- Email: [email protected]
High temperature proton NMR study of yttrium doped barium cerates
Solid State Communications 130 (2004) 73–77 www.elsevier.com/locate/ssc
High temperature proton NMR study of yttrium doped barium cerates Hideki Maekawaa,b,c,*, Yoshitaka Ukeia,1, Kai Morotad, Naohito Kashiid, Junichi Kawamurad,2, Tsutomu Yamamuraa a
Department of Metallurgy, Graduate School of Engineering, Tohoku University, Aramaki 02, Aoba, Aoba-ku, Sendai 980-8579, Japan b Center for Interdisciplinary Research, Tohoku University, 980-8578, Japan c PRESTO, Japan Science and Technology Corporation, Kawaguchi 3320012, Japan d Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 9 July 2003; received in revised form 30 November 2003; accepted 8 January 2004 by S. Ushioda
Abstract The mobility of protons in BaCe12xYxO32d perovskite (x ¼ 0:01 to 0.10) was investigated by high temperature 1H NMR spin-lattice relaxation time ðT1 Þ and ac conductivity measurements. The temperature variation of T1 was obtained from room temperature to 1073 K. The absolute magnitude of T1 shows a complex dependence on doping concentration. However, the shape of the temperature dependence of T1 was independent of the doping concentration, suggesting the absence of major differences of the proton hopping mechanism on doping level of this material. The measured conductivity was well reproduced by a simple hopping model using correlation times of proton migration and proton concentrations estimated from the NMR measurement. q 2004 Published by Elsevier Ltd. PACS: 81 Keywords: A. Nanostructures; B. Chemical synthesis; D. Electronic transport; E. Neutron scattering; E. Nuclear resonance
1. Introduction High temperature solid-state proton conductors have been developed in a wide variety of oxides [1 – 5]. Their properties and possible applications are reviewed [6 –8]. Among the proton conducting oxides, alkaline earth cerates have attracted the most attention. The fundamental scientific research of this material is increasing its importance in * Corresponding author. Address: Department of Metallurgy, Graduate School of Engineering, Tohoku University, Aramaki 02, Aoba, Aoba-ku, Sendai 980-8579, Japan. Tel.: þ81-22-217-7311; fax: þ 81-22-217-7310. E-mail address: [email protected] (H. Maekawa). 1 Present address: Chuo Spring Co., Ltd, 68 Kamishiota, Narumicho, Midori-ku, Nagoya 458-8505, Japan. 2 Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577, Japan. 0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ssc.2004.01.005
relation to its mechanism of proton conduction. The electrochemical properties [9,10], isotope effects on conductivity [11,12], 18O tracer diffusion [13,14] and thermogravimetric properties [15,16] have been investigated in relation to the proton diffusivity and the dissolution rates of protons from the atmosphere. The spectroscopic techniques and theoretical calculations have been employed for investigations of proton hopping mechanism in this material, such as quasielastic neutron scattering [17 – 21], optical spectra of rare earth metal ions [22 – 24] and quantum mechanical calculations [25 – 29]. However, direct observation technique for proton dynamics of this material has been quite limited. High temperature 1H NMR can provide an opportunity for straightforward observation of the proton migration of the material. In the previous paper, we have investigated the proton diffusion in SrCe0.95Y0.05 O32d by 1H NMR method [30]. The introduction of dopant ions such as yttrium into
74
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
barium cerates was proposed to cause negatively charged defects that act as traps for protons, suggested from the neutron diffraction observation by Hempelman et al. [17]. However, thermogravimetric measurements by Yajima et al. [31,32] and pulsed field gradient NMR measurements by Kreuer et al. [14] showed no dopant concentration dependence on the diffusivities of protons. These authors concluded that there is no indication of a significant interaction with the acceptor dopant, as opposed to the neutron diffraction. In the present investigation, a high resolution NMR was applied to various doped barium cerates to evaluate the effect of dopant concentration on proton migration by 1H NMR relaxation time measurements at temperatures up to 1073 K. 2. Experimental Perovskite type oxide BaCe12xYxO32d with x ¼ 0:01; 0.03, 0.05 and 0.10, were synthesized from 99.99% purity BaCO3, CeO2 and Y2O3. Calculated amount of powders were mixed with motor and pressed to a pellet at 1000 kg/cm2. Calcination of the pellet was performed at 1573 K for 15 h in the air. The sample was crushed to a powder, and pressed again to a pellet. Additional calcination was made at 1573 K for 24 h. Protons are introduced into the ceramics from the atmosphere. Samples for the hightemperature NMR experiment and conductivity measurement have a cylindrical form with 10 mm diameter, 8 mm thick. High temperature NMR measurements were made with Bruker MSL-200 spectrometer with operating frequency of 200.13 MHz. A newly assembled home built probe was utilized for the high temperature NMR measurements from room temperature to 1073 K. Spin-lattice relaxation times were obtained by inversion-recovery pulse sequence ð1808 – t – 908Þ: The dissolved proton concentration of samples after high temperature NMR measurements was determined from 1H NMR peak intensities at room temperature. Bruker MSL-300 NMR spectrometer with operating frequency of 300.13 MHz was utilized for room temperature 1H NMR measurements. X-ray diffraction measurements (Cu Ka) were made using Rigaku RINT-2000 diffractometer. Complex impedance spectra were measured by Solartron SI 1260 impedance analyzer from room temperature to 1073 K. The atmosphere of the samples was kept at the saturated vapor pressure of water at 6.6 8C, corresponding to 103 Pa. The frequency range was from 10 MHz to 1 Hz and the ac excitation voltage 1 V. The sample after high temperature NMR measurements was used for the impedance measurement.
