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NMR and conductivity study of the protonic conductor HPb2Nb3O10·nH2O
Solid State Ionics 136–137 (2000) 243–247 www.elsevier.com / locate / ssi
NMR and conductivity study of the protonic conductor HPb 2 Nb 3 O 10 ? nH 2 O a b, a b c ˆ C.E. Tambelli , J.P. Donoso *, C.J. Magon , A.C.D. Angelo , A.O. Florentino , M.J. Saeki b a
˜ Carlos, Universidade de Sao ˜ Paulo, C.P. 369, 13560 -970, Sao ˜ Paulo, Brazil ´ Instituto de Fısica de Sao b ˜ Paulo, Brazil ´ Departamento de Quımica , UNESP, C.P. 473, 17033 -360 Bauru, Sao c ˜ Paulo, Brazil ´ ´ , UNESP, C.P. 510, 18618 -000 Botucatu, Sao Departamento de Quımica e Bioquımica
Abstract Electrical conductivity and 1 H Nuclear Magnetic Resonance (NMR) techniques were used to investigate the ionexchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ? nH 2 O, over the temperature range 90–350 K. Compounds were synthesized by the sol–gel method and calcinated at 6508C. Analysis of the NMR data gives activation energies for the proton motion in the range 0.14–0.40 eV, which are dependent on the water content. The frequency and temperature dependencies of the proton spin–lattice relaxation times show that the character of the motion of the water molecules is essentially two-dimensional, reflecting the layered structure of the material. The 1 H line-narrowing transition and the single spin–lattice relaxation rate maximum, observed in the hydrated compounds, are consistent with a Grotthuss-like mechanism for the proton diffusion. 2000 Elsevier Science B.V. All rights reserved. Keywords: NMR; Impedance spectroscopy; Proton conductor; Sol–gel; Perovskite Materials: 76.60; 66.30.D; 71.20.T
1. Introduction Ion-exchangeable layered perovskites based on octahedral NbO 6 , TiO 6 or TaO 6 have been the subject of several studies because of their excellent ion-exchange ability [1–3], acidity for intercalation reactions [4,5], optical properties [6] and noticeable activity for the photocatalytic reactions [7]. Many of the compounds exhibiting ion-exchange reactivity also show ion conductivity [2,8–10]. The ionic
properties of hydrous oxides are mainly attributed to surface phenomena involving adsorbed water molecule and its dissociation forms [11]. The water adsorption and dissociation occur at the oxide–water interface generating fully hydroxilated oxides, M– OH. The M–OH groups can undergo acidic or basic properties by further dissociation reaction: M–OH 1 H 2 O → M–O 2 1 H 3 O 1 2 M–OH 1 H 2 O → M–OH 1 2 1 OH
*Corresponding author. Fax: 155-16-272-2218. E-mail address: [email protected] (J.P. Donoso).
Acid dissociation is promoted by highly charged
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00316-7
244
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
small central cations and base dissociation by large metal cations of low charge. These properties depend not only on the atomic and bonding properties of the central species, but also on the properties of the aggregate surface and on the state of aggregation [9]. The H 1 -exchanged forms of hydrous oxides are known to be easily hydrated even under atmospheric conditions [7], exhibiting peculiar behavior of fast protonic conductivity (s .10 26 V21 cm 21 ) [10,11]. Since the proton conduction through these materials depends upon the relative humidity, this property has been applied in the development of humidity sensors [12]. In the present work, the ion-exchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ?nH 2 O synthesized by sol–gel method is investigated by impedance spectroscopy and NMR, as a function of the watervapor pressure.
