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Structural phase transition and thermal expansion in Bi1−2.5xPr1.5xBaxFeO3 ceramics
Journal of Alloys and Compounds 566 (2013) 235–238
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Structural phase transition and thermal expansion in Bi12.5xPr1.5xBaxFeO3 ceramics G.F. Cheng a,b, Y.J. Ruan b, Y.H. Huang b, X.S. Wu a,⇑ a b
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, PR China Analysis & Testing Center for Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 22 December 2012 Accepted 19 February 2013 Available online 7 March 2013 Keywords: Bi12.5xPr1.5xBaxFeO3 Structural phase transition Thermal expansion In situ XRD
a b s t r a c t The temperature dependence of the crystal structure for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) polycrystallines is studied by X-ray powder diffraction with Rietveld refinements in the temperature range of 25– 800 °C. A structural phase transition of Rhombohedral-to-Cubic occurs for Bi0.875Pr0.075Ba0.05FeO3 sample in the temperature of 600–700 °C, which may relate to its unstable rhombohedra distorted structure with the space group R3c. The rarely decomposition of these samples indicates that the Pr, Ba co-doped make the BiFeO3 ceramics more stable. The thermal expansion determined by the temperature dependence of the unit-cell lattice parameters and volumes for Bi12.5xPr1.5xBaxFeO3 samples is also investigated, which shows an isotropic and positive behavior. The average thermal expansion coefficient decreases with the increasing x. We argue that the Cubic crystal structure with the high symmetrical of the space group Pm3m may be more stable for Bi0.75Pr0.15Ba0.1FeO3 sample, which may explain the reason why no phase transition occurs and its lower thermal expansion efficiencies. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction BiFeO3 (BFO) is one of the most important single-phase multiferroic materials, which show spontaneous electric and magnetic ordering simultaneously [1–3]. It has an antiferromagnetic behavior with a relatively high Néel temperature (TN 370 °C) and ferroelectric behavior with a high Curie temperature (TC 810 °C) and possesses a rhombohedra distorted perovskite-like structure with the space group of R3c at room temperature [4–6]. However, the spiral antiferromagnetic spin order and large leakage current hinder the technological application of the coupling between magnetism and electricity in BFO [7–10]. Recently a lot of efforts to modify its magnetic and electric properties have been reported, such as ions substitution, epitaxial films, single crystals and so on [10–13]. But due to the narrow temperature range of phase stabilization, the preparation of pure BFO is very difficult [6]. The presence of impurities (such as Bi25FeO39 and Bi2Fe4O9) results in high leakage current, which leading to poor ferroelectric behavior [6]. Selbach et al. [14] have investigated the thermodynamic stability of BFO by high temperature X-ray diffraction and discussed the stability of BFO and related Bi-based perovskites in relation to Goldschmidt tolerance factor and the influence of pressure and chemical substitution. Chen et al. [15] investigated the structure ⇑ Corresponding author. Tel./fax: +86 25 83594402. E-mail address: [email protected] (X.S. Wu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.138
and negative thermal expansion in the PbTiO3–BiFeO3 system. Bhattacharjee et al. [16] reported an unusual ferroelectric to ferroelectric isostructural phase transition and associated giant negative thermal expansion(NTE) for the tetragonal composition x = 0.31 closest to the morphotropic phase boundary(MPB) of the multiferroic (1x) BiFeO3–xPbTiO3 (BF–xPT) solid solution system. Chen et al. [17] have investigated the phase relations of the (1x) BiFeO3–xLaFeO3 system and their temperature dependence of the lattice parameters, unit-cell volumes and expansion coefficients. However, the study of unit cell thermal expansion about the BFO and doped BFO is very scarce. Thermal expansion is a very important evaluation indicator of the thermodynamic stability, which is affected by the increasing amplitude of the phonon during the heating process and in turn results in dilation of the crystal lattice [18]. The study of correlation between the structural phase transition and thermal expansion behavior is proposed to understand the thermodynamic stability. In the present, we synthesized the Bi12.5xPr1.5xBaxFeO3 ceramic samples. Structural phase transition and thermal expansion behavior are investigated by high temperature powder X-ray diffraction (HT-XRD). 2. Experimental The Bi12.5xPr1.5xBaxFeO3 ceramic samples are prepared by the conventional solid state reaction technique using Bi2O3, Fe2O3, BaCO3 and Pr6O11 reagents. The mixtures of reagents taken in desired cation ratios and pressed into pellets are annealed at 880 °C for 1 h in air. The crystal structure of the samples is characterized by X-ray
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diffraction at high temperature using Rigaku D/max 2550 V X-ray diffractometer equipped with SHT-1500 multipurpose high temperature attachment, Cu ka radiation (40 kV, 250 mA). The heating rate of the high temperature attachment is set to be 10 °C min1 in air. Before experiments the sample is remained at a specified temperature for 30 min to reach a thermal equilibrium. Rietveld refinements are used to analyze the XRD data with the TOPAS software.