single phase. The lattice parameters are under Vegard’s law until 5% yttrium doping. A small deviation from the law was observed for 10% doped sample. The proton concentration of BaCe12xYxO32d samples after high temperature NMR measurements was determined from 1H NMR absolute intensity at room temperature. Adamantane was utilized as an external reference of the proton concentration. The result was summarized in Table 1. The proton concentration was almost proportional to the dopant concentration except for the 10 mol% yttrium sample. 3.2. High temperature NMR line shapes High temperature 1H NMR line shape and relaxation time measurements were made for BaCe12xYxO3 samples with x ¼ 0:01; 0.03, 0.05 and 0.10. Fig. 1 represents temperature dependence of the 1H NMR spectra for x ¼ 0:05 sample. The spectrum at room temperature is consisted of a single slightly distorted broad line. At elevated temperature, the line width decreased with an increasing temperature, consistent with an increase of the proton mobility with temperature. More than a single line was observed above 640 K, suggesting an existence of protons in different chemical environments in the sample. However, samples other than x ¼ 0:05 did not show vigorous evidence of existing these plural peaks. The reason for these plural peaks is not clear at the present stage of the investigation. 3.3. Relaxation times 1 H NMR spin-lattice relaxation times ðT1 sÞ were measured by an inversion-recovery method for samples with x ¼ 0:01; 0.03, 0.05 and 0.10. Because of a limited quality of the spectrum, we do not perform a peak separation to determine the T1 values separately for each of the components. The whole integral of the spectrum was used for the determination of averaged T1 values. The T1 values for x ¼ 0:01; 0.03, 0.05 and 0.10 samples are shown in Fig. 2(a). The absolute value is scattered more than one order of magnitude. The absolute value of T1 is proportional to the magnitude of the interaction of which relaxation of the 1H magnetization was produced. The major relaxation mechanism expected for this system include proton– proton dipolar interaction and proton – electron spin interaction. The present result suggests the magnitude of the
Table 1 Proton concentration of BaCe12xYxO32d determined from room temperature 1H NMR absolute intensities Composition, x in BaCe12xYxO32d
Proton concentration/mol%
0.01 0.03 0.05 0.10
2.0 3.3 6.3 7.4
3. Results and discussion 3.1. Proton concentrations X-ray diffraction confirmed all samples investigated are
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
75
Fig. 1. In situ high temperature 1H NMR spectra for BaCe0.95Y0.05 O32d.
interaction was not simply proportional to the amount of protons in the sample. The interaction with electron spins in Ce3þ may be responsible as a major relaxation mechanism of this system. For the aim of comparison of T1 curves with different doped samples, translation of each T1 data was made so that the absolute value at T1 minimum to coincide. In Fig. 2(b), data in Fig. 2(a) is moved in the transversal axis to minimize the difference with reflect to the x ¼ 0:05 data by multiplying constants to the data for x ¼ 0:01; x ¼ 0:03 and x ¼ 0:10 samples. The T1 curves almost collapse into one line independent of the doping ion concentration except for the higher temperature side of the T1 minimum. The unusual temperature dependence at high temperature may be relating to thermodynamic phase transitions of BaCeO3 and its complex [33]. There are some reports on the existence of structural phase transitions in BaCeO3 [33,34]. Knight reported the detailed analysis of the temperature dependence of the crystal structure of BaCeO3 and concluded there are three structural phase transitions in the temperature range from 373 to 1273 K. The room temperature orthorhombic Pmcn phase (I) is transferred to another orthorhombic Incn phase (II) at 290 8C; this phase transition is reported to be second order and the structural change is rather small. There are no significant changes in T1 at the corresponding temperature; the separation of the two NMR peaks mentioned above becomes clear near the temperature. The orthorhombic Incn phase (II) changes to rhombohedral F3 – 2=n at 673 K and rhombohedral to cubic
Fig. 2. (a) Temperature dependence of 1H NMR spin-lattice relaxation time ðT1 Þ for BaCe12xYxO32d. (A) x ¼ 0:10; (X) x ¼ 0:05; (W) x ¼ 0:03; and (K) x ¼ 0:01; respectively. (b) Temperature dependence of 1H NMR spin-lattice relaxation time ðT1 Þ for BaCe12xYxO32d (BCY) and SrCe0.95Y0.05O32d. Data in (a) for BCY is moved in the transversal axis to minimize the difference with reflect to the x ¼ 0:05 data by multiplying constants to the data for x ¼ 0:01; x ¼ 0:03 and x ¼ 0:10 samples. (A) x ¼ 0:10; (X) x ¼ 0:05; (W) x ¼ 0:03; and (K) x ¼ 0:01; respectively. Solid line is a fitting to the data for x ¼ 0:05 using Eq. (1). (B) 1H NMR T1 for SrCe0.95Y0.05O32d [30]. Solid line is a fitting to the data using Eq. (1).