3. Results and discussion Fig. 1 shows the temperature dependence of the conductivity for a sample exposed to water vapor pressure in a saturated container (79% r.h.). Above 280 K the conductivity increases with increasing temperature, until a maximum value is attained at approximately 310 K. Assuming that the conductivity results from a thermally activated process, expressed by an Arrhenius function, an activation energy of 0.22 eV can be extracted from the data. A similar behavior was observed in HTiNbO 7 ?nH 2 O [14] and HLaNb 2 O 7 ?nH 2 O [10]. In the layered leadniobate perovskite studied here, a large part of the proton conduction is due to surface conductivity of the water adsorbed in the grain boundaries and micro-crystal surfaces, which are removed upon heating above 300 K, thereby removing the conduction medium. Fig. 2 shows the temperature dependence of the
2. Experimental HPb 2 Nb 3 O 10 ?nH 2 O was initially synthesized in alkali form by the sol–gel method, which consists in dissolving the stoichiometric quantities of NaO(CH 3 ) 3 and Nb(OC 4 H 9 ) 5 in butanol. Proton exchange of Na 1 / H 1 was carried out in 0.3 N HNO 3 , at room temperature, for two days. Analysis of the protonated oxides, Pb and Nb contents and ratio was carried out by elemental analysis. The final product was characterized by powder X-ray diffraction, scanning electronic microscopy and BET analysis. Details of the preparation procedure and characterization will be published elsewhere [13]. Conductivity measurements were performed on goldmetallized pellets of polycrystalline material, pressed at 2 ton cm 22 . The complex impedance technique was used over the frequency range of 1–6 MHz (Solartron Analyser 1260). Proton NMR line shapes and spin–lattice relaxation times (T 1 ) were measured in powder samples of HPb 2 Nb 3 O 10 ?nH 2 O, for n5 1.2, 0.3 and ,0.3, with a pulsed NMR spectrometer operating at 36 MHz, in the temperature range of 90–350 K. Samples were always pre-equilibrated at the water-vapor pressure at 258C during 24 h. The amount of water absorbed was determined by weighing the sample in an analytical balance.
Fig. 1. Temperature dependence of the conductivity of HPb 2 Nb 3 O 10 ?nH 2 O calcinated at 6508C. Sample exposed to 79% r.h.
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
245
Fig. 2. Temperature dependence of the 1 H linewidth of the central line in HPb 2 Nb 3 O 10 ?nH 2 O for n51.2 (s), 0.3 (m) and ,0.3 (h). In the insert is shown the spectrum of sample with n51.2 at 123 K, where the solid line represents the simulation of the experimental spectrum, resulting from a superposition of a central narrow lorentzian line and a Pake doublet.
linewidth of the central component of the 1 H NMR spectrum (insert). The experimental lineshape at 120 K was simulated with a narrow line of lorentzian shape flanked by a pair of peaks, of gaussian shape, separated by 11 Gauss. The side peaks conform a Pake doublet [15–17] attributed to paired protons in H 2 O with a proton–proton separation of 1.5560.05 ˚ The central line has been attributed to the A. superposition of the 1 H signals belonging to the OH 2 and to the central component of the oxonium ion (H 3 O 1 ) spectrum [15,16,18,19]. Below 125 K the ‘rigid lattice’ nuclear dipole–dipole interaction is the main source of line broadening. Above 140 K,
the high mobility of the protons averages out the inter-molecular dipolar interactions and the line narrows (Fig. 2). From the analysis of the line narrowing data we obtain activation energies between 0.18 eV for n51.2 and 0.40 eV for n,0.3 (Table 1). These values are in good agreement with those reported in Cs 5 H 3 (SO 4 ) 4 ?H 2 O (0.34 eV) [15] and in HLa 2 NbTi 2 O 10 ?1.5H 2 O (0.27 eV) [16]. Fig. 3 shows the temperature dependence of the proton spin–lattice relaxation rates (T 121 ). A single maximum, at 250–273 K, was observed for the three investigated samples (Fig. 3a). The relaxation data can be interpreted in terms of the motional modula-
Table 1 1 H NMR linewidth and spin–lattice relaxation data in HPb 2 Nb 3 O 10 ?nH 2 O. T trans is the temperature of the line narrowing transition; T 21 1 max is the temperature at the spin–lattice relaxation rate maximum, EA and E A9 are the activation energies obtained from the line-narrowing and the relaxation data, respectively Sample n ,0.3 0.3 1.2
Linewidth data
Relaxation data
T trans (K)
EA (eV)
(T 21 1 )
21065 18565 17065
0.4060.05 0.3560.05 0.1860.05
330610 245610 230610
max
(K)
E 9A (eV)
to (s)
0.3060.03 0.2160.05 0.1460.05
1.6 10 214 1.5 10 213 7.0 10 212
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
246
Fig. 3. Temperature dependence of the 1 H NMR spin–lattice relaxation rate (T 21 1 ) in HPb 2 Nb 3 O 10 ?nH 2 O (a) measurements made at 36 MHz for samples n51.2 (s), 0.3 (m) and ,0.3 (h) and (b) measurements of T 21 as a function of Larmor frequency, 1 36 MHz (m), 45 MHz (d) and 60 MHz (j) for the sample n50.3. The solid lines are fits to Eq. (1).
