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Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 studied by 23Na NMR
PERGAMON
Solid State Communications 117 (2001) 65±68
www.elsevier.com/locate/ssc
Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 studied by 23Na NMR O.H. Han a,*, J.K. Jung b, M.-Y. Yi b, J.H. Kwak b, Y.J. Shin b b
a Magnetic Resonance Team, Korea Basic Science Institute, Taejeon 305-333, South Korea Division of Natural Science, The Catholic University of Korea, Pucheon City 420-743, South Korea
Received 6 August 2000; received in revised form 12 October 2000; accepted 15 October 2000 by H. Akai
Abstract Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 powder was studied by 23Na NMR techniques. Even though 23Na is a quadrupole nucleus, the spectra are dominantly governed by magnetic susceptibility anisotropy of paramagnetism rather than quadrupole interaction. The static resonance line becomes narrower at higher temperature manifesting motional narrowing. The line shape is not axially symmetric below 270 K but becomes symmetric at higher temperature. This indicates that the Na 1 motion is faster than 10 28 s and isotropic in two-dimension above 270 K resulting in averaging out the anisotropy in the ab-planes. Two different prismatically coordinated Na sites of the sample are reported by XRD but the fast and isotropic Na motion in the ab-planes averages out the locational difference in the NMR spectra. At lower temperature, the Na 1 motion slows down and becomes anisotropic in the ab-planes enough to see the anisotropy in the plane. q 2000 Published by Elsevier Science Ltd. Keywords: A. Layer-type oxides; C. Nonstoichiometry; D. Ion dynamics; E. Nuclear resonances PACS: 66.10.Ed; 61.72.Ji; 32.30.Dx
1. Introduction Nuclear magnetic resonance (NMR) technique is a powerful method for investigating the local structure and dynamics of ions in compounds. In this study, we present the ®rst report on the dynamic phenomena associated with the local structures at Na sites in the Na0.67Ni0.33Ti0.67O2 powder by employing 23Na NMR. A lot of ternary oxides A IB IIIO2 exhibit layer-type structure derived from ordered rock-salt one [1]. Here, A 1 and B 31 represent monovalent ion such as alkali metal ions and diverse transition metal ions, respectively. Sometimes coupled allovalent cations like B 21/B 41 can also enter into the position of B 31 ion [2]. The structure of layer-type ABO2 may be characterized by two-dimensionally developed BO2 sheets in the ab-plane, which are formed out of the BO6 octahedra connected to one another by edge-sharing. BO2 sheets are piled up along c-axis, giving rise to various * Corresponding author. Tel.: 182-42-865-3436; fax: 182-42865-3419. E-mail address: [email protected] (O.H. Han).
coordination sites for A 1 ions: octahedral, tetrahedral and prismatic ones, among which the ®rst one is the most frequently occupied. The prismatic coordination of A 1 has been reported for a limited number of compounds with nonstoichiometric compositions AxBO2 where x is usually smaller than 0.75 [3±5]. The reasons for the appearance of prismatic coordination in oxides have been proposed, including the increased covalency of B±O bond and the electrostatic repulsion between negatively charged BO2 sheets [6,12]. From this viewpoint, large A 1 ions can be advantageous to prismatic coordination, in that they can reduce more effectively the repulsion of the BO2 sheets and their low electronegativity and polarizing power should lead to weak A±O bonds so that the covalency of the competitive B±O bonds would be increased. In fact, several phases have been reported when A 1 is potassium [7,8], while only a few phases have been known with Na 1 [3,9] except for the electrochemically obtained ones [10]. A 1 ions in trigonal prisms are expected to have higher mobility than those in octahedra or tetrahedra because they can migrate more easily in the ab-plane through wider
0038-1098/01/$ - see front matter q 2000 Published by Elsevier Science Ltd. PII: S 0038-109 8(00)00431-2
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O.H. Han et al. / Solid State Communications 117 (2001) 65±68
Fig. 1. Two different prismatic coordinations of Na in the Na0.67Ni0.33Ti0.67O2 sample. Occupation ratio of Na1 and Na2 sites was found to be 0.20:0.47 by Rietveld re®nement of the XRD data.
