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Vibrational properties of proton conducting double perovskites
Solid State Ionics 176 (2005) 2971 – 2974 www.elsevier.com/locate/ssi
Vibrational properties of proton conducting double perovskites Maths Karlsson a,*, Aleksandar Matic a, Pedro Berastegui b, Lars Bo¨rjesson a a
b
Department of Applied Physics, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
Abstract Raman and infrared spectroscopy results of protonated and dry samples of the proton conducting double perovskites Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 (BCN18) are presented. As confirmed by the Raman spectra, the incorporation of protons leads to only small distortions in the host lattice of the perovskites. However, the intensity growth of a band at approximately 650 cm 1 in the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75 after protonation may be related to a symmetry reduction towards a monoclinic structure. The O – H stretch region in the infrared spectra of Sr3CaZr0.5Ta1.5O8.75 and BCN18 are quite different. Stronger bands in the 2000 – 2500 cm 1 range in Sr3CaZr0.5Ta1.5O8.75 may be related to the higher concentration of oxygen vacancies in this material after protonation. D 2005 Elsevier B.V. All rights reserved. PACS: 78.30.-j Keywords: Perovskites; Raman; Infrared spectroscopy; Proton conducting; BCN18; Sr3CaZr0.5Ta1.5O8.75
1. Introduction A large number of perovskite-type oxides (A2+B4+O32 ) are found to be proton conducting when protonated [1]. These materials are of great interest due to their actual and potential applications in modern science and technology, e.g. as electrolytes for high-temperature fuel cells [1]. Prerequisites for high proton conduction in perovskites are a high concentration and a high mobility of protons in the perovskite structure. Protons can be incorporated into the structure by forming oxygen vacancies and then protonate the material. Oxygen-deficient perovskites can be formed during synthesis as a charge-compensating effect by introducing a lower-valent dopant to the B-site. Protonation is subsequently performed in water vapor at elevated temperatures, where water molecules dissociate into hydroxide ions which fill oxygen vacancies while the remaining protons form covalent bonds with neighboring lattice oxygens. The concentration of protons in the perovskite therefore depends on the concentration of oxygen vacancies and hence on the dopant level. As discovered by Liang and coworkers [2,3], several complex perovskites of the form A3(B2+C25+)O9 show proton conductivities that are comparable or even superior to that of many ‘‘classical’’ * Corresponding author. E-mail address: [email protected] (M. Karlsson). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.033
doped perovskites ABO3. Within this class of materials, the oxygen vacancies are formed by changing the stoichiometric ratio of the divalent B ion and the pentavalent C ion. In order to systematically improve the proton conductivity in perovskites, a detailed knowledge about the conduction mechanism is necessary. The principal mechanism for proton transport is known, and can be divided into thermally activated proton transfer between neighboring oxygens and reorientation of the hydroxide ion at the oxygen site [1]. The rates of these two processes are partly influenced by the vibrational dynamics of the proton. These vibrations may be investigated experimentally by use of Raman and infrared (IR) spectroscopy, and are manifested as bands in the corresponding vibrational spectrum. Normally, bands related to protons are found above 1500 cm 1. Particularly strong are the O – H stretch bands, which most commonly are observed between 3000 and 3500 cm 1 in protonated perovskites. However, one should note that bands at considerable lower frequencies have also been assigned to fundamental O – H stretch vibrations in perovskites, see for instance the recent paper by Omata et al. [5]. Apart from vibrations directly related to protons, the host lattice vibrations of the perovskite, below 1500 cm 1, also influence the proton transport. One example is the vibrations of the oxygen sub-lattice, which strongly affect the probability for proton transfer between adjacent oxygens, one of the processes in the proton conduction mechanism.