3. Results and discussion 3.1. Structural phase transition Fig. 1 shows the XRD patterns of the Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples at room temperature (25 °C). Both samples are found to be single phase. For sample with x = 0.05, the symmetry is a typical Rhombohedral with the space group of R3c, as that for BiFeO3 [19,20]. The upper inset of Fig. 1 show some separated peaks in the 2h range of 31.0–33.0°, which stand for two peaks with the index of (1 0 4) and (1 1 0) for Bi12.5xPr1.5xBaxFeO3 (x = 0.05) sample in the space group of R3c. The symmetry elements increase with increasing the dopants of Ba and Pr in the BiFeO3 unit cell due to the FeO6 octahedral trends to uniform and the separated peaks move together even become overlap in the XRD patterns, which indicates the structural phase transition from Rhombohedral R3c to Cubic Pm3m [21,22] occurs in Bi12.5xPr1.5xBaxFeO3 (x = 0.1) sample as shown in the inset. To further study the detailed subtle structure changes of the sample, Rietveld refinements are performed using the TOPAS software [23–25]. The typical refined structural parameters for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples at room temperature (25 °C) are shown in Table 1. It can be seen the criteria R-factors are very small, which suggests that the refinements are successfully performed. The final results confirm that the Bi+3 ion is substituted by the Ba2+, Pr3+ and Pr4+ ions. To investigate the thermodynamic stability of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples, HT-XRD is performed in the temperature range of 25–800 °C in air. None decomposition of Bi12.5xPr1.5xBaxFeO3 samples is observed between 25 and 800 °C as shown in Fig. 2, which confirm thermodynamically stable. This is very different from that of the BiFeO3, which is metastable with the formation of Bi2Fe4O9 and Bi25FeO39 impurity phases during 720 < T < 1040 K [26]. Sverre et al. observed the decomposition of BiFeO3 to Bi25FeO39 and Bi2Fe4O9 between 878 and 1090 K, but no decomposition of BiFeO3 happened between 1090 and 1199 K [26]. We argue that the substitution of Ba and Pr may increase the stability of BiFeO3 with respect to the binary oxides and possibly also with respect to the Bi2Fe4O9 and Bi25FeO39
phase [26]. However, some separated peaks move together even becoming overlap during the heating process, which indicates the change in the symmetry of the lattice and the structural phase transit from Rhombohedral R3c to Cubic Pm3m in Bi12.5xPr1.5xBaxFeO3 (x = 0.05) sample at about 700 °C as shown in the upper inset of Fig. 2. The Rhombohedral to Cubic transition is accompani ed with a decrease in cell volume from 380.82 to 0 63.64 Å A [3]. The structural phase transition would be related to its unstable low-symmetry rhombohedra distorted perovskitelike structure with the space group R3c. In addition to, the lines of the HT-XRD patterns shifting to smaller Bragg angles indicate the lattice constants transition and thermal expansion behavior as discussed below. 3.2. Lattice constants transition and thermal expansion The temperature dependences of lattice parameters (a, c) and unit-cell volumes for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples are shown in Figs. 3 and 4, respectively. The variation of the lattice parameters (a, c) and unit-cell volumes with temperature could be fitted to a third order polynomial function of the form y = y0 + AT + BT2 + CT3 as shown with red dash lines in Figs. 3 and 4 respectively, where A, B, C are constants and T denotes the temperature (°C) [19,27,28]. It can be observed that the lattice parameters (a, c) and unit-cell volumes of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) increase during the heating process in the temperature range 25–600 °C and a structural phase transition occurs in the temperature range 700–800 °C. However, the lattice parameter (a) and unit-cell volumes of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) increasing monotonically indicate no phase transition occurred in the temperature range 25–800 °C. Figs. 5 and 6 show the relative changes in the lattice parameters and cell volumes. The blue and red dash lines represent the values calculated from the linear fit of the lattice parameters and cell volumes, which suggest two regions (below 400 °C and above 400 °C) with distinct temperature dependence [19]. The thermal linear expansion coefficients of the samples are calculated with the formula as follows [18,27,29,30]:
aaT ¼
a a0 Da ¼ a0 ðT T 0 Þ a0 DT
ð1Þ
acT ¼
c c0 Dc ¼ c0 ðT T 0 Þ c0 DT
ð2Þ
And the thermal volume expansion coefficient is defined as [18,27,29,30]:
aVT ¼
Fig. 1. Observed (black squares), calculated (red circles) and difference (blue triangles) XRD patterns of the Bi12.5xPr1.5xBaxFeO3 samples at room temperature (25 °C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
V V0 DV ¼ V 0 ðT T 0 Þ V 0 DT
ð3Þ
where a0, c0, V0 denote the lattice parameter a, c, and the cell volume V at 25 °C respectively. The thermal linear and volume expansion coefficients in different temperature regions are listed in Table 2, which show an isotropic and positive behavior. The average values of thermal expansion coefficient decrease with the increase of x. It suggest that the high symmetrical Cubic crystal structure with the space group Pm3m is very stable in the Bi0.75Pr0.15Ba0.1FeO3 sample, which could be the possible mechanism for their non-phase transition and lower thermal expansion behavior during the heating process. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. In order to prove such hypothesis, we measure the specific heat Cp of the samples at the temperature range
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G.F. Cheng et al. / Journal of Alloys and Compounds 566 (2013) 235–238 Table 1 The typical refined structural parameters for the Bi12.5xPr1.5xBaxFeO3 samples obtained by Rietveld refinements from the XRD data at room temperature (25 °C). Sample Space group
x = 0.05 R3c
x = 0.1 Pm3m
Cell a (Å) c (Å) V (Å3)
5.57888(91) 13.8491(23) 373.29(14)
3.9584(95)
Atom Position x y z Cry size Gaussian (nm) Strain L Crystal Density (g/cm3) R factors (%) Rexp Rwp Rp GOF Wt% – Rietveld
Bi/Pr/Ba 6a 0 0 0
Fe 6a 0 0 0.2124(18) 179(65) 0.226(24) 8.1175(30) 3.19 3.54 2.77 1.11 100.000
62.03(45) O 18b 0.5(27) 0.027(17) 0.9634(46)
Bi/Pr/Ba 1b 0.500000 0.500000 0.500000
Fe 1a 0 0 0 63(59) 0.16(77) 7.910(57)
O 3d 0 0 0.50000
3.12 3.32 2.63 1.06 100.000
Fig. 4. Lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) in the temperature range 25–800 °C.
Fig. 2. XRD patterns of the Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples measured by HT-XRD at different temperatures.
Fig. 5. Relative changes in the lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) as a function of temperature.
Fig. 3. Lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) in the temperature range 25–800 °C.
from 25 °C to 450 °C as it is shown in the Fig. 7.As it is expected, the variation of Cp as a function of temperature shows a strong peak, which suggests a magnetic order–disorder phase transition
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G.F. Cheng et al. / Journal of Alloys and Compounds 566 (2013) 235–238
4. Conclusions
Fig. 6. Relative changes in the lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) as a function of temperature.
Table 2 Thermal expansion coefficients of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples. Sample
Temperature region
aaT 106
acT 106
aVT 106
(/°C)
(/°C)
(/°C)
x = 0.05
Below 400 °C Above 400 °C
12.2 11.7
15.2 2.73
37.7 37.7
x = 0.1
Below 400 °C Above 400 °C
5.63 11.6
16.6 35.3
Fig. 7. Specific heat (Cp) of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples as a function of temperature.
happened. The point of heat anomaly indicates the Néel temperature of the samples. Such an anomaly has been also observed in other compound such as YMnO3 and BaNi2V2O8 [31,32]. The variation of thermal expansion coefficient is very flexible, which may be infected by lots of factors such as crystal structure, lattice parameter, cell volume, oxygen vacancy, doped ionic radius, density, porosity, crystallite size, lattice distortion and bulk modulus [31,32]. In Bi12.5xPr1.5xBaxFeO3 samples, the different substitution has change the structure from Rhombohedral-to-Cubic and their lattice parameter, cell volume and bulk modulus. And it also induces the magnetic phase transition Néel temperature alerting. These parameters in turn affect their thermal expansion coefficients.