rectangular windows and that a suf®ciently large concentration of vacancy as accepting sites are present in the structure. For instance, studies on the ionic conductivity of KxInxSn12xO2 have con®rmed that the K 1 ions in the prismatic sites exhibit much larger ionic conductivity than those in the octahedral sites [11]. However, Na or Li phases are not so well studied despite their much larger potential for application, probably due to the lack of appropriate materials to work on. Recently, we have successfully prepared the nonstoichioII IV metric oxides NaxNix/2 Ti12x/2 O2 over a wide range of x 0:60 # x # 1:0: Among them, Na0.67Ni0.33Ti0.67O2 has been proved to have Na 1 ions in the prismatic sites and also to exhibit fairly high ionic conductivity s 700 K 1022 V 21 cm21 [12]. Therefore, we employed 23Na NMR to comprehend the dynamics of Na 1 ions in the prismatic coordination of the compound. 2. Experimental Powder sample of Na0.67Ni0.33Ti0.67O2 was prepared by a solid state reaction from a mixture of Na2CO3 (.99%), NiO (.99%), and TiO2 (.99.9%). The mixture was preheated at 8008C for 12 h to ensure decarbonation of Na2CO3. The calcined powder was ground, pressed into a disk-like pellet with a 13 mm diameter under a pressure of 75 MPa and
Fig. 2. 23Na MAS spectrum obtained at 11.7 T and 34.5 kHz spinning rate. The pulse length of 0.5 ms was employed when 908 pulse length was 3 ms. The spinning side bands are marked by p. The small peak near 0 ppm is from an impurity with Na 1 in octahedral coordination.
Fig. 3. Representative 23Na static spectra at various temperatures. The small peak near 0 ppm is from an impurity with Na 1 in octahedral coordination.
®nally heated at 9508C for 30 h. The reaction was performed in Ar ¯ow to avoid the eventual oxidation of Ni 21 and Na2CO3 was added at 15% excess due to its volatility. Powder X-ray diffraction analysis was carried out using a Siemens D5005 X-ray diffractometer with CuKa1 radiation equipped with a graphite monochromater. All 1:5405 A) the diffraction lines have been successfully indexed with c 11:42 A; primitive hexagonal lattice; a 2:981 A; and consistent with the prismatic phase [1]. The 23Na I 3=2 NMR spectra of the Na0.67Ni0.33Ti0.67O2 sample were obtained with 105.8 MHz Larmor frequency at static magnetic ®eld strength B0 9:4 T with a Bruker DSX400 NMR spectrometer unless stated otherwise. The wideline spectra were acquired in the temperature range of 180±480 K with the p1 ±tau±p2 echo pulse sequence where p1 1 ms; p2 2 ms; and tau delay 13 ms. The pulse repetition delay and dwell time was 2 s and 0.5 ms, respectively. The magic angle spinning (MAS) spectra were obtained with a single pulse of 1 ms and repetition delay of 1 s at various spinning rates of
O.H. Han et al. / Solid State Communications 117 (2001) 65±68
67
Fig. 5. Reciprocal magnetic susceptibility versus temperature.
Fig. 4. Plots of: (a) isotropic chemical shift (d i); (b) asymmetric parameter (h ); (c) anisotropy (Dd ) values obtained from the simulation of the spectra versus temperature.
