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Determination of deuterium location in Ba3Ca1.18Nb1.82O8.73
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Solid State Ionics 179 (2008) 231 – 235 www.elsevier.com/locate/ssi
Determination of deuterium location in Ba3Ca1.18Nb1.82O8.73 Tomotaka Shimoyama a , Takeo Tojo a , Hitoshi Kawaji a,⁎, Tooru Atake a , Naoki Igawa b , Yoshinobu Ishii b a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503 Japan b Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195 Japan Received 14 October 2007; received in revised form 18 January 2008; accepted 21 January 2008
Abstract The neutron diffraction study was carried out to determine the location of protons in the crystal of a proton conducting mixed perovskite oxide Ba3Ca1.18Nb1.82O8.73. The sample was synthesized by a method of solid state reaction, and then the sintered sample was heated for the two experiments; for dried and deuterated samples. The neutron diffraction experiments were performed using HRPD at JAEA for the dried and deuterated samples at 300 K. The obtained data were analyzed by Rietveld method and maximum entropy method. It was concluded that deuterium existed at 96j site, which was shifted to the Ca/Nb mixed site. © 2008 Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; Proton conductor; Mixed perovskite oxide; Location of proton
1. Introduction Since the discovery of proton conduction in some perovskite-type oxides by Iwahara and Takahashi in 1978 [1], a variety of proton conductors with perovskite-type structure have been studied extensively [2–6]. The high proton conductivity of 10− 2–10− 3 Scm− 1 has been attained in some perovskite-type oxide compounds at about 600 °C in the atmosphere including hydrogen source such as the hydrogen gas, and steam, etc. These proton conductors are called high-temperature protonic conductors (HTPC), and have been of great interest for their potential application to solid state fuel cells as well as to sensors and steam electrolyzers. The mother compounds of the perovskite HTPC have a common formula ABO3, in which A and B ions are usually divalent and tetravalent, respectively. In order to introduce oxygen vacancies, the B ion is substituted partially by lower-valence dopant B' as AB1 − xB'xO3 − δ. In a
⁎ Corresponding author. E-mail address: [email protected] (H. Kawaji). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.064
humid atmosphere, the oxygen vacancies react with water vapor, and protons are introduced into the crystal as follows, : Vo þ H2 OYOo þ 2HI The protons can move through the crystal by a hopping mechanism [7]. More recently, it was found that some complex perovskites with formula A3B'1B”2O9 could be mother compounds for high proton conductors [8,9], where A was an alkali earth ion, Ca, Sr, Ba, etc., B' was a divalent or trivalent ion, and B" is a pentavalent ion. A mother compound, Ba3CaNb2O9, has no oxygen vacancy and no proton solubility, but the oxygen vacancies are formed by substituting pentavalent Nb ions by divalent Ca as Ba3Ca1 + xNb2 − xO9 − 3x / 2, and the proton solubility increases with increasing x from 0 to 0.18, and thus the proton conductivity increases [10,11]. Ba3Ca1.18Nb1.82O8.73 (BCN18) has the highest proton conductivity and proton solubility in the Ba3Ca1 + xNb2 − xO9 − δ system, and the properties have been widely studied as a promising high-temperature proton conductor [12–14]. The crystal structure of Ba3Ca1 + xNb2 − xO9 − 3x / 2 changes with x. When x = 0, Ba3CaNb2O9 forms the 1:2 order phase, in which Ca and Nb ions are distributed on three (111) planes of
232
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Fig. 1. Neutron diffraction intensity obtained for dry BCN18 and BCN18-D at 300 K. The wavelength of the incident neutrons was 1.8233 Å.
