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Synthesis of an oxygen nonstoichiometric Sr6Co5O15 phase
Materials Research Bulletin 41 (2006) 732–739 www.elsevier.com/locate/matresbu
Synthesis of an oxygen nonstoichiometric Sr6Co5O15 phase Kouta Iwasaki a,*, Tsuyoshi Ito a, Tsuneo Matsui a,b, Takanori Nagasaki b, Shingo Ohta c, Kunihito Koumoto c a
Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan c Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan Received 19 May 2005; received in revised form 6 October 2005; accepted 10 October 2005 Available online 8 November 2005
Abstract Sr6Co5O15, a member of the (A3Co2O6)m(A3Co3O9)n [A = Ca, Sr, Ba] series, showed oxygen nonstoichiometry. Sr6Co5O15.12, Sr6Co5O14.98, Sr6Co5O14.45 and Sr6Co5O14.26 were prepared by a solid state reaction in air. The Sr6Co5O15 phase is stable in the temperature range of 873–973 K in air, there are structure transitions at 773 K and above 1023 K. With the loss of oxygen, the a-axis ˚ (Sr6Co5O15.12) to 9.4390(4) A ˚ (Sr6Co5O14.26) and the c-axis parameter of the Sr6Co5O15 phase decreased from 9.4988(3) A ˚ (Sr6Co5O15.12) to 12.5066(4) A ˚ (Sr6Co5O14.26) with decreasing oxygen content. The parameter increased from 12.3772(3) A Rietveld analysis of the powder X-ray diffraction data suggested that the increase in the c-axis was mainly due to the increase in the Co–Co distance between the CoO6 trigonal prism and CoO6 octahedron. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Oxides; C. Thermogravimetric analysis; C. X-ray diffraction; D. Crystal structure
1. Introduction The oxygen content in the SrCoO3 x system is one of the factors determining the phase stability and crystal structure; the orthorhombic (brownmillerite-type), cubic (perovskite-type) and rhombohedral phases are the main phases that have been studied [1–12]. The rhombohedral phase, which is thought to have a hexagonal BaNiO3-type structure, is currently known as Sr6Co5O15. The crystal structure of Sr6Co5O15 was analyzed by Harrison et al. using powder neutron diffraction data [13]. Members of the (A3Co2O6)m(A3Co3O9)n (or A3m+3nCo2m+3nO6m+9n) and (A8Co6O18)a(A8Co8O24)b (or A4a+4bCo3a+4bO9a+12b) [A = Ca, Sr, Ba] series have pseudo-one-dimensional structures consisting of Co–O chains and alkaline earth atoms, in which CoO6 octahedra and CoO6 trigonal prisms are connected sharing their faces and alkaline earth atoms isolate the Co–O chains [13–27]. Sr6Co5O15 (m = 1, n = 1) is a member of the (A3Co2O6)m(A3Co3O9)n series, and four consecutive CoO6 octahedra and one CoO6 trigonal prism are connected ˚ and c = 12.3966(4) A ˚ [13]), as shown in Fig. 1. in the unit cell (space group R32, a = 9.5035(2) A
* Corresponding author. Tel.: +81 52 789 4689; fax: +81 52 789 3779. E-mail address: [email protected] (K. Iwasaki). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.10.012
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Fig. 1. Crystal structure of Sr6Co5O15.
One of the problems in the (A3Co2O6)m(A3Co3O9)n and (A8Co6O18)a(A8Co8O24)b series is that other members are synthesized as byproducts in addition to the target member because of the similarity of the sample compositions. Sr6Co5O15 can be obtained by annealing at 1148 K in air [13]. Our previous study on Sr6Co5O15 showed that Sr6Co5O15 was not always stable below 1148 K. For example, an unidentified Sr–Co–O phase was obtained below 773 K from Sr6Co5O15, and Sr6Co5O15 exhibited weight loss at high temperatures [28]. Understanding the stability and nonstoichiometry of Sr6Co5O15 is important since contamination by an impurity phase and the variation of sample composition often affect the physical properties. In this study, we investigated the relation between the synthesis conditions and formation of the Sr6Co5O15 phase. Four samples of the Sr6Co5O15 phase with different oxygen contents were prepared by a solid state reaction, and the crystal structures were refined. Throughout this paper, we will refer to the Sr6Co5O15 phase obtained by heating at 1173 K in air simply as Sr6Co5O15. 2. Experimental SrCO3 (99.99+%, Rare Metallic Co.) and Co3O4 (99.95%, Kanto Chemical Co.) powders were used as starting materials. The powders were mixed in the appropriate proportion (molar ratio of Sr:Co was 6:5) in an agate mortar with ethanol, and pressed into a pellet form. The pellet was put on a Pt sheet and heated (Yamato, muffle furnace FO310) at 1173 K for 210 h in air with intermediate grindings every 70 h to obtain single phase Sr6Co5O15. In this study, four samples of the Sr6Co5O15 phase with different oxygen contents were prepared as follows: (a) Sr6Co5O15 was heated at 1123 K for 70 h in air and then cooled in the furnace (it took about 0.6 h to decrease the temperature inside the furnace from 1123 K to 773 K and 2.5 h from 773 K to 373 K); (b) Sr6Co5O15 was heated at 1123 K for 15 h in air and cooled outside the furnace at room temperature in air; (c) Sr6Co5O15 was heated at 873 K for 15 h in air and quenched in liquid nitrogen; (d) Sr6Co5O15 was heated at 973 K for 15 h in air and quenched in liquid nitrogen. The X-ray powder diffraction (XRD) patterns were recorded by a diffractometer (Rigaku, RINT2200) using Cu Ka radiation with a pyrolytic graphite monochromator. The crystal structures were refined by the Rietveld method using the program RIETAN-2000 [29] with XRD data in the range 108 2u 1208 (scan step 0.038). Thermogravimetry and differential thermal analysis (TG-DTA) were carried out in air at a heating and cooling rate of 10 K/min (Rigaku, TG8120). Quantitative analysis of oxygen was performed by the He carrier melting–infrared absorption method (LECO, TC-436). 3. Results and discussion Upon heating at 1173 K in air, an unidentified impurity phase formed on the surface of the pellet in contacted with the Pt sheet. Single phase Sr6Co5O15 was obtained by scraping away the surface of the pellet. In the energy dispersion X-ray spectroscopy analysis (EDS, JEOL JSM-6460LA), no Pt contamination was detected in Sr6Co5O15. Fig. 2 shows the TG-DTA curves for Sr6Co5O15 measured over the temperature range 300–1173 K in air at a heating and cooling rate of 10 K/min. Sr6Co5O15 showed weight loss above 650 K; the sample weight decreased with increasing temperature. The weight loss was about 1.2% at 1000 K. No endothermic and exothermic peaks were
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Fig. 2. TG and DTA curves for Sr6Co5O15 in air at a heating and cooling rate of 10 K/min. Black line and gray line show the data from 300 K to 1173 K and from 1173 K to 300 K, respectively.
