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3)O3
Journal of Alloys and Compounds 472 (2009) 502–506
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Crystallographic and Raman spectroscopic studies of microwave dielectric ceramics Ba(Ca1/3 Nb2/3 )O3 Jinxia Deng a , Jun Chen a , Ranbo Yu a , Guirong Liu a , Xianran Xing a,b,∗ a b
Department of Physical Chemistry, University of Science & Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 6 November 2007 Received in revised form 2 May 2008 Accepted 2 May 2008 Available online 16 June 2008 Keywords: Ceramics X-ray diffraction Order–disorder effects Transmission electron microscopy Crystal structure
a b s t r a c t The crystal structure of the microwave dielectric ceramics Ba(Ca1/3 Nb2/3 )O3 synthesized by the molten salt 3 ¯ method has been refined, from X-ray diffraction, in a trigonal order–disorder lattice with the P 3m1(D 3d ) space group. The lattice distortion and cation ordering are attributed to the large Ca2+ ions at B -site, which induces a large deviation of bond valence sums of the B-site ions. The ordering degree determined by the variations of the occupancies of Ca and Nb ions is 0.44. High-resolution transmission electron microscopy (HRTEM) and Raman spectrum show the evidence in support of the 1:2 B-site order–disorder model. The complete Raman mode assignment for Ba(Ca1/3 Nb2/3 )O3 with order–disorder structure is also presented. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Complex perovskite ceramics have attracted considerable attention for their remarkable microwave dielectric properties [1,2]. The low microwave losses are observed in alkaline-earth based niobate and tantalate members of the 1:2 type A(B 1/3 B 2/3 )O3 family of mixed-metal perovskite (A = Ca, Sr, Ba; B = Mg, Ca, Sr, Mn, Fe, Ni, Cu, Zn, Cd; and B = Nb, Ta) [3–5]. From a structural point of view, these materials can be classified into ordered and disordered types with respect to the ordering degree of the divalent B and pentavalent B cations [6–8]. For these systems, the ordering degree on the octahedral sites plays a critical role in mediating the dielectric response, and the highest quality factors are associated with a fully ordered arrangement of the B and B cations along each of the pseudo-cubic [1 1 1] directions [8] (Fig. 1). The probability for an ordered arrangement of the B and B cations is essentially controlled by the differences in their ionic charges and radii [9,10]. Large Cd2+ ions on B -site were confirmed with a trigonal order–disorder structure [5]. Alkalineearth element Ca with larger ionic radius could occupy both the A2+ site and B2+ site [10–12] (RCa2+ = 0.134 nm, CN = 12; RCa2+ = 0.100 nm > RCd2+ = 0.095 nm > RZn2+ = 0.074 nm > RMg2+ = 0.072 nm > RNb5+ = 0.064 nm, CN = 6 [13]). By the occupation
∗ Corresponding author. Tel.: +86 10 62334200; fax: +86 10 62332525. E-mail address: [email protected] (X. Xing). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.05.006
of a larger Ca2+ cation in the B -site, the crystal structure of A(B 1/3 B 2/3 )O3 perovskite will be distorted, and tend to be an more ordered structure. In this work, an order–disorder structure model based on the detailed quantitative Rietveld analysis of the X-ray diffraction (XRD) patterns is developed. Besides clarifying the order trigonal structure of Ba(Ca1/3 Nb2/3 )O3 , the ordering degree can also be calculated. This is verified by the high-resolution transmission electron microscopy (HRTEM) investigation. Furthermore, Raman scattering spectrum is performed to determine ordering [14]. 2. Experimental Ba(Ca1/3 Nb2/3 )O3 compound was synthesized by a molten salt method. The starting materials were BaCO3 (A.R.), CaO (A.R.), Nb2 O5 (A.R.) and NaCl (A.R.). The mole ratio of the salts NaCl to Ba(Ca1/3 Nb2/3 )O3 was 10. Stoichiometric starting reactants were thoroughly ground with ethanol in an agate mortar for 30 min. Dried powders were then calcined at 850 ◦ C for 10 h. The chlorides were removed from the products by hot deionized water washing for several times until the filtrate gave no reaction with silver nitrate solution. The as-prepared material was finally dried at 100 ◦ C in a drying oven. For structure refinement, XRD data were collected at room temperature using a 21 kW extra-power powder XRD (Model M21XVHF22, Mac Science, Yokohama, Japan), operating with Cu K␣ radiation and equipped with a graphite monochromator. A step-scan mode was employed with a step width of 0.02◦ . The program Fullprof [15] was used for the Rietveld refinement. HRTEM observation and the corresponding selected area electron diffraction (SAED) patterns were carried out on a JEM-2010 electron microscope (JEOL Ltd., Japan). For HRTEM observation, the synthesized powders were ultrasonically dis-
J. Deng et al. / Journal of Alloys and Compounds 472 (2009) 502–506
503
Fig. 2. Observed (circles), calculated (line) and difference (bottom of figure) X-ray diffraction profiles for Ba(Ca1/3 Nb2/3 )O3 at room temperature. The tick marks below the X-ray pattern give the position of all possible Bragg reflections. The inset is the enlarged pattern from 14◦ to 27◦ .