Pm – 3m at 1173 K. The phase transition from orthorhombic Incn to rhombohedral phase at 637 K is close to the temperature where the T1 curvature deviates from Arrhenius law at high temperature. The doping of yttrium to BaCeO3 may modify these phase transition temperatures. However, there is not enough information on the composition dependencies to discuss the effect in detail here. 3.4. Analysis of T1 data The present T1 data has been analyzed by introducing the distribution of activation energy for Arrhenius temperature dependence of the hopping frequency for protons, as have
76
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
been applied in the previous paper [30], based on the theoretical treatments in the case of the asymmetric 1=T versus T1 plot frequently observed for the solid state ionic conductors. The distribution of the activation energies Ea leads the ions to hop with different rates to the nearest empty wells, and give a distribution of T1 : The measured T1 will be an average, which has been defined by Svare et al. [35] as, ð1 1 tc 4tc ¼ þ ð1Þ ZNMR dEa ; 2 2 T1 1 þ 442 t2c 21 1 þ 4 tc where v is a frequency of NMR spectrometer, tc is a correlation time of ionic motion of protons and C is a constant which is related to the magnitude of the dipole – dipole interactions. Here, we have assumed the number of the next nearest hopping sites to be 6. ZNMR represents a Gaussian distribution of Ea ; " # 1 ðEm 2 Ea Þ2 exp 2 ; ð2Þ ZNMR ðEa Þ ¼ ð2pÞ1=2 Eb 2Eb2 where Ea is an activation energy, Em is the center of the distribution of Ea ; and Eb is the width of the distribution of Ea . The central value of the activation energy ðEm Þ was set to coincide to that obtained from conductivity data (2 52 kJ/ mol). The result of fitting of Eq. (1) for x ¼ 0:05 sample are shown as a solid curve in Fig. 2(b). The experimental and fitted curves for previous SrCe0.95Y0.05O3 sample [30] are also shown as a comparison. The distribution Eb is obtained as 6.5 kJ/mol, which is larger than that of SrCe0.95Y0.05O3 sample. When we treat the data with the approach proposed by Ngai and Kanart [36] using a KWW correlation function, the coupling parameter bð¼ 1 2 nÞ is evaluated about 0.70 from the present data. Again this b value is larger in comparison with the previous SrCe0.95Y0.05O3 sample, where b value is obtained as 0.83 [30]. The larger value of b; corresponding to stronger many body interaction or broader distribution, is probably due to the higher concentration of protons in this system. 3.5. Comparison of NMR and impedance results A comparison of the NMR correlation time and the dc conductivity was made. According to the simple jump model without considering correlation effect, the dc conductivity can be calculated from proton concentration n; its jumping distance d; and its average jump rate n; as,
s¼
ne2 d 2 n : 6kB T
ð3Þ
The magnitude of the proton jump rate n can be estimated to be equal to the inverse of the average correlation time tAV from T1 measurements. tAV could be calculated as, ð1 tAV ðTÞ ¼ tc ZNMR dEa : ð4Þ 0
Fig. 3. Measured dc conductivities for BaCe12xYxO32d (BCY) and SrCe0.95Y0.05O32d (SCY) [30]. (A) BCY x ¼ 0:10; (X) BCY x ¼ 0:05; (W) BCY x ¼ 0:03; (K) BCY x ¼ 0:01 and (B) SCY. Dashed and dotted lines are calculated conductivities for BCY using Eq. (3) with the fitting to 1H NMR T1 data as shown in Fig. 2(b), for the proton concentrations of 7.4 mol% (corresponding to x ¼ 0:10) and 2.0 mol% (corresponding to x ¼ 0:01), respectively. Solid line is a previous result for SCY sample calculated from 1H NMR T1 using Eq. (3) [30].