10±15 kHz. The chemical shift was referenced to the external 1 N aqueous NaCl solution. The powder patterns were simulated with the Bruker win®t program [13]. 3. Results and discussion Na0.67Ni0.33Ti0.67O2 sample was observed to have b-RbScO2 type structure by XRD and sodium ions were found to be distributed at two different prismatic sites, as
shown in Fig. 1, with the occupation ratio 0.20:0.47 from Rietveld re®nement [12]. Thus two different center peaks were expected in the 23Na MAS NMR, however, only one center peak appeared in the MAS spectrum taken at 9.4 T and 14.5 kHz spinning rate. There was a small peak near 0 ppm, which is from an impurity of sodium ions in octahedral coordination which is not detected even in the XRD data. To con®rm the number of center peaks, a MAS spectrum was taken with a 2.5 mm rotor at 11.7 T and 34.5 kHz spinning rate as shown in Fig. 2. It shows only one center peak within a given experimental error. The peak shapes in the MAS spectra are symmetric and not showing any feature of the second order quadrupole powder pattern. The static line shape in ppm was independent of magnetic ®eld strength and governed by chemical shift or magnetic susceptibility anisotropy. Thus the main interaction determining the line shapes is chemical shift or magnetic susceptibility anisotropy rather than quadrupole interaction although 23Na is a quadrupole nucleus. It is quite contrary to the previously reported results of 23Na NMR study on NaxCoO2 [14]. A series of representative static spectra were obtained at various temperatures as in Fig. 3. From the simulation of the spectra, isotropic chemical shift (d i), asymmetric parameter (h ), anisotropy (Dd ) values at various temperatures were determined (Fig. 4). The relationship among d i, h , and, Dd is as follows [15]:
di 1=3 d11 1 d22 1 d33 Dd d33 2 1=2 d11 1 d22
h d22 2 d11 = 2=3Dd with ud33 2 di u $ ud11 2 di u $ ud22 2 di u: The d i values are the same with the gravimetric centers (d GM) of the powder patterns if the pattern is simulated for a single site. If d11 d22 ; h 0 and Dd equals the distance between two edges of the line shape. If d11 ± d22 ; h $ 0 and
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O.H. Han et al. / Solid State Communications 117 (2001) 65±68
lation ratio are expected from XRD, above 270 K the fast and isotropic two-dimensional motion of Na 1 within the ab planes averages out the difference between the two sites as well as any locational difference in the planes. On the other hand, below 270 K, the motion is slowed down and not isotropic anymore so that the orientational anisotropy in the ab plane appears in the NMR spectra. Further study is in progress to clarify the correlation time and activation energy of the Na 1 motions. Our study demonstrates that solid-state NMR techniques are very powerful to probe the characteristics of ion dynamics, especially in the nonstoichiometric samples.
Fig. 6. Plot of isotropic chemical shift versus magnetic susceptibility.
Dd becomes smaller than the distance between two edges of the line shape. The d i values are down®eld shifted as temperature is lowered, which is explained by paramagnetic shift due to Ni 21 ions. The magnetic susceptibility of the sample measured by SQUID also shows clear paramagnetizm as shown in Fig. 5 and a linear relationship between susceptibility and d i is monitored in Fig. 6. Therefore, it is apparent that the 23Na NMR spectra are strongly in¯uenced by magnetic susceptibility anisotropy. Motional narrowing is detected by smaller Dd at higher temperature. The line shapes for h < 0 observed above 270 K result from the relatively fast and isotropic two-dimensional motion of Na 1 ions within the ab-planes [12] where all prismatic coordinated Na 1 ions are located. The fast (in NMR time scale, shorter than 10 28 s) and isotropic two-dimensional motion averages out the difference of Na1 and Na2 and locational differences in the ab planes resulting in d11 d22 and h 0: But this two-dimensional motion does not average out the difference between the magnetic susceptibilities along c-axis and that of a or b axis, resulting in d33 ± d11 or d 22 and the asymmetric line shape, which is typical for h 0 and has been reported previously [16,17]. The line shape is broadened and changes to that for h ± 0 at lower temperatures. It implies that the motion is slowed down and not isotropic within the ab planes anymore, and as a result, sodium ions start to feel orientational anisotropy even in the ab plane. 4. Conclusion In summary, 23Na NMR was employed to study dynamics of Na 1 in Na0.67Ni0.33Ti0.67O2 in which all Na 1s are prismatically coordinated. The main interaction governing the line shape of the spectra is the magnetic susceptibilities anisotropy of paramagnetizm even though 23Na is a quadrupole nucleus. Although two Na 1 sites with a different popu-
Acknowledgements This work was partially supported by the KBSI for the expenses of the NMR experiments in 1999. We thank Dr Stefan Steuernagel at the Bruker Analytik GmbH in Germany for the 23Na spectra at 11.7 T.