the elementary cubic structure; one plane containing Ca ions and other two planes containing Nb ions [11]. When 0 b x b 0.18, 1:2 order phase and 1:1 order phase coexist, and the amount of 1:1 order phase increases with x. In the 1:1 order phase, (111) planes of the Ca and Nb ions alternate. When x = 0.18, Ba3Ca1 + xNb2 − xO9 − 3x / 2 (BCN18) is in the 1:1 order phase, and − [2,3] or R3m − [15]. It has the space group is reported to be Fm3m been suggested that the proton exists in the vicinity of oxide ions by X-ray and neutron diffraction measurements [16–18]. Sosnowska et al. [17] reported that deuterium was located at 96j site. However, they analyzed the diffraction data under a restricted condition for the position of deuterium, and the resultant reliable factors (R factors) were extremely large [17]. In addition, they reported a structural change caused by the absorption of heavy water and the structural change was ignored in the analysis of crystal structure [17]. Such analysis might be insufficient to specify the position of deuterium. The objective of this study is to determine precisely the location of proton in BCN18. The neutron diffraction data were collected on dried
and deuterated samples at room temperature, and the location of the deuterium in deuterated BCN18 was determined. 2. Experimental Polycrystalline sample of Ba3Ca1.18Nb1.82O8.73 was synthesized by a method of solid state reaction [18,19]. BaCO3 (RARE METALLIC Co., Ltd., 99.99%), Nb2O5 (MITSUI MINING & SMELTING Co., Ltd., 99.99%) and CaCO3 (RARE METALLIC Co., Ltd., 99.99%) were weighed and mixed in an alumina motor with small amount of ethanol. The mixture was dried in air, and calcined in an alumina crucible at 1000 °C for 20 h. The product was ground, pressed into pellets, and then sintered at 1500 °C for 24 h in air. The dried sample (denoted as dry BCN18) was prepared by heating the specimen at 900 °C for 20 h under vacuum with a rotary pump and a liquid nitrogen trap. The deuterated sample was prepared as follows. First, 13.4 mmol of the sample was put in a pyrex glass vacuum line and then heated at 900 °C in vacuo to make dry BCN18. After the dry process, the
Fig. 2. Result of Rietveld analysis of the neutron diffraction intensity of dry BCN18. The plus symbol denotes the experimental data, and the solid line shows the refined curve. The calculated peak positions of dry BCN18 are indicated by tick marks below the diffraction pattern.
T. Shimoyama et al. / Solid State Ionics 179 (2008) 231–235
233
Fig. 3. Result of Rietveld analysis of the neutron diffraction intensity of BCN18-D.
sample was hold at 350 °C, and exposed to the heavy water vapor, which was 2.7 mmol in a glass flask. With decreasing the temperature from 350 °C to room temperature in four days, the heavy water was absorbed into the sample (this sample is denoted as BCN18-D). The amount of the heavy water in the BCN18-D was determined to be 0.053 g (about 2.6 mmol) by the mass measurement. Thus the composition of BCN18-D was represented as Ba3Ca1.18Nb1.82O8.73 (D2O)0.20. The powder neutron diffraction measurements were carried out at room temperature using the high resolution powder diffractometer (HRPD) installed at the Japan Research Reactor 3 at JAEA (the Japan Atomic Energy Agency) with a wavelength of 1.8233 Å. The amounts of dry BCN18 and BCN18-D samples used for the measurements were about 10 g. The sample was packed into a cylindrical vanadium cell (inner diameter: 9.4 mm, thickness: 0.025 mm, depth: 38 mm) in He atmosphere. The detector bank of the HRPD is comprised of 64 3 He-detectors. The neutron diffraction data were collected with constant monitor counts and a step angle of 0.05° over the 2θ range of 2.5°–160°. The diffraction patterns were analyzed by the Rietveld method using RIETAN-2000 [20,21]. After the Rietveld analysis, maximum entropy method (MEM) was carried out using PRIMA [22] with 128 × 128 × 128 pixels for a unit cell. The 3D graphic image of the density distribution was drawn with VENUS [23]. 3. Results and discussion The diffraction patterns of dry BCN18 and BCN18-D are shown in Fig. 1. No reflection due to impurities is detected in both samples. No structural change due to absorption of D2O is observed in BCN18-D; d = 2.6 Å peak reported by Sosnowska et al. [17] is not detected, which might be the main peak of Ca (OH)2 formed by decomposition of the sample. All peaks of BCN18-D are shifted to lower angles comparing with dry BCN18. This is attributed to the expansion of the lattice due to the introduction of oxide and deuterium ions into the oxygen vacancies in BCN18. The difference in the relative intensity is also observed clearly between dry BCN18 and BCN18-D. The