tion of the inter-proton dipolar interactions. Measurements carried out at different Larmor frequencies show that the relaxation rate is frequency dependent in the high-temperature side of the T 21 maximum 1 (Fig. 3b). Accordingly, data were fitted with the relaxation expression for two-dimensional motions, which is consistent with the structural feature of the layered perovskites compound studied here [16,20]:
FS
D S
1 1 1 ] ~ t ln 1 1 ]] 1 4ln 1 1 ]] 2 2 T1 v ot 4v 2ot 2
DG
(1)
where vo is the Larmor frequency and t is the
correlation time, expressed by an Arrhenius function [t 5to exp(EA /kT )]. In contrast to three-dimensional diffusion, where in the extreme narrowing limit vot <1, T 121 is independent of vo , a logarithmic frequency dependence is predicted from Eq. (1) for a two-dimensional motion. Indeed, the fit of the data of Fig. 3b indicates that the molecular motion has a pronounced two-dimensional character. The parameters obtained from the fit are listed in Table 1. The single line-narrowing transition observed in the 1 H NMR spectrum, in the temperature range 90–350 K, and the single spin–lattice relaxation rate maximum observed in Fig. 3a, are consistent with a Grotthuss-like mechanism for the proton diffusion. This process consist of a succession of molecular reorientations of H 3 O 1 ions or H 2 O molecules combined with proton jumps on a hydrogen-bond network [17–19]. The parameters obtained from the relaxation data allows us to estimate the proton diffusion coefficient by means of a two-dimensional random walk expression, D 5 l 2 / 4t. Here l is the minimum distance of approach, which may be simply calculated from the ˚ [16,19]. The temperaaverage H–H distance (1.7 A) ture dependence of the proton diffusion coefficient has been measured in some hydrated layered compounds by the pulse field gradient NMR method. In Cs 5 H 3 (SO 4 ) 4 ?H 2 O, for example, D (cm 2 / s)5(2.93 10 24 ) exp(20.33 /kT ) [15] and in CsOH?H 2 O, D (cm 2 / s)5(1.2310 25 ) exp(20.15 /kT ) [21]. For our sample, HPb 2 Nb 3 O 10 ?1.2H 2 O, the diffusion coefficient estimated with the parameters obtained from the relaxation data, Ea 5 0.14 eV and to ¯7310 212 s, yields D¯5310 28 cm 2 / s at room temperature. Taking into account that conductivity and other motional parameters are strongly dependent on water content [13,19], we may consider our results consistent to those measured by the pulse field gradient NMR technique mentioned above.
4. Conclusions The temperature dependencies for conductivity, 1 H NMR linewidth and spin–lattice relaxation time of the ion-exchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ?nH 2 O have been investigated over the temperature range 90–350 K. Rigid-lattice spectra
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
exhibit a narrow line of lorentzian shape, attributed to the superposition of the 1 H signals belonging to the OH 2 and to the central component of the H 3 O 1 spectrum, flanked by a pair of gaussian peaks separated by 11 Gauss, which were attributed to paired protons in H 2 O. Activation energies, in the range of 0.14–0.40 eV, were obtained from the NMR line narrowing data. The single spin–lattice relaxation rate maximum and line-narrowing transition, observed in the NMR results, are consistent with a Grotthuss-like mechanism for the proton diffusion. The conductivity and NMR experimental data indicate that HPb 2 Nb 3 O 10 ?nH 2 O is a fairly good protonic conductor.
Acknowledgements This research is supported by Fapesp, CNPq and Finep Brazilian sponsoring agencies.
References [1] M.A. Sbramanian, J. Gopalakrishman, A.W. Sleight, Mat. Res. Bull. 23 (1988) 837. [2] M. Sato, Y. Kono, T. Jin, J. Ceram. Soc. Jap. 101 (1993) 954. [3] M. Sato, J. Abo, T. Jin, Solid State Ionics 57 (1992) 285. [4] J. Gopalakrishman, V. Bhat, Mat. Res. Bull. 22 (1987) 413.