Fig. 1. Structure of 1:2 B-site ordered A(B 1/3 B 2/3 )O3 viewed along the [1 0 0] direction. B O6 octahedra are dark blue, B O6 octahedra are line gray and A ions are omitted for clarity. (For interpretation of the reference to color in this figure legend, the reader is referred to the web version of the article.)
persed in ethanol, and a drop of the suspension was placed on a Cu grid coated with carbon film. Raman scattering data were collected at room temperature on a JYT64000 Raman spectrometer (Jobin Yvon, France).
3. Results and discussion 3.1. Structure refinement The XRD data of the Ba(Ca1/3 Nb2/3 )O3 powder synthesized by the molten salt method for the 2 region of 10–110◦ were used for the refinement. It could be completely indexed on the basis ¯ and ordered trigof both the disordered cubic space group Pm3m ¯ onal space group P 3m1 (based on the Ba(Mg1/3 Ta2/3 )O3 [16] see Table 1). In our initial refinement, these two structural models were both introduced. However, in the disordered model, weak superlattice reflection is present at the 2 angle 17.3◦ which could be identified as a superstructure reflecting the ordering of Ca/Nb ions at B /B sites. We could also observe the larger preferred orienTable 1 ¯ and P 3m1 ¯ Structure model based on the space groups Pm3m Atom
Site
x
y
z
SOF
1 ¯ Pm3m(O h) Ba Ca Nb O
1a 1b 1b 3c
0 1/2 1/2 0
0 1/2 1/2 1/2
0 1/2 1/2 1/2
1 1/3 2/3 1
3 ¯ P 3m1(D 3d ) Ba1 Ba2 Ca Nb O1 O2
1a 2d 1b 1b 6i 3e
0 1/3 0 1/3 x 1/2
0 2/3 0 2/3 y 0
0 z 1/2 z z 0
1 1 1 2 1 1
tation along the cubic (1 1 1) direction. Moreover the refinement of the isotropic thermal parameters of Ca and Nb, which related to the displacements of atoms, leaded to slightly higher values about 2.0 A˚ 2 . Large R-factors were obtained for the sample refined by the disordered model. And the R-factors in the ordered model (Rp = 8.55, Rwp = 12.0, RB = 6.39, S = 2.20) were smaller than those of disordered model. However isotropic thermal parameters of the O atoms (Biso(O) = 1.7812 A˚ 2 ) were slightly higher. In the final refinement, the crystallographic model was based on ¯ the order trigonal space group P 3m1, which allows the Ca and Nb to occupy the two symmetry-inequivalent sites, 1b and 2d sites. The structure parameters are the same for the Ca1 and Nb1 on 1b sites and also for the Ca2 and Nb2 on 2d sites except for the occupation factors (Occ.). Furthermore the occupation factors of the Ca1, Nb1, Ca2, and Nb2 sites have the linear constraints: OccCa2 = OccNb1 , and 2(OccCa1 + OccNb1 ) = OccCa2 + OccNb2 . It was found that the splitting of the 1b site and 2d site was successful in terms of the refinement: introduce of the disordering of Ca/Nb over the two available sites lead to a remarkable improvement of the profile fitting. An excellent fitting result was obtained for this order–disorder model, as illustrated in Fig. 2. Table 2 lists the final atomic coordinates and agreement factors. Table 3 lists the selected interatiomic distances and angles of the trigonal order–disorder Ba(Ca1/3 Nb2/3 )O3 . The bond angle of Ca1/Nb1-O1-Ca2/Nb2 are reduced to 176.480◦ . The deviation from 180◦ reduces the imbalance of local valance, and could therefore stabilize the perovskite structure. It is essential for the ordering assignment and the charge transfer between the Ca1(Nb1)O6 layer Table 2 Refined structural parameters of Ba(Ca1/3 Nb2/3 )O3 a based on the trigonal order–disorder model Atom
Site
x
y
z
˚ B (A)
SOF
Ba1 Ba2 Ca1/Nb1 Nb2/Ca2 O1 O2
1a 2d 1b 2d 6i 3e
0 1/3 0 1/3 0.17337(15) 0.5
0 2/3 0 2/3 0.82663(15) 0
0 0.67271(4) 1/2 0.16576(6) 0.33702(23) 0
1.251(0) 1.251(0) 0.236(8) 0.883(0) 1.692(6) 1.692(6)
1 1 0.63/0.37 0.81/0.19 1 1
a ¯ ˚ ch = 7.2636(3) A; ˚ ˛r = 4.1809(6) A, ˚ ˛r = Space group P 3m1; ah = 5.9037(7) A, 89.825(7)◦ ; Rp = 7.65, Rwp = 11.0, RB = 5.29, S = 2.01.