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In this work we have combined infrared and Raman spectroscopy to investigate the O – H stretch region and the response of the host lattice vibrational bands on protonation of the two double perovskites, Ba 3 Ca 1.18 Nb 1.82 O 8.73 (BCN18) and Sr3CaZr0.5Ta1.5O8.75. BCN18 is a well-established high-temperature proton conductor that exhibits the ˚ [6]. The incorpocubic Fm3m symmetry with a = 8.3846 A ration of protons into this material leads to an increase in the lattice parameter as observed in X-ray diffraction [6]. Sr3CaZr0.5Ta1.5O8.75 is recently reported as proton conducting in wet atmospheres. The conductivity below 550 -C was assumed to be almost pure protonic and the bulk conductivity was found to be 4.64 * 10 4 S cm 1 at 300 -C [7]. As-prepared samples were found to have a tetragonal unit ˚ and c = 8.220(1) A ˚ . High-resolution cell with a = 5.8049(6) A neutron diffraction indicated a slight decrease of crystal symmetry towards a monoclinic structure, P1a1-symmetry, ˚ (unique axis), with a = 8.2200(2) A˚ , b = 5.8002(6) A ˚ and b = 89.922(9)-, due to a small displacec = 5.8268(9) A ment in the oxygen sub-lattice (not seen in X-ray diffraction) after protonation [7]. Thermal gravimetric analysis (TGA) showed that 55% of the oxygen vacancies were filled after protonation of a vacuum-treated sample in 5% H2/Ar atmosphere, at 380 -C for 24 h [7].
(Graseby Specac, ‘‘Selector’’). A wrinkled aluminium foil was used as reference. The vibrational spectra were derived by taking the logarithm of the ratio between the reference spectra and the sample spectra. 3. Results 3.1. Structure The XRD experiments reveal a cubic symmetry for the ˚ and a tetragonal structure BCN18 sample with a = 8.4099(1) A ˚ and c = 8.242(3) A ˚, for Sr3CaZr0.5Ta1.5O8.75 with a = 5.834(1) A in agreement with previously reported structural investigations [6,7]. 3.2. Vibrational properties Fig. 1 shows the high-frequency IR-spectra of protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73. The main difference between the spectra of the protonated and un-protonated samples is the presence of strong absorbance bands at around 1500 cm 1 and above after protonation. This is expected since vibrational modes at these high frequencies in perovskites mainly involve protons [8].
2. Experimental 2.1. Sample preparation The samples were prepared through a conventional solidstate synthesis. Since as-prepared samples are well known to contain protons, the samples were heated at 600 -C in vacuum for 1 h to remove as many protons from the samples as possible. Protonated samples were prepared by annealing in a flow of water saturated Ar(g) at 250 -C for 2 weeks. XRD experiments were performed using a Guinier camera to reveal phase purity and obtain unit cell parameters. 2.2. Raman spectroscopy The Raman spectroscopy experiments were performed on a DILOR XY800 triple-grating spectrometer, in double subtractive mode. The spectrometer was equipped with a liquid nitrogen cooled charge coupled device detector. The measurements were performed at room temperature in back-scattering geometry with a spot size about 30 Am in diameter. The 488 nm line from an Ar+ laser was used for excitation. The 514.5 nm Ar+-line was used to separate vibrational bands from luminescence contribution. 2.3. Infrared spectroscopy The infrared spectroscopy experiments were performed in an inert atmosphere at room temperature in diffuse reflectance mode using a Bruker VECTOR 22 FT-IR spectrometer, equipped with a KBr beam splitter, a DTGS (deuterated triglycerine sulfate) detector and a diffuse reflectance device
Fig. 1. Infrared spectra of dry (a) and protonated (b) samples of Ba3Ca1.18Nb1.82O8.73 and Sr3CaZr0.5Ta1.5O8.75. The spectra have been vertically offset for clarity.
