7Li and 51V NMR study on Li+ ionic diffusion in lithium intercalated LixV2O5

Solid State Ionics 177 (2006) 129 – 135 www.elsevier.com/locate/ssi

7

Li and

51

V NMR study on Li+ ionic diffusion in lithium intercalated Lix V2O5

Koichi Nakamura a,*, Daisuke Nishioka a, Yoshitaka Michihiro a, M. Vijayakumar b,1, S. Selvasekarapandian b, Tatsuo Kanashiro a a

Department of Physics, Faculty of Engineering, The University of Tokushima, 2-1 Minami-Josanjima-Cho, Tokushima 770-8506, Japan Solid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore-641 046, Tamilnadu, India

b

Received 19 June 2005; received in revised form 5 September 2005; accepted 9 September 2005

Abstract Lix V2O5 (0.4 < x < 1.4) prepared by solid-state reaction were studied by 7Li and 51V NMR spectroscopy. 7Li NMR spectra showed a narrowing of the line width in relation to Li+ ionic diffusion. Analysis of Lix V2O5 using a Debye-type relaxation model showed a low activation energy ¨0.07 eV in the sample of x = 0.4 below room temperature, and revealed a Li+ionic diffusion with larger activation energy ¨0.5 eV above 450 K in lithium-rich samples. The latter is ascribed to the existence of a multi-phase system comprising stable q- and g-phases, resulting from complicated phase transitions at high temperature. These shapes and shifts enable the classification of the h-, q-, y-, and g-phases. The ionic diffusion of Li+ ions is discussed in relation to the complicated phase transitions. D 2005 Elsevier B.V. All rights reserved. PACS: 66.30; 76.60 Keywords: NMR; Li+ ion diffusion; Lithium vanadium bronze; Phase transition

1. Introduction The development of advanced cathode materials to replace the use of LiCoO2, in commercial products, has been particularly intensive over the last decade. Lithium intercalated Lix V2O5 is attractive for the cathode material of 3 V lithium rechargeable batteries. It is known that V2O5 acts as a host, able to intercalate a maximum of 2 Li+ ions per V2O5 reversibly. The various valence states of vanadium and the ability of the oxide (V2O5) to accept large amounts of Li+ ions through intercalation, causes complicated phase transitions [1,2]. The host oxide V2O5 has an orthorhombic structure and forms [VO5] V layers parallel to (001), consisting of edge- and corner-sharing VO5 square pyramids. It is generally considered that in this structure, ravines shaped by the square pyramids along the b-axis, form a one-dimensional thoroughfare for the Li+ ions to pass through. However, Li+ionic diffusion is not * Corresponding author. E-mail address: [email protected] (K. Nakamura). 1 Present address: Department of Physics, College of William and Mary, Williamsburg-23187, Virginia, USA. 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.012

well known in this system. The bronzes a-A x V2O5 (A = Li, Na, Ag, K, Cu; 0
x
0.10) are isostructural. The chemical lithium insertion in V2O5 gives rise to several structural modifications: a-phase for 0 < x < 0.13, q-phase for 0.32 < x < 0.88, y-phase for 0.88 < x < 1.0 and g-phase for x > 1.0 [3]. In addition, Lix V2O5 prepared at high temperature by solid-state reaction exhibits the h/h V-phase in the range of 0.2 < x < 0.6 [4,5]. The composition limits in the solution are not defined and the neighboring phases are usually coexistent. Lix V2O5 exhibits a very similar framework with only a slight puckering of V2O5 up to x = 1.0, while also showing good reversibility. Beyond x = 1.0 however, this puckered structure is not particularly stable and is typically replaced by the g-phase, which exhibits a remarkably distorted structure. Such a phase transition is closely related to Li+ionic motion. Since a study on the Li+ionic diffusion of the h-phase by Gendell [6 –11], intensive NMR studies on lithiated V2O5 have revealed that the crystalline material maintains several structural phases depending on the lithium content as mentioned above. Although the solid-state reaction method is often used for sample synthesis, a heating/melting process at high temperature can easily cause the loss of oxygen, resulting in non-