504
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Table 3 ˚ selected bond angles (◦ ) and bond valence sum calculations of trigonal order–disorder Ba(Ca1/3 Nb2/3 )O3 a Derived interatomic distances (A), Ba1-O1 Ba1-O2
3.023 × 6 2.952 × 6
Ca1/Nb1-O1 Nb2/Ca2-O1
2.132 × 6 2.055 × 3
Ba2-O1
2.936 × 3 2.954 × 6
Nb2/Ca2-O2 Ca1/Nb1-O1-Nb2/Ca2
2.087 × 3 176.48
Ba2-O2 Ca1/Nb1-O1-Nb2/Ca2
2.925 × 3 176.48
Nb2/Ca2-O2-Nb2/Ca2 Ca2/Nb2-O2-Nb2/Ca2
180 180
Ba1 Ba2 1.96 2.21 a
vBa(av) 2.13
Ca1 Nb1 3.96 3.43
B (av) 3.76
Ca2 Nb2 4.67 4.04
B (av)
B(av)
4.16
4.03
O1 O2 2.04 2.07
O(av) 2.05
(av) are the average valence of the ion.
and Nb2(Ca2)O6 layer. The bond valence sum calculations for atoms are also listed in Table 3 [17]. It could be seen that a large valence deviation of the B-site cations, where the B cation has a valence larger than the ideal value of 2, while the B cation has a valence less than the ideal value of 5. The mutual compensation between the surplus of B and the deficiency of B cation suggests that two cations are coupled with large ions motions for their stabilization in the complex perovskite structure. It can be deduced that the nearest neighbor environments of B and B cations are optimized, which would be likely induced by the occupation of larger Ca ion in B -site. The larger Ca ions would be compressed when they are
placed within a regular oxygen octahedron, which could creates a thermodynamic driving force towards the ordered distribution of B O6 and B O6 octahedra. From a crystallographic point of view [18,19], based on the detailed quantitative Rietveld analysis of the XRD patterns, the ordering degree in ordered trigonal perovskite Ba(Ca1/3 Nb2/3 )O3 can be determined by = CCa − CNb = NNb − NCa , where CCa or NCa is the fraction of Ca in 1b or 2d sites, and CNb or NNb is the fraction of Nb in 1b or 2d sites. It should be noted that 62.7% Ca and 37.3% Nb occupies the B -site, while 18.7% Ca and 81.3% Nb occupies the B . The final calculated ordering degree is 0.44, which
¯ and (d) [1 1¯ 2] pseudo-cubic zone axes, respectively. Fig. 3. (a) HRTEM image of trigonal Ba(Ca1/3 Nb2/3 )O3 and the corresponding SAED patterns taken along (b) [5 2¯ 3], (c) [1 1¯ 1] Weak superlattice reflections are indicated by arrows.
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are comparable with the Ba(Cd1/3 Nb2/3 )O3 system [5]. Therefore, we believe that the order–disorder arrangement of large B ions would benefit structure stabilization of complex perovskite. Though Ba(B 1/3 Nb2/3 )O3 system have the uniform ordering degree, the superlattice reflections which also reflects the ordering of B /Nb ions at B sites are different because of the different difference of X-ray scattering factors of B and Nb. 3.2. High-resolution transmission electron microscopy (HRTEM) HRTEM lattice image and the SAED patterns (Fig. 3) showed only reflections attributable to trigonal unit cell with a pseudo-cubic ˚ ˛r = 89.825(7)◦ ). The extincperovskite subcell (˛r = 4.1809(6) A, ¯ tion conditions were consistent with the space group P 3m1, which is confirmed for the 1:2 order–disorder structure based on the Rietveld refined result. SAED is particularly sensitive to superstructural features. Diffuse streaks and weak Bragg spots, which are indicated by the arrowheads in Fig. 3(b), can be observed along the 1 1 1∗pc (pseudo-cubic) reciprocal directions. These extra reflections of 1:2 ordering are characterized by the (h ± 1/3, k ± 1/3, l ± 1/3)-type reflections in the pseudo-cubic cell description. So it can be concluded that the present structure determined by the Rietveld refinement is reliable. 3.3. Raman spectroscopic studies Raman spectroscopy was employed to identify the small structural distortions in cubic lattices, which was a very sensitive technique for probing order–disorder and for determining structures of symmetry lower than cubic. Theoretically, the ideal cubic disorder perovskite structure belongs to the space group 1 ¯ Pm3m(O h ). The zone center phonons classify this cubic system into 3 infrared-active modes, and there should be no first-order Ramanactive mode for this symmetry, whereas perovskites with a loss of translational symmetries or that with less symmetric have Ramanactive modes [20]. The 1:2 trigonal ordered structure of A(B 1/3 B 2/3 )O3 ceramics 3 ¯ belongs to P 3m1(D ) space group and its primitive cell has 15 3d