M. Karlsson et al. / Solid State Ionics 176 (2005) 2971 – 2974
However, one should note that there are signatures also in the un-protonated samples at these frequencies, a result of the difficulty to remove all protons from the structure. Focusing on the IR-spectra of the protonated samples (Fig. 1b), we can resolve several bands. Both samples exhibit quite distinct bands at around 1500, 1800 and 2500 cm 1 while a broad band is found between 2600 and 3600 cm 1. Only Sr3CaZr0.5Ta1.5O8.75 exhibits a band at around 2000 cm 1 while the IR-spectrum of Ba3Ca1.18Nb1.82O8.73 shows some bands above 3700 cm 1. The IR-spectrum of Ba3Ca1.18Nb1.82O8.73 agrees well with previously reported results [4]. The origin of the 1500 cm 1 band is not clear but may be related to O – H wag vibrations as discussed by Colomban et al. in their study of various double perovskites [9]. The broad band between 2600 and 3600 cm 1 is referred to as O –H stretch vibrations. The distinct absorptions at around 1800, 2000 and 2500 cm 1 may also be due to O – H stretch vibrations, as discussed further below. The sharp band at around 3700 cm 1 in BCN18 is a signature of the presence of non-hydrogen-bonded protons in the structure. In order to investigate the response of the host lattice vibrational modes on protonation, Raman spectra were measured before and after protonation. Backgrounds due to luminescence were subtracted to make the analysis of the spectra clearer. Fig. 2 shows the Raman spectra of protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75. We observe two distinct bands at around 780 and 550 cm 1 while two broad regions of bands are found at 50 –200 and 200 –450 cm 1, respectively. The strong band at 780 cm 1 is characteristic of the symmetrical stretch vibrations of the octahedron units in the perovskite structure [9]. Its shoulder at 830 cm 1 has previously been assigned to collinear B – O –B (B = Ca, Zr, Ta) stretch vibrations and is observed in the Raman spectra of several oxygen-deficient perovskites [9]. The bands between 200 and 450 cm 1 are attributed to bend modes of the octahedron units while the low-frequency bands between 50 and 200 cm 1 mainly involve vibrational motions of the heavy cations [9– 11]. On protonation, we observe several changes in the spectrum. There is a slight redshift of the 780 cm 1 band
2973
Fig. 3. Background-subtracted Raman spectra of Ba3Ca1.18Nb1.82O8.73, before and after protonation.
with 7 cm 1 and an intensity increase of a band at 650 cm 1. The 650 cm 1 band is barely visible in the un-protonated sample. We also observe some small intensity changes of the bands below 500 cm 1. Fig. 3 shows the Raman spectra of protonated and unprotonated samples of Ba3Ca1.18Nb1.82O8.73. Although the crystal structure of this perovskite is different from that of Sr3CaZr0.5Ta1.5O8.75, the Raman spectra mainly show the same features. We observe three distinct bands at around 550, 750 and 820 cm 1 while two broad regions of bands are found at 50– 200 and 200 –450 cm 1. In accordance to the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75, we assign the 750 cm 1 band to symmetric stretch vibrations of the octahedron units, the 820 cm 1 band to collinear B – O – B (B = Ca, Nb) stretch vibrations and the broad bands at 200 – 450 cm 1 and 50– 200 cm 1 to bend modes and vibrational motions of the cations, respectively. After protonation there are only minor changes in the Raman spectrum. Firstly, the relative intensity of the 750 and 820 cm 1 bands changes, where the intensity of the 820 cm 1 band is higher compared to the intensity of the 750 cm 1 band in the protonated sample. In addition, there is a redshift of 5 cm 1 of the 750 cm 1 band (cf. the redshift of the 780 cm 1 band for Sr3CaZr0.5Ta1.5O8.75). Finally, we observe how the intensity of the peak at 110 cm 1 has increased after protonation. However, the appearance of this peak is not evident since it can be a result of an intensity decrease of the peak at around 90 cm 1 as well. 4. Discussion
Fig. 2. Background-subtracted Raman spectra of Sr3CaZr0.5Ta1.5O8.75, before and after protonation.