diffraction data of dry BCN18 were analyzed on the basis of the − cubic structure (Fm3m) [17]. Fig. 2 shows the results of the Rietveld analysis for dry BCN18. The result of calculation is in good agreement with that of the experiments; RI = 1.67%, and RF = 0.87%. The obtained crystal parameters of dry BCN18 agree closely with the results of Sosnowska et al. [17]. At the first stage of the determination of the location of deuterium in BCN18-D, we tried to refine the crystal structure of BCN18-D Table 1 Crystal parameters, R factor of dry BCN18 and BCN18-D obtained by the Rietveld analysis Sample
Dry BCN18
BCN18-D
a/Å Ba 8c g x(=y = z) B / Å2 Ca/Nb 4a g x(= y = z) B / Å2 Nb 4b g x(= y = z) B / Å2 O 24e g x B / Å2 y(= z) B / Å2 D 96j g x y z B / Å2 Rwp / % Re / % RI / % RF / % S
8.4169 (4)
8.4293 (2)
1.0 0.25 1.033 (28)
1.0 0.25 1.063 (29)
1.0 0.0 0.797 (54)
1.0 0.0 0.780 (56)
1.0 0.5 0.432 (38)
1.0 0.5 0.514 (41)
0.97 0.2635 (1) 0.0 2.247 (24)
0.9918 0.2601 (57) 0.0 2.121 (27)
– – – – – 7.45 3.89 1.67 0.87 1.91
0.0109 0.2258 (55) 0.0 0.0849 (42) 2.909 (793) 7.06 3.54 3.22 1.92 1.99
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T. Shimoyama et al. / Solid State Ionics 179 (2008) 231–235
Fig. 4. Distribution of nuclear scattering length density of dry BCN18 at 300 K. (a) and (b) show the 3-dimensional distributions (isosurfaces at 1.0 fm/Å3) in dry BCN18 and BCN18-D, respectively.
neglecting the deuterium. The obtained R values (RI = 3.49%, RF = 2.21%) are relatively large due to the neglection of the deuterium. The result of MEM shows that the scattering length density around 96j site near oxide ion is relatively high. It indicates that deuterium ion may occupy the 96j site. Then, we refined the crystal structure of BCN18-D using the structural model in which deuterium occupies 96j site. The occupancy parameters g of the deuterium and oxide ion were fixed to the value determined by the mass measurement as Ba3Ca1.18Nb1.82O8.73(D2O)0.20. In the previous report [17] the atomic coordinate z of deuterium was fixed, and thus the distance of OD was 1.0 Å. In the present analysis, however, we refined the z value as follows. Fig. 3 shows the results of Rietveld analysis for BCN18-D. The obtained crystal parameters and R factors are given in Table 1. As a result, the R factors based on the Bragg intensities, RI and RF were reduced by 8% and 13% from the values without deuterium refinement, respectively. The obtained S value is smaller than that of Sosnowska [17]. The scattering length density distribution of oxide ions in dry BCN18 is isotropic and that of BCN18-D is anisotropic as shown in Fig. 4(a) and (b), respectively, in which the same isosurface level (1.0 fm/Å3) of the scattering length density distribution is drawn. The deuterium is located at the apex of the lozenge of the anisotropic distribution. The 96j site is shifted to the Ca/Nb mixed site from the O–O straight line. The cause of the shift is considered as follows. The average charge of the Ca/ Nb mixed site is lower than that of single the Nb site due to the random substitution of Nb by lower valent Ca. Therefore, the lower electric repulsive force of the mixed site than single Nb site should cause the shift of deuterium to the mixed sites. Such a shift to low average charge sites has been also reported in the other proton conductors BaInxZr1 − xO3 − x / 2 by IR [24], neutron diffraction experiments [25] and the first-principles calculation [26]. On the other hand, the deuterium is not shifted in BaSn0.5In0.5O2.75 + α [27], in which the both sites have the same average charge. The calculated interatomic distances in dry BCN18 and BCN18-D are tabulated in Table 2. Many atomic distances in BCN18-D are larger than those in dry BCN18 except for the distance between Ca/Nb mixed site and O site. The distance between oxide ion site (24e) and deuterium one (96j) is about