247
[5] P. Batamack, R. Vincent, J. Fraissard, Catal. Today 28 (1996) 31. [6] C.E. Rice, J. Solid State Chem. 64 (1986) 188. [7] J. Yoshimura, Y. Ebina, J. Kondo, K. Domen, A. Tanaka, J. Phys. Chem. 97 (1993) 1970. [8] K. Toda, Y. Kameo, M. Fijimoto, M. Sato, J. Ceram. Soc. Jap. 102 (1994) 735. [9] P. Colomban (Ed.), Proton Conductors: Solids, Membranes and Gels–Materials and Devices, Cambridge University Press, Cambridge, 1992. [10] M. Sato, T. Jin, K. Uematsu, J. Solid State Chem. 102 (1993) 557. [11] J. Livage, Solid State Ionics 50 (1992) 307. [12] V. Bondarenka, S. Grebinsky, S. Mickevicius, V. Volkov, G. Zacharova, Sensors and Actuators B 28 (1995) 227. ˆ [13] M.J. Saeki, E. Tonhy, A.C.D. Angelo, A.O. Florentino, C.E. Tambelli, J.P. Donoso, C.J. Magon. Solid State Ionics (submitted for publication). [14] M. Lal, A.T. Howe, J. Solid State Chem. 51 (1984) 355. [15] A.M. Fajdiga-Bulat, G. Lahajnar, J. Dolinsek, J. Slak, B. Lohar, B. Zalar, L.A. Shuvalov, R. Blinc, Solid State Ionics 77 (1995) 101. [16] G. Mangamma, V. Bhat, J. Gopalakrishnan, S.V. Bhat, Solid State Ionics 58 (1992) 303. [17] M. Durand-Le Floch, J. Pannetier, C. Doremieux-Morin, H. Arribart, J. Chem. Phys. 84 (1986) 4760. [18] H. Arribart, Y. Piffard, Solid State Communication 45 (1983) 571. [19] N. Binesh, V. Bhat, S.V. Bhat, Solid State Ionics 86–88 (1996) 609. [20] H. Arribart, Y. Piffard, C. Doremieux-Morin, Solid State Ionics 7 (1982) 91. [21] J. Gallier, B. Toudic, M. Stahn, R.E. Lechner, H. Dachs, J. Physique 49 (1988) 949.
NMR and conductivity study of the protonic conductor HPb 2 Nb 3 O 10 ? nH 2 O a b, a b c ˆ C.E. Tambelli , J.P. Donoso *, C.J. Magon , A.C.D. Angelo , A.O. Florentino , M.J. Saeki b a
˜ Carlos, Universidade de Sao ˜ Paulo, C.P. 369, 13560 -970, Sao ˜ Paulo, Brazil ´ Instituto de Fısica de Sao b ˜ Paulo, Brazil ´ Departamento de Quımica , UNESP, C.P. 473, 17033 -360 Bauru, Sao c ˜ Paulo, Brazil ´ ´ , UNESP, C.P. 510, 18618 -000 Botucatu, Sao Departamento de Quımica e Bioquımica
Abstract Electrical conductivity and 1 H Nuclear Magnetic Resonance (NMR) techniques were used to investigate the ionexchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ? nH 2 O, over the temperature range 90–350 K. Compounds were synthesized by the sol–gel method and calcinated at 6508C. Analysis of the NMR data gives activation energies for the proton motion in the range 0.14–0.40 eV, which are dependent on the water content. The frequency and temperature dependencies of the proton spin–lattice relaxation times show that the character of the motion of the water molecules is essentially two-dimensional, reflecting the layered structure of the material. The 1 H line-narrowing transition and the single spin–lattice relaxation rate maximum, observed in the hydrated compounds, are consistent with a Grotthuss-like mechanism for the proton diffusion. 2000 Elsevier Science B.V. All rights reserved. Keywords: NMR; Impedance spectroscopy; Proton conductor; Sol–gel; Perovskite Materials: 76.60; 66.30.D; 71.20.T
1. Introduction Ion-exchangeable layered perovskites based on octahedral NbO 6 , TiO 6 or TaO 6 have been the subject of several studies because of their excellent ion-exchange ability [1–3], acidity for intercalation reactions [4,5], optical properties [6] and noticeable activity for the photocatalytic reactions [7]. Many of the compounds exhibiting ion-exchange reactivity also show ion conductivity [2,8–10]. The ionic
properties of hydrous oxides are mainly attributed to surface phenomena involving adsorbed water molecule and its dissociation forms [11]. The water adsorption and dissociation occur at the oxide–water interface generating fully hydroxilated oxides, M– OH. The M–OH groups can undergo acidic or basic properties by further dissociation reaction: M–OH 1 H 2 O → M–O 2 1 H 3 O 1 2 M–OH 1 H 2 O → M–OH 1 2 1 OH
*Corresponding author. Fax: 155-16-272-2218. E-mail address: [email protected] (J.P. Donoso).