observed. As the temperature decreased from 1173 K to 300 K, the sample weight increased up to the starting weight of the sample. The reversible change of the sample weight indicates the oxygen nonstoichiometry of Sr6Co5O15. The weight loss of 1.2% at 1000 K corresponds to the sample composition of Sr6Co5O14.21 assuming the starting composition of the sample is Sr:Co:O = 6:5:15. When Sr6Co5O15 was heated in air at various temperatures and quenched in liquid nitrogen, single phase Sr6Co5O15 d, which was confirmed to have a Sr6Co5O15 structure by the Rietveld method as described later, was obtained in the temperature range of 873–973 K. On the other hand, the single phase was obtained between 873 K and 1173 K when the sample was cooled in the furnace (Table 1). Fig. 3 shows the XRD patterns of samples prepared under several different conditions. The Miller indices are described according to the hexagonal axes. When Sr6Co5O15 was heated at 1223 K and cooled in the furnace, single phase Sr6Co5O15 was not obtained, as shown in Fig. 3(a). The XRD pattern was similar to that of Sr6Co5O15, however, the peaks were not indexed with Sr6Co5O15. There were also some peaks around 2u = 298, where the (1 1 3) peak of Sr6Co5O15 appears, suggesting that the sample contained more than two phases. Single phase Sr6Co5O15 was obtained at 1173 K and 873 K (see Fig. 3(b) and (c), respectively), and the (1 1 3) peak tended to broaden with decreasing temperature. When Sr6Co5O15 was heated at 773 K, a shoulder peak appeared at lower angles of the (1 1 3) peak of Sr6Co5O15, as reported in our previous study [28]. The intensity of the peak at 2u = 28.28 increased upon additional heating at 773 K for 420 h (Fig. 3(d)). The XRD pattern was similar to that of Sr6Co5O15, but not identified as Sr6Co5O15. The similarity of the XRD patterns suggested that the compound obtained at 773 K was a member of the (A3Co2O6)m(A3Co3O9)n series, and lattice ˚ , c1 = 2.454 A ˚ and q = 0.5806 A ˚ (c2 = 4.227 A ˚ ), could parameters with an incommensurate phase, a = 9.492 A explain the XRD pattern. Some unidentified peaks were also included in the XRD pattern; the small peak around 2u = 28.68 was attributed to the (1 1 3) peak of Sr6Co5O15. Extra heating did not reduce the intensity of the unidentified peaks, suggesting that the Sr/Co ratio of the compound obtained at 773 K is not 6/5. The change of the crystal structure of Sr6Co5O15 at 773 K is mainly attributable to the change of the atomic arrangement in the c-axis Table 1 Phases obtained by cooling of Sr6Co5O15 after heating at various temperatures Temperature (K)
Heated for 70 h in air and cooled in the furnace
Heated for 15 h in air and quenched in liquid N2
1223 1173 1123 1023 973 873 773
Sr6Co5O15 Sr6Co5O15 – Sr6Co5O15 Sr6Co5O15
– – Sr6Co5O15 Sr6Co5O15 –
‘’, not single phase of Sr6Co5O15; ‘–’, uninvestigated.
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Fig. 3. XRD patterns of Sr6Co5O15 cooled in a heating furnace after heating at (a) 1223 K for 70 h, (b) 1173 K for 70 h, (c) 873 K for 70 h, (d) 773 K for 420 h and quenched after heating at (e) 1023 K for 15 h.
˚ for Sr6Co5O15.12 as described direction since the a-axis was similar to that of the Sr6Co5O15 phase (a = 9.4988 A later). The broadening in the (1 1 3) peak observed at 873 K (Fig. 3(c)) also signals the beginning of the change in atomic arrangement. The change of the Sr6Co5O15 phase was also shown at 573 K; a shoulder peak was found at lower angles of the (1 1 3) peak when Sr6Co5O15 was heated at 573 K for 420 h in air. These results indicate that the Sr6Co5O15 phase is unstable at 573–773 K in air. However, single phase Sr6Co5O15 can be obtained without quenching since the transformation of the crystal structure is not rapid. In the case of the sample that was quenched in liquid N2 after heating, single phase Sr6Co5O15 d was obtained at 873 K and 973 K. When Sr6Co5O15 was heated above 1023 K, the single phase was not obtained, and at least two XRD peaks appeared at around 2u = 32.88, as shown in Fig. 3(e). When the sample was cooled in Ar gas from 1023 K to 773 K at a cooling rate of 125 K/h after heating at 1023 K, the XRD pattern obtained was almost the same as that of the sample quenched in liquid N2. This indicates that the cooling rate does not influence the products. Thus, the change of the crystal structure of Sr6Co5O15 above 1023 K is attributable to the decrease in oxygen content. These results show that the Sr6Co5O15 phase is stable between 873 K and 973 K in air and that the absorption of oxygen is necessary for the formation of the Sr6Co5O15 phase when the sample is cooled after heating at 1023 K or higher. In our previous study, the Seebeck coefficient of Sr6Co5O15 was measured over the temperature range 300–1120 K in air [28]. The Seebeck coefficient of Sr6Co5O15 exhibited a complex temperature dependence: the Seebeck coefficient decreased at 300–750 K (130–110 mV/K), increased at 750–1000 K (110–138 mV/K) and decreased at 1000–1120 K (138–129 mV/K) with increasing temperature. The decrease in the oxygen content of Sr6Co5O15 above 650 K and the change of the Sr6Co5O15 phase above 1023 K seem to be related to the increase in the Seebeck coefficient between 750 K and 1000 K and the decrease between 1000 K and 1120 K, respectively. The oxygen content of the Sr6Co5O15 phase could be controlled by varying the cooling conditions since Sr6Co5O15 showed oxygen nonstoichiometry above 650 K. In this study, four Sr6Co5O15 samples with oxygen contents of (a) 15.12 0.08, (b) 14.98 0.09, (c) 14.45 0.04 and (d) 14.26 0.12 were prepared. The oxygen content of 15.12 for sample (a) exceeded the 15 for stoichiometric Sr6Co5O15, suggesting the presence of cation defects. However, the