atoms with 45 degrees of freedom. The lattice vibration modes can be predicted by group theory analysis, based on the occupied Wyckoff sites. The totally ordered Ba(Ca1/3 Nb2/3 )O3 cell with 15 atoms can be divided into 6 groups, which have 9 Raman-active modes, 4A1g + 5Eg , 16 infrared-active modes, 7A2u + 9Eu , 3 silent modes, A2g + 2A1u , and 2 acoustic modes, A2u + Eu [20]. Fig. 4 present the Raman spectra of Ba(Ca1/3 Nb2/3 )O3 . For the highly ordered Ba(Ca1/3 Nb2/3 )O3 , nine bands can be easily identified and assigned [21,22]. Apparently, the sample has a highly ordered structure for the sharper Raman peaks [14,23]. As can be seen, two overlapping bands around 90 cm−1 correspond to A1g (Ba) and Eg (Ba) symmetry modes (the motion of Ba ions against the oxygen octahedral), while the bands associated with the internal vibrations of oxygen octahedra near 360 and 410 cm−1 are A1g (O) and 2Eg (O), respectively. Bands at 138, 228 and 246 cm−1 are related to Eg (O), A1g (Nb) and Eg (Nb) in trigonal order structure. The phonon around 800 cm−1 is assigned to the oxygen octahedral stretching mode A1g (O). The relatively large width of A1g (O) can be attributed to the co-existence of the two octahedrons (Ca2 and Nb2 octahedrons in 2d site). The fitted Lorentzian curves near 800 cm−1 are inserted in Fig. 4. As shown, the dominant peak near 820 cm−1 is the NbO6 stretching vibration and a weak peak at 779 cm−1 is assigned the CaO6 stretching mode. Based on the Rietveld refinement result that the 2d site is occupied by 18.7% Ca and 81.3% Nb, the intensity of the CaO6 stretching mode would be weaker than that of NbO6 stretching mode. Furthermore, the weaker polarizability of Ca2+ ions (3.16 A´˚ 3 ) compared with Nb5+ ions (3.97 A´˚ 3 ) results in
Fig. 4. Raman spectra of the Ba(Ca1/3 Nb2/3 )O3 . The inset shows the typical fitting result of the A1g (O) near 800 cm−1 .
the smaller energy of CaO6 stretching mode [24,25]. Similar studies were also found in other literatures [26,27]. 4. Conclusion Crystal structure of Ba(Ca1/3 Nb2/3 )O3 synthesized by the molten salt method with 1:2 ordering structure is determined based on Rietveld refinements of X-ray powder diffraction data. It belongs to 3 ¯ trigonal space group P 3m1(D 3d ) with an order–disorder arrangement along pseudo-cubic 1 1 1 planes. This order–disorder structure is attributed to the large Ca ions at B -site. And the coordination environments of Ca ions are compressed, resulting in larger bond valence sums of Ca2+ and smaller ones of Nb5+ than the formal oxidation states, whereas the O2− bond valence sums approach the ideal values. Both SAED and Raman spectrum of Ba(Ca1/3 Nb2/3 )O3 show the evidence in support of the 1:2 B-site order–disorder. The calculated ordering degree is 0.44, which are comparable with that of the crystal Ba(Cd1/3 Nb2/3 )O3 . This extent of order–disorder arrangement for large divalent cations in B -site is considered to benefit the structure stabilization of complex perovskite. Acknowledgments This work was supported by National Natural Science Foundation of China (nos. 20731001, 50725415) and the National Key Program for Basic Research (973 Program, Grants No. 2007CB613601). R. Yu thanks the “Beijing Nova Program” (No.: 2005B20) and NCET. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
T.A. Vanderah, Science 298 (2002) 1182. R.J. Cava, J. Mater. Chem. 11 (2001) 54. S. Kawashima, M. Nishida, I. Ueda, H. Ouchi, J. Am. Ceram. Soc. 66 (1983) 421. H. Tamura, T. Konoike, Y. Sakabe, K. Wakino, J. Am. Ceram. Soc. 67 (1984) C59. J. Deng, X. Xing, J. Chen, R. Yu, G. Liu, Scripta Mater. 56 (2007) 65. F. Galasso, J.R. Barrante, L. Katz, J. Am. Chem. Soc. 83 (1961) 2830. F. Galasso, J. Pyle, Inorg. Chem. 2 (1963) 482. J. Jacobson, B.M. Collins, B.E. Fender, Acta Crystallogr. B 32 (1976) 1083. N. Setter, L.E. Cross, J. Mater. Sci. 15 (1980) 2478. F. Galasso, J. Pyle, J. Phys. Chem. 67 (1963) 1561. F. Galasso, J. Pyle, J. Phys. Chem. 67 (1963) 533. J.Y. Levin, R.G. Chan, J.E. Geyer, T.A. Maslar, J. Vanderah, Solid State Chem. 156 (2001) 122. [13] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.