In Fig. 3, the calculated proton conductivity for both of the x ¼ 0:10 and x ¼ 0:01; using the measured proton concentration (Table 1) of 7.4 mol% and 2.0 mol%, respectively, are plotted. The proton jump distance was assumed as 0.1 nm [30]. The calculated conductivity reproduced well the measured conductivity in a wide temperature range. This behavior had been observed for the previous measurement for SrCe0.95Y0.05O3 as also shown in the figure. The present result suggests that the doping concentration dependence of barium cerates is not large within the experimental uncertainty of the 1H T1 measurement. The conductivity can be understood with a single proton migration process for different level of doping, thus at different proton concentrations.
Acknowledgements This work was supported under Grant-in Aid for Scientific Research No. 13750681 from the Ministry of Education, Science, Sports, and Culture. Supports from ‘Research for the Future’ program from the Japan Society for the Promotion of Science: ‘Preparation and Application of Newly Designed Solid Electrolytes’ JSPS-RFTF 96P00102 and Precursory Research for Embryonic Science and Technology of Japan Science and Technology Corporation is also acknowledged.
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
References [1] K. Amezawa, S. Kjelstrup, T. Norby, Y. Ito, J. Electrochem. Soc. 145 (1998) 3313. [2] T. Fukui, S. Ohara, S. Kawatsu, J. Power Sources 71 (1998) 164. [3] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [4] A.S. Nowick, Y. Du, Solid State Ionics 77 (1995) 137. [5] T. Omata, K. Okuda, S. Tsugimoto, S. Otsuka-Matsuo-Yao, Solid State Ionics 104 (1997) 249. [6] H. Iwahara, Solid State Ionics 86-8 (1996) 9. [7] K.D. Kreuer, Solid State Ionics 97 (1997) 1. [8] T. Norby, Solid State Ionics 125 (1999) 1. [9] U. Reichel, R.R. Arons, W. Schilling, Solid State Ionics 86-8 (1996) 639. [10] T. Schober, J. Friedrich, J.B. Condon, Solid State Ionics 77 (1995) 175. [11] K.D. Kreuer, A. Fuchs, J. Maier, Solid State Ionics 77 (1995) 157. [12] A.S. Nowick, A.V. Vaysleyb, Solid State Ionics 97 (1997) 17. [13] R.A. De Souza, J.A. Kilner, C. Jeynes, Solid State Ionics 97 (1997) 409. [14] K.D. Kreuer, T. Dippel, Y.M. Baikov, J. Maier, Solid State Ionics 86-8 (1996) 613. [15] T. Hibino, K. Mizutani, T. Yajima, H. Iwahara, Solid State Ionics 57 (1992) 303. [16] J. Muller, K.D. Kreuer, J. Maier, S. Matsuo, M. Ishigame, Solid State Ionics 97 (1997) 421. [17] R. Hempelmann, C. Karmonik, T. Matzke, M. Cappadonia, U. Stimming, T. Springer, M.A. Adams, Solid State Ionics 77 (1995) 152.
77
[18] R. Hempelmann, Physica B 226 (1996) 72. [19] T. Matzke, U. Stimming, C. Karmonik, M. Soetratmo, R. Hempelmann, F. Guthoff, Solid State Ionics 86-8 (1996) 621. [20] N. Sata, K. Hiramoto, M. Ishigame, S. Hosoya, N. Niimura, S. Shin, Phys. Rev. B—Condens. Matter 54 (1996) 15795. [21] K.S. Knight, Solid State Ionics 127 (2000) 43. [22] H. Huck, P. Ehrhart, W. Schilling, J. Eur. Ceram. Soc. 19 (1999) 939. [23] N. Sata, M. Ishigame, S. Shin, Solid State Ionics 86-8 (1996) 629. [24] H. Yugami, Y. Chiba, M. Ishigame, Solid State Ionics 77 (1995) 201. [25] M. Cherry, M.S. Islam, J.D. Gale, C.R.A. Catlow, Solid State Ionics 77 (1995) 207. [26] R. Glockner, M.S. Islam, T. Norby, Solid State Ionics 122 (1999) 145. [27] E. Matsushita, A. Tanase, Solid State Ionics 97 (1997) 45. [28] W. Munch, G. Seifert, K.D. Kreuer, J. Maier, Solid State Ionics 86-8 (1996) 647. [29] F. Shimojo, K. Hoshino, H. Okazaki, J. Phys. Soc. Jpn 65 (1996) 1143. [30] H. Maekawa, N. Kashii, J.I. Kawamura, Y. Hinatsu, T. Yamamura, Solid State Ionics 122 (1999) 231. [31] T. Yajima, H. Iwahara, Solid State Ionics 53-6 (1992) 983. [32] T. Yajima, H. Iwahara, Solid State Ionics 50 (1992) 281. [33] T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State Ionics 61 (1993) 93. [34] K.S. Knight, Solid State Ionics 74 (1994) 109. [35] I. Svare, F. Borsa, D.R. Torgeson, S.W. Martin, Phys. Rev. B—Condens. Matter 48 (1993) 9336. [36] K.L. Ngai, O. Kanert, Solid State Ionics 53-6 (1992) 936.