details of this remain unclear. The crystal structures of the (A3Co2O6)m(A3Co3O9)n series are sensitive to the sample composition: subtle variations of the composition often lead to different arrangements of the CoO6 octahedra and CoO6 trigonal prisms. Crystal structure refinement was performed by the Rietveld method (RIETAN-2000 program [29]) to investigate whether the crystal structures of Sr6Co5O15.12, Sr6Co5O14.98, Sr6Co5O14.45 and Sr6Co5O14.26 were influenced by differences in oxygen content. In the refinement, the crystallographic parameters of Sr6Co5O15 [13] were used as the initial structure model. The oxygen occupancies were allocated equally to each oxygen site. The oxygen occupancies for Sr6Co5O15.12 were replaced with 1. This assumption was that this approximation would not significantly affect the structure refinement since the atomic scattering factor of oxygen is smaller than those of Sr and Co atoms. The results of the crystal structure refinement are summarized in Table 2, and the profile-fit of the XRD data for Sr6Co5O15.12 and
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Table 2 Crystallographic parameters and results of crystal structure refinement for Sr6Co5O15 phase Atom
9d 9e 3b 6c 6c 9d 18f 18f
Sr6Co5O15.12
Sr6Co5O14.98
g
x
y
z
B
g
x
y
z
B
1 1 1 1 1 1 1 1
0.3232(3) 0.6423(3) 0 0 0 0.845(2) 0.4965(16) 0.8434(12)
0 0 0 0 0 0 0.6816(13) 0.9742(13)
0 0.5 0.5 0.0963(7) 0.2934(4) 0 0.4776(8) 0.6109(9)
0.80(11)
1 1 1 1 1 0.9987 0.9987 0.9987
0.3230(3) 0.6421(2) 0 0 0 0.847(2) 0.4957(16) 0.8461(12)
0 0 0 0 0 0 0.6793(13) 0.9793(13)
0 0.5 0.5 0.0970(6) 0.2914(4) 0 0.4798(8) 0.6082(9)
0.87(11)
˚) a-Axis (A ˚) c-Axis (A ˚ 3) vol. (A Rwp (%) RI (%) RF (%) S Atom
Sr1 Sr2 Co1 Co2 Co3 O1 O2 O3 ˚) a-Axis (A ˚) c-Axis (A ˚ 3) vol. (A Rwp (%) RI (%) RF (%) S
0.34(16)
0.6(2)
9.4988(3) 12.3772(3) 967.13(4) 12.12 3.03 2.62 1.29 Site
9d 9e 3b 6c 6c 9d 18f 18f
0.56(16)
0.6(2)
9.4890(2) 12.4024(2) 967.11(3) 12.28 3.55 3.00 1.33
Sr6Co5O14.45
Sr6Co5O14.26
g
x
y
z
B
g
x
y
z
B
1 1 1 1 1 0.9633 0.9633 0.9633
0.3208(3) 0.6398(3) 0 0 0 0.843(2) 0.4921(16) 0.8654(15)
0 0 0 0 0 0 0.6679(14) 0.0180(16)
0 0.5 0.5 0.0983(5) 0.2873(4) 0 0.4836(9) 0.6127(9)
0.88(11)
1 1 1 1 1 0.9507 0.9507 0.9507
0.3226(3) 0.6384(3) 0 0 0 0.841(2) 0.498(2) 0.8484(16)
0 0 0 0 0 0 0.6843(15) 0.980(2)
0 0.5 0.5 0.0981(5) 0.2864(4) 0 0.4832(9) 0.6147(10)
0.67(11)
0.43(16)
0.9(2)
9.4468(3) 12.4801(3) 964.52(4) 13.23 4.07 3.79 1.44
Space group: R32 [No. 155]; linear constraint: B [Sr1] = B [Sr2], B [Co1] = B [Co2] = B [Co3], B [O1] = B [O2] = B [O3].
9.4390(4) 12.5066(4) 964.98(6) 12.65 3.97 3.11 1.37
0.16(16)
0.5(2)
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Sr1 Sr2 Co1 Co2 Co3 O1 O2 O3
Site
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Fig. 4. X-ray Rietveld refinement profiles for (a) Sr6Co5O15.12 and (b) Sr6Co5O14.26. Points are the observed data and solid line is the calculated profile. Marks below the profile show the allowed reflections. The difference between observed and calculated data is presented beneath.
Sr6Co5O14.26 are shown in Fig. 4. The crystal structure of each sample could be refined, suggesting that the oxygen nonstoichiometric phases have the Sr6Co5O15 structure. The interatomic distances (Table 3) also corresponded to those of related compounds, Sr6Co5O15 reported by Harrison et al. [13] and Sr14Co11O33 [16]. The comparisons of Co–Co and Co–O distances were as follows: (i) The ˚ for Sr6Co5O14.26, Co–Co (Co1–Co3) distance between the CoO6 trigonal prism and CoO6 octahedron was 2.672 A ˚ ˚ which was larger than 2.50 A for Sr6Co5O15 and 2.61 A for Sr14Co11O33; (ii) The average values of Co–Co (Co2–Co2 ˚ for Sr6Co5O15.12, 2.410 A ˚ for and Co2–Co3) distances between the CoO6 octahedra in the unit cell were 2.421 A ˚ ˚ ˚ for Sr6Co5O14.98, 2.390 A for Sr6Co5O14.45 and 2.387 A for Sr6Co5O14.26. These values were smaller than 2.467 A ˚ ˚, Sr6Co5O15 and 2.44 A for Sr14Co11O33; (iii) the Co–O (Co1–O3) distances in the CoO6 trigonal prism, 1.920–1.968 A ˚ ˚ were smaller than 1.973 A for Sr6Co5O15 and 2.05 A for Sr14Co11O33; (iv) in the CoO6 octahedra, the average Co2–O ˚ ) and the average Co3–O distances (1.894–1.916 A ˚ ) were almost constant, i.e. independent distances (1.867–1.881 A ˚ ˚ for the of oxygen content. These values agreed with those for Sr6Co5O15 (1.89 A for the Co2–O distance and 1.92 A ˚ Co3–O distance) and Sr14Co11O33 (average Co–O distance of 1.90 A). Fig. 5 shows the relation between the lattice parameters and the oxygen content for the Sr6Co5O15 phase. The a-axis ˚ (Sr6Co5O15.12) to 9.4390(4) A ˚ (Sr6Co5O14.26) and the c-axis parameter parameter decreased from 9.4988(3) A Table 3 ˚ ) of Sr6Co5O15 phase Interatomic distances (A
Co1–O3 6 Co2–O1 3 Co2–O2 3 Co3–O2 3 Co3–O3 3 Co1–Co3 2 Co2–Co2 1 Co2–Co3 2 Sr1–O1 2 Sr1–O2 2 Sr1–O2 2 Sr1–O3 2 Sr1–O3 2 Sr2–O1 2 Sr2–O2 2 Sr2–O2 2 Sr2–O3 2 Sr2–O3 2
Sr6Co5O15.12
Sr6Co5O14.98
Sr6Co5O14.45
Sr6Co5O14.26
1.947(10) 1.895(14) 1.877(14) 1.967(13) 1.820(13) 2.558(5) 2.383(16) 2.440(12) 2.659(2) 2.498(13) 2.833(12) 2.641(11) 3.120(14) 2.605(11) 2.637(11) 2.688(11) 2.462(12) 3.228(12)
1.920(10) 1.877(13) 1.857(14) 1.971(13) 1.853(13) 2.587(5) 2.405(16) 2.412(12) 2.656(2) 2.517(12) 2.809(12) 2.688(11) 3.086(11) 2.625(11) 2.651(10) 2.662(11) 2.443(12) 3.261(13)
1.959(10) 1.927(14) 1.830(14) 1.982(13) 1.850(13) 2.654(5) 2.455(13) 2.358(10) 2.625(2) 2.543(13) 2.753(12) 2.770(13) 2.971(14) 2.595(11) 2.534(11) 2.730(11) 2.487(12) 3.239(13)
1.968(11) 1.937(16) 1.825(16) 1.962(15) 1.830(14) 2.672(5) 2.454(14) 2.354(10) 2.637(3) 2.557(14) 2.785(13) 2.667(14) 3.049(14) 2.587(12) 2.594(12) 2.657(13) 2.527(13) 3.207(13)
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Fig. 5. Lattice parameters of the Sr6Co5O15 phase.