Acid dissociation is promoted by highly charged
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00316-7
244
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
small central cations and base dissociation by large metal cations of low charge. These properties depend not only on the atomic and bonding properties of the central species, but also on the properties of the aggregate surface and on the state of aggregation [9]. The H 1 -exchanged forms of hydrous oxides are known to be easily hydrated even under atmospheric conditions [7], exhibiting peculiar behavior of fast protonic conductivity (s .10 26 V21 cm 21 ) [10,11]. Since the proton conduction through these materials depends upon the relative humidity, this property has been applied in the development of humidity sensors [12]. In the present work, the ion-exchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ?nH 2 O synthesized by sol–gel method is investigated by impedance spectroscopy and NMR, as a function of the watervapor pressure.
3. Results and discussion Fig. 1 shows the temperature dependence of the conductivity for a sample exposed to water vapor pressure in a saturated container (79% r.h.). Above 280 K the conductivity increases with increasing temperature, until a maximum value is attained at approximately 310 K. Assuming that the conductivity results from a thermally activated process, expressed by an Arrhenius function, an activation energy of 0.22 eV can be extracted from the data. A similar behavior was observed in HTiNbO 7 ?nH 2 O [14] and HLaNb 2 O 7 ?nH 2 O [10]. In the layered leadniobate perovskite studied here, a large part of the proton conduction is due to surface conductivity of the water adsorbed in the grain boundaries and micro-crystal surfaces, which are removed upon heating above 300 K, thereby removing the conduction medium. Fig. 2 shows the temperature dependence of the
2. Experimental HPb 2 Nb 3 O 10 ?nH 2 O was initially synthesized in alkali form by the sol–gel method, which consists in dissolving the stoichiometric quantities of NaO(CH 3 ) 3 and Nb(OC 4 H 9 ) 5 in butanol. Proton exchange of Na 1 / H 1 was carried out in 0.3 N HNO 3 , at room temperature, for two days. Analysis of the protonated oxides, Pb and Nb contents and ratio was carried out by elemental analysis. The final product was characterized by powder X-ray diffraction, scanning electronic microscopy and BET analysis. Details of the preparation procedure and characterization will be published elsewhere [13]. Conductivity measurements were performed on goldmetallized pellets of polycrystalline material, pressed at 2 ton cm 22 . The complex impedance technique was used over the frequency range of 1–6 MHz (Solartron Analyser 1260). Proton NMR line shapes and spin–lattice relaxation times (T 1 ) were measured in powder samples of HPb 2 Nb 3 O 10 ?nH 2 O, for n5 1.2, 0.3 and ,0.3, with a pulsed NMR spectrometer operating at 36 MHz, in the temperature range of 90–350 K. Samples were always pre-equilibrated at the water-vapor pressure at 258C during 24 h. The amount of water absorbed was determined by weighing the sample in an analytical balance.
Fig. 1. Temperature dependence of the conductivity of HPb 2 Nb 3 O 10 ?nH 2 O calcinated at 6508C. Sample exposed to 79% r.h.
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
245
Fig. 2. Temperature dependence of the 1 H linewidth of the central line in HPb 2 Nb 3 O 10 ?nH 2 O for n51.2 (s), 0.3 (m) and ,0.3 (h). In the insert is shown the spectrum of sample with n51.2 at 123 K, where the solid line represents the simulation of the experimental spectrum, resulting from a superposition of a central narrow lorentzian line and a Pake doublet.