0.72 Å, which is much shorter than the normal distance of OD group. If the atomic coordinate z (= 0.115) of deuterium is fixed, the distance of OD is 1.0 Å [17], and the thermal parameter of deuterium becomes about 5 Å, which is unacceptably large value. In addition, RI and RF are 10% and 8%, respectively, which are larger than the values obtained without fixing z. On the other hand, it is clearly shown that the distance of OD in BCN18-D is about 1.0 Å from Raman and IR experiments [28]. One possibility is that oxygen in OD group is shifted from the 24e site slightly toward the opposite side of deuterium. It is difficult to determine the precise position of oxygen in OD group by diffraction experiments, because the amount of oxygen in OD group is much smaller than that of oxide ion at 24e site and the shift is only about 0.3 Å. 4. Conclusion Neutron diffraction data have been obtained for the dried sample of Ba3Ca1.18Nb1.82O8.73 (dry BCN18), and the deuterated sample (BCN18-D) at 300 K. The data have been analyzed by the Rietveld method and MEM. The crystal structure of these samples at 300 K is successfully refined on the basis of the − cubic structure with the space group Fm3m. The distance between oxide ion site (24e) and deuterium (96j) is about 0.72 Å, which is shorter than the normal distance of OD group. The oxygen in OD group is shifted from 24e site slightly toward the opposite side of deuterium. The deuterium is located at 96j site that is shifted to the Ca and Nb mixed site. The shift of deuterium should be caused by the lower average charge at single Nb site due to random existence of Ca and Nb.
Table 2 Interatomic distance in dry BCN18 and BCN18-D
Ba–O Ca/Nb–O Nb–O O–D1 O–D2 O–O
Dry BCN18
BCN18-D
2.977 (1) 2.217 (1) 1.991 (1) – – 2.815 (1)
2.982 (1) 2.210 (12) 2.004 (12) 0.716 (35) 2.398 (43) 2.835 (17)
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Solid State Ionics 179 (2008) 231 – 235 www.elsevier.com/locate/ssi
Determination of deuterium location in Ba3Ca1.18Nb1.82O8.73 Tomotaka Shimoyama a , Takeo Tojo a , Hitoshi Kawaji a,⁎, Tooru Atake a , Naoki Igawa b , Yoshinobu Ishii b a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503 Japan b Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195 Japan Received 14 October 2007; received in revised form 18 January 2008; accepted 21 January 2008
Abstract The neutron diffraction study was carried out to determine the location of protons in the crystal of a proton conducting mixed perovskite oxide Ba3Ca1.18Nb1.82O8.73. The sample was synthesized by a method of solid state reaction, and then the sintered sample was heated for the two experiments; for dried and deuterated samples. The neutron diffraction experiments were performed using HRPD at JAEA for the dried and deuterated samples at 300 K. The obtained data were analyzed by Rietveld method and maximum entropy method. It was concluded that deuterium existed at 96j site, which was shifted to the Ca/Nb mixed site. © 2008 Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; Proton conductor; Mixed perovskite oxide; Location of proton
1. Introduction Since the discovery of proton conduction in some perovskite-type oxides by Iwahara and Takahashi in 1978 [1], a variety of proton conductors with perovskite-type structure have been studied extensively [2–6]. The high proton conductivity of 10− 2–10− 3 Scm− 1 has been attained in some perovskite-type oxide compounds at about 600 °C in the atmosphere including hydrogen source such as the hydrogen gas, and steam, etc. These proton conductors are called high-temperature protonic conductors (HTPC), and have been of great interest for their potential application to solid state fuel cells as well as to sensors and steam electrolyzers. The mother compounds of the perovskite HTPC have a common formula ABO3, in which A and B ions are usually divalent and tetravalent, respectively. In order to introduce oxygen vacancies, the B ion is substituted partially by lower-valence dopant B' as AB1 − xB'xO3 − δ. In a