Fig. 6. Relation between the Co–Co distances and c-axis for the Sr6Co5O15 phase.
˚ (Sr6Co5O15.12) to 12.5066(4) A ˚ (Sr6Co5O14.26) with decreasing oxygen content. The increased from 12.3772(3) A decrease in oxygen content leads to the decrease in the average valence state of Co ions (e.g. +3.65 for Sr6Co5O15.12 and +3.30 for Sr6Co5O14.26), thus, the average radius of Co ions increases with decreasing oxygen content. The increase in the c-axis corresponds to the increase in the ionic radius, however, the decrease in the a-axis is not consistent with the increase in the ionic radius. The Co1 site is in the CoO6 trigonal prism, and the Co2 and Co3 sites are in the CoO6 octahedra. Each Co site is located on the z-axis, thus, the changes in Co–Co distances are directly related to the changes in the c-axis length. The relation between the Co–Co distances and the c-axis is shown in Fig. 6. The Co1–Co3 and Co2–Co2 distances tended to increase with increasing the c-axis, while the Co2–Co3 distance decreased. The Co2–O2 distance ˚ to 1.825 A ˚ with decreasing oxygen content, while the Co3–O2 distance remained almost decreased from 1.877 A constant, which indicates that the Co atom in the Co2 site approached the Co3 site with decreasing oxygen content. The increase in the c-axis was mainly due to the increase in the Co1–Co3 distance between the CoO6 trigonal prism ˚ , and the increase in the and CoO6 octahedron (the difference between Sr6Co5O15.12 and Sr6Co5O14.26 was 0.114 A ˚ ˚) unit cell was 0.114 2 = 0.228 A) relative to the increase in the Co2–Co2 distance (the difference was 0.071 A between CoO6 octahedra.
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4. Conclusion The oxygen nonstoichiometric Sr6Co5O15 phases were prepared. The Sr6Co5O15 phase was stable over the temperature range from 873 K to 973 K in air, however, weight loss due to oxygen release occurred above 650 K. The oxygen release and absorption were reversible, thus, the Sr6Co5O15 phase can be obtained upon cooling from 1023 K or higher. The decrease in the oxygen content resulted in a decrease in the a-axis parameter and an increase in the caxis parameter. The presence of an oxygen nonstoichiometric Sr6Co5O15 phases suggest that oxygen nonstoichiometry may also be observed for other members of the (A3Co2O6)m(A3Co3O9)n and (A8Co6O18)a(A8Co8O24)b series. Acknowledgements The authors wish to express their gratitude to Dr. Teiichi Kimura and Dr. Kunio Takada (Institute for Materials Research, Tohoku University) for their cooperation in the analysis of the oxygen content. This work was partly supported by The Foundation ‘‘Hattori-Hokokai’’. References [1] H. Watanabe, T. Takeda, in: Y. Hoshino (Ed.), FERRITES: Proceedings of the International Conference on Ferrites, Kyoto, Japan, July 1970, University of Tokyo Press, Tokyo, 1971, pp. 588–590. [2] T. Takeda, H. Watanabe, J. Phys. Soc. Jpn. 33 (1972) 973–978. [3] J.C. Grenier, S. Ghodbane, G. Demazeau, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 831–839. [4] Y. Takeda, R. Kanno, T. Takada, O. Yamamoto, M. Takano, Y. Bando, Z. Anorg. Allg. Chem. 540/541 (1986) 259–270. [5] J. Rodrı´guez, J.M. Gonza´lez-Calbet, Mater. Res. Bull. 21 (1986) 429–439. [6] J.C. Grenier, L. Fourne´s, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 21 (1986) 441–449. [7] J. Rodrı´guez, J.M. Gonza´lez-Calbet, J.C. Grenier, J. Pannertier, M. Anne, Solid State Commun. 62 (1987) 231–234. [8] P.D. Battle, T.C. Gibb, J. Chem. Soc. Dalton Trans. (1987) 667–671. [9] P.D. Battle, T.C. Gibb, A.T. Steel, J. Chem. Soc. Dalton Trans. (1987) 2359–2363. [10] P.D. Battle, T.C. Gibb, A.T. Steel, J. Chem. Soc. Dalton Trans. (1988) 83–87. [11] V.V. Vashook, M.V. Zinkevich, H. Ullmann, J. Paulsen, N. Trofimenko, K. Teske, Solid State Ionics 99 (1997) 23–32. [12] V.V. Vashook, M.V. Zinkevich, Y.Z. Zonov, Solid State Ionics 116 (1999) 129–138. [13] W.T.A. Harrison, S.L. Hegwood, A.J. Jacobson, J. Chem. Soc. Chem. Commun. (1995) 1953–1954. [14] H. Taguchi, Y. Takeda, F. Kanamaru, M. Shimada, M. Koizumi, Acta Cryst. B33 (1977) 1298–1299. [15] H. Fjellvag, E. Gulbrandsen, S. Aasland, A. Olsen, B.C. Hauback, J. Solid State Chem. 124 (1996) 190–194. [16] O. Gourdon, V. Petricek, M. Dusek, P. Bezdicka, S. Durovic, D. Gyepesova, M. Evain, Acta Cryst. B55 (1999) 841–848. [17] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, J. Solid State Chem. 142 (1999) 419–427. [18] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, J. Solid State Chem. 145 (1999) 116–127. [19] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, A. Vegas, J. Solid State Chem. 151 (2000) 77–84. [20] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, Chem. Mater. 12 (2000) 25–32. [21] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, Chem. Mater. 12 (2000) 2727–2735. [22] M.-H. Whangbo, H.-J. Koo, K.-S. Lee, O. Gourdon, M. Evain, S. Jobic, R. Brec, J. Solid State Chem. 160 (2001) 239–246. [23] A. El Abed, S.E. Elqebbaj, M. Zakhour, M. Champeaux, J.M. Perez-Mato, J. Darriet, J. Solid State Chem. 161 (2001) 300–306. [24] J.M. Gonza´lez-Calbet, K. Boulahya, M.L. Ruiz, M. Parras, J. Solid State Chem. 162 (2001) 322–326. [25] J. Darriet, L. Elcoro, A. El Abed, E. Gaudin, J.M. Perez-Mato, Chem. Mater. 14 (2002) 3349–3363. [26] J.M. Perez-Mato, M. Zakhour-Nakhl, F. Weill, J. Darriet, J. Mater. Chem. 9 (1999) 2795–2808. [27] L. Elcoro, J.M. Perez-Mato, J. Darriet, A. El Abed, Acta Cryst. B59 (2003) 217–233. [28] K. Iwasaki, M. Shimada, H. Yamane, J. Takahashi, S. Kubota, T. Nagasaki, Y. Arita, J. Yuhara, Y. Nishi, T. Matsui, J. Alloys Compd. 377 (2004) 272–276. [29] F. Izumi, T. Ikeda, Mater. Sci. Forum 198 (2000) 321–324.