K. Nakamura et al. / Solid State Ionics 177 (2006) 129 – 135

2. Experimental procedures The raw materials Li2CO3 (Aldrich, USA, 99.99%) and V2O5 (Aldrich, USA, 99.99%) have been used to fabricate lithium vanadate Lix V2O5 (x = 0.4 –1.4). The reactions were carried out in silicon crucibles within an open-air atmosphere. The chemical reaction during this process has been explained by Hirschinger et al. [9]. The raw materials were mixed in an appropriate molar ratio. A TG/DTA analysis of these raw materials shows that the thermal reaction is complete by 750 -C. The Fourier Transform Infra Red (FTIR) spectroscopy of these raw materials heated at different temperature also confirms the reaction temperature (750 -C) for the formation of lithium vanadium bronze [12]. Hence, the mixed materials were heat-treated at 750 -C, with heating and cooling rates of 1 -C per minute. The final product was crushed into a fine powder using a mortar and pestle, and sintered at 400 -C for 5 h to afford a homogenous product. The resultant powder samples were then subjected to X-ray diffraction (XRD) and Auger Electron Spectroscopy (AES), and the phase formation with different lithium composition was determined [12]. These phases are in good agreement with the phase diagram of Lix V2O5 reported by Galy [1]. The broad line 7Li-NMR spectrum was measured over the temperature range from 77 to around 600 K at 10.08 MHz, using a custom-made conventional pulse NMR spectrometer. Each powdered polycrystalline sample was packed into a 9 mm Pyrex tube. To confirm the existence of multi-phases within the Lix V2O5 samples, 7Li MAS-NMR measurements at room temperature were performed using a Bruker Avance 300 spectrometer at 116.672 MHz. Each sample was packed into a 4 mm zirconium rotor and rotated with a spin speed of 7 kHz. 51 V MAS-NMR measurements at room temperature were performed at 78.9426 MHz with a 2.5 mm zirconium rotor at a spin speed of 25 kHz. The NMR shift is defined as the center peak position of the 7Li and 51V MAS-NMR spectra. Aqueous solutions of LiCl (1.0M) and NaVO3 (0.16M) were used as 7Li and 51V references, respectively. 3. Experimental results 3.1. Broad line NMR Spectrum of 7Li nucleus For the 7Li nucleus (I = 3 / 2) of Lix V2O5, a single central transition line was observed due to the quadrupole broadening of the satellite lines. Fig. 1 shows the broad line 7Li-NMR spectra of Lix V2O5 at room temperature. It is seen that the central lines are broad for the Li-rich samples, becoming

LixV2O5

10.08MHz R. T.

Li-NMR Intensity ( arb. units )

stoichiometrical oxides and a multi-phase system. In this study, conventional broadband NMR and MAS-NMR measurements were performed on Lix V2O5 in order to elucidate the Li+ionic motion in the multi-phase and the structural changes in this compound. The temperature dependence of the line width and MAS-NMR spectra, and their dependence on the Li content, are discussed below.

x=0.4 x=0.6

x=0.8

x=1.0

x=1.2

7

130

x=1.4

-20000

-10000

0

10000

20000

Frequency ( Hz ) Fig. 1. Broad line 7Li-NMR spectrum of Lix V2O5 at room temperature.