High temperature proton NMR study of yttrium doped barium cerates Hideki Maekawaa,b,c,*, Yoshitaka Ukeia,1, Kai Morotad, Naohito Kashiid, Junichi Kawamurad,2, Tsutomu Yamamuraa a
Department of Metallurgy, Graduate School of Engineering, Tohoku University, Aramaki 02, Aoba, Aoba-ku, Sendai 980-8579, Japan b Center for Interdisciplinary Research, Tohoku University, 980-8578, Japan c PRESTO, Japan Science and Technology Corporation, Kawaguchi 3320012, Japan d Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 9 July 2003; received in revised form 30 November 2003; accepted 8 January 2004 by S. Ushioda
Abstract The mobility of protons in BaCe12xYxO32d perovskite (x ¼ 0:01 to 0.10) was investigated by high temperature 1H NMR spin-lattice relaxation time ðT1 Þ and ac conductivity measurements. The temperature variation of T1 was obtained from room temperature to 1073 K. The absolute magnitude of T1 shows a complex dependence on doping concentration. However, the shape of the temperature dependence of T1 was independent of the doping concentration, suggesting the absence of major differences of the proton hopping mechanism on doping level of this material. The measured conductivity was well reproduced by a simple hopping model using correlation times of proton migration and proton concentrations estimated from the NMR measurement. q 2004 Published by Elsevier Ltd. PACS: 81 Keywords: A. Nanostructures; B. Chemical synthesis; D. Electronic transport; E. Neutron scattering; E. Nuclear resonance
1. Introduction High temperature solid-state proton conductors have been developed in a wide variety of oxides [1 – 5]. Their properties and possible applications are reviewed [6 –8]. Among the proton conducting oxides, alkaline earth cerates have attracted the most attention. The fundamental scientific research of this material is increasing its importance in * Corresponding author. Address: Department of Metallurgy, Graduate School of Engineering, Tohoku University, Aramaki 02, Aoba, Aoba-ku, Sendai 980-8579, Japan. Tel.: þ81-22-217-7311; fax: þ 81-22-217-7310. E-mail address: [email protected] (H. Maekawa). 1 Present address: Chuo Spring Co., Ltd, 68 Kamishiota, Narumicho, Midori-ku, Nagoya 458-8505, Japan. 2 Present address: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577, Japan. 0038-1098/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ssc.2004.01.005
relation to its mechanism of proton conduction. The electrochemical properties [9,10], isotope effects on conductivity [11,12], 18O tracer diffusion [13,14] and thermogravimetric properties [15,16] have been investigated in relation to the proton diffusivity and the dissolution rates of protons from the atmosphere. The spectroscopic techniques and theoretical calculations have been employed for investigations of proton hopping mechanism in this material, such as quasielastic neutron scattering [17 – 21], optical spectra of rare earth metal ions [22 – 24] and quantum mechanical calculations [25 – 29]. However, direct observation technique for proton dynamics of this material has been quite limited. High temperature 1H NMR can provide an opportunity for straightforward observation of the proton migration of the material. In the previous paper, we have investigated the proton diffusion in SrCe0.95Y0.05 O32d by 1H NMR method [30]. The introduction of dopant ions such as yttrium into
74
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
barium cerates was proposed to cause negatively charged defects that act as traps for protons, suggested from the neutron diffraction observation by Hempelman et al. [17]. However, thermogravimetric measurements by Yajima et al. [31,32] and pulsed field gradient NMR measurements by Kreuer et al. [14] showed no dopant concentration dependence on the diffusivities of protons. These authors concluded that there is no indication of a significant interaction with the acceptor dopant, as opposed to the neutron diffraction. In the present investigation, a high resolution NMR was applied to various doped barium cerates to evaluate the effect of dopant concentration on proton migration by 1H NMR relaxation time measurements at temperatures up to 1073 K. 2. Experimental Perovskite type oxide BaCe12xYxO32d with x ¼ 0:01; 0.03, 0.05 and 0.10, were synthesized from 99.99% purity BaCO3, CeO2 and Y2O3. Calculated amount of powders were mixed with motor and pressed to a pellet at 1000 kg/cm2. Calcination of the pellet was performed at 1573 K for 15 h in the air. The sample was crushed to a powder, and pressed again to a pellet. Additional calcination was made at 1573 K for 24 h. Protons are introduced into the ceramics from the atmosphere. Samples for the hightemperature NMR experiment and conductivity measurement have a cylindrical form with 10 mm diameter, 8 mm thick. High temperature NMR measurements were made with Bruker MSL-200 spectrometer with operating frequency of 200.13 MHz. A newly assembled home built probe was utilized for the high temperature NMR measurements from room temperature to 1073 K. Spin-lattice relaxation times were obtained by inversion-recovery pulse sequence ð1808 – t – 908Þ: The dissolved proton concentration of samples after high temperature NMR measurements was determined from 1H NMR peak intensities at room temperature. Bruker MSL-300 NMR spectrometer with operating frequency of 300.13 MHz was utilized for room temperature 1H NMR measurements. X-ray diffraction measurements (Cu Ka) were made using Rigaku RINT-2000 diffractometer. Complex impedance spectra were measured by Solartron SI 1260 impedance analyzer from room temperature to 1073 K. The atmosphere of the samples was kept at the saturated vapor pressure of water at 6.6 8C, corresponding to 103 Pa. The frequency range was from 10 MHz to 1 Hz and the ac excitation voltage 1 V. The sample after high temperature NMR measurements was used for the impedance measurement.