References [1] C. Delmas, C. Fouassier, P. Hagenmuller, Physica B 99 (1980) 81. [2] Y.J. Shin, J.P. Doumerc, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 28 (1993) 159. [3] C. Fouassier, G. Matejka, J.M. Reau, P. Hagenmuller, J. Solid State Chem. 6 (1973) 532. [4] C. Delmas, M. Devalette, C. Fouassier, P. Hagenmuller, Mater. Res. Bull. 10 (1975) 393. [5] C. Delmas, P. Werner, Acta Chem. Scand. A32 (1978) 329. [6] C. Delmas, C. Fouassier, P. Hagenmuller, Mater. Res. Bull. 11 (1976) 1483. [7] C. Delmas, C. Fouassier, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 11 (1976) 1081. [8] M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 97. [9] M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 104. [10] C. Delmas, J.J. Braconnier, C. Fouassier, P. Hagenmuller, Solid State Ionics 3/4 (1981) 165. [11] C. Delmas, A. Maazaz, C. Fouassier, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 329. [12] Y.-J. Shin, M.Y. Yi, Solid State Ionics 132 (1±2) (2000) 131. [13] D. Massiot, H. Thiele, A. Germanus, Bruker Report 140 (1994) 43. [14] P. Stallworth, S. Greenbaum, Y. Ma, L. Ding, M. Doeff, S. Visco, Solid State Ionics 86±88 (1996) 797. [15] A.-R. Grimmer, B. Blumich, in: P. Diehl, E. Fluck, H. Gunther, R. Kosfeld, J. Seelig (Eds.), NMR Basic Principles and Progress, Vol. 30, Spinger, Berlin, 1994, p. 14 and references therein. [16] A. Pines, M.G. Gibby, J.S. Waugh, Chem. Phys. Lett. 15 (1972) 373. [17] D.E. Wemmer, D.J. Ruben, A. Pines, J. Am. Chem. Soc. 103 (1981) 28.
Solid State Communications 117 (2001) 65±68
www.elsevier.com/locate/ssc
Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 studied by 23Na NMR O.H. Han a,*, J.K. Jung b, M.-Y. Yi b, J.H. Kwak b, Y.J. Shin b b
a Magnetic Resonance Team, Korea Basic Science Institute, Taejeon 305-333, South Korea Division of Natural Science, The Catholic University of Korea, Pucheon City 420-743, South Korea
Received 6 August 2000; received in revised form 12 October 2000; accepted 15 October 2000 by H. Akai
Abstract Sodium ion dynamics in the nonstoichiometric layer-type oxide Na0.67Ni0.33Ti0.67O2 powder was studied by 23Na NMR techniques. Even though 23Na is a quadrupole nucleus, the spectra are dominantly governed by magnetic susceptibility anisotropy of paramagnetism rather than quadrupole interaction. The static resonance line becomes narrower at higher temperature manifesting motional narrowing. The line shape is not axially symmetric below 270 K but becomes symmetric at higher temperature. This indicates that the Na 1 motion is faster than 10 28 s and isotropic in two-dimension above 270 K resulting in averaging out the anisotropy in the ab-planes. Two different prismatically coordinated Na sites of the sample are reported by XRD but the fast and isotropic Na motion in the ab-planes averages out the locational difference in the NMR spectra. At lower temperature, the Na 1 motion slows down and becomes anisotropic in the ab-planes enough to see the anisotropy in the plane. q 2000 Published by Elsevier Science Ltd. Keywords: A. Layer-type oxides; C. Nonstoichiometry; D. Ion dynamics; E. Nuclear resonances PACS: 66.10.Ed; 61.72.Ji; 32.30.Dx
1. Introduction Nuclear magnetic resonance (NMR) technique is a powerful method for investigating the local structure and dynamics of ions in compounds. In this study, we present the ®rst report on the dynamic phenomena associated with the local structures at Na sites in the Na0.67Ni0.33Ti0.67O2 powder by employing 23Na NMR. A lot of ternary oxides A IB IIIO2 exhibit layer-type structure derived from ordered rock-salt one [1]. Here, A 1 and B 31 represent monovalent ion such as alkali metal ions and diverse transition metal ions, respectively. Sometimes coupled allovalent cations like B 21/B 41 can also enter into the position of B 31 ion [2]. The structure of layer-type ABO2 may be characterized by two-dimensionally developed BO2 sheets in the ab-plane, which are formed out of the BO6 octahedra connected to one another by edge-sharing. BO2 sheets are piled up along c-axis, giving rise to various * Corresponding author. Tel.: 182-42-865-3436; fax: 182-42865-3419. E-mail address: [email protected] (O.H. Han).