steadily narrower with decreasing Li content x. A sharp line width is observed for sample x = 0.4 at about 480 Hz. Fig. 2 (a), (b), (c), (d), (e) and (f) illustrate the broad line spectra for x = 0.4, 0.6, 0.8, 1.0, 1.2 and 1.4 at varying temperatures from 90 to 600 K, respectively. Each broad line at low temperature changes into sufficiently narrowed lines with increasing temperature in each sample. This phenomenon is considered to be the result of motional narrowing originating from Li+ ionic diffusion at high temperature. 3.2. MAS-NMR spectrum of 7Li nucleus The solid-state MAS-NMR spectra of Lix V2O5 reveal a fine structure arising from the existence of the multi-phase. Indeed, valuable information relating to Lix V2O5 has been reported in previous NMR studies [7– 11]. Fig. 3 shows the 7Li MAS-NMR spectra of Lix V2O5 at room temperature. When x = 0.8, the corresponding MASNMR spectrum shows a symmetrically shaped signal at  9 ppm, while in the sample of x = 1.0, a small shoulder appears to the left of this signal around  1 ppm. The intensity of the shoulder is observed to increase as x increases, becoming dominant when x = 1.4. In sample x = 0.6 however, a new peak appears at  25 ppm and increases remarkably in the sample of x = 0.4, accompanied by a concomitant decrease in the peak at  9 ppm. Comparison of these complicated MAS-NMR spectra of Lix V2O5 with the results of the XRD measurements reveals some interesting results. First, the sharp peak in sample x = 0.4 around  25 ppm is tentatively ascribed to the h-phase, since according to XRD studies, this same phase is reported to be dominant in the sample of x = 0.4, prepared by solid-state reaction at high temperature. The peak around  9 ppm observed in the MASNMR spectra of samples x = 0.4 and 0.6 is tentatively assigned to the q-phase. Here, sample x = 0.6 consists mainly of the qphase, since the intensity of this peak is considerably larger than the h-phase peak at  25 ppm.

K. Nakamura et al. / Solid State Ionics 177 (2006) 129 – 135

-20

-10

(b)x=0.6

0

(c)x=0.8

450K

600K

630K

300K

500K

500K

150K

340K

370K

90K

203K

220K

10

20 -20

(d)x=1.0

-10

0

10

20 -20

(e)x=1.2

-10

0

10

20

(f)x=1.4

630K

630K

500K

520K

445K

440K

276K

295K

360K

160K

135K

288K

7

Li-NMR Intensity ( arb. units )

(a)x=0.4

131

-20

-10

0

10

20 -20

-10

0

10

20 -20

-10

0

10

20

Frequency ( kHz ) Fig. 2. Broad line 7Li-NMR spectra of Lix V2O5.

The single peak in sample x = 0.8 is mainly ascribed to the yphase, because the XRD results show an existence of the yphase and a small amount of the q-phase [12,13]. However, the difference between the two phases is elusive in these spectra. As reported in recent work [14,15], the peak at  1 ppm in x = 1.0 and higher, is tentatively assigned to the g-phase, because the corresponding XRD profiles indicate that this phase is dominant above x = 1.0 and is coexistent with the yphase and a small amount of the q-phase in x = 1.0 [12,14]. The small shoulder seen in samples x = 1.0 and higher corresponds to the signal with larger chemical shift reported in previous work [7,10]. However, the value here is not necessarily consistent with that previously reported. Actually, the phase

content is strongly dependent on the fabrication procedure; chemical intercalation, either via an electrochemical or high temperature reaction, affords variations in the MAS-NMR data for each phase [7 –11]. 3.3. MAS-NMR spectrum of

51

V nucleus

Lithium intercalation causes a change in both the arrangement of the VO5 square pyramids and the oxidation state of the vanadium. The NMR spectrum of the 51V nucleus (I = 7 / 2) reflects the local environment around the vanadium LixV2O5

Li-NMR Intensity ( arb. units )

x=1.4

x=1.2 γ

x=1.0

x=0.8 β/β'

x=1.4

V MAS-NMR Intensity ( arb. units )

ε/δ

LixV2O5

x=1.0

x=0.8

x=0.6

7

51

x=0.6

x=1.2

x=0.4 x=0.4

2000 100

50

0

-50

-100

Shift ( ppm ) Fig. 3. 7Li MAS-NMR spectra of Lix V2O5.

1000

0

-1000

-2000

-3000

Shift ( ppm ) 51

Fig. 4. V MAS-NMR spectrum of Lix V2O5. Solid circles represent the peaks corresponding to the h-phase.