linewidth of the central component of the 1 H NMR spectrum (insert). The experimental lineshape at 120 K was simulated with a narrow line of lorentzian shape flanked by a pair of peaks, of gaussian shape, separated by 11 Gauss. The side peaks conform a Pake doublet [15–17] attributed to paired protons in H 2 O with a proton–proton separation of 1.5560.05 ˚ The central line has been attributed to the A. superposition of the 1 H signals belonging to the OH 2 and to the central component of the oxonium ion (H 3 O 1 ) spectrum [15,16,18,19]. Below 125 K the ‘rigid lattice’ nuclear dipole–dipole interaction is the main source of line broadening. Above 140 K,
the high mobility of the protons averages out the inter-molecular dipolar interactions and the line narrows (Fig. 2). From the analysis of the line narrowing data we obtain activation energies between 0.18 eV for n51.2 and 0.40 eV for n,0.3 (Table 1). These values are in good agreement with those reported in Cs 5 H 3 (SO 4 ) 4 ?H 2 O (0.34 eV) [15] and in HLa 2 NbTi 2 O 10 ?1.5H 2 O (0.27 eV) [16]. Fig. 3 shows the temperature dependence of the proton spin–lattice relaxation rates (T 121 ). A single maximum, at 250–273 K, was observed for the three investigated samples (Fig. 3a). The relaxation data can be interpreted in terms of the motional modula-
Table 1 1 H NMR linewidth and spin–lattice relaxation data in HPb 2 Nb 3 O 10 ?nH 2 O. T trans is the temperature of the line narrowing transition; T 21 1 max is the temperature at the spin–lattice relaxation rate maximum, EA and E A9 are the activation energies obtained from the line-narrowing and the relaxation data, respectively Sample n ,0.3 0.3 1.2
Linewidth data
Relaxation data
T trans (K)
EA (eV)
(T 21 1 )
21065 18565 17065
0.4060.05 0.3560.05 0.1860.05
330610 245610 230610
max
(K)
E 9A (eV)
to (s)
0.3060.03 0.2160.05 0.1460.05
1.6 10 214 1.5 10 213 7.0 10 212
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
246
Fig. 3. Temperature dependence of the 1 H NMR spin–lattice relaxation rate (T 21 1 ) in HPb 2 Nb 3 O 10 ?nH 2 O (a) measurements made at 36 MHz for samples n51.2 (s), 0.3 (m) and ,0.3 (h) and (b) measurements of T 21 as a function of Larmor frequency, 1 36 MHz (m), 45 MHz (d) and 60 MHz (j) for the sample n50.3. The solid lines are fits to Eq. (1).
tion of the inter-proton dipolar interactions. Measurements carried out at different Larmor frequencies show that the relaxation rate is frequency dependent in the high-temperature side of the T 21 maximum 1 (Fig. 3b). Accordingly, data were fitted with the relaxation expression for two-dimensional motions, which is consistent with the structural feature of the layered perovskites compound studied here [16,20]:
FS
D S
1 1 1 ] ~ t ln 1 1 ]] 1 4ln 1 1 ]] 2 2 T1 v ot 4v 2ot 2
DG
(1)
where vo is the Larmor frequency and t is the
correlation time, expressed by an Arrhenius function [t 5to exp(EA /kT )]. In contrast to three-dimensional diffusion, where in the extreme narrowing limit vot <1, T 121 is independent of vo , a logarithmic frequency dependence is predicted from Eq. (1) for a two-dimensional motion. Indeed, the fit of the data of Fig. 3b indicates that the molecular motion has a pronounced two-dimensional character. The parameters obtained from the fit are listed in Table 1. The single line-narrowing transition observed in the 1 H NMR spectrum, in the temperature range 90–350 K, and the single spin–lattice relaxation rate maximum observed in Fig. 3a, are consistent with a Grotthuss-like mechanism for the proton diffusion. This process consist of a succession of molecular reorientations of H 3 O 1 ions or H 2 O molecules combined with proton jumps on a hydrogen-bond network [17–19]. The parameters obtained from the relaxation data allows us to estimate the proton diffusion coefficient by means of a two-dimensional random walk expression, D 5 l 2 / 4t. Here l is the minimum distance of approach, which may be simply calculated from the ˚ [16,19]. The temperaaverage H–H distance (1.7 A) ture dependence of the proton diffusion coefficient has been measured in some hydrated layered compounds by the pulse field gradient NMR method. In Cs 5 H 3 (SO 4 ) 4 ?H 2 O, for example, D (cm 2 / s)5(2.93 10 24 ) exp(20.33 /kT ) [15] and in CsOH?H 2 O, D (cm 2 / s)5(1.2310 25 ) exp(20.15 /kT ) [21]. For our sample, HPb 2 Nb 3 O 10 ?1.2H 2 O, the diffusion coefficient estimated with the parameters obtained from the relaxation data, Ea 5 0.14 eV and to ¯7310 212 s, yields D¯5310 28 cm 2 / s at room temperature. Taking into account that conductivity and other motional parameters are strongly dependent on water content [13,19], we may consider our results consistent to those measured by the pulse field gradient NMR technique mentioned above.