Synthesis of an oxygen nonstoichiometric Sr6Co5O15 phase Kouta Iwasaki a,*, Tsuyoshi Ito a, Tsuneo Matsui a,b, Takanori Nagasaki b, Shingo Ohta c, Kunihito Koumoto c a
Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan c Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan Received 19 May 2005; received in revised form 6 October 2005; accepted 10 October 2005 Available online 8 November 2005
Abstract Sr6Co5O15, a member of the (A3Co2O6)m(A3Co3O9)n [A = Ca, Sr, Ba] series, showed oxygen nonstoichiometry. Sr6Co5O15.12, Sr6Co5O14.98, Sr6Co5O14.45 and Sr6Co5O14.26 were prepared by a solid state reaction in air. The Sr6Co5O15 phase is stable in the temperature range of 873–973 K in air, there are structure transitions at 773 K and above 1023 K. With the loss of oxygen, the a-axis ˚ (Sr6Co5O15.12) to 9.4390(4) A ˚ (Sr6Co5O14.26) and the c-axis parameter of the Sr6Co5O15 phase decreased from 9.4988(3) A ˚ (Sr6Co5O15.12) to 12.5066(4) A ˚ (Sr6Co5O14.26) with decreasing oxygen content. The parameter increased from 12.3772(3) A Rietveld analysis of the powder X-ray diffraction data suggested that the increase in the c-axis was mainly due to the increase in the Co–Co distance between the CoO6 trigonal prism and CoO6 octahedron. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Oxides; C. Thermogravimetric analysis; C. X-ray diffraction; D. Crystal structure
1. Introduction The oxygen content in the SrCoO3 x system is one of the factors determining the phase stability and crystal structure; the orthorhombic (brownmillerite-type), cubic (perovskite-type) and rhombohedral phases are the main phases that have been studied [1–12]. The rhombohedral phase, which is thought to have a hexagonal BaNiO3-type structure, is currently known as Sr6Co5O15. The crystal structure of Sr6Co5O15 was analyzed by Harrison et al. using powder neutron diffraction data [13]. Members of the (A3Co2O6)m(A3Co3O9)n (or A3m+3nCo2m+3nO6m+9n) and (A8Co6O18)a(A8Co8O24)b (or A4a+4bCo3a+4bO9a+12b) [A = Ca, Sr, Ba] series have pseudo-one-dimensional structures consisting of Co–O chains and alkaline earth atoms, in which CoO6 octahedra and CoO6 trigonal prisms are connected sharing their faces and alkaline earth atoms isolate the Co–O chains [13–27]. Sr6Co5O15 (m = 1, n = 1) is a member of the (A3Co2O6)m(A3Co3O9)n series, and four consecutive CoO6 octahedra and one CoO6 trigonal prism are connected ˚ and c = 12.3966(4) A ˚ [13]), as shown in Fig. 1. in the unit cell (space group R32, a = 9.5035(2) A
* Corresponding author. Tel.: +81 52 789 4689; fax: +81 52 789 3779. E-mail address: [email protected] (K. Iwasaki). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.10.012
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Fig. 1. Crystal structure of Sr6Co5O15.
One of the problems in the (A3Co2O6)m(A3Co3O9)n and (A8Co6O18)a(A8Co8O24)b series is that other members are synthesized as byproducts in addition to the target member because of the similarity of the sample compositions. Sr6Co5O15 can be obtained by annealing at 1148 K in air [13]. Our previous study on Sr6Co5O15 showed that Sr6Co5O15 was not always stable below 1148 K. For example, an unidentified Sr–Co–O phase was obtained below 773 K from Sr6Co5O15, and Sr6Co5O15 exhibited weight loss at high temperatures [28]. Understanding the stability and nonstoichiometry of Sr6Co5O15 is important since contamination by an impurity phase and the variation of sample composition often affect the physical properties. In this study, we investigated the relation between the synthesis conditions and formation of the Sr6Co5O15 phase. Four samples of the Sr6Co5O15 phase with different oxygen contents were prepared by a solid state reaction, and the crystal structures were refined. Throughout this paper, we will refer to the Sr6Co5O15 phase obtained by heating at 1173 K in air simply as Sr6Co5O15. 2. Experimental SrCO3 (99.99+%, Rare Metallic Co.) and Co3O4 (99.95%, Kanto Chemical Co.) powders were used as starting materials. The powders were mixed in the appropriate proportion (molar ratio of Sr:Co was 6:5) in an agate mortar with ethanol, and pressed into a pellet form. The pellet was put on a Pt sheet and heated (Yamato, muffle furnace FO310) at 1173 K for 210 h in air with intermediate grindings every 70 h to obtain single phase Sr6Co5O15. In this study, four samples of the Sr6Co5O15 phase with different oxygen contents were prepared as follows: (a) Sr6Co5O15 was heated at 1123 K for 70 h in air and then cooled in the furnace (it took about 0.6 h to decrease the temperature inside the furnace from 1123 K to 773 K and 2.5 h from 773 K to 373 K); (b) Sr6Co5O15 was heated at 1123 K for 15 h in air and cooled outside the furnace at room temperature in air; (c) Sr6Co5O15 was heated at 873 K for 15 h in air and quenched in liquid nitrogen; (d) Sr6Co5O15 was heated at 973 K for 15 h in air and quenched in liquid nitrogen. The X-ray powder diffraction (XRD) patterns were recorded by a diffractometer (Rigaku, RINT2200) using Cu Ka radiation with a pyrolytic graphite monochromator. The crystal structures were refined by the Rietveld method using the program RIETAN-2000 [29] with XRD data in the range 108 2u 1208 (scan step 0.038). Thermogravimetry and differential thermal analysis (TG-DTA) were carried out in air at a heating and cooling rate of 10 K/min (Rigaku, TG8120). Quantitative analysis of oxygen was performed by the He carrier melting–infrared absorption method (LECO, TC-436). 3. Results and discussion Upon heating at 1173 K in air, an unidentified impurity phase formed on the surface of the pellet in contacted with the Pt sheet. Single phase Sr6Co5O15 was obtained by scraping away the surface of the pellet. In the energy dispersion X-ray spectroscopy analysis (EDS, JEOL JSM-6460LA), no Pt contamination was detected in Sr6Co5O15. Fig. 2 shows the TG-DTA curves for Sr6Co5O15 measured over the temperature range 300–1173 K in air at a heating and cooling rate of 10 K/min. Sr6Co5O15 showed weight loss above 650 K; the sample weight decreased with increasing temperature. The weight loss was about 1.2% at 1000 K. No endothermic and exothermic peaks were
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Fig. 2. TG and DTA curves for Sr6Co5O15 in air at a heating and cooling rate of 10 K/min. Black line and gray line show the data from 300 K to 1173 K and from 1173 K to 300 K, respectively.