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[14] K.P. Surendran, M.T. Sebastian, P. Mohanan, R.L. Moreira, A. Dias, Chem. Mater. 17 (2005) 142. [15] J. Rodriguez-Carvajal, Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the IUCr, Toulouse, France, 1990, p. 127. [16] S. Janaswamy, G.S. Murthy, E.D. Dias, V.R.K. Murthy, Mater. Lett. 55 (2002) 414. [17] M. O’Keeffe, N.E. Brese, J. Am. Chem. Soc. 113 (1991) 3226. [18] B.E. Warren, X-ray Diffraction, Addison-Wesley, London, 1969. [19] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Amsterdam,1978. [20] H. Tamura, D.A. Sagala, K. Wakino, Jpn. J. Appl. Phys. 25 (1986) 787.
[21] C.-T. Chia, Y.C. Chen, H.F. Cheng, I.N. Lin, J. Appl. Phys. 94 (2003) 3360. [22] A. Dias, F.M. Matinaga, R.L. Moreira, Chem. Mater. 19 (2007) 2335. [23] H.-F. Cheng, C.-T. Chia, H.-L. Liu, M.-Y. Chen, Y.-T. Tzeng, I.-N. Lin, J. Eur. Ceram. Soc. 27 (2007) 2893. [24] N.W. Grimes, R.W. Grimes, J. Phys. Condens. Matter 10 (1998) 3029. [25] C.-T. Chia, P.-J. Chang, M.-Y. Chen, I.-N. Lin, H. Ikawa, L.-J. Lin, J. Appl. Phys. 101 (2007) 084115. [26] R.L. Moreira, F.M. Matinaga, A. Dias, Appl. Phys. Lett. 78 (2001) 428. [27] H. Ikawa, M. Takemoto, M. Katouno, M. Takamura, J. Eur. Ceram. Soc. 23 (2007) 2511.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Crystallographic and Raman spectroscopic studies of microwave dielectric ceramics Ba(Ca1/3 Nb2/3 )O3 Jinxia Deng a , Jun Chen a , Ranbo Yu a , Guirong Liu a , Xianran Xing a,b,∗ a b
Department of Physical Chemistry, University of Science & Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science & Technology Beijing, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 6 November 2007 Received in revised form 2 May 2008 Accepted 2 May 2008 Available online 16 June 2008 Keywords: Ceramics X-ray diffraction Order–disorder effects Transmission electron microscopy Crystal structure
a b s t r a c t The crystal structure of the microwave dielectric ceramics Ba(Ca1/3 Nb2/3 )O3 synthesized by the molten salt 3 ¯ method has been refined, from X-ray diffraction, in a trigonal order–disorder lattice with the P 3m1(D 3d ) space group. The lattice distortion and cation ordering are attributed to the large Ca2+ ions at B -site, which induces a large deviation of bond valence sums of the B-site ions. The ordering degree determined by the variations of the occupancies of Ca and Nb ions is 0.44. High-resolution transmission electron microscopy (HRTEM) and Raman spectrum show the evidence in support of the 1:2 B-site order–disorder model. The complete Raman mode assignment for Ba(Ca1/3 Nb2/3 )O3 with order–disorder structure is also presented. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Complex perovskite ceramics have attracted considerable attention for their remarkable microwave dielectric properties [1,2]. The low microwave losses are observed in alkaline-earth based niobate and tantalate members of the 1:2 type A(B 1/3 B 2/3 )O3 family of mixed-metal perovskite (A = Ca, Sr, Ba; B = Mg, Ca, Sr, Mn, Fe, Ni, Cu, Zn, Cd; and B = Nb, Ta) [3–5]. From a structural point of view, these materials can be classified into ordered and disordered types with respect to the ordering degree of the divalent B and pentavalent B cations [6–8]. For these systems, the ordering degree on the octahedral sites plays a critical role in mediating the dielectric response, and the highest quality factors are associated with a fully ordered arrangement of the B and B cations along each of the pseudo-cubic [1 1 1] directions [8] (Fig. 1). The probability for an ordered arrangement of the B and B cations is essentially controlled by the differences in their ionic charges and radii [9,10]. Large Cd2+ ions on B -site were confirmed with a trigonal order–disorder structure [5]. Alkalineearth element Ca with larger ionic radius could occupy both the A2+ site and B2+ site [10–12] (RCa2+ = 0.134 nm, CN = 12; RCa2+ = 0.100 nm > RCd2+ = 0.095 nm > RZn2+ = 0.074 nm > RMg2+ = 0.072 nm > RNb5+ = 0.064 nm, CN = 6 [13]). By the occupation
∗ Corresponding author. Tel.: +86 10 62334200; fax: +86 10 62332525. E-mail address: [email protected] (X. Xing). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.05.006
of a larger Ca2+ cation in the B -site, the crystal structure of A(B 1/3 B 2/3 )O3 perovskite will be distorted, and tend to be an more ordered structure. In this work, an order–disorder structure model based on the detailed quantitative Rietveld analysis of the X-ray diffraction (XRD) patterns is developed. Besides clarifying the order trigonal structure of Ba(Ca1/3 Nb2/3 )O3 , the ordering degree can also be calculated. This is verified by the high-resolution transmission electron microscopy (HRTEM) investigation. Furthermore, Raman scattering spectrum is performed to determine ordering [14]. 