coordination sites for A 1 ions: octahedral, tetrahedral and prismatic ones, among which the ®rst one is the most frequently occupied. The prismatic coordination of A 1 has been reported for a limited number of compounds with nonstoichiometric compositions AxBO2 where x is usually smaller than 0.75 [3±5]. The reasons for the appearance of prismatic coordination in oxides have been proposed, including the increased covalency of B±O bond and the electrostatic repulsion between negatively charged BO2 sheets [6,12]. From this viewpoint, large A 1 ions can be advantageous to prismatic coordination, in that they can reduce more effectively the repulsion of the BO2 sheets and their low electronegativity and polarizing power should lead to weak A±O bonds so that the covalency of the competitive B±O bonds would be increased. In fact, several phases have been reported when A 1 is potassium [7,8], while only a few phases have been known with Na 1 [3,9] except for the electrochemically obtained ones [10]. A 1 ions in trigonal prisms are expected to have higher mobility than those in octahedra or tetrahedra because they can migrate more easily in the ab-plane through wider
0038-1098/01/$ - see front matter q 2000 Published by Elsevier Science Ltd. PII: S 0038-109 8(00)00431-2
66
O.H. Han et al. / Solid State Communications 117 (2001) 65±68
Fig. 1. Two different prismatic coordinations of Na in the Na0.67Ni0.33Ti0.67O2 sample. Occupation ratio of Na1 and Na2 sites was found to be 0.20:0.47 by Rietveld re®nement of the XRD data.
rectangular windows and that a suf®ciently large concentration of vacancy as accepting sites are present in the structure. For instance, studies on the ionic conductivity of KxInxSn12xO2 have con®rmed that the K 1 ions in the prismatic sites exhibit much larger ionic conductivity than those in the octahedral sites [11]. However, Na or Li phases are not so well studied despite their much larger potential for application, probably due to the lack of appropriate materials to work on. Recently, we have successfully prepared the nonstoichioII IV metric oxides NaxNix/2 Ti12x/2 O2 over a wide range of x 0:60 # x # 1:0: Among them, Na0.67Ni0.33Ti0.67O2 has been proved to have Na 1 ions in the prismatic sites and also to exhibit fairly high ionic conductivity s 700 K 1022 V 21 cm21 [12]. Therefore, we employed 23Na NMR to comprehend the dynamics of Na 1 ions in the prismatic coordination of the compound. 2. Experimental Powder sample of Na0.67Ni0.33Ti0.67O2 was prepared by a solid state reaction from a mixture of Na2CO3 (.99%), NiO (.99%), and TiO2 (.99.9%). The mixture was preheated at 8008C for 12 h to ensure decarbonation of Na2CO3. The calcined powder was ground, pressed into a disk-like pellet with a 13 mm diameter under a pressure of 75 MPa and
Fig. 2. 23Na MAS spectrum obtained at 11.7 T and 34.5 kHz spinning rate. The pulse length of 0.5 ms was employed when 908 pulse length was 3 ms. The spinning side bands are marked by p. The small peak near 0 ppm is from an impurity with Na 1 in octahedral coordination.
Fig. 3. Representative 23Na static spectra at various temperatures. The small peak near 0 ppm is from an impurity with Na 1 in octahedral coordination.