The temperature dependence of the crystal structure and thermal expansion for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) polycrystallines are studied by powder X-ray diffraction at high temperature in the range of 25–800 °C in air. A structural phase transition of Rhombohedral-to-Cubic occurs for Bi0.875Pr0.075Ba0.05FeO3 sample in the temperature range from 600 to 700 °C. The thermal expansion for the Bi12.5xPr1.5xBaxFeO3 samples shows an isotropic and positive behavior. The average thermal expansion coefficient decreases with the increasing x. The Cubic crystal structure with the high symmetrical of the space group Pm3m may be more stable for Bi0.75Pr0.15Ba0.1FeO3 sample, which may explain why no phase transition occurs, and its lower thermal expansion efficiencies in high temperature. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. Acknowledgments This work is supported by the National Key Projects for Basic Researches of China (2010CB923404), the Natural Science Foundation of China (10974081, 10979017, 11274153, and 51202280). References [1] Qingyu Xu, Shengqiang Zhou, D.Wu, et al., Schmidt, J. Appl. Phys. 107 (2010) 093920. [2] D.H. Wang, W.C. Goh, M. Ning, C.K.Ong, Appl. Phys. Lett. 88 (2006) 212907. [3] Neeraj. Kumara, Neeraj. Panwarb, Bhasker. Gahtoria, et al., J. Alloys Comp. 501 (2010) L29–L32. [4] K.S. Nalwa, A. Garg, A. Upadhyaya, Mater. Lett. 62 (2008) 878–881. [5] Jun Zhang, M.A. Gondal, Wei Wei, et al., J. Alloys Comp. (2012) 530107– 530110. [6] Dinesh Varshney, Ashwini Kumar, Kavita. Verma, J. Alloys Comp. 509 (2011) 8421–8426. [7] D. Lebeuge, D. Colson, A. Forget, M. Viret, Phys. Rev. B 76 (2007) 024116. [8] Y.P. Wang, L. Zhou, M.F. Zhang, et al., Appl. Phys. Lett. 84 (2004) 1731. [9] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401(R). [10] Preetam Singh, J.H. Jung, Physica B. 405 (2010) 1086–1089. [11] J.J. Ge, X.B. Xue, G.F. Cheng, et al., J. Magn. Magn. Mater. 324 (2012) 200–204. [12] Y.C. Hu, Z.Z. Jiang, K.G. Gao, G.F. Cheng, et al., Chem. Phys. Lett. 534 (2012) 62– 66. [13] G.F. Cheng, Y.H. Huang, J.J. Ge, B. Lv, X.S. Wu, J. Appl. Phys. 111 (2012) 07C707. [14] Sverre M. Selbach, Mari-Ann Einarsrud, Tor Grande, Chem. Mater. 21 (2009) 169–173. [15] J. Chen, X.R. Xing, et al., Appl. Phys. Lett 89 (2006) 101914. [16] S. Bhattacharjee, K. Taji, et al., Phys. Rev. B 84 (2011) 104116. [17] J.R. Chen, W.L. Wang, J.B. Li, G.H. Rao, J. Alloys Comp. 459 (2008) 66–70. [18] S.N. Achary, S.J. Patwe, A.K. Tyagi, J. Alloys Comp. 461 (2008) 474–480. [19] V.A. Khomchenko, D.A. Kiselev, M. Kopcewicz, et al., J. Magn. Magn. Mater. 321 (2009) 1692–1698. [20] M. Mahesh Kumar, S. Srinath, et al., J. Magn. Magn. Mater. (1998) 188–203. [21] J. Li, Y. Duan, H. He, D. Song, J. Alloys Comp. 315 (2001) 259. [22] B. Kundys, A. Maignan, et al., Appl. Phys. Lett. 92 (2008) 112905. [23] Q.Y. Xie, B. Lv, P.F. Wang, P. Song, X.S. Wu, Mater. Chem. Phys. 114 (2009) 636. [24] X.S. Wu, J. Gao, Physica. C 313 (1999) 49. [25] Dinesh Varshney, Ashwini Kumar, Kavita Verma, J. Alloys Comp. 509 (2011) 8421–8426. [26] Sverre M. Selbach et al., Chem. Mater. 21 (2009) 169–173. [27] Meera Keskar, K. Krishnan, N.D. Dahale, J. Alloys Comp. 458 (2008) 104–108. [28] R.V. Wandekar, B.N. Wani, S.R. Bharadwaj, J. Alloys Comp. 433 (2007) 84–90. [29] Marita Kerstan, Christian Rüssel, J. Power Sour. 196 (2011) 7578–7584. [30] M. Halvarsson, V. Langer, S. Vuorinen, Surf. Coat. Technol. 76–77 (1995) 358– 362. [31] Hui. Shen, Xu. Jiayue, et al., Mater. Sci. Eng. B 157 (2009) 77–80. [32] W. Knafo, C. Meingast, K. Grube, et al., J. Magn. Magn. Mater. 310 (2007) 1248– 1250.