2. Experimental Ba(Ca1/3 Nb2/3 )O3 compound was synthesized by a molten salt method. The starting materials were BaCO3 (A.R.), CaO (A.R.), Nb2 O5 (A.R.) and NaCl (A.R.). The mole ratio of the salts NaCl to Ba(Ca1/3 Nb2/3 )O3 was 10. Stoichiometric starting reactants were thoroughly ground with ethanol in an agate mortar for 30 min. Dried powders were then calcined at 850 ◦ C for 10 h. The chlorides were removed from the products by hot deionized water washing for several times until the filtrate gave no reaction with silver nitrate solution. The as-prepared material was finally dried at 100 ◦ C in a drying oven. For structure refinement, XRD data were collected at room temperature using a 21 kW extra-power powder XRD (Model M21XVHF22, Mac Science, Yokohama, Japan), operating with Cu K␣ radiation and equipped with a graphite monochromator. A step-scan mode was employed with a step width of 0.02◦ . The program Fullprof [15] was used for the Rietveld refinement. HRTEM observation and the corresponding selected area electron diffraction (SAED) patterns were carried out on a JEM-2010 electron microscope (JEOL Ltd., Japan). For HRTEM observation, the synthesized powders were ultrasonically dis-
J. Deng et al. / Journal of Alloys and Compounds 472 (2009) 502–506
503
Fig. 2. Observed (circles), calculated (line) and difference (bottom of figure) X-ray diffraction profiles for Ba(Ca1/3 Nb2/3 )O3 at room temperature. The tick marks below the X-ray pattern give the position of all possible Bragg reflections. The inset is the enlarged pattern from 14◦ to 27◦ .
Fig. 1. Structure of 1:2 B-site ordered A(B 1/3 B 2/3 )O3 viewed along the [1 0 0] direction. B O6 octahedra are dark blue, B O6 octahedra are line gray and A ions are omitted for clarity. (For interpretation of the reference to color in this figure legend, the reader is referred to the web version of the article.)
persed in ethanol, and a drop of the suspension was placed on a Cu grid coated with carbon film. Raman scattering data were collected at room temperature on a JYT64000 Raman spectrometer (Jobin Yvon, France).
3. Results and discussion 3.1. Structure refinement The XRD data of the Ba(Ca1/3 Nb2/3 )O3 powder synthesized by the molten salt method for the 2 region of 10–110◦ were used for the refinement. It could be completely indexed on the basis ¯ and ordered trigof both the disordered cubic space group Pm3m ¯ onal space group P 3m1 (based on the Ba(Mg1/3 Ta2/3 )O3 [16] see Table 1). In our initial refinement, these two structural models were both introduced. However, in the disordered model, weak superlattice reflection is present at the 2 angle 17.3◦ which could be identified as a superstructure reflecting the ordering of Ca/Nb ions at B /B sites. We could also observe the larger preferred orienTable 1 ¯ and P 3m1 ¯ Structure model based on the space groups Pm3m Atom
Site
x
y
z
SOF
1 ¯ Pm3m(O h) Ba Ca Nb O
1a 1b 1b 3c
0 1/2 1/2 0
0 1/2 1/2 1/2
0 1/2 1/2 1/2
1 1/3 2/3 1
3 ¯ P 3m1(D 3d ) Ba1 Ba2 Ca Nb O1 O2
1a 2d 1b 1b 6i 3e
0 1/3 0 1/3 x 1/2
0 2/3 0 2/3 y 0
0 z 1/2 z z 0
1 1 1 2 1 1
tation along the cubic (1 1 1) direction. Moreover the refinement of the isotropic thermal parameters of Ca and Nb, which related to the displacements of atoms, leaded to slightly higher values about 2.0 A˚ 2 . Large R-factors were obtained for the sample refined by the disordered model. And the R-factors in the ordered model (Rp = 8.55, Rwp = 12.0, RB = 6.39, S = 2.20) were smaller than those of disordered model. However isotropic thermal parameters of the O atoms (Biso(O) = 1.7812 A˚ 2 ) were slightly higher. In the final refinement, the crystallographic model was based on ¯ the order trigonal space group P 3m1, which allows the Ca and Nb to occupy the two symmetry-inequivalent sites, 1b and 2d sites. The structure parameters are the same for the Ca1 and Nb1 on 1b sites and also for the Ca2 and Nb2 on 2d sites except for the occupation factors (Occ.). Furthermore the occupation factors of the Ca1, Nb1, Ca2, and Nb2 sites have the linear constraints: OccCa2 = OccNb1 , and 2(OccCa1 + OccNb1 ) = OccCa2 + OccNb2 . It was found that the splitting of the 1b site and 2d site was successful in terms of the refinement: introduce of the disordering of Ca/Nb over the two available sites lead to a remarkable improvement of the profile fitting. An excellent fitting result was obtained for this order–disorder model, as illustrated in Fig. 2. Table 2 lists the final atomic coordinates and agreement factors. Table 3 lists the selected interatiomic distances and angles of the trigonal order–disorder Ba(Ca1/3 Nb2/3 )O3 . The bond angle of Ca1/Nb1-O1-Ca2/Nb2 are reduced to 176.480◦ . The deviation from 180◦ reduces the imbalance of local valance, and could therefore stabilize the perovskite structure. It is essential for the ordering assignment and the charge transfer between the Ca1(Nb1)O6 layer Table 2 Refined structural parameters of Ba(Ca1/3 Nb2/3 )O3 a based on the trigonal order–disorder model Atom
Site
x
y
z
˚ B (A)
SOF
Ba1 Ba2 Ca1/Nb1 Nb2/Ca2 O1 O2
1a 2d 1b 2d 6i 3e
0 1/3 0 1/3 0.17337(15) 0.5
0 2/3 0 2/3 0.82663(15) 0
0 0.67271(4) 1/2 0.16576(6) 0.33702(23) 0
1.251(0) 1.251(0) 0.236(8) 0.883(0) 1.692(6) 1.692(6)
1 1 0.63/0.37 0.81/0.19 1 1
a ¯ ˚ ch = 7.2636(3) A; ˚ ˛r = 4.1809(6) A, ˚ ˛r = Space group P 3m1; ah = 5.9037(7) A, 89.825(7)◦ ; Rp = 7.65, Rwp = 11.0, RB = 5.29, S = 2.01.