⁎ Corresponding author. E-mail address: [email protected] (H. Kawaji). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.064
humid atmosphere, the oxygen vacancies react with water vapor, and protons are introduced into the crystal as follows, : Vo þ H2 OYOo þ 2HI The protons can move through the crystal by a hopping mechanism [7]. More recently, it was found that some complex perovskites with formula A3B'1B”2O9 could be mother compounds for high proton conductors [8,9], where A was an alkali earth ion, Ca, Sr, Ba, etc., B' was a divalent or trivalent ion, and B" is a pentavalent ion. A mother compound, Ba3CaNb2O9, has no oxygen vacancy and no proton solubility, but the oxygen vacancies are formed by substituting pentavalent Nb ions by divalent Ca as Ba3Ca1 + xNb2 − xO9 − 3x / 2, and the proton solubility increases with increasing x from 0 to 0.18, and thus the proton conductivity increases [10,11]. Ba3Ca1.18Nb1.82O8.73 (BCN18) has the highest proton conductivity and proton solubility in the Ba3Ca1 + xNb2 − xO9 − δ system, and the properties have been widely studied as a promising high-temperature proton conductor [12–14]. The crystal structure of Ba3Ca1 + xNb2 − xO9 − 3x / 2 changes with x. When x = 0, Ba3CaNb2O9 forms the 1:2 order phase, in which Ca and Nb ions are distributed on three (111) planes of
232
T. Shimoyama et al. / Solid State Ionics 179 (2008) 231–235
Fig. 1. Neutron diffraction intensity obtained for dry BCN18 and BCN18-D at 300 K. The wavelength of the incident neutrons was 1.8233 Å.
the elementary cubic structure; one plane containing Ca ions and other two planes containing Nb ions [11]. When 0 b x b 0.18, 1:2 order phase and 1:1 order phase coexist, and the amount of 1:1 order phase increases with x. In the 1:1 order phase, (111) planes of the Ca and Nb ions alternate. When x = 0.18, Ba3Ca1 + xNb2 − xO9 − 3x / 2 (BCN18) is in the 1:1 order phase, and − [2,3] or R3m − [15]. It has the space group is reported to be Fm3m been suggested that the proton exists in the vicinity of oxide ions by X-ray and neutron diffraction measurements [16–18]. Sosnowska et al. [17] reported that deuterium was located at 96j site. However, they analyzed the diffraction data under a restricted condition for the position of deuterium, and the resultant reliable factors (R factors) were extremely large [17]. In addition, they reported a structural change caused by the absorption of heavy water and the structural change was ignored in the analysis of crystal structure [17]. Such analysis might be insufficient to specify the position of deuterium. The objective of this study is to determine precisely the location of proton in BCN18. The neutron diffraction data were collected on dried
and deuterated samples at room temperature, and the location of the deuterium in deuterated BCN18 was determined. 2. Experimental Polycrystalline sample of Ba3Ca1.18Nb1.82O8.73 was synthesized by a method of solid state reaction [18,19]. BaCO3 (RARE METALLIC Co., Ltd., 99.99%), Nb2O5 (MITSUI MINING & SMELTING Co., Ltd., 99.99%) and CaCO3 (RARE METALLIC Co., Ltd., 99.99%) were weighed and mixed in an alumina motor with small amount of ethanol. The mixture was dried in air, and calcined in an alumina crucible at 1000 °C for 20 h. The product was ground, pressed into pellets, and then sintered at 1500 °C for 24 h in air. The dried sample (denoted as dry BCN18) was prepared by heating the specimen at 900 °C for 20 h under vacuum with a rotary pump and a liquid nitrogen trap. The deuterated sample was prepared as follows. First, 13.4 mmol of the sample was put in a pyrex glass vacuum line and then heated at 900 °C in vacuo to make dry BCN18. After the dry process, the
Fig. 2. Result of Rietveld analysis of the neutron diffraction intensity of dry BCN18. The plus symbol denotes the experimental data, and the solid line shows the refined curve. The calculated peak positions of dry BCN18 are indicated by tick marks below the diffraction pattern.