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Structural phase transition and thermal expansion in Bi12.5xPr1.5xBaxFeO3 ceramics G.F. Cheng a,b, Y.J. Ruan b, Y.H. Huang b, X.S. Wu a,⇑ a b
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, PR China Analysis & Testing Center for Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e
i n f o
Article history: Received 4 September 2012 Received in revised form 22 December 2012 Accepted 19 February 2013 Available online 7 March 2013 Keywords: Bi12.5xPr1.5xBaxFeO3 Structural phase transition Thermal expansion In situ XRD
a b s t r a c t The temperature dependence of the crystal structure for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) polycrystallines is studied by X-ray powder diffraction with Rietveld refinements in the temperature range of 25– 800 °C. A structural phase transition of Rhombohedral-to-Cubic occurs for Bi0.875Pr0.075Ba0.05FeO3 sample in the temperature of 600–700 °C, which may relate to its unstable rhombohedra distorted structure with the space group R3c. The rarely decomposition of these samples indicates that the Pr, Ba co-doped make the BiFeO3 ceramics more stable. The thermal expansion determined by the temperature dependence of the unit-cell lattice parameters and volumes for Bi12.5xPr1.5xBaxFeO3 samples is also investigated, which shows an isotropic and positive behavior. The average thermal expansion coefficient decreases with the increasing x. We argue that the Cubic crystal structure with the high symmetrical of the space group Pm3m may be more stable for Bi0.75Pr0.15Ba0.1FeO3 sample, which may explain the reason why no phase transition occurs and its lower thermal expansion efficiencies. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction BiFeO3 (BFO) is one of the most important single-phase multiferroic materials, which show spontaneous electric and magnetic ordering simultaneously [1–3]. It has an antiferromagnetic behavior with a relatively high Néel temperature (TN 370 °C) and ferroelectric behavior with a high Curie temperature (TC 810 °C) and possesses a rhombohedra distorted perovskite-like structure with the space group of R3c at room temperature [4–6]. However, the spiral antiferromagnetic spin order and large leakage current hinder the technological application of the coupling between magnetism and electricity in BFO [7–10]. Recently a lot of efforts to modify its magnetic and electric properties have been reported, such as ions substitution, epitaxial films, single crystals and so on [10–13]. But due to the narrow temperature range of phase stabilization, the preparation of pure BFO is very difficult [6]. The presence of impurities (such as Bi25FeO39 and Bi2Fe4O9) results in high leakage current, which leading to poor ferroelectric behavior [6]. Selbach et al. [14] have investigated the thermodynamic stability of BFO by high temperature X-ray diffraction and discussed the stability of BFO and related Bi-based perovskites in relation to Goldschmidt tolerance factor and the influence of pressure and chemical substitution. Chen et al. [15] investigated the structure ⇑ Corresponding author. Tel./fax: +86 25 83594402. E-mail address: [email protected] (X.S. Wu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.02.138
and negative thermal expansion in the PbTiO3–BiFeO3 system. Bhattacharjee et al. [16] reported an unusual ferroelectric to ferroelectric isostructural phase transition and associated giant negative thermal expansion(NTE) for the tetragonal composition x = 0.31 closest to the morphotropic phase boundary(MPB) of the multiferroic (1x) BiFeO3–xPbTiO3 (BF–xPT) solid solution system. Chen et al. [17] have investigated the phase relations of the (1x) BiFeO3–xLaFeO3 system and their temperature dependence of the lattice parameters, unit-cell volumes and expansion coefficients. However, the study of unit cell thermal expansion about the BFO and doped BFO is very scarce. Thermal expansion is a very important evaluation indicator of the thermodynamic stability, which is affected by the increasing amplitude of the phonon during the heating process and in turn results in dilation of the crystal lattice [18]. The study of correlation between the structural phase transition and thermal expansion behavior is proposed to understand the thermodynamic stability. In the present, we synthesized the Bi12.5xPr1.5xBaxFeO3 ceramic samples. Structural phase transition and thermal expansion behavior are investigated by high temperature powder X-ray diffraction (HT-XRD). 2. Experimental The Bi12.5xPr1.5xBaxFeO3 ceramic samples are prepared by the conventional solid state reaction technique using Bi2O3, Fe2O3, BaCO3 and Pr6O11 reagents. The mixtures of reagents taken in desired cation ratios and pressed into pellets are annealed at 880 °C for 1 h in air. The crystal structure of the samples is characterized by X-ray
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G.F. Cheng et al. / Journal of Alloys and Compounds 566 (2013) 235–238
diffraction at high temperature using Rigaku D/max 2550 V X-ray diffractometer equipped with SHT-1500 multipurpose high temperature attachment, Cu ka radiation (40 kV, 250 mA). The heating rate of the high temperature attachment is set to be 10 °C min1 in air. Before experiments the sample is remained at a specified temperature for 30 min to reach a thermal equilibrium. Rietveld refinements are used to analyze the XRD data with the TOPAS software.