By combining the results from IR and Raman spectroscopy on protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 we can reveal some effects by incorporation of protons into these double perovskites. As observed for both samples, there are only small changes in the Raman spectra after protonation. The protonation of these perovskites leads to only small distortions in the host lattice of the perovskites, which may be surprising. However, the change in relative intensity of the two oxygen modes at 750 and 820
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M. Karlsson et al. / Solid State Ionics 176 (2005) 2971 – 2974
cm 1 in BCN18 indicates a local distortion in the oxygen sublattice. Furthermore, the intensity increase of the band at 650 cm 1 in Sr3CaZr0.5Ta1.5O8.75, which is barely visible in the unprotonated sample, may indicate a decrease in crystal symmetry in the protonated sample. This is consistent with the high-resolution neutron diffraction by Corcoran et al. who observed a slight distortion towards a monoclinic structure in the protonated sample [7]. The redshifts of the bands attributed to octahedron stretch vibrations at around 780 cm 1 in Sr3CaZr0.5Ta1.5O8.75 and 750 cm 1 in Ba3Ca1.18Nb1.82O8.73 are expected by the increase in unit cell dimensions as reported elsewhere [6,7]. The IR-spectroscopy experiments on Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 have shown that several bands appear between 1500 and 4500 cm 1 after protonation. By comparing the IR-spectra of the protonated perovskites, it becomes clear that the low-frequency bands at around 2000 and 2500 cm 1 are much more pronounced in Sr3CaZr0.5Ta1.5O8.75. The presence of the low-frequency components in the O – H stretch region in this sample shows that there are a larger number of protons in strongly hydrogen-bonding configurations. To obtain strong hydrogen-bonding in perovskite structures the oxygen sub-lattice needs to be distorted to shorten the O – O distance and thus facilitate the formation of hydrogen bonds. Such a distortion of the lattice can be induced either by the dopant atoms introduced in the structure and/or by the presence of oxygen vacancies. An oxygen vacancy in the structure will act as a positively charged defect and repel the proton, thus forcing it towards another lattice oxygen forming a hydrogen bond. Indeed, Sr3CaZr0.5Ta1.5O8.75 contains a large number of oxygen vacancies also after protonation [7], causing a distortion of the oxygen sub-lattice and thus creating local configurations resulting in hydrogen-bonded protons. On the contrary, BCN18 contains only few vacancies after protonation [6,12], in agreement with the few protons in strongly hydrogen-bonded configurations. This result concurs well with a recent study of the vibrational proton dynamics in the proton conducting perovskite system BaInx Zr1 x O3 x/2, where the low-frequency O –H stretch bands were assigned to protons in strongly non-symmetrical local environments, close to oxygen vacancies [13]. The presence of low-frequency O –H stretch vibrations, and thus protons experiencing strong hydrogen-bonding, indicates loosely bound protons. Such configurations are beneficial for the proton transfer step between neighboring oxygens. However, the reported conductivity of BCN18 is higher than that of Sr3CaZr0.5Ta1.5O8.75 having more loosely bound protons [7]. This should indicate that the rate-limiting step in Sr3CaZr0.5Ta1.5O8.75 is reorientation and not proton transfer. Proton transfer is though likely the rate limiting process in
BCN18, due to the few strongly hydrogen-bonded protons in this material. That is, the rate limiting process for longrange diffusion of protons may be different in these two proton conducting perovskite materials. 5. Conclusions Using Raman and IR-spectroscopy we have investigated the O –H stretch region and the response of the host lattice vibrational bands on protonation of the two proton conducting double perovskites, Ba3 Ca1.18Nb 1.82O 8.73 (BCN18) and Sr3CaZr0.5Ta1.5O8.75. We found only small changes in the Raman spectra of both materials after protonation showing that the distortion of the host lattice must be small. However, the intensity increase of the 650 cm 1 band in the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75 may confirm a slight decrease in crystal symmetry as observed through neutron diffraction as reported elsewhere. In the IR-spectra there is a clear difference in the O –H stretch region between the two perovskites after protonation. The intensity of low-frequency O – H stretch modes in Sr3CaZr0.5Ta1.5O8.75 compared to BCN18 is much larger, indicating a larger number of protons in strongly hydrogen-bonded configurations in Sr3CaZr0.5Ta1.5O8.75. We suggest that these configurations are a result of the larger number of oxygen vacancies in this material compared to BCN18 after protonation. Acknowledgments This work was supported by the National Graduate School in Material Science and the Research Council, Sweden. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
K.D. Kreuer, Annual Review of Materials Research 33 (2003) 333. K. Liang, A. Novick, Solid State Ionics 61 (1993) 77. K. Liang, Y. Du, A. Novick, Solid State Ionics 69 (1994) 117. K.D. Kreuer, Solid State Ionics 125 (1999) 285. T. Omata, M. Takagi, S. Otsuka-Uao-Matsuo, Solid State Ionics 168 (2004) 99. H. Bohn, T. Schober, T. Mono, W. Schilling, Solid State Ionics 117 (1999) 219. D.J. Corcoran, J.T. Irvine, Solid State Ionics 145 (2001) 307. P. Colomban, Proton Conductors, Cambridge University Press, 1992. P. Colomban, F. Romain, A. Neiman, I. Animitsa, Solid State Ionics 145 (2001) 339. I. Kosacki, J. Schoonman, Solid State Ionics 57 (1992) 345. T. Scherban, R. Villeneuve, L. Abello, G. Lucazeau, Solid State Ionics 61 (1993) 93. T. Schober, J. Friedrich, D. Triefenbach, F. Tietz, Solid State Ionics 100 (1997) 173. M. Karlsson, M.E. Bjo¨rketun, P.G. Sundell, I. Ahmed, A. Matic, G. Wahnstro¨m, D. Engberg, S. Eriksson, P. Berastegui, L. Bo¨rjesson, Physical Review B 72 (2005) 094303.