K. Nakamura et al. / Solid State Ionics 177 (2006) 129 – 135

-700

51

V shift ( ppm )

-600

-800 0

0.2 0.4 0.6 0.8 1.0 1.2 1.4

Li content, x Fig. 5. V2O5.

51

V shift of Lix V2O5. Solid triangles represent the

51

V nucleus shift in

site as influenced by the five apical oxygen atoms. Fig. 4 illustrates the 51V MAS-NMR spectra for x = 0.4, 0.6, 0.8, 1.0, 1.2 and 1.4 at room temperature. Each spectrum consists of a resonant line at approximately  600 ppm, and a series of sidebands spread between the range  2000 to 2000 ppm, even in multi-phase (the q-, y- and g-phases) systems. Of all the spectra, only sample x = 0.8 exhibits a very broad spectrum around  800 ppm. The center peak position of the 51V signal for each sample is shown in Fig. 5. Here, the shifts for x = 0.4 and 0.6 (denoted by open circles in Fig. 5) are quite similar to that of V2O5, which is represented by solid triangles. These shifts, which are associated with the electronic shielding effects resulting from both the surrounding electrons and the oxygen environment around V, show enhanced negativity as x increases. For samples above x = 1.0, their peaks are clearly resolved, and the signal shifts (open squares) exhibit only a slight decrease. These shifts are decidedly different from those of samples x = 0.8 and below, and are more comparable with values previously reported for polycrystalline Li1.16V2O5 and Li1.48V2O5 [10,11]. The small peak around  783 ppm observed for sample x = 1.0 corresponds to the value of  797 ppm for the y-phase of x = 0.8, which is denoted by solid circles. Thus, no significant change in the shift is observed even in samples above x = 1.0. It is likely that the VO5 square pyramidal structure is retained without drastic structural modification, e.g., distortion of the pyramid, etc. although phase transition occurs after lithium intercalation. 4. Discussion The results of the 7Li and 51V MAS-NMR spectra highlight the features, asymmetric spectra and distinct peak shifts, of multi-phase systems over a wide range of Li content within the samples prepared by solid-state reaction. Here, samples with a small amount of Li (below x = 0.6) are clearly multi-phase systems, showing the existence of both h- and q-phases. The yphase is dominant for samples x = 0.8, while above x = 1.0, it is the g-phase which dominates, with the additional presence of both the y-phase and a small amount of the q-phase. Indeed, a detailed XRD analysis by Satto et al. revealed significant structural changes in the polyhedra surrounding the Li site in each phase. Here, a bicapped tetrahedron, a bicapped trigonal

prism, and a LiO6 octahedron were determined for the y-, q-, and g-phases, respectively [16,17]. These various Li environments correspond to the specific 7Li MAS-NMR peaks seen in Fig. 3. Lithium intercalation is found to partially reduce the vanadium oxidation state to V4+, as a result of electron transfer from the lithium 2s orbital to the vanadium 3d orbitals through the overlapped bonding of Li(s) – O( p) – V(d) bands. The increase in the amount of magnetic V4+ ions with a localized 3d spin induces a magnetic interaction between the V5+ –V4+ ions, and a peak shift as seen in the broad 51V MAS-NMR spectrum of sample x = 0.8. However, the broadening of the signals in the 7Li MAS-NMR spectra is minimal, and their shifts are small. This implies that the hyperfine coupling between the Li and V4+ species through the Li– O – V bonds is not particularly efficient. On the other hand, the observed V shift is close to the values reported for other lithiated vanadium oxides such as Li1+x V3O8 and Lix V2O5 [10,11]. A rigid V polyhedron produces clearly resolved peaks and no drastic signal shifts in the 51V MASNMR spectra of Lix V2O5, particularly during the complicated structural transitions associated with samples x = 0.8 and above. Furthermore, there are no drastic signal losses or spectral broadening due to electronic transitions arising from localization/delocalization of electrons on the V sites, as seen in Li1+x V3O8. The observed negative V shift is thought to be induced by core polarization of the inner orbitals due to d electrons donated from electronic charge transfer between the 3d 1 and 3d 0 configurations, or diamagnetic electron currents of closed V orbitals. Localization of electrons on the V site is expected to proceed with the increase of x and produce a broad spectrum for x = 0.8. However, further Li intercalation does not result in more remarkable modification in the spectrum or in the signal shift. It is speculated that the delocalization due to