4. Conclusions The temperature dependencies for conductivity, 1 H NMR linewidth and spin–lattice relaxation time of the ion-exchanged layered lead-niobate perovskite HPb 2 Nb 3 O 10 ?nH 2 O have been investigated over the temperature range 90–350 K. Rigid-lattice spectra
C.E. Tambelli et al. / Solid State Ionics 136 – 137 (2000) 243 – 247
exhibit a narrow line of lorentzian shape, attributed to the superposition of the 1 H signals belonging to the OH 2 and to the central component of the H 3 O 1 spectrum, flanked by a pair of gaussian peaks separated by 11 Gauss, which were attributed to paired protons in H 2 O. Activation energies, in the range of 0.14–0.40 eV, were obtained from the NMR line narrowing data. The single spin–lattice relaxation rate maximum and line-narrowing transition, observed in the NMR results, are consistent with a Grotthuss-like mechanism for the proton diffusion. The conductivity and NMR experimental data indicate that HPb 2 Nb 3 O 10 ?nH 2 O is a fairly good protonic conductor.
Acknowledgements This research is supported by Fapesp, CNPq and Finep Brazilian sponsoring agencies.
References [1] M.A. Sbramanian, J. Gopalakrishman, A.W. Sleight, Mat. Res. Bull. 23 (1988) 837. [2] M. Sato, Y. Kono, T. Jin, J. Ceram. Soc. Jap. 101 (1993) 954. [3] M. Sato, J. Abo, T. Jin, Solid State Ionics 57 (1992) 285. [4] J. Gopalakrishman, V. Bhat, Mat. Res. Bull. 22 (1987) 413.
247
[5] P. Batamack, R. Vincent, J. Fraissard, Catal. Today 28 (1996) 31. [6] C.E. Rice, J. Solid State Chem. 64 (1986) 188. [7] J. Yoshimura, Y. Ebina, J. Kondo, K. Domen, A. Tanaka, J. Phys. Chem. 97 (1993) 1970. [8] K. Toda, Y. Kameo, M. Fijimoto, M. Sato, J. Ceram. Soc. Jap. 102 (1994) 735. [9] P. Colomban (Ed.), Proton Conductors: Solids, Membranes and Gels–Materials and Devices, Cambridge University Press, Cambridge, 1992. [10] M. Sato, T. Jin, K. Uematsu, J. Solid State Chem. 102 (1993) 557. [11] J. Livage, Solid State Ionics 50 (1992) 307. [12] V. Bondarenka, S. Grebinsky, S. Mickevicius, V. Volkov, G. Zacharova, Sensors and Actuators B 28 (1995) 227. ˆ [13] M.J. Saeki, E. Tonhy, A.C.D. Angelo, A.O. Florentino, C.E. Tambelli, J.P. Donoso, C.J. Magon. Solid State Ionics (submitted for publication). [14] M. Lal, A.T. Howe, J. Solid State Chem. 51 (1984) 355. [15] A.M. Fajdiga-Bulat, G. Lahajnar, J. Dolinsek, J. Slak, B. Lohar, B. Zalar, L.A. Shuvalov, R. Blinc, Solid State Ionics 77 (1995) 101. [16] G. Mangamma, V. Bhat, J. Gopalakrishnan, S.V. Bhat, Solid State Ionics 58 (1992) 303. [17] M. Durand-Le Floch, J. Pannetier, C. Doremieux-Morin, H. Arribart, J. Chem. Phys. 84 (1986) 4760. [18] H. Arribart, Y. Piffard, Solid State Communication 45 (1983) 571. [19] N. Binesh, V. Bhat, S.V. Bhat, Solid State Ionics 86–88 (1996) 609. [20] H. Arribart, Y. Piffard, C. Doremieux-Morin, Solid State Ionics 7 (1982) 91. [21] J. Gallier, B. Toudic, M. Stahn, R.E. Lechner, H. Dachs, J. Physique 49 (1988) 949.