Vibrational properties of proton conducting double perovskites Maths Karlsson a,*, Aleksandar Matic a, Pedro Berastegui b, Lars Bo¨rjesson a a
b
Department of Applied Physics, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden
Abstract Raman and infrared spectroscopy results of protonated and dry samples of the proton conducting double perovskites Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 (BCN18) are presented. As confirmed by the Raman spectra, the incorporation of protons leads to only small distortions in the host lattice of the perovskites. However, the intensity growth of a band at approximately 650 cm 1 in the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75 after protonation may be related to a symmetry reduction towards a monoclinic structure. The O – H stretch region in the infrared spectra of Sr3CaZr0.5Ta1.5O8.75 and BCN18 are quite different. Stronger bands in the 2000 – 2500 cm 1 range in Sr3CaZr0.5Ta1.5O8.75 may be related to the higher concentration of oxygen vacancies in this material after protonation. D 2005 Elsevier B.V. All rights reserved. PACS: 78.30.-j Keywords: Perovskites; Raman; Infrared spectroscopy; Proton conducting; BCN18; Sr3CaZr0.5Ta1.5O8.75
1. Introduction A large number of perovskite-type oxides (A2+B4+O32 ) are found to be proton conducting when protonated [1]. These materials are of great interest due to their actual and potential applications in modern science and technology, e.g. as electrolytes for high-temperature fuel cells [1]. Prerequisites for high proton conduction in perovskites are a high concentration and a high mobility of protons in the perovskite structure. Protons can be incorporated into the structure by forming oxygen vacancies and then protonate the material. Oxygen-deficient perovskites can be formed during synthesis as a charge-compensating effect by introducing a lower-valent dopant to the B-site. Protonation is subsequently performed in water vapor at elevated temperatures, where water molecules dissociate into hydroxide ions which fill oxygen vacancies while the remaining protons form covalent bonds with neighboring lattice oxygens. The concentration of protons in the perovskite therefore depends on the concentration of oxygen vacancies and hence on the dopant level. As discovered by Liang and coworkers [2,3], several complex perovskites of the form A3(B2+C25+)O9 show proton conductivities that are comparable or even superior to that of many ‘‘classical’’ * Corresponding author. E-mail address: [email protected] (M. Karlsson). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.033
doped perovskites ABO3. Within this class of materials, the oxygen vacancies are formed by changing the stoichiometric ratio of the divalent B ion and the pentavalent C ion. In order to systematically improve the proton conductivity in perovskites, a detailed knowledge about the conduction mechanism is necessary. The principal mechanism for proton transport is known, and can be divided into thermally activated proton transfer between neighboring oxygens and reorientation of the hydroxide ion at the oxygen site [1]. The rates of these two processes are partly influenced by the vibrational dynamics of the proton. These vibrations may be investigated experimentally by use of Raman and infrared (IR) spectroscopy, and are manifested as bands in the corresponding vibrational spectrum. Normally, bands related to protons are found above 1500 cm 1. Particularly strong are the O – H stretch bands, which most commonly are observed between 3000 and 3500 cm 1 in protonated perovskites. However, one should note that bands at considerable lower frequencies have also been assigned to fundamental O – H stretch vibrations in perovskites, see for instance the recent paper by Omata et al. [5]. Apart from vibrations directly related to protons, the host lattice vibrations of the perovskite, below 1500 cm 1, also influence the proton transport. One example is the vibrations of the oxygen sub-lattice, which strongly affect the probability for proton transfer between adjacent oxygens, one of the processes in the proton conduction mechanism.