Li0.4V2O5

Li-NMR Intensity ( arb. units )

LixV2O5

-500

450K

-4000

-2000

0

2000

7

132

4000

172K

-15000

-7500

0

7500

15000

Frequency ( Hz ) Fig. 6. Broad line NMR spectra of Li0.4V2O5. Upper and lower lines at each temperature show the observed and calculated line shapes, respectively.

K. Nakamura et al. / Solid State Ionics 177 (2006) 129 – 135

Li-NMR Intensity ( arb. units )

Li0.8V2O5

600K

-4000

-2000

0

2000

4000

220K

7

electronic charge transfer through the Li –O – V overlap is promoted because of an increase in the oxygen coordination around the Li center; from LiO4 (y-phase) to LiO6 polyhedra (q- and g-phases) with increasing x above x = 0.8. As such, further delocalization would result in the degradation or suppression of V – O – V4+ coupling. Indeed, the V spectra and peak shifts for samples above x = 1.0 appear to be comparable to those for samples with a small quantity of Li, as shown in Figs. 4 and 5. The following section concerns the temperature dependence of the 7Li broad line NMR spectra. As shown in Fig. 3, a simple powder pattern is observed in the 7Li MAS spectra of samples below x = 0.8. It is possible to represent the broad line NMR spectrum with a Gaussian or Lorentzian line fitting. Figs. 6 –8 show the observed and calculated spectra for x = 0.4, 0.6, and 0.8, respectively. The upper and lower curves in each spectrum correspond to the curves obtained at high and low temperatures, respectively. For each sample, the curves obtained at higher temperatures exhibit typical single Lorentzian character, while at lower temperatures, the sample curves reveal a broad Gaussian shape. Sample x = 0.6 however, reveals a broad spectrum which can be represented by the superposition of a broad and narrow Gaussian shape, as indicated by I and II, respectively (Fig. 7). Fig. 9 shows an Arrhenius plot of the line width of the samples of x = 0.4 to 1.4, where the image inset illustrates the linear temperature dependence of the line width. Here, the line width is defined as the full width at half maximum (FWHM) of the spectrum. The line width of x = 0.4 is equal to the narrow line (II) of x = 0.6. Line-II has been ascribed to the h-phase and is consistent with the result of the MAS-NMR spectra. On the other hand, the second line width (I) of x = 0.6 is in a good

133

-20000

-10000

0

10000

20000

Frequency ( Hz ) Fig. 8. Broad line NMR spectra of Li0.8V2O5. Upper and lower lines at each temperature show observed and calculated line shapes, respectively.

agreement with the line widths of x = 0.8 and 1.0 and as such, is deemed to correspond to the q-phase. The values of the line widths for samples x = 0.8 and 1.0 are almost the same as those of the samples above x = 1.2. The abrupt decrease in the temperature dependent line width for samples above 450 K is thought to be attributed to the Li+ionic motion. According to a simple ion-hopping picture, we assume that the motional narrowed line width obeys the following expression: DW ðT Þ ¼ AD expðEm =kB T Þ þ DW0 where E m is the activation energy for the diffusion of Li+ ions, A D is the prefactor and DW 0 is the temperature independent residual line width [18]. The first term expresses the line width due to the dipole – dipole coupling, while the second term contains the other

550K

104 0

2000

4000

290K I+II

II -10000

-5000

0

I

10

102

101 5000

8 6 4 2 0

Fig. 7. Broad line NMR spectra of Li0.6V2O5. Upper and lower lines at each temperature show observed and calculated line shapes, respectively. The line width of x = 0.6 is decomposed into the broad and narrow line denoted by I and II, respectively.