single phase. The lattice parameters are under Vegard’s law until 5% yttrium doping. A small deviation from the law was observed for 10% doped sample. The proton concentration of BaCe12xYxO32d samples after high temperature NMR measurements was determined from 1H NMR absolute intensity at room temperature. Adamantane was utilized as an external reference of the proton concentration. The result was summarized in Table 1. The proton concentration was almost proportional to the dopant concentration except for the 10 mol% yttrium sample. 3.2. High temperature NMR line shapes High temperature 1H NMR line shape and relaxation time measurements were made for BaCe12xYxO3 samples with x ¼ 0:01; 0.03, 0.05 and 0.10. Fig. 1 represents temperature dependence of the 1H NMR spectra for x ¼ 0:05 sample. The spectrum at room temperature is consisted of a single slightly distorted broad line. At elevated temperature, the line width decreased with an increasing temperature, consistent with an increase of the proton mobility with temperature. More than a single line was observed above 640 K, suggesting an existence of protons in different chemical environments in the sample. However, samples other than x ¼ 0:05 did not show vigorous evidence of existing these plural peaks. The reason for these plural peaks is not clear at the present stage of the investigation. 3.3. Relaxation times 1 H NMR spin-lattice relaxation times ðT1 sÞ were measured by an inversion-recovery method for samples with x ¼ 0:01; 0.03, 0.05 and 0.10. Because of a limited quality of the spectrum, we do not perform a peak separation to determine the T1 values separately for each of the components. The whole integral of the spectrum was used for the determination of averaged T1 values. The T1 values for x ¼ 0:01; 0.03, 0.05 and 0.10 samples are shown in Fig. 2(a). The absolute value is scattered more than one order of magnitude. The absolute value of T1 is proportional to the magnitude of the interaction of which relaxation of the 1H magnetization was produced. The major relaxation mechanism expected for this system include proton– proton dipolar interaction and proton – electron spin interaction. The present result suggests the magnitude of the
Table 1 Proton concentration of BaCe12xYxO32d determined from room temperature 1H NMR absolute intensities Composition, x in BaCe12xYxO32d
Proton concentration/mol%
0.01 0.03 0.05 0.10
2.0 3.3 6.3 7.4
3. Results and discussion 3.1. Proton concentrations X-ray diffraction confirmed all samples investigated are
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
75
Fig. 1. In situ high temperature 1H NMR spectra for BaCe0.95Y0.05 O32d.
interaction was not simply proportional to the amount of protons in the sample. The interaction with electron spins in Ce3þ may be responsible as a major relaxation mechanism of this system. For the aim of comparison of T1 curves with different doped samples, translation of each T1 data was made so that the absolute value at T1 minimum to coincide. In Fig. 2(b), data in Fig. 2(a) is moved in the transversal axis to minimize the difference with reflect to the x ¼ 0:05 data by multiplying constants to the data for x ¼ 0:01; x ¼ 0:03 and x ¼ 0:10 samples. The T1 curves almost collapse into one line independent of the doping ion concentration except for the higher temperature side of the T1 minimum. The unusual temperature dependence at high temperature may be relating to thermodynamic phase transitions of BaCeO3 and its complex [33]. There are some reports on the existence of structural phase transitions in BaCeO3 [33,34]. Knight reported the detailed analysis of the temperature dependence of the crystal structure of BaCeO3 and concluded there are three structural phase transitions in the temperature range from 373 to 1273 K. The room temperature orthorhombic Pmcn phase (I) is transferred to another orthorhombic Incn phase (II) at 290 8C; this phase transition is reported to be second order and the structural change is rather small. There are no significant changes in T1 at the corresponding temperature; the separation of the two NMR peaks mentioned above becomes clear near the temperature. The orthorhombic Incn phase (II) changes to rhombohedral F3 – 2=n at 673 K and rhombohedral to cubic
Fig. 2. (a) Temperature dependence of 1H NMR spin-lattice relaxation time ðT1 Þ for BaCe12xYxO32d. (A) x ¼ 0:10; (X) x ¼ 0:05; (W) x ¼ 0:03; and (K) x ¼ 0:01; respectively. (b) Temperature dependence of 1H NMR spin-lattice relaxation time ðT1 Þ for BaCe12xYxO32d (BCY) and SrCe0.95Y0.05O32d. Data in (a) for BCY is moved in the transversal axis to minimize the difference with reflect to the x ¼ 0:05 data by multiplying constants to the data for x ¼ 0:01; x ¼ 0:03 and x ¼ 0:10 samples. (A) x ¼ 0:10; (X) x ¼ 0:05; (W) x ¼ 0:03; and (K) x ¼ 0:01; respectively. Solid line is a fitting to the data for x ¼ 0:05 using Eq. (1). (B) 1H NMR T1 for SrCe0.95Y0.05O32d [30]. Solid line is a fitting to the data using Eq. (1).