3. Results and discussion 3.1. Structural phase transition Fig. 1 shows the XRD patterns of the Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples at room temperature (25 °C). Both samples are found to be single phase. For sample with x = 0.05, the symmetry is a typical Rhombohedral with the space group of R3c, as that for BiFeO3 [19,20]. The upper inset of Fig. 1 show some separated peaks in the 2h range of 31.0–33.0°, which stand for two peaks with the index of (1 0 4) and (1 1 0) for Bi12.5xPr1.5xBaxFeO3 (x = 0.05) sample in the space group of R3c. The symmetry elements increase with increasing the dopants of Ba and Pr in the BiFeO3 unit cell due to the FeO6 octahedral trends to uniform and the separated peaks move together even become overlap in the XRD patterns, which indicates the structural phase transition from Rhombohedral R3c to Cubic Pm3m [21,22] occurs in Bi12.5xPr1.5xBaxFeO3 (x = 0.1) sample as shown in the inset. To further study the detailed subtle structure changes of the sample, Rietveld refinements are performed using the TOPAS software [23–25]. The typical refined structural parameters for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples at room temperature (25 °C) are shown in Table 1. It can be seen the criteria R-factors are very small, which suggests that the refinements are successfully performed. The final results confirm that the Bi+3 ion is substituted by the Ba2+, Pr3+ and Pr4+ ions. To investigate the thermodynamic stability of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples, HT-XRD is performed in the temperature range of 25–800 °C in air. None decomposition of Bi12.5xPr1.5xBaxFeO3 samples is observed between 25 and 800 °C as shown in Fig. 2, which confirm thermodynamically stable. This is very different from that of the BiFeO3, which is metastable with the formation of Bi2Fe4O9 and Bi25FeO39 impurity phases during 720 < T < 1040 K [26]. Sverre et al. observed the decomposition of BiFeO3 to Bi25FeO39 and Bi2Fe4O9 between 878 and 1090 K, but no decomposition of BiFeO3 happened between 1090 and 1199 K [26]. We argue that the substitution of Ba and Pr may increase the stability of BiFeO3 with respect to the binary oxides and possibly also with respect to the Bi2Fe4O9 and Bi25FeO39
phase [26]. However, some separated peaks move together even becoming overlap during the heating process, which indicates the change in the symmetry of the lattice and the structural phase transit from Rhombohedral R3c to Cubic Pm3m in Bi12.5xPr1.5xBaxFeO3 (x = 0.05) sample at about 700 °C as shown in the upper inset of Fig. 2. The Rhombohedral to Cubic transition is accompani ed with a decrease in cell volume from 380.82 to 0 63.64 Å A [3]. The structural phase transition would be related to its unstable low-symmetry rhombohedra distorted perovskitelike structure with the space group R3c. In addition to, the lines of the HT-XRD patterns shifting to smaller Bragg angles indicate the lattice constants transition and thermal expansion behavior as discussed below. 3.2. Lattice constants transition and thermal expansion The temperature dependences of lattice parameters (a, c) and unit-cell volumes for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples are shown in Figs. 3 and 4, respectively. The variation of the lattice parameters (a, c) and unit-cell volumes with temperature could be fitted to a third order polynomial function of the form y = y0 + AT + BT2 + CT3 as shown with red dash lines in Figs. 3 and 4 respectively, where A, B, C are constants and T denotes the temperature (°C) [19,27,28]. It can be observed that the lattice parameters (a, c) and unit-cell volumes of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) increase during the heating process in the temperature range 25–600 °C and a structural phase transition occurs in the temperature range 700–800 °C. However, the lattice parameter (a) and unit-cell volumes of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) increasing monotonically indicate no phase transition occurred in the temperature range 25–800 °C. Figs. 5 and 6 show the relative changes in the lattice parameters and cell volumes. The blue and red dash lines represent the values calculated from the linear fit of the lattice parameters and cell volumes, which suggest two regions (below 400 °C and above 400 °C) with distinct temperature dependence [19]. The thermal linear expansion coefficients of the samples are calculated with the formula as follows [18,27,29,30]:
