb and J.P. Boilot, J. Solid State Chem. 94 (1991) 260. WI M. Smaihi, D. Petit, F. GourbiIleau, F. Chaput and J.P. Boilot, Sol. State Ionics 48 (1991) 213. 1131 K. Jackowska and A.R. West, J. Mat. Sci. 18 (1983) 2380. r141 I.M. Hodge, M.D. Ingram and A.R. West, J. Am. Ceram. Sot. 59 (1976) 360.
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State Ionics 78 (1995) Ll-L8 [24] I. Faman and J.F. Stebbins, J. Am. Chem. Sot. 112 (1990) 32. [25] M. Smaihi, D. Petit, J.P. Boilot, F.M. Botter, J. Mougin and M.J. Boncoeur, in: Fusion Technology 1990, eds. B.E. Keen, M. Hugnet and R. Hemsworth (Elsevier, New York, 1990) p. 817. [26] H. Ohno, S. Konishi, T. Nagasaki, T. Kurasawa, H. Katsuta and H. Watanabe, J. Nucl. Mater. 133/134 (1985) 181. [27] J.F. Stebbins, I. Farnan, E.H. Williams and J. Roux, Phys. Chem. Miner. 16 (1989) 763. [28] D.R. Spearing, J.F. Stebbins and I. Faman, Phys. Chem. Miner. 21 (1994) 373. [29] I. Faman and J.F. Stebbins, J. Non-Cryst. Solids 124 (1990) 207. [30] I. Faman and J.F. Stebbins, Science 265 (1994) 1206.