T. Shimoyama et al. / Solid State Ionics 179 (2008) 231–235
233
Fig. 3. Result of Rietveld analysis of the neutron diffraction intensity of BCN18-D.
sample was hold at 350 °C, and exposed to the heavy water vapor, which was 2.7 mmol in a glass flask. With decreasing the temperature from 350 °C to room temperature in four days, the heavy water was absorbed into the sample (this sample is denoted as BCN18-D). The amount of the heavy water in the BCN18-D was determined to be 0.053 g (about 2.6 mmol) by the mass measurement. Thus the composition of BCN18-D was represented as Ba3Ca1.18Nb1.82O8.73 (D2O)0.20. The powder neutron diffraction measurements were carried out at room temperature using the high resolution powder diffractometer (HRPD) installed at the Japan Research Reactor 3 at JAEA (the Japan Atomic Energy Agency) with a wavelength of 1.8233 Å. The amounts of dry BCN18 and BCN18-D samples used for the measurements were about 10 g. The sample was packed into a cylindrical vanadium cell (inner diameter: 9.4 mm, thickness: 0.025 mm, depth: 38 mm) in He atmosphere. The detector bank of the HRPD is comprised of 64 3 He-detectors. The neutron diffraction data were collected with constant monitor counts and a step angle of 0.05° over the 2θ range of 2.5°–160°. The diffraction patterns were analyzed by the Rietveld method using RIETAN-2000 [20,21]. After the Rietveld analysis, maximum entropy method (MEM) was carried out using PRIMA [22] with 128 × 128 × 128 pixels for a unit cell. The 3D graphic image of the density distribution was drawn with VENUS [23]. 3. Results and discussion The diffraction patterns of dry BCN18 and BCN18-D are shown in Fig. 1. No reflection due to impurities is detected in both samples. No structural change due to absorption of D2O is observed in BCN18-D; d = 2.6 Å peak reported by Sosnowska et al. [17] is not detected, which might be the main peak of Ca (OH)2 formed by decomposition of the sample. All peaks of BCN18-D are shifted to lower angles comparing with dry BCN18. This is attributed to the expansion of the lattice due to the introduction of oxide and deuterium ions into the oxygen vacancies in BCN18. The difference in the relative intensity is also observed clearly between dry BCN18 and BCN18-D. The
diffraction data of dry BCN18 were analyzed on the basis of the − cubic structure (Fm3m) [17]. Fig. 2 shows the results of the Rietveld analysis for dry BCN18. The result of calculation is in good agreement with that of the experiments; RI = 1.67%, and RF = 0.87%. The obtained crystal parameters of dry BCN18 agree closely with the results of Sosnowska et al. [17]. At the first stage of the determination of the location of deuterium in BCN18-D, we tried to refine the crystal structure of BCN18-D Table 1 Crystal parameters, R factor of dry BCN18 and BCN18-D obtained by the Rietveld analysis Sample
Dry BCN18
BCN18-D
a/Å Ba 8c g x(=y = z) B / Å2 Ca/Nb 4a g x(= y = z) B / Å2 Nb 4b g x(= y = z) B / Å2 O 24e g x B / Å2 y(= z) B / Å2 D 96j g x y z B / Å2 Rwp / % Re / % RI / % RF / % S
8.4169 (4)
8.4293 (2)
1.0 0.25 1.033 (28)
1.0 0.25 1.063 (29)
1.0 0.0 0.797 (54)
1.0 0.0 0.780 (56)
1.0 0.5 0.432 (38)
1.0 0.5 0.514 (41)
0.97 0.2635 (1) 0.0 2.247 (24)
0.9918 0.2601 (57) 0.0 2.121 (27)
– – – – – 7.45 3.89 1.67 0.87 1.91
0.0109 0.2258 (55) 0.0 0.0849 (42) 2.909 (793) 7.06 3.54 3.22 1.92 1.99
234
T. Shimoyama et al. / Solid State Ionics 179 (2008) 231–235
Fig. 4. Distribution of nuclear scattering length density of dry BCN18 at 300 K. (a) and (b) show the 3-dimensional distributions (isosurfaces at 1.0 fm/Å3) in dry BCN18 and BCN18-D, respectively.