aaT ¼
a a0 Da ¼ a0 ðT T 0 Þ a0 DT
ð1Þ
acT ¼
c c0 Dc ¼ c0 ðT T 0 Þ c0 DT
ð2Þ
And the thermal volume expansion coefficient is defined as [18,27,29,30]:
aVT ¼
Fig. 1. Observed (black squares), calculated (red circles) and difference (blue triangles) XRD patterns of the Bi12.5xPr1.5xBaxFeO3 samples at room temperature (25 °C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
V V0 DV ¼ V 0 ðT T 0 Þ V 0 DT
ð3Þ
where a0, c0, V0 denote the lattice parameter a, c, and the cell volume V at 25 °C respectively. The thermal linear and volume expansion coefficients in different temperature regions are listed in Table 2, which show an isotropic and positive behavior. The average values of thermal expansion coefficient decrease with the increase of x. It suggest that the high symmetrical Cubic crystal structure with the space group Pm3m is very stable in the Bi0.75Pr0.15Ba0.1FeO3 sample, which could be the possible mechanism for their non-phase transition and lower thermal expansion behavior during the heating process. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. In order to prove such hypothesis, we measure the specific heat Cp of the samples at the temperature range
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G.F. Cheng et al. / Journal of Alloys and Compounds 566 (2013) 235–238 Table 1 The typical refined structural parameters for the Bi12.5xPr1.5xBaxFeO3 samples obtained by Rietveld refinements from the XRD data at room temperature (25 °C). Sample Space group
x = 0.05 R3c
x = 0.1 Pm3m
Cell a (Å) c (Å) V (Å3)
5.57888(91) 13.8491(23) 373.29(14)
3.9584(95)
Atom Position x y z Cry size Gaussian (nm) Strain L Crystal Density (g/cm3) R factors (%) Rexp Rwp Rp GOF Wt% – Rietveld
Bi/Pr/Ba 6a 0 0 0
Fe 6a 0 0 0.2124(18) 179(65) 0.226(24) 8.1175(30) 3.19 3.54 2.77 1.11 100.000
62.03(45) O 18b 0.5(27) 0.027(17) 0.9634(46)
Bi/Pr/Ba 1b 0.500000 0.500000 0.500000
Fe 1a 0 0 0 63(59) 0.16(77) 7.910(57)
O 3d 0 0 0.50000
3.12 3.32 2.63 1.06 100.000
Fig. 4. Lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) in the temperature range 25–800 °C.
Fig. 2. XRD patterns of the Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples measured by HT-XRD at different temperatures.
Fig. 5. Relative changes in the lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) as a function of temperature.
Fig. 3. Lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.05) in the temperature range 25–800 °C.
from 25 °C to 450 °C as it is shown in the Fig. 7.As it is expected, the variation of Cp as a function of temperature shows a strong peak, which suggests a magnetic order–disorder phase transition
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G.F. Cheng et al. / Journal of Alloys and Compounds 566 (2013) 235–238
4. Conclusions
Fig. 6. Relative changes in the lattice parameters and cell volume of Bi12.5xPr1.5xBaxFeO3 (x = 0.1) as a function of temperature.
Table 2 Thermal expansion coefficients of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples. Sample
Temperature region
aaT 106
acT 106
aVT 106
(/°C)
(/°C)
(/°C)
x = 0.05
Below 400 °C Above 400 °C
12.2 11.7
15.2 2.73
37.7 37.7
x = 0.1
Below 400 °C Above 400 °C
5.63 11.6
16.6 35.3
Fig. 7. Specific heat (Cp) of Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) samples as a function of temperature.
happened. The point of heat anomaly indicates the Néel temperature of the samples. Such an anomaly has been also observed in other compound such as YMnO3 and BaNi2V2O8 [31,32]. The variation of thermal expansion coefficient is very flexible, which may be infected by lots of factors such as crystal structure, lattice parameter, cell volume, oxygen vacancy, doped ionic radius, density, porosity, crystallite size, lattice distortion and bulk modulus [31,32]. In Bi12.5xPr1.5xBaxFeO3 samples, the different substitution has change the structure from Rhombohedral-to-Cubic and their lattice parameter, cell volume and bulk modulus. And it also induces the magnetic phase transition Néel temperature alerting. These parameters in turn affect their thermal expansion coefficients.