®nally heated at 9508C for 30 h. The reaction was performed in Ar ¯ow to avoid the eventual oxidation of Ni 21 and Na2CO3 was added at 15% excess due to its volatility. Powder X-ray diffraction analysis was carried out using a Siemens D5005 X-ray diffractometer with CuKa1 radiation equipped with a graphite monochromater. All 1:5405 A) the diffraction lines have been successfully indexed with c 11:42 A; primitive hexagonal lattice; a 2:981 A; and consistent with the prismatic phase [1]. The 23Na I 3=2 NMR spectra of the Na0.67Ni0.33Ti0.67O2 sample were obtained with 105.8 MHz Larmor frequency at static magnetic ®eld strength B0 9:4 T with a Bruker DSX400 NMR spectrometer unless stated otherwise. The wideline spectra were acquired in the temperature range of 180±480 K with the p1 ±tau±p2 echo pulse sequence where p1 1 ms; p2 2 ms; and tau delay 13 ms. The pulse repetition delay and dwell time was 2 s and 0.5 ms, respectively. The magic angle spinning (MAS) spectra were obtained with a single pulse of 1 ms and repetition delay of 1 s at various spinning rates of
O.H. Han et al. / Solid State Communications 117 (2001) 65±68
67
Fig. 5. Reciprocal magnetic susceptibility versus temperature.
Fig. 4. Plots of: (a) isotropic chemical shift (d i); (b) asymmetric parameter (h ); (c) anisotropy (Dd ) values obtained from the simulation of the spectra versus temperature.
10±15 kHz. The chemical shift was referenced to the external 1 N aqueous NaCl solution. The powder patterns were simulated with the Bruker win®t program [13]. 3. Results and discussion Na0.67Ni0.33Ti0.67O2 sample was observed to have b-RbScO2 type structure by XRD and sodium ions were found to be distributed at two different prismatic sites, as
shown in Fig. 1, with the occupation ratio 0.20:0.47 from Rietveld re®nement [12]. Thus two different center peaks were expected in the 23Na MAS NMR, however, only one center peak appeared in the MAS spectrum taken at 9.4 T and 14.5 kHz spinning rate. There was a small peak near 0 ppm, which is from an impurity of sodium ions in octahedral coordination which is not detected even in the XRD data. To con®rm the number of center peaks, a MAS spectrum was taken with a 2.5 mm rotor at 11.7 T and 34.5 kHz spinning rate as shown in Fig. 2. It shows only one center peak within a given experimental error. The peak shapes in the MAS spectra are symmetric and not showing any feature of the second order quadrupole powder pattern. The static line shape in ppm was independent of magnetic ®eld strength and governed by chemical shift or magnetic susceptibility anisotropy. Thus the main interaction determining the line shapes is chemical shift or magnetic susceptibility anisotropy rather than quadrupole interaction although 23Na is a quadrupole nucleus. It is quite contrary to the previously reported results of 23Na NMR study on NaxCoO2 [14]. A series of representative static spectra were obtained at various temperatures as in Fig. 3. From the simulation of the spectra, isotropic chemical shift (d i), asymmetric parameter (h ), anisotropy (Dd ) values at various temperatures were determined (Fig. 4). The relationship among d i, h , and, Dd is as follows [15]:
di 1=3 d11 1 d22 1 d33 Dd d33 2 1=2 d11 1 d22
h d22 2 d11 = 2=3Dd with ud33 2 di u $ ud11 2 di u $ ud22 2 di u: The d i values are the same with the gravimetric centers (d GM) of the powder patterns if the pattern is simulated for a single site. If d11 d22 ; h 0 and Dd equals the distance between two edges of the line shape. If d11 ± d22 ; h $ 0 and
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O.H. Han et al. / Solid State Communications 117 (2001) 65±68
lation ratio are expected from XRD, above 270 K the fast and isotropic two-dimensional motion of Na 1 within the ab planes averages out the difference between the two sites as well as any locational difference in the planes. On the other hand, below 270 K, the motion is slowed down and not isotropic anymore so that the orientational anisotropy in the ab plane appears in the NMR spectra. Further study is in progress to clarify the correlation time and activation energy of the Na 1 motions. Our study demonstrates that solid-state NMR techniques are very powerful to probe the characteristics of ion dynamics, especially in the nonstoichiometric samples.