observed. As the temperature decreased from 1173 K to 300 K, the sample weight increased up to the starting weight of the sample. The reversible change of the sample weight indicates the oxygen nonstoichiometry of Sr6Co5O15. The weight loss of 1.2% at 1000 K corresponds to the sample composition of Sr6Co5O14.21 assuming the starting composition of the sample is Sr:Co:O = 6:5:15. When Sr6Co5O15 was heated in air at various temperatures and quenched in liquid nitrogen, single phase Sr6Co5O15 d, which was confirmed to have a Sr6Co5O15 structure by the Rietveld method as described later, was obtained in the temperature range of 873–973 K. On the other hand, the single phase was obtained between 873 K and 1173 K when the sample was cooled in the furnace (Table 1). Fig. 3 shows the XRD patterns of samples prepared under several different conditions. The Miller indices are described according to the hexagonal axes. When Sr6Co5O15 was heated at 1223 K and cooled in the furnace, single phase Sr6Co5O15 was not obtained, as shown in Fig. 3(a). The XRD pattern was similar to that of Sr6Co5O15, however, the peaks were not indexed with Sr6Co5O15. There were also some peaks around 2u = 298, where the (1 1 3) peak of Sr6Co5O15 appears, suggesting that the sample contained more than two phases. Single phase Sr6Co5O15 was obtained at 1173 K and 873 K (see Fig. 3(b) and (c), respectively), and the (1 1 3) peak tended to broaden with decreasing temperature. When Sr6Co5O15 was heated at 773 K, a shoulder peak appeared at lower angles of the (1 1 3) peak of Sr6Co5O15, as reported in our previous study [28]. The intensity of the peak at 2u = 28.28 increased upon additional heating at 773 K for 420 h (Fig. 3(d)). The XRD pattern was similar to that of Sr6Co5O15, but not identified as Sr6Co5O15. The similarity of the XRD patterns suggested that the compound obtained at 773 K was a member of the (A3Co2O6)m(A3Co3O9)n series, and lattice ˚ , c1 = 2.454 A ˚ and q = 0.5806 A ˚ (c2 = 4.227 A ˚ ), could parameters with an incommensurate phase, a = 9.492 A explain the XRD pattern. Some unidentified peaks were also included in the XRD pattern; the small peak around 2u = 28.68 was attributed to the (1 1 3) peak of Sr6Co5O15. Extra heating did not reduce the intensity of the unidentified peaks, suggesting that the Sr/Co ratio of the compound obtained at 773 K is not 6/5. The change of the crystal structure of Sr6Co5O15 at 773 K is mainly attributable to the change of the atomic arrangement in the c-axis Table 1 Phases obtained by cooling of Sr6Co5O15 after heating at various temperatures Temperature (K)
Heated for 70 h in air and cooled in the furnace
Heated for 15 h in air and quenched in liquid N2
1223 1173 1123 1023 973 873 773
Sr6Co5O15 Sr6Co5O15 – Sr6Co5O15 Sr6Co5O15
– – Sr6Co5O15 Sr6Co5O15 –
‘’, not single phase of Sr6Co5O15; ‘–’, uninvestigated.
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Fig. 3. XRD patterns of Sr6Co5O15 cooled in a heating furnace after heating at (a) 1223 K for 70 h, (b) 1173 K for 70 h, (c) 873 K for 70 h, (d) 773 K for 420 h and quenched after heating at (e) 1023 K for 15 h.
˚ for Sr6Co5O15.12 as described direction since the a-axis was similar to that of the Sr6Co5O15 phase (a = 9.4988 A later). The broadening in the (1 1 3) peak observed at 873 K (Fig. 3(c)) also signals the beginning of the change in atomic arrangement. The change of the Sr6Co5O15 phase was also shown at 573 K; a shoulder peak was found at lower angles of the (1 1 3) peak when Sr6Co5O15 was heated at 573 K for 420 h in air. These results indicate that the Sr6Co5O15 phase is unstable at 573–773 K in air. However, single phase Sr6Co5O15 can be obtained without quenching since the transformation of the crystal structure is not rapid. In the case of the sample that was quenched in liquid N2 after heating, single phase Sr6Co5O15 d was obtained at 873 K and 973 K. When Sr6Co5O15 was heated above 1023 K, the single phase was not obtained, and at least two XRD peaks appeared at around 2u = 32.88, as shown in Fig. 3(e). When the sample was cooled in Ar gas from 1023 K to 773 K at a cooling rate of 125 K/h after heating at 1023 K, the XRD pattern obtained was almost the same as that of the sample quenched in liquid N2. This indicates that the cooling rate does not influence the products. Thus, the change of the crystal structure of Sr6Co5O15 above 1023 K is attributable to the decrease in oxygen content. These results show that the Sr6Co5O15 phase is stable between 873 K and 973 K in air and that the absorption of oxygen is necessary for the formation of the Sr6Co5O15 phase when the sample is cooled after heating at 1023 K or higher. In our previous study, the Seebeck coefficient of Sr6Co5O15 was measured over the temperature range 300–1120 K in air [28]. The Seebeck coefficient of Sr6Co5O15 exhibited a complex temperature dependence: the Seebeck coefficient decreased at 300–750 K (130–110 mV/K), increased at 750–1000 K (110–138 mV/K) and decreased at 1000–1120 K (138–129 mV/K) with increasing temperature. The decrease in the oxygen content of Sr6Co5O15 above 650 K and the change of the Sr6Co5O15 phase above 1023 K seem to be related to the increase in the Seebeck coefficient between 750 K and 1000 K and the decrease between 1000 K and 1120 K, respectively. The oxygen content of the Sr6Co5O15 phase could be controlled by varying the cooling conditions since Sr6Co5O15 showed oxygen nonstoichiometry above 650 K. In this study, four Sr6Co5O15 samples with oxygen contents of (a) 15.12 0.08, (b) 14.98 0.09, (c) 14.45 0.04 and (d) 14.26 0.12 were prepared. The oxygen content of 15.12 for sample (a) exceeded the 15 for stoichiometric Sr6Co5O15, suggesting the presence of cation defects. However, the