The temperature dependence of the crystal structure and thermal expansion for Bi12.5xPr1.5xBaxFeO3 (x = 0.05, 0.1) polycrystallines are studied by powder X-ray diffraction at high temperature in the range of 25–800 °C in air. A structural phase transition of Rhombohedral-to-Cubic occurs for Bi0.875Pr0.075Ba0.05FeO3 sample in the temperature range from 600 to 700 °C. The thermal expansion for the Bi12.5xPr1.5xBaxFeO3 samples shows an isotropic and positive behavior. The average thermal expansion coefficient decreases with the increasing x. The Cubic crystal structure with the high symmetrical of the space group Pm3m may be more stable for Bi0.75Pr0.15Ba0.1FeO3 sample, which may explain why no phase transition occurs, and its lower thermal expansion efficiencies in high temperature. An obvious change in the slope of the linear fitted lines between 300 °C and 400 °C suggests a possible antiferromagnetic–paramagnetic transition, which occurs around the Néel temperature of the Bi12.5xPr1.5xBaxFeO3 samples. Acknowledgments This work is supported by the National Key Projects for Basic Researches of China (2010CB923404), the Natural Science Foundation of China (10974081, 10979017, 11274153, and 51202280). References [1] Qingyu Xu, Shengqiang Zhou, D.Wu, et al., Schmidt, J. Appl. Phys. 107 (2010) 093920. [2] D.H. Wang, W.C. Goh, M. Ning, C.K.Ong, Appl. Phys. Lett. 88 (2006) 212907. [3] Neeraj. Kumara, Neeraj. Panwarb, Bhasker. Gahtoria, et al., J. Alloys Comp. 501 (2010) L29–L32. [4] K.S. Nalwa, A. Garg, A. Upadhyaya, Mater. Lett. 62 (2008) 878–881. [5] Jun Zhang, M.A. Gondal, Wei Wei, et al., J. Alloys Comp. (2012) 530107– 530110. [6] Dinesh Varshney, Ashwini Kumar, Kavita. Verma, J. Alloys Comp. 509 (2011) 8421–8426. [7] D. Lebeuge, D. Colson, A. Forget, M. Viret, Phys. Rev. B 76 (2007) 024116. [8] Y.P. Wang, L. Zhou, M.F. Zhang, et al., Appl. Phys. Lett. 84 (2004) 1731. [9] C. Ederer, N.A. Spaldin, Phys. Rev. B 71 (2005) 060401(R). [10] Preetam Singh, J.H. Jung, Physica B. 405 (2010) 1086–1089. [11] J.J. Ge, X.B. Xue, G.F. Cheng, et al., J. Magn. Magn. Mater. 324 (2012) 200–204. [12] Y.C. Hu, Z.Z. Jiang, K.G. Gao, G.F. Cheng, et al., Chem. Phys. Lett. 534 (2012) 62– 66. [13] G.F. Cheng, Y.H. Huang, J.J. Ge, B. Lv, X.S. Wu, J. Appl. Phys. 111 (2012) 07C707. [14] Sverre M. Selbach, Mari-Ann Einarsrud, Tor Grande, Chem. Mater. 21 (2009) 169–173. [15] J. Chen, X.R. Xing, et al., Appl. Phys. Lett 89 (2006) 101914. [16] S. Bhattacharjee, K. Taji, et al., Phys. Rev. B 84 (2011) 104116. [17] J.R. Chen, W.L. Wang, J.B. Li, G.H. Rao, J. Alloys Comp. 459 (2008) 66–70. [18] S.N. Achary, S.J. Patwe, A.K. Tyagi, J. Alloys Comp. 461 (2008) 474–480. [19] V.A. Khomchenko, D.A. Kiselev, M. Kopcewicz, et al., J. Magn. Magn. Mater. 321 (2009) 1692–1698. [20] M. Mahesh Kumar, S. Srinath, et al., J. Magn. Magn. Mater. (1998) 188–203. [21] J. Li, Y. Duan, H. He, D. Song, J. Alloys Comp. 315 (2001) 259. [22] B. Kundys, A. Maignan, et al., Appl. Phys. Lett. 92 (2008) 112905. [23] Q.Y. Xie, B. Lv, P.F. Wang, P. Song, X.S. Wu, Mater. Chem. Phys. 114 (2009) 636. [24] X.S. Wu, J. Gao, Physica. C 313 (1999) 49. [25] Dinesh Varshney, Ashwini Kumar, Kavita Verma, J. Alloys Comp. 509 (2011) 8421–8426. [26] Sverre M. Selbach et al., Chem. Mater. 21 (2009) 169–173. [27] Meera Keskar, K. Krishnan, N.D. Dahale, J. Alloys Comp. 458 (2008) 104–108. [28] R.V. Wandekar, B.N. Wani, S.R. Bharadwaj, J. Alloys Comp. 433 (2007) 84–90. [29] Marita Kerstan, Christian Rüssel, J. Power Sour. 196 (2011) 7578–7584. [30] M. Halvarsson, V. Langer, S. Vuorinen, Surf. Coat. Technol. 76–77 (1995) 358– 362. [31] Hui. Shen, Xu. Jiayue, et al., Mater. Sci. Eng. B 157 (2009) 77–80. [32] W. Knafo, C. Meingast, K. Grube, et al., J. Magn. Magn. Mater. 310 (2007) 1248– 1250.