Fig. 6. Plot of isotropic chemical shift versus magnetic susceptibility.
Dd becomes smaller than the distance between two edges of the line shape. The d i values are down®eld shifted as temperature is lowered, which is explained by paramagnetic shift due to Ni 21 ions. The magnetic susceptibility of the sample measured by SQUID also shows clear paramagnetizm as shown in Fig. 5 and a linear relationship between susceptibility and d i is monitored in Fig. 6. Therefore, it is apparent that the 23Na NMR spectra are strongly in¯uenced by magnetic susceptibility anisotropy. Motional narrowing is detected by smaller Dd at higher temperature. The line shapes for h < 0 observed above 270 K result from the relatively fast and isotropic two-dimensional motion of Na 1 ions within the ab-planes [12] where all prismatic coordinated Na 1 ions are located. The fast (in NMR time scale, shorter than 10 28 s) and isotropic two-dimensional motion averages out the difference of Na1 and Na2 and locational differences in the ab planes resulting in d11 d22 and h 0: But this two-dimensional motion does not average out the difference between the magnetic susceptibilities along c-axis and that of a or b axis, resulting in d33 ± d11 or d 22 and the asymmetric line shape, which is typical for h 0 and has been reported previously [16,17]. The line shape is broadened and changes to that for h ± 0 at lower temperatures. It implies that the motion is slowed down and not isotropic within the ab planes anymore, and as a result, sodium ions start to feel orientational anisotropy even in the ab plane. 4. Conclusion In summary, 23Na NMR was employed to study dynamics of Na 1 in Na0.67Ni0.33Ti0.67O2 in which all Na 1s are prismatically coordinated. The main interaction governing the line shape of the spectra is the magnetic susceptibilities anisotropy of paramagnetizm even though 23Na is a quadrupole nucleus. Although two Na 1 sites with a different popu-
Acknowledgements This work was partially supported by the KBSI for the expenses of the NMR experiments in 1999. We thank Dr Stefan Steuernagel at the Bruker Analytik GmbH in Germany for the 23Na spectra at 11.7 T.
References [1] C. Delmas, C. Fouassier, P. Hagenmuller, Physica B 99 (1980) 81. [2] Y.J. Shin, J.P. Doumerc, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 28 (1993) 159. [3] C. Fouassier, G. Matejka, J.M. Reau, P. Hagenmuller, J. Solid State Chem. 6 (1973) 532. [4] C. Delmas, M. Devalette, C. Fouassier, P. Hagenmuller, Mater. Res. Bull. 10 (1975) 393. [5] C. Delmas, P. Werner, Acta Chem. Scand. A32 (1978) 329. [6] C. Delmas, C. Fouassier, P. Hagenmuller, Mater. Res. Bull. 11 (1976) 1483. [7] C. Delmas, C. Fouassier, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 11 (1976) 1081. [8] M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 97. [9] M. Jansen, R. Hoppe, Z. Anorg. Allg. Chem. 408 (1974) 104. [10] C. Delmas, J.J. Braconnier, C. Fouassier, P. Hagenmuller, Solid State Ionics 3/4 (1981) 165. [11] C. Delmas, A. Maazaz, C. Fouassier, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 329. [12] Y.-J. Shin, M.Y. Yi, Solid State Ionics 132 (1±2) (2000) 131. [13] D. Massiot, H. Thiele, A. Germanus, Bruker Report 140 (1994) 43. [14] P. Stallworth, S. Greenbaum, Y. Ma, L. Ding, M. Doeff, S. Visco, Solid State Ionics 86±88 (1996) 797. [15] A.-R. Grimmer, B. Blumich, in: P. Diehl, E. Fluck, H. Gunther, R. Kosfeld, J. Seelig (Eds.), NMR Basic Principles and Progress, Vol. 30, Spinger, Berlin, 1994, p. 14 and references therein. [16] A. Pines, M.G. Gibby, J.S. Waugh, Chem. Phys. Lett. 15 (1972) 373. [17] D.E. Wemmer, D.J. Ruben, A. Pines, J. Am. Chem. Soc. 103 (1981) 28.