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Table 3 ˚ selected bond angles (◦ ) and bond valence sum calculations of trigonal order–disorder Ba(Ca1/3 Nb2/3 )O3 a Derived interatomic distances (A), Ba1-O1 Ba1-O2
3.023 × 6 2.952 × 6
Ca1/Nb1-O1 Nb2/Ca2-O1
2.132 × 6 2.055 × 3
Ba2-O1
2.936 × 3 2.954 × 6
Nb2/Ca2-O2 Ca1/Nb1-O1-Nb2/Ca2
2.087 × 3 176.48
Ba2-O2 Ca1/Nb1-O1-Nb2/Ca2
2.925 × 3 176.48
Nb2/Ca2-O2-Nb2/Ca2 Ca2/Nb2-O2-Nb2/Ca2
180 180
Ba1 Ba2 1.96 2.21 a
vBa(av) 2.13
Ca1 Nb1 3.96 3.43
B (av) 3.76
Ca2 Nb2 4.67 4.04
B (av)
B(av)
4.16
4.03
O1 O2 2.04 2.07
O(av) 2.05
(av) are the average valence of the ion.
and Nb2(Ca2)O6 layer. The bond valence sum calculations for atoms are also listed in Table 3 [17]. It could be seen that a large valence deviation of the B-site cations, where the B cation has a valence larger than the ideal value of 2, while the B cation has a valence less than the ideal value of 5. The mutual compensation between the surplus of B and the deficiency of B cation suggests that two cations are coupled with large ions motions for their stabilization in the complex perovskite structure. It can be deduced that the nearest neighbor environments of B and B cations are optimized, which would be likely induced by the occupation of larger Ca ion in B -site. The larger Ca ions would be compressed when they are
placed within a regular oxygen octahedron, which could creates a thermodynamic driving force towards the ordered distribution of B O6 and B O6 octahedra. From a crystallographic point of view [18,19], based on the detailed quantitative Rietveld analysis of the XRD patterns, the ordering degree in ordered trigonal perovskite Ba(Ca1/3 Nb2/3 )O3 can be determined by = CCa − CNb = NNb − NCa , where CCa or NCa is the fraction of Ca in 1b or 2d sites, and CNb or NNb is the fraction of Nb in 1b or 2d sites. It should be noted that 62.7% Ca and 37.3% Nb occupies the B -site, while 18.7% Ca and 81.3% Nb occupies the B . The final calculated ordering degree is 0.44, which
¯ and (d) [1 1¯ 2] pseudo-cubic zone axes, respectively. Fig. 3. (a) HRTEM image of trigonal Ba(Ca1/3 Nb2/3 )O3 and the corresponding SAED patterns taken along (b) [5 2¯ 3], (c) [1 1¯ 1] Weak superlattice reflections are indicated by arrows.