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M. Karlsson et al. / Solid State Ionics 176 (2005) 2971 – 2974
In this work we have combined infrared and Raman spectroscopy to investigate the O – H stretch region and the response of the host lattice vibrational bands on protonation of the two double perovskites, Ba 3 Ca 1.18 Nb 1.82 O 8.73 (BCN18) and Sr3CaZr0.5Ta1.5O8.75. BCN18 is a well-established high-temperature proton conductor that exhibits the ˚ [6]. The incorpocubic Fm3m symmetry with a = 8.3846 A ration of protons into this material leads to an increase in the lattice parameter as observed in X-ray diffraction [6]. Sr3CaZr0.5Ta1.5O8.75 is recently reported as proton conducting in wet atmospheres. The conductivity below 550 -C was assumed to be almost pure protonic and the bulk conductivity was found to be 4.64 * 10 4 S cm 1 at 300 -C [7]. As-prepared samples were found to have a tetragonal unit ˚ and c = 8.220(1) A ˚ . High-resolution cell with a = 5.8049(6) A neutron diffraction indicated a slight decrease of crystal symmetry towards a monoclinic structure, P1a1-symmetry, ˚ (unique axis), with a = 8.2200(2) A˚ , b = 5.8002(6) A ˚ and b = 89.922(9)-, due to a small displacec = 5.8268(9) A ment in the oxygen sub-lattice (not seen in X-ray diffraction) after protonation [7]. Thermal gravimetric analysis (TGA) showed that 55% of the oxygen vacancies were filled after protonation of a vacuum-treated sample in 5% H2/Ar atmosphere, at 380 -C for 24 h [7].
(Graseby Specac, ‘‘Selector’’). A wrinkled aluminium foil was used as reference. The vibrational spectra were derived by taking the logarithm of the ratio between the reference spectra and the sample spectra. 3. Results 3.1. Structure The XRD experiments reveal a cubic symmetry for the ˚ and a tetragonal structure BCN18 sample with a = 8.4099(1) A ˚ and c = 8.242(3) A ˚, for Sr3CaZr0.5Ta1.5O8.75 with a = 5.834(1) A in agreement with previously reported structural investigations [6,7]. 3.2. Vibrational properties Fig. 1 shows the high-frequency IR-spectra of protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73. The main difference between the spectra of the protonated and un-protonated samples is the presence of strong absorbance bands at around 1500 cm 1 and above after protonation. This is expected since vibrational modes at these high frequencies in perovskites mainly involve protons [8].
2. Experimental 2.1. Sample preparation The samples were prepared through a conventional solidstate synthesis. Since as-prepared samples are well known to contain protons, the samples were heated at 600 -C in vacuum for 1 h to remove as many protons from the samples as possible. Protonated samples were prepared by annealing in a flow of water saturated Ar(g) at 250 -C for 2 weeks. XRD experiments were performed using a Guinier camera to reveal phase purity and obtain unit cell parameters. 2.2. Raman spectroscopy The Raman spectroscopy experiments were performed on a DILOR XY800 triple-grating spectrometer, in double subtractive mode. The spectrometer was equipped with a liquid nitrogen cooled charge coupled device detector. The measurements were performed at room temperature in back-scattering geometry with a spot size about 30 Am in diameter. The 488 nm line from an Ar+ laser was used for excitation. The 514.5 nm Ar+-line was used to separate vibrational bands from luminescence contribution. 2.3. Infrared spectroscopy The infrared spectroscopy experiments were performed in an inert atmosphere at room temperature in diffuse reflectance mode using a Bruker VECTOR 22 FT-IR spectrometer, equipped with a KBr beam splitter, a DTGS (deuterated triglycerine sulfate) detector and a diffuse reflectance device
Fig. 1. Infrared spectra of dry (a) and protonated (b) samples of Ba3Ca1.18Nb1.82O8.73 and Sr3CaZr0.5Ta1.5O8.75. The spectra have been vertically offset for clarity.