neglecting the deuterium. The obtained R values (RI = 3.49%, RF = 2.21%) are relatively large due to the neglection of the deuterium. The result of MEM shows that the scattering length density around 96j site near oxide ion is relatively high. It indicates that deuterium ion may occupy the 96j site. Then, we refined the crystal structure of BCN18-D using the structural model in which deuterium occupies 96j site. The occupancy parameters g of the deuterium and oxide ion were fixed to the value determined by the mass measurement as Ba3Ca1.18Nb1.82O8.73(D2O)0.20. In the previous report [17] the atomic coordinate z of deuterium was fixed, and thus the distance of OD was 1.0 Å. In the present analysis, however, we refined the z value as follows. Fig. 3 shows the results of Rietveld analysis for BCN18-D. The obtained crystal parameters and R factors are given in Table 1. As a result, the R factors based on the Bragg intensities, RI and RF were reduced by 8% and 13% from the values without deuterium refinement, respectively. The obtained S value is smaller than that of Sosnowska [17]. The scattering length density distribution of oxide ions in dry BCN18 is isotropic and that of BCN18-D is anisotropic as shown in Fig. 4(a) and (b), respectively, in which the same isosurface level (1.0 fm/Å3) of the scattering length density distribution is drawn. The deuterium is located at the apex of the lozenge of the anisotropic distribution. The 96j site is shifted to the Ca/Nb mixed site from the O–O straight line. The cause of the shift is considered as follows. The average charge of the Ca/ Nb mixed site is lower than that of single the Nb site due to the random substitution of Nb by lower valent Ca. Therefore, the lower electric repulsive force of the mixed site than single Nb site should cause the shift of deuterium to the mixed sites. Such a shift to low average charge sites has been also reported in the other proton conductors BaInxZr1 − xO3 − x / 2 by IR [24], neutron diffraction experiments [25] and the first-principles calculation [26]. On the other hand, the deuterium is not shifted in BaSn0.5In0.5O2.75 + α [27], in which the both sites have the same average charge. The calculated interatomic distances in dry BCN18 and BCN18-D are tabulated in Table 2. Many atomic distances in BCN18-D are larger than those in dry BCN18 except for the distance between Ca/Nb mixed site and O site. The distance between oxide ion site (24e) and deuterium one (96j) is about
0.72 Å, which is much shorter than the normal distance of OD group. If the atomic coordinate z (= 0.115) of deuterium is fixed, the distance of OD is 1.0 Å [17], and the thermal parameter of deuterium becomes about 5 Å, which is unacceptably large value. In addition, RI and RF are 10% and 8%, respectively, which are larger than the values obtained without fixing z. On the other hand, it is clearly shown that the distance of OD in BCN18-D is about 1.0 Å from Raman and IR experiments [28]. One possibility is that oxygen in OD group is shifted from the 24e site slightly toward the opposite side of deuterium. It is difficult to determine the precise position of oxygen in OD group by diffraction experiments, because the amount of oxygen in OD group is much smaller than that of oxide ion at 24e site and the shift is only about 0.3 Å. 4. Conclusion Neutron diffraction data have been obtained for the dried sample of Ba3Ca1.18Nb1.82O8.73 (dry BCN18), and the deuterated sample (BCN18-D) at 300 K. The data have been analyzed by the Rietveld method and MEM. The crystal structure of these samples at 300 K is successfully refined on the basis of the − cubic structure with the space group Fm3m. The distance between oxide ion site (24e) and deuterium (96j) is about 0.72 Å, which is shorter than the normal distance of OD group. The oxygen in OD group is shifted from 24e site slightly toward the opposite side of deuterium. The deuterium is located at 96j site that is shifted to the Ca and Nb mixed site. The shift of deuterium should be caused by the lower average charge at single Nb site due to random existence of Ca and Nb.
Table 2 Interatomic distance in dry BCN18 and BCN18-D
Ba–O Ca/Nb–O Nb–O O–D1 O–D2 O–O
Dry BCN18
BCN18-D
2.977 (1) 2.217 (1) 1.991 (1) – – 2.815 (1)
2.982 (1) 2.210 (12) 2.004 (12) 0.716 (35) 2.398 (43) 2.835 (17)
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