Pm – 3m at 1173 K. The phase transition from orthorhombic Incn to rhombohedral phase at 637 K is close to the temperature where the T1 curvature deviates from Arrhenius law at high temperature. The doping of yttrium to BaCeO3 may modify these phase transition temperatures. However, there is not enough information on the composition dependencies to discuss the effect in detail here. 3.4. Analysis of T1 data The present T1 data has been analyzed by introducing the distribution of activation energy for Arrhenius temperature dependence of the hopping frequency for protons, as have
76
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
been applied in the previous paper [30], based on the theoretical treatments in the case of the asymmetric 1=T versus T1 plot frequently observed for the solid state ionic conductors. The distribution of the activation energies Ea leads the ions to hop with different rates to the nearest empty wells, and give a distribution of T1 : The measured T1 will be an average, which has been defined by Svare et al. [35] as, ð1 1 tc 4tc ¼ þ ð1Þ ZNMR dEa ; 2 2 T1 1 þ 442 t2c 21 1 þ 4 tc where v is a frequency of NMR spectrometer, tc is a correlation time of ionic motion of protons and C is a constant which is related to the magnitude of the dipole – dipole interactions. Here, we have assumed the number of the next nearest hopping sites to be 6. ZNMR represents a Gaussian distribution of Ea ; " # 1 ðEm 2 Ea Þ2 exp 2 ; ð2Þ ZNMR ðEa Þ ¼ ð2pÞ1=2 Eb 2Eb2 where Ea is an activation energy, Em is the center of the distribution of Ea ; and Eb is the width of the distribution of Ea . The central value of the activation energy ðEm Þ was set to coincide to that obtained from conductivity data (2 52 kJ/ mol). The result of fitting of Eq. (1) for x ¼ 0:05 sample are shown as a solid curve in Fig. 2(b). The experimental and fitted curves for previous SrCe0.95Y0.05O3 sample [30] are also shown as a comparison. The distribution Eb is obtained as 6.5 kJ/mol, which is larger than that of SrCe0.95Y0.05O3 sample. When we treat the data with the approach proposed by Ngai and Kanart [36] using a KWW correlation function, the coupling parameter bð¼ 1 2 nÞ is evaluated about 0.70 from the present data. Again this b value is larger in comparison with the previous SrCe0.95Y0.05O3 sample, where b value is obtained as 0.83 [30]. The larger value of b; corresponding to stronger many body interaction or broader distribution, is probably due to the higher concentration of protons in this system. 3.5. Comparison of NMR and impedance results A comparison of the NMR correlation time and the dc conductivity was made. According to the simple jump model without considering correlation effect, the dc conductivity can be calculated from proton concentration n; its jumping distance d; and its average jump rate n; as,
s¼
ne2 d 2 n : 6kB T
ð3Þ
The magnitude of the proton jump rate n can be estimated to be equal to the inverse of the average correlation time tAV from T1 measurements. tAV could be calculated as, ð1 tAV ðTÞ ¼ tc ZNMR dEa : ð4Þ 0
Fig. 3. Measured dc conductivities for BaCe12xYxO32d (BCY) and SrCe0.95Y0.05O32d (SCY) [30]. (A) BCY x ¼ 0:10; (X) BCY x ¼ 0:05; (W) BCY x ¼ 0:03; (K) BCY x ¼ 0:01 and (B) SCY. Dashed and dotted lines are calculated conductivities for BCY using Eq. (3) with the fitting to 1H NMR T1 data as shown in Fig. 2(b), for the proton concentrations of 7.4 mol% (corresponding to x ¼ 0:10) and 2.0 mol% (corresponding to x ¼ 0:01), respectively. Solid line is a previous result for SCY sample calculated from 1H NMR T1 using Eq. (3) [30].
In Fig. 3, the calculated proton conductivity for both of the x ¼ 0:10 and x ¼ 0:01; using the measured proton concentration (Table 1) of 7.4 mol% and 2.0 mol%, respectively, are plotted. The proton jump distance was assumed as 0.1 nm [30]. The calculated conductivity reproduced well the measured conductivity in a wide temperature range. This behavior had been observed for the previous measurement for SrCe0.95Y0.05O3 as also shown in the figure. The present result suggests that the doping concentration dependence of barium cerates is not large within the experimental uncertainty of the 1H T1 measurement. The conductivity can be understood with a single proton migration process for different level of doping, thus at different proton concentrations.