details of this remain unclear. The crystal structures of the (A3Co2O6)m(A3Co3O9)n series are sensitive to the sample composition: subtle variations of the composition often lead to different arrangements of the CoO6 octahedra and CoO6 trigonal prisms. Crystal structure refinement was performed by the Rietveld method (RIETAN-2000 program [29]) to investigate whether the crystal structures of Sr6Co5O15.12, Sr6Co5O14.98, Sr6Co5O14.45 and Sr6Co5O14.26 were influenced by differences in oxygen content. In the refinement, the crystallographic parameters of Sr6Co5O15 [13] were used as the initial structure model. The oxygen occupancies were allocated equally to each oxygen site. The oxygen occupancies for Sr6Co5O15.12 were replaced with 1. This assumption was that this approximation would not significantly affect the structure refinement since the atomic scattering factor of oxygen is smaller than those of Sr and Co atoms. The results of the crystal structure refinement are summarized in Table 2, and the profile-fit of the XRD data for Sr6Co5O15.12 and
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Table 2 Crystallographic parameters and results of crystal structure refinement for Sr6Co5O15 phase Atom
9d 9e 3b 6c 6c 9d 18f 18f
Sr6Co5O15.12
Sr6Co5O14.98
g
x
y
z
B
g
x
y
z
B
1 1 1 1 1 1 1 1
0.3232(3) 0.6423(3) 0 0 0 0.845(2) 0.4965(16) 0.8434(12)
0 0 0 0 0 0 0.6816(13) 0.9742(13)
0 0.5 0.5 0.0963(7) 0.2934(4) 0 0.4776(8) 0.6109(9)
0.80(11)
1 1 1 1 1 0.9987 0.9987 0.9987
0.3230(3) 0.6421(2) 0 0 0 0.847(2) 0.4957(16) 0.8461(12)
0 0 0 0 0 0 0.6793(13) 0.9793(13)
0 0.5 0.5 0.0970(6) 0.2914(4) 0 0.4798(8) 0.6082(9)
0.87(11)
˚) a-Axis (A ˚) c-Axis (A ˚ 3) vol. (A Rwp (%) RI (%) RF (%) S Atom
Sr1 Sr2 Co1 Co2 Co3 O1 O2 O3 ˚) a-Axis (A ˚) c-Axis (A ˚ 3) vol. (A Rwp (%) RI (%) RF (%) S
0.34(16)
0.6(2)
9.4988(3) 12.3772(3) 967.13(4) 12.12 3.03 2.62 1.29 Site
9d 9e 3b 6c 6c 9d 18f 18f
0.56(16)
0.6(2)
9.4890(2) 12.4024(2) 967.11(3) 12.28 3.55 3.00 1.33
Sr6Co5O14.45
Sr6Co5O14.26
g
x
y
z
B
g
x
y
z
B
1 1 1 1 1 0.9633 0.9633 0.9633
0.3208(3) 0.6398(3) 0 0 0 0.843(2) 0.4921(16) 0.8654(15)
0 0 0 0 0 0 0.6679(14) 0.0180(16)
0 0.5 0.5 0.0983(5) 0.2873(4) 0 0.4836(9) 0.6127(9)
0.88(11)
1 1 1 1 1 0.9507 0.9507 0.9507
0.3226(3) 0.6384(3) 0 0 0 0.841(2) 0.498(2) 0.8484(16)
0 0 0 0 0 0 0.6843(15) 0.980(2)
0 0.5 0.5 0.0981(5) 0.2864(4) 0 0.4832(9) 0.6147(10)
0.67(11)
0.43(16)
0.9(2)
9.4468(3) 12.4801(3) 964.52(4) 13.23 4.07 3.79 1.44
Space group: R32 [No. 155]; linear constraint: B [Sr1] = B [Sr2], B [Co1] = B [Co2] = B [Co3], B [O1] = B [O2] = B [O3].
9.4390(4) 12.5066(4) 964.98(6) 12.65 3.97 3.11 1.37
0.16(16)
0.5(2)
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Sr1 Sr2 Co1 Co2 Co3 O1 O2 O3
Site
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Fig. 4. X-ray Rietveld refinement profiles for (a) Sr6Co5O15.12 and (b) Sr6Co5O14.26. Points are the observed data and solid line is the calculated profile. Marks below the profile show the allowed reflections. The difference between observed and calculated data is presented beneath.
Sr6Co5O14.26 are shown in Fig. 4. The crystal structure of each sample could be refined, suggesting that the oxygen nonstoichiometric phases have the Sr6Co5O15 structure. The interatomic distances (Table 3) also corresponded to those of related compounds, Sr6Co5O15 reported by Harrison et al. [13] and Sr14Co11O33 [16]. The comparisons of Co–Co and Co–O distances were as follows: (i) The ˚ for Sr6Co5O14.26, Co–Co (Co1–Co3) distance between the CoO6 trigonal prism and CoO6 octahedron was 2.672 A ˚ ˚ which was larger than 2.50 A for Sr6Co5O15 and 2.61 A for Sr14Co11O33; (ii) The average values of Co–Co (Co2–Co2 ˚ for Sr6Co5O15.12, 2.410 A ˚ for and Co2–Co3) distances between the CoO6 octahedra in the unit cell were 2.421 A ˚ ˚ ˚ for Sr6Co5O14.98, 2.390 A for Sr6Co5O14.45 and 2.387 A for Sr6Co5O14.26. These values were smaller than 2.467 A ˚ ˚, Sr6Co5O15 and 2.44 A for Sr14Co11O33; (iii) the Co–O (Co1–O3) distances in the CoO6 trigonal prism, 1.920–1.968 A ˚ ˚ were smaller than 1.973 A for Sr6Co5O15 and 2.05 A for Sr14Co11O33; (iv) in the CoO6 octahedra, the average Co2–O ˚ ) and the average Co3–O distances (1.894–1.916 A ˚ ) were almost constant, i.e. independent distances (1.867–1.881 A ˚ ˚ for the of oxygen content. These values agreed with those for Sr6Co5O15 (1.89 A for the Co2–O distance and 1.92 A ˚ Co3–O distance) and Sr14Co11O33 (average Co–O distance of 1.90 A). Fig. 5 shows the relation between the lattice parameters and the oxygen content for the Sr6Co5O15 phase. The a-axis ˚ (Sr6Co5O15.12) to 9.4390(4) A ˚ (Sr6Co5O14.26) and the c-axis parameter parameter decreased from 9.4988(3) A Table 3 ˚ ) of Sr6Co5O15 phase Interatomic distances (A
Co1–O3 6 Co2–O1 3 Co2–O2 3 Co3–O2 3 Co3–O3 3 Co1–Co3 2 Co2–Co2 1 Co2–Co3 2 Sr1–O1 2 Sr1–O2 2 Sr1–O2 2 Sr1–O3 2 Sr1–O3 2 Sr2–O1 2 Sr2–O2 2 Sr2–O2 2 Sr2–O3 2 Sr2–O3 2
Sr6Co5O15.12
Sr6Co5O14.98
Sr6Co5O14.45
Sr6Co5O14.26
1.947(10) 1.895(14) 1.877(14) 1.967(13) 1.820(13) 2.558(5) 2.383(16) 2.440(12) 2.659(2) 2.498(13) 2.833(12) 2.641(11) 3.120(14) 2.605(11) 2.637(11) 2.688(11) 2.462(12) 3.228(12)
1.920(10) 1.877(13) 1.857(14) 1.971(13) 1.853(13) 2.587(5) 2.405(16) 2.412(12) 2.656(2) 2.517(12) 2.809(12) 2.688(11) 3.086(11) 2.625(11) 2.651(10) 2.662(11) 2.443(12) 3.261(13)
1.959(10) 1.927(14) 1.830(14) 1.982(13) 1.850(13) 2.654(5) 2.455(13) 2.358(10) 2.625(2) 2.543(13) 2.753(12) 2.770(13) 2.971(14) 2.595(11) 2.534(11) 2.730(11) 2.487(12) 3.239(13)
1.968(11) 1.937(16) 1.825(16) 1.962(15) 1.830(14) 2.672(5) 2.454(14) 2.354(10) 2.637(3) 2.557(14) 2.785(13) 2.667(14) 3.049(14) 2.587(12) 2.594(12) 2.657(13) 2.527(13) 3.207(13)
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Fig. 5. Lattice parameters of the Sr6Co5O15 phase.