100 200 300 400 500 600

Temperature ( K )

10000

Frequency ( Hz )

x=0.4 x=0.6(I) x=0.6(II) x=0.8 x=1.0 x=1.2 x=1.4

3

FWHM ( kHz )

-2000

FWHM ( Hz )

-4000

7

Li-NMR Intensity ( arb. units )

Li0.6V2O5

0

2

4

6

8

10

12

14

1000/T ( K-1 ) Fig. 9. Arrhenius diagram of the line width of x = 0.4, 0.6, 0.8, 1.0, 1.2 and 1.4 in Lix V2O5. The image inset shows the temperature dependence of the line widths.

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contributions including the external magnetic inhomogeneity and the quadrupole interaction. From the relationship of ln(DW  DW 0) vs. T  1, we can evaluate the corresponding activation energies for the diffusion of Li+ ions, and compare them with values evaluated from the data set obtained below 400 K in previous work [6– 8]. In our study, the activation energies for x = 0.4 and the line width (II) of x = 0.6 are estimated to be 0.07 T 0.01 eV above 150 K. These values are consistent with that of 1.55 kcal/mol (0.067 eV) for the hphase [6]. As seen in Fig. 9 however, the steep slopes above 450 K yield larger activation energies than those evaluated below 400 K. The activation energies of the sample of x = 0.8 and the linewidth (I) for x = 0.6 are evaluated to be 0.57 T 0.02 eV above 450 K. On the other hand, the activation energies for x = 1.0, 1.2 and 1.4 above 450 K are approximately 0.58 T 0.03 eV, which is generally regarded as the averaged value for coexisting q- and g-phases. In the h-phase, a small activation energy of 0.07 eV is obtained above 150K, while in the middle range (200 to 400K), and in the Li rich samples (x = 1.0– 1.4), the activation energies are evaluated to be 0.08 to 0.10 eV. In the h-phase, it would be possible for the Li+ ions to migrate with low activation energy, because of the many unoccupied stable Li sites located in the tunnel along the b-axis. The activation energies of the Li rich samples in the middle temperature range however, are much less than those of other lithium ionic conductors, for example 0.30 eV in LiCoO2 [19], 0.59 eV in LiNiO2 [19], 0.23 eV in LiCuO2 [20], 0.5 eV in LiMn2O4 [21] etc. and consistent with the reported value of 0.08 eV for the g-phase [7]. Therefore, it is considered that the decrease in the line width in the middle temperature range is not directly connected to the Li+ionic diffusion with such low activation energies. It seems that in the Li rich samples, the temperature range of 200 – 400 K corresponds to a transient stage in which a remarkable ionic diffusion state is gradually developing. Contrary to this, the higher values evaluated above 450 K are close to values obtained from conductivity measurements [22]. The intrinsic activation energy related to the Li+ionic diffusion of Li x V2O 5 should be defined in the high temperature range. The larger activation energy of the yphase of x = 0.8 at the high temperature range is expected to be connected with the high temperature phase transition. In a previous report, Satto et al. revealed the occurrence of both the y- 6 q-phase transition around 110 to 130 -C, and the transformation from the q- Y g-phase between 175 and 220 -C [16]. As such, the q-phase is stable over a wide range of x å 0.3 to 1.0, and temperatures from 130 to 175 -C [16,17]. It is conceivable therefore, that the high activation energy above 0.5 eV would be ascribed to the mixing of the q- and g-phases, or the major g-phase above 500 K. The polyhedron defined by the VO5 square pyramids surrounding the Li site is expected to keep the diffusion path after undergoing a phase transition. Indeed, the interlayer distances, even in the g-phase with a disturbed up-and-down pyramidal structure, are longer than those in the q-phase, so that tunnels along the b-axis which allow for Li+ ion migration,