M. Karlsson et al. / Solid State Ionics 176 (2005) 2971 – 2974
However, one should note that there are signatures also in the un-protonated samples at these frequencies, a result of the difficulty to remove all protons from the structure. Focusing on the IR-spectra of the protonated samples (Fig. 1b), we can resolve several bands. Both samples exhibit quite distinct bands at around 1500, 1800 and 2500 cm 1 while a broad band is found between 2600 and 3600 cm 1. Only Sr3CaZr0.5Ta1.5O8.75 exhibits a band at around 2000 cm 1 while the IR-spectrum of Ba3Ca1.18Nb1.82O8.73 shows some bands above 3700 cm 1. The IR-spectrum of Ba3Ca1.18Nb1.82O8.73 agrees well with previously reported results [4]. The origin of the 1500 cm 1 band is not clear but may be related to O – H wag vibrations as discussed by Colomban et al. in their study of various double perovskites [9]. The broad band between 2600 and 3600 cm 1 is referred to as O –H stretch vibrations. The distinct absorptions at around 1800, 2000 and 2500 cm 1 may also be due to O – H stretch vibrations, as discussed further below. The sharp band at around 3700 cm 1 in BCN18 is a signature of the presence of non-hydrogen-bonded protons in the structure. In order to investigate the response of the host lattice vibrational modes on protonation, Raman spectra were measured before and after protonation. Backgrounds due to luminescence were subtracted to make the analysis of the spectra clearer. Fig. 2 shows the Raman spectra of protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75. We observe two distinct bands at around 780 and 550 cm 1 while two broad regions of bands are found at 50 –200 and 200 –450 cm 1, respectively. The strong band at 780 cm 1 is characteristic of the symmetrical stretch vibrations of the octahedron units in the perovskite structure [9]. Its shoulder at 830 cm 1 has previously been assigned to collinear B – O –B (B = Ca, Zr, Ta) stretch vibrations and is observed in the Raman spectra of several oxygen-deficient perovskites [9]. The bands between 200 and 450 cm 1 are attributed to bend modes of the octahedron units while the low-frequency bands between 50 and 200 cm 1 mainly involve vibrational motions of the heavy cations [9– 11]. On protonation, we observe several changes in the spectrum. There is a slight redshift of the 780 cm 1 band
2973
Fig. 3. Background-subtracted Raman spectra of Ba3Ca1.18Nb1.82O8.73, before and after protonation.
with 7 cm 1 and an intensity increase of a band at 650 cm 1. The 650 cm 1 band is barely visible in the un-protonated sample. We also observe some small intensity changes of the bands below 500 cm 1. Fig. 3 shows the Raman spectra of protonated and unprotonated samples of Ba3Ca1.18Nb1.82O8.73. Although the crystal structure of this perovskite is different from that of Sr3CaZr0.5Ta1.5O8.75, the Raman spectra mainly show the same features. We observe three distinct bands at around 550, 750 and 820 cm 1 while two broad regions of bands are found at 50– 200 and 200 –450 cm 1. In accordance to the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75, we assign the 750 cm 1 band to symmetric stretch vibrations of the octahedron units, the 820 cm 1 band to collinear B – O – B (B = Ca, Nb) stretch vibrations and the broad bands at 200 – 450 cm 1 and 50– 200 cm 1 to bend modes and vibrational motions of the cations, respectively. After protonation there are only minor changes in the Raman spectrum. Firstly, the relative intensity of the 750 and 820 cm 1 bands changes, where the intensity of the 820 cm 1 band is higher compared to the intensity of the 750 cm 1 band in the protonated sample. In addition, there is a redshift of 5 cm 1 of the 750 cm 1 band (cf. the redshift of the 780 cm 1 band for Sr3CaZr0.5Ta1.5O8.75). Finally, we observe how the intensity of the peak at 110 cm 1 has increased after protonation. However, the appearance of this peak is not evident since it can be a result of an intensity decrease of the peak at around 90 cm 1 as well. 4. Discussion
Fig. 2. Background-subtracted Raman spectra of Sr3CaZr0.5Ta1.5O8.75, before and after protonation.