Fig. 6. Relation between the Co–Co distances and c-axis for the Sr6Co5O15 phase.
˚ (Sr6Co5O15.12) to 12.5066(4) A ˚ (Sr6Co5O14.26) with decreasing oxygen content. The increased from 12.3772(3) A decrease in oxygen content leads to the decrease in the average valence state of Co ions (e.g. +3.65 for Sr6Co5O15.12 and +3.30 for Sr6Co5O14.26), thus, the average radius of Co ions increases with decreasing oxygen content. The increase in the c-axis corresponds to the increase in the ionic radius, however, the decrease in the a-axis is not consistent with the increase in the ionic radius. The Co1 site is in the CoO6 trigonal prism, and the Co2 and Co3 sites are in the CoO6 octahedra. Each Co site is located on the z-axis, thus, the changes in Co–Co distances are directly related to the changes in the c-axis length. The relation between the Co–Co distances and the c-axis is shown in Fig. 6. The Co1–Co3 and Co2–Co2 distances tended to increase with increasing the c-axis, while the Co2–Co3 distance decreased. The Co2–O2 distance ˚ to 1.825 A ˚ with decreasing oxygen content, while the Co3–O2 distance remained almost decreased from 1.877 A constant, which indicates that the Co atom in the Co2 site approached the Co3 site with decreasing oxygen content. The increase in the c-axis was mainly due to the increase in the Co1–Co3 distance between the CoO6 trigonal prism ˚ , and the increase in the and CoO6 octahedron (the difference between Sr6Co5O15.12 and Sr6Co5O14.26 was 0.114 A ˚ ˚) unit cell was 0.114 2 = 0.228 A) relative to the increase in the Co2–Co2 distance (the difference was 0.071 A between CoO6 octahedra.
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4. Conclusion The oxygen nonstoichiometric Sr6Co5O15 phases were prepared. The Sr6Co5O15 phase was stable over the temperature range from 873 K to 973 K in air, however, weight loss due to oxygen release occurred above 650 K. The oxygen release and absorption were reversible, thus, the Sr6Co5O15 phase can be obtained upon cooling from 1023 K or higher. The decrease in the oxygen content resulted in a decrease in the a-axis parameter and an increase in the caxis parameter. The presence of an oxygen nonstoichiometric Sr6Co5O15 phases suggest that oxygen nonstoichiometry may also be observed for other members of the (A3Co2O6)m(A3Co3O9)n and (A8Co6O18)a(A8Co8O24)b series. Acknowledgements The authors wish to express their gratitude to Dr. Teiichi Kimura and Dr. Kunio Takada (Institute for Materials Research, Tohoku University) for their cooperation in the analysis of the oxygen content. This work was partly supported by The Foundation ‘‘Hattori-Hokokai’’. References [1] H. Watanabe, T. Takeda, in: Y. Hoshino (Ed.), FERRITES: Proceedings of the International Conference on Ferrites, Kyoto, Japan, July 1970, University of Tokyo Press, Tokyo, 1971, pp. 588–590. [2] T. Takeda, H. Watanabe, J. Phys. Soc. Jpn. 33 (1972) 973–978. [3] J.C. Grenier, S. Ghodbane, G. Demazeau, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 831–839. [4] Y. Takeda, R. Kanno, T. Takada, O. Yamamoto, M. Takano, Y. Bando, Z. Anorg. Allg. Chem. 540/541 (1986) 259–270. [5] J. Rodrı´guez, J.M. Gonza´lez-Calbet, Mater. Res. Bull. 21 (1986) 429–439. [6] J.C. Grenier, L. Fourne´s, M. Pouchard, P. Hagenmuller, Mater. Res. Bull. 21 (1986) 441–449. [7] J. Rodrı´guez, J.M. Gonza´lez-Calbet, J.C. Grenier, J. Pannertier, M. Anne, Solid State Commun. 62 (1987) 231–234. [8] P.D. Battle, T.C. Gibb, J. Chem. Soc. Dalton Trans. (1987) 667–671. [9] P.D. Battle, T.C. Gibb, A.T. Steel, J. Chem. Soc. Dalton Trans. (1987) 2359–2363. [10] P.D. Battle, T.C. Gibb, A.T. Steel, J. Chem. Soc. Dalton Trans. (1988) 83–87. [11] V.V. Vashook, M.V. Zinkevich, H. Ullmann, J. Paulsen, N. Trofimenko, K. Teske, Solid State Ionics 99 (1997) 23–32. [12] V.V. Vashook, M.V. Zinkevich, Y.Z. Zonov, Solid State Ionics 116 (1999) 129–138. [13] W.T.A. Harrison, S.L. Hegwood, A.J. Jacobson, J. Chem. Soc. Chem. Commun. (1995) 1953–1954. [14] H. Taguchi, Y. Takeda, F. Kanamaru, M. Shimada, M. Koizumi, Acta Cryst. B33 (1977) 1298–1299. [15] H. Fjellvag, E. Gulbrandsen, S. Aasland, A. Olsen, B.C. Hauback, J. Solid State Chem. 124 (1996) 190–194. [16] O. Gourdon, V. Petricek, M. Dusek, P. Bezdicka, S. Durovic, D. Gyepesova, M. Evain, Acta Cryst. B55 (1999) 841–848. [17] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, J. Solid State Chem. 142 (1999) 419–427. [18] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, J. Solid State Chem. 145 (1999) 116–127. [19] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, A. Vegas, J. Solid State Chem. 151 (2000) 77–84. [20] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, Chem. Mater. 12 (2000) 25–32. [21] K. Boulahya, M. Parras, J.M. Gonza´lez-Calbet, Chem. Mater. 12 (2000) 2727–2735. [22] M.-H. Whangbo, H.-J. Koo, K.-S. Lee, O. Gourdon, M. Evain, S. Jobic, R. Brec, J. Solid State Chem. 160 (2001) 239–246. [23] A. El Abed, S.E. Elqebbaj, M. Zakhour, M. Champeaux, J.M. Perez-Mato, J. Darriet, J. Solid State Chem. 161 (2001) 300–306. [24] J.M. Gonza´lez-Calbet, K. Boulahya, M.L. Ruiz, M. Parras, J. Solid State Chem. 162 (2001) 322–326. [25] J. Darriet, L. Elcoro, A. El Abed, E. Gaudin, J.M. Perez-Mato, Chem. Mater. 14 (2002) 3349–3363. [26] J.M. Perez-Mato, M. Zakhour-Nakhl, F. Weill, J. Darriet, J. Mater. Chem. 9 (1999) 2795–2808. [27] L. Elcoro, J.M. Perez-Mato, J. Darriet, A. El Abed, Acta Cryst. B59 (2003) 217–233. [28] K. Iwasaki, M. Shimada, H. Yamane, J. Takahashi, S. Kubota, T. Nagasaki, Y. Arita, J. Yuhara, Y. Nishi, T. Matsui, J. Alloys Compd. 377 (2004) 272–276. [29] F. Izumi, T. Ikeda, Mater. Sci. Forum 198 (2000) 321–324.