has been formed, although the puckering of the square pyramids is not preferable to Li+ ionic diffusion. In the qphase, the Li+ ions coordinated by O2 ions would be strongly affected by the O – O interaction between the shortest interlayer distances in the y-, q-and g-phases. Such difference in correlation between O2 and Li+ may be connected with the large activation energy of the coexisting phase at high temperature. The diffusion mechanism involved for large activation energies at high temperature has yet to be ascertained. 5. Summary Vanadium oxide intercalated with lithium ions (Lix V2O5) has been prepared by solid-state reaction, and has been shown to exhibit multi-phase regions depending on the Li content. In the h-phase, the Li+ ions can diffuse sufficiently at low temperature with low activation energy. The 51V NMR spectrum of the y-phase is strongly affected by the magnetic V5+ – V4+ interaction arising from the 3d spin of the V atom. Lithium intercalation is widely expected to promote localization of the electrons on the V site, because of the increase in the effective Li – O – V overlapping, which corresponds to the characteristic modification in the MAS-NMR spectra and peak shifts of the V nucleus. However, further Li insertion beyond x = 1.0 results in delocalization of the electrons on the V site because of a change in the configuration of the polyhedral groups around the Li atom. The ionic diffusion of Li+ ions with relative high activation energy ¨0.5 eV observed at high temperature would be connected with the stable q- and gphases resulting from the complicated phase transition of yY q- Y g-phase. These results show an intrinsic ionic diffusion in these systems at high temperature. References [1] J. Galy, J. Solid State Chem. 100 (1992) 229. [2] C. Delmas, H. Cognac-Abrade, J.M. Cocciantelli, M. Me`ne`trier, J.P. Doumerc, Solid State Ionics 69 (1994) 257. [3] B. Pecquenard, D. Gourier, N. Baffier, Solid State Ionics 78 (1995) 287. [4] A.D. Wadsley, Acta Crystallogr. 8 (1955) 695. [5] J. Galy, J. Darriet, P. Hagenmuller, Rev. Chim. Miner. 8 (1971) 509. [6] J. Gendell, R.M. Cotts, M.J. Sienko, J. Chem. Phys 37 (1962) 220. [7] J.M. Cocciantelli, K.S. Suh, J. Senegas, J.P. Doumerc, J.L. Soubeyroux, M. Pouchard, P. Hagenmuller, J. Phys. Chem. Solids 53 (1992) 51. [8] J.M. Cocciantelli, K.S. Suh, J. Senegas, J.P. Doumerc, M. Pouchard, J. Phys. Chem. Solids 53 (1992) 57. [9] J. Hirschinger, T. Mongrelet, P. Granger, J.-M. Savariault, E. De´ramond, J. Galy, J. Phys. Chem. 97 (1993) 10301. [10] P.E. Stallworth, F.S. Johnson, S.G. Greenbaum, S. Passerini, J. Flowers, W. Smyrl, J.J. Fontanella, Solid State Ionics 146 (2002) 43. [11] P.E. Stallworth, F.S. Johnson, S.G. Greenbaum, S. Passerini, J. Flowers, W. Smyrl, J.J. Fontanella, J. Appl. Phys. 92 (2002) 3839. [12] M. Vijayakumar, Doctoral thesis, Bharathiar Univ., India, 2003. [13] M. Vijayakumar, S. Selvasekarapandian, R. Kesavamoorthy, K. Nakamura, T. Kanashiro, Mater. Lett. 57 (2003) 3618. [14] M. Vijayakumar, S. Selvasekarapandian, K. Nakamura, T. Kanashiro, R. Kesavamoorthy, Solid State Ionics 167 (2004) 41. [15] K. Nakamura, M. Vijayakumar, S. Selvasekarapandian, T. Kanashiro, in press. Proceedings of 1st international discussion and meeting for superionic conductors edited by S. Yoshikado, Kyoto, Japan, 2004.

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