By combining the results from IR and Raman spectroscopy on protonated and un-protonated samples of Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 we can reveal some effects by incorporation of protons into these double perovskites. As observed for both samples, there are only small changes in the Raman spectra after protonation. The protonation of these perovskites leads to only small distortions in the host lattice of the perovskites, which may be surprising. However, the change in relative intensity of the two oxygen modes at 750 and 820
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cm 1 in BCN18 indicates a local distortion in the oxygen sublattice. Furthermore, the intensity increase of the band at 650 cm 1 in Sr3CaZr0.5Ta1.5O8.75, which is barely visible in the unprotonated sample, may indicate a decrease in crystal symmetry in the protonated sample. This is consistent with the high-resolution neutron diffraction by Corcoran et al. who observed a slight distortion towards a monoclinic structure in the protonated sample [7]. The redshifts of the bands attributed to octahedron stretch vibrations at around 780 cm 1 in Sr3CaZr0.5Ta1.5O8.75 and 750 cm 1 in Ba3Ca1.18Nb1.82O8.73 are expected by the increase in unit cell dimensions as reported elsewhere [6,7]. The IR-spectroscopy experiments on Sr3CaZr0.5Ta1.5O8.75 and Ba3Ca1.18Nb1.82O8.73 have shown that several bands appear between 1500 and 4500 cm 1 after protonation. By comparing the IR-spectra of the protonated perovskites, it becomes clear that the low-frequency bands at around 2000 and 2500 cm 1 are much more pronounced in Sr3CaZr0.5Ta1.5O8.75. The presence of the low-frequency components in the O – H stretch region in this sample shows that there are a larger number of protons in strongly hydrogen-bonding configurations. To obtain strong hydrogen-bonding in perovskite structures the oxygen sub-lattice needs to be distorted to shorten the O – O distance and thus facilitate the formation of hydrogen bonds. Such a distortion of the lattice can be induced either by the dopant atoms introduced in the structure and/or by the presence of oxygen vacancies. An oxygen vacancy in the structure will act as a positively charged defect and repel the proton, thus forcing it towards another lattice oxygen forming a hydrogen bond. Indeed, Sr3CaZr0.5Ta1.5O8.75 contains a large number of oxygen vacancies also after protonation [7], causing a distortion of the oxygen sub-lattice and thus creating local configurations resulting in hydrogen-bonded protons. On the contrary, BCN18 contains only few vacancies after protonation [6,12], in agreement with the few protons in strongly hydrogen-bonded configurations. This result concurs well with a recent study of the vibrational proton dynamics in the proton conducting perovskite system BaInx Zr1 x O3 x/2, where the low-frequency O –H stretch bands were assigned to protons in strongly non-symmetrical local environments, close to oxygen vacancies [13]. The presence of low-frequency O –H stretch vibrations, and thus protons experiencing strong hydrogen-bonding, indicates loosely bound protons. Such configurations are beneficial for the proton transfer step between neighboring oxygens. However, the reported conductivity of BCN18 is higher than that of Sr3CaZr0.5Ta1.5O8.75 having more loosely bound protons [7]. This should indicate that the rate-limiting step in Sr3CaZr0.5Ta1.5O8.75 is reorientation and not proton transfer. Proton transfer is though likely the rate limiting process in
BCN18, due to the few strongly hydrogen-bonded protons in this material. That is, the rate limiting process for longrange diffusion of protons may be different in these two proton conducting perovskite materials. 5. Conclusions Using Raman and IR-spectroscopy we have investigated the O –H stretch region and the response of the host lattice vibrational bands on protonation of the two proton conducting double perovskites, Ba3 Ca1.18Nb 1.82O 8.73 (BCN18) and Sr3CaZr0.5Ta1.5O8.75. We found only small changes in the Raman spectra of both materials after protonation showing that the distortion of the host lattice must be small. However, the intensity increase of the 650 cm 1 band in the Raman spectrum of Sr3CaZr0.5Ta1.5O8.75 may confirm a slight decrease in crystal symmetry as observed through neutron diffraction as reported elsewhere. In the IR-spectra there is a clear difference in the O –H stretch region between the two perovskites after protonation. The intensity of low-frequency O – H stretch modes in Sr3CaZr0.5Ta1.5O8.75 compared to BCN18 is much larger, indicating a larger number of protons in strongly hydrogen-bonded configurations in Sr3CaZr0.5Ta1.5O8.75. We suggest that these configurations are a result of the larger number of oxygen vacancies in this material compared to BCN18 after protonation. Acknowledgments This work was supported by the National Graduate School in Material Science and the Research Council, Sweden. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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