Acknowledgements This work was supported under Grant-in Aid for Scientific Research No. 13750681 from the Ministry of Education, Science, Sports, and Culture. Supports from ‘Research for the Future’ program from the Japan Society for the Promotion of Science: ‘Preparation and Application of Newly Designed Solid Electrolytes’ JSPS-RFTF 96P00102 and Precursory Research for Embryonic Science and Technology of Japan Science and Technology Corporation is also acknowledged.
H. Maekawa et al. / Solid State Communications 130 (2004) 73–77
References [1] K. Amezawa, S. Kjelstrup, T. Norby, Y. Ito, J. Electrochem. Soc. 145 (1998) 3313. [2] T. Fukui, S. Ohara, S. Kawatsu, J. Power Sources 71 (1998) 164. [3] H. Iwahara, T. Esaka, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359. [4] A.S. Nowick, Y. Du, Solid State Ionics 77 (1995) 137. [5] T. Omata, K. Okuda, S. Tsugimoto, S. Otsuka-Matsuo-Yao, Solid State Ionics 104 (1997) 249. [6] H. Iwahara, Solid State Ionics 86-8 (1996) 9. [7] K.D. Kreuer, Solid State Ionics 97 (1997) 1. [8] T. Norby, Solid State Ionics 125 (1999) 1. [9] U. Reichel, R.R. Arons, W. Schilling, Solid State Ionics 86-8 (1996) 639. [10] T. Schober, J. Friedrich, J.B. Condon, Solid State Ionics 77 (1995) 175. [11] K.D. Kreuer, A. Fuchs, J. Maier, Solid State Ionics 77 (1995) 157. [12] A.S. Nowick, A.V. Vaysleyb, Solid State Ionics 97 (1997) 17. [13] R.A. De Souza, J.A. Kilner, C. Jeynes, Solid State Ionics 97 (1997) 409. [14] K.D. Kreuer, T. Dippel, Y.M. Baikov, J. Maier, Solid State Ionics 86-8 (1996) 613. [15] T. Hibino, K. Mizutani, T. Yajima, H. Iwahara, Solid State Ionics 57 (1992) 303. [16] J. Muller, K.D. Kreuer, J. Maier, S. Matsuo, M. Ishigame, Solid State Ionics 97 (1997) 421. [17] R. Hempelmann, C. Karmonik, T. Matzke, M. Cappadonia, U. Stimming, T. Springer, M.A. Adams, Solid State Ionics 77 (1995) 152.
77
[18] R. Hempelmann, Physica B 226 (1996) 72. [19] T. Matzke, U. Stimming, C. Karmonik, M. Soetratmo, R. Hempelmann, F. Guthoff, Solid State Ionics 86-8 (1996) 621. [20] N. Sata, K. Hiramoto, M. Ishigame, S. Hosoya, N. Niimura, S. Shin, Phys. Rev. B—Condens. Matter 54 (1996) 15795. [21] K.S. Knight, Solid State Ionics 127 (2000) 43. [22] H. Huck, P. Ehrhart, W. Schilling, J. Eur. Ceram. Soc. 19 (1999) 939. [23] N. Sata, M. Ishigame, S. Shin, Solid State Ionics 86-8 (1996) 629. [24] H. Yugami, Y. Chiba, M. Ishigame, Solid State Ionics 77 (1995) 201. [25] M. Cherry, M.S. Islam, J.D. Gale, C.R.A. Catlow, Solid State Ionics 77 (1995) 207. [26] R. Glockner, M.S. Islam, T. Norby, Solid State Ionics 122 (1999) 145. [27] E. Matsushita, A. Tanase, Solid State Ionics 97 (1997) 45. [28] W. Munch, G. Seifert, K.D. Kreuer, J. Maier, Solid State Ionics 86-8 (1996) 647. [29] F. Shimojo, K. Hoshino, H. Okazaki, J. Phys. Soc. Jpn 65 (1996) 1143. [30] H. Maekawa, N. Kashii, J.I. Kawamura, Y. Hinatsu, T. Yamamura, Solid State Ionics 122 (1999) 231. [31] T. Yajima, H. Iwahara, Solid State Ionics 53-6 (1992) 983. [32] T. Yajima, H. Iwahara, Solid State Ionics 50 (1992) 281. [33] T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State Ionics 61 (1993) 93. [34] K.S. Knight, Solid State Ionics 74 (1994) 109. [35] I. Svare, F. Borsa, D.R. Torgeson, S.W. Martin, Phys. Rev. B—Condens. Matter 48 (1993) 9336. [36] K.L. Ngai, O. Kanert, Solid State Ionics 53-6 (1992) 936.