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are comparable with the Ba(Cd1/3 Nb2/3 )O3 system [5]. Therefore, we believe that the order–disorder arrangement of large B ions would benefit structure stabilization of complex perovskite. Though Ba(B 1/3 Nb2/3 )O3 system have the uniform ordering degree, the superlattice reflections which also reflects the ordering of B /Nb ions at B sites are different because of the different difference of X-ray scattering factors of B and Nb. 3.2. High-resolution transmission electron microscopy (HRTEM) HRTEM lattice image and the SAED patterns (Fig. 3) showed only reflections attributable to trigonal unit cell with a pseudo-cubic ˚ ˛r = 89.825(7)◦ ). The extincperovskite subcell (˛r = 4.1809(6) A, ¯ tion conditions were consistent with the space group P 3m1, which is confirmed for the 1:2 order–disorder structure based on the Rietveld refined result. SAED is particularly sensitive to superstructural features. Diffuse streaks and weak Bragg spots, which are indicated by the arrowheads in Fig. 3(b), can be observed along the 1 1 1∗pc (pseudo-cubic) reciprocal directions. These extra reflections of 1:2 ordering are characterized by the (h ± 1/3, k ± 1/3, l ± 1/3)-type reflections in the pseudo-cubic cell description. So it can be concluded that the present structure determined by the Rietveld refinement is reliable. 3.3. Raman spectroscopic studies Raman spectroscopy was employed to identify the small structural distortions in cubic lattices, which was a very sensitive technique for probing order–disorder and for determining structures of symmetry lower than cubic. Theoretically, the ideal cubic disorder perovskite structure belongs to the space group 1 ¯ Pm3m(O h ). The zone center phonons classify this cubic system into 3 infrared-active modes, and there should be no first-order Ramanactive mode for this symmetry, whereas perovskites with a loss of translational symmetries or that with less symmetric have Ramanactive modes [20]. The 1:2 trigonal ordered structure of A(B 1/3 B 2/3 )O3 ceramics 3 ¯ belongs to P 3m1(D ) space group and its primitive cell has 15 3d
atoms with 45 degrees of freedom. The lattice vibration modes can be predicted by group theory analysis, based on the occupied Wyckoff sites. The totally ordered Ba(Ca1/3 Nb2/3 )O3 cell with 15 atoms can be divided into 6 groups, which have 9 Raman-active modes, 4A1g + 5Eg , 16 infrared-active modes, 7A2u + 9Eu , 3 silent modes, A2g + 2A1u , and 2 acoustic modes, A2u + Eu [20]. Fig. 4 present the Raman spectra of Ba(Ca1/3 Nb2/3 )O3 . For the highly ordered Ba(Ca1/3 Nb2/3 )O3 , nine bands can be easily identified and assigned [21,22]. Apparently, the sample has a highly ordered structure for the sharper Raman peaks [14,23]. As can be seen, two overlapping bands around 90 cm−1 correspond to A1g (Ba) and Eg (Ba) symmetry modes (the motion of Ba ions against the oxygen octahedral), while the bands associated with the internal vibrations of oxygen octahedra near 360 and 410 cm−1 are A1g (O) and 2Eg (O), respectively. Bands at 138, 228 and 246 cm−1 are related to Eg (O), A1g (Nb) and Eg (Nb) in trigonal order structure. The phonon around 800 cm−1 is assigned to the oxygen octahedral stretching mode A1g (O). The relatively large width of A1g (O) can be attributed to the co-existence of the two octahedrons (Ca2 and Nb2 octahedrons in 2d site). The fitted Lorentzian curves near 800 cm−1 are inserted in Fig. 4. As shown, the dominant peak near 820 cm−1 is the NbO6 stretching vibration and a weak peak at 779 cm−1 is assigned the CaO6 stretching mode. Based on the Rietveld refinement result that the 2d site is occupied by 18.7% Ca and 81.3% Nb, the intensity of the CaO6 stretching mode would be weaker than that of NbO6 stretching mode. Furthermore, the weaker polarizability of Ca2+ ions (3.16 A´˚ 3 ) compared with Nb5+ ions (3.97 A´˚ 3 ) results in
Fig. 4. Raman spectra of the Ba(Ca1/3 Nb2/3 )O3 . The inset shows the typical fitting result of the A1g (O) near 800 cm−1 .
the smaller energy of CaO6 stretching mode [24,25]. Similar studies were also found in other literatures [26,27]. 4. Conclusion Crystal structure of Ba(Ca1/3 Nb2/3 )O3 synthesized by the molten salt method with 1:2 ordering structure is determined based on Rietveld refinements of X-ray powder diffraction data. It belongs to 3 ¯ trigonal space group P 3m1(D 3d ) with an order–disorder arrangement along pseudo-cubic 1 1 1 planes. This order–disorder structure is attributed to the large Ca ions at B -site. And the coordination environments of Ca ions are compressed, resulting in larger bond valence sums of Ca2+ and smaller ones of Nb5+ than the formal oxidation states, whereas the O2− bond valence sums approach the ideal values. Both SAED and Raman spectrum of Ba(Ca1/3 Nb2/3 )O3 show the evidence in support of the 1:2 B-site order–disorder. The calculated ordering degree is 0.44, which are comparable with that of the crystal Ba(Cd1/3 Nb2/3 )O3 . This extent of order–disorder arrangement for large divalent cations in B -site is considered to benefit the structure stabilization of complex perovskite. Acknowledgments This work was supported by National Natural Science Foundation of China (nos. 20731001, 50725415) and the National Key Program for Basic Research (973 Program, Grants No. 2007CB613601). R. Yu thanks the “Beijing Nova Program” (No.: 2005B20) and NCET. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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