Thermodynamic Stability and Thermal Expansion of Pure-phase BiFeO3

CHENG Guo-Feng, RUAN Yin-Jie, SUN Yue, YIN Han-Di

Thermodynamic Stability and Thermal Expansion of Pure-phase BiFeO3

CHENG Guo-Feng, RUAN Yin-Jie, SUN Yue, YIN Han-Di

(Analysis and Testing Center for Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050)

The thermodynamic stability and thermal expansion of pure-phase BiFeO3multiferroic are studied usinghigh temperature X-ray diffraction (HT-XRD), and high temperature Raman spectroscopy (HT-Raman). BiFeO3keeps its rhombohedral structure with space group ofin heating processHowever, minor BiFeO3could decompose to Bi2Fe4O9and Bi25FeO39in cooling process, which may be induced by its oxygen octahedron tilt distortion. In addition, the thermal expansion coefficients of BiFeO3are also investigated, showing an isotropic and positive behavior. These results are further confirmed by Raman spectrum analysis. This work may benefit for the preparation of pure-phase BiFeO3.

BiFeO3; phase transition; thermal expansion coefficients; HT-XRD; HT-Raman

Multiferroic materials couple ferromagnetic and ferroelectric properties simultaneously, in which BiFeO3(BFO) is one of the well-known single-phase materi-als[1-2]. It has a G-type antiferromagnetic and ferroelectric behavior with higher Néel temperature (N~370 ℃) and Curie temperature (C~810 ℃)[3]. It is reported to have a rhombohedrally distorted perovskite structure withspace group[4]. Recently, many attempts have been made to improve the ferroelectricity and magneti-zation in BFO, such as ion-doping, epitaxial films, single crystals and nanoparticles[5-7], it is very difficult to syn-thesize pure BFO ceramic for its narrow phase stabilized temperature range and low peritectic decomposition temperature[8-9]. In order to remove the Fe-rich (Bi2Fe4O9) and Bi-rich (Bi25FeO39) phases, many efforts have been done such as rapid liquid phase sintering, quenching process, leaching with dilute nitric acid[10-13]. Although these methods are very effective, the thermodynamic stability and decomposition of BiFeO3are currently still not clarified. Selbach,[14]reported the decomposition of BiFeO3to Bi25FeO39and Bi2Fe4O9in the temperature interval 447–767 ℃ for its metastable property. In our previous study, BiFeO3, Bi25FeO39and Bi2Fe4O9phases are not in thermodynamic stable status during the cooling sintering process[15].In addition, many works focus on the thermal expansion and transformation of BiFeO3solid solutions. Chen,[16]reported the negative ther-mal expansion and unusual transformation in PbTiO3- BiFeO3. Bhattacharjee,[17]studied the associated giant negative thermal expansion for (1–)BiFeO3–PbTiO3(=0.31) solid solution system. Klyndyuk and Chen,[18-19]investigated the linear thermal expansion coeffi-cient of BiFeO3-NdMnO3and (1–) BiFeO3–LaFeO3,respectively. Nevertheless, the clear pictures on thermal stability and thermal expansion characteristics for either BFO or doped BFO are very limited.

To address the issue, we synthesized pure BFO powder, studied its thermody-namic stability and thermal ex-pan-sion byHT- XRD and HT-Raman. This work would provide a useful experimental guidance for the preparation of pure-phase BFO by clarifying its thermo--dynamic stability and ther-mal expansion characters.

1 Experimental

The BFO ceramic was prepared by a combined method of solid state reaction and quenching using Bi2O3and Fe2O3. Minor Bi25FeO39phase in the ceramic was leached by dilute nitric acid. The crystal structure of pure BFO powder was characterized by HT-XRD using Bruker D8 ADVANCE X-ray diffractometer with the heating rate 30 ℃?min–1in air. Data were collected from 20° to 110° (2) in the step scanning mode (step size=0.02°, FT= 1.5 s) in the temperature range of 25–800 ℃ and 700– 25 ℃. The lattice parameters and unit-cell volumes were calculated by Rietveld refinements based on TOPAS software. This refinement modeled the background and peak shape by a fifth order polynomial and a Pseudo- Voigt II function, respectively. Backscattered Electron (BSE) images and Energy-dispersive Spectrometry (EDS) quantitative measurement were carried out by a Scanning Electron Microscope (FEI MAGELLAN 400). The Raman spectra of pure BFO powder at the temperature range 25–750 ℃were examined using a Renishaw confocal Raman spectroscopy (model inVia). The laser power was maintained at 0.2 mW with semiconductor laser excitation at 532 nm.

2 Results and discussion

The thermodynamic stability of pure BFO powder is performed by theHT-XRD. Fig. 1(a–b) show the X-ray diffraction patterns at selected temperatures in the heating and cooling process, respectively. The upper inserts show the two main peaks of BFO as index of (104) and (110). During the heating process, BFO keeps rhom-bohedralstructure and no phase transition occurs. The shift of 2-values towards smaller Bragg angles is observed in the XRD patterns, which means that heating could make the lattice parameters transition, indicating the thermal expansion behavior. However, the main peak of Bi25FeO39as index of (201) is observed during the cooling process from 700 ℃ to 25 ℃ as shown in the upper left insert in Fig. 1(b), which demonstrates the decomposition of metastable BiFeO3.

BSE images and EDS quantitative analysis results of the sample before and after HT-XRD analyzing are shown in Fig. 2. It is clearly seen that some white phases (Bi-rich Bi25FeO39and Fe-rich Bi2Fe4O9) are dispersed on the boundary of gray phases (BiFeO3). The decomposition equation of BiFeO3will be[13,15]:

49BiFeO3→Bi25FeO39+12Bi2Fe4O9(1)

This decomposition suggests that BiFeO3shows thermodynamic instability behavior in the cooling process, which is different from that in the heating process. The decomposition reaction could be assimilated to a first- order transition, and the nucleation is required[20]. In ad-dition, the shift of 2-values towards bigger Bragg angles in the cooling process could indicate the negative thermal expansion behavior. To further study the detailed changes of the lattice constants transition, Rietveld refinements are performed using TOPAS software.

Fig. 3 shows the typical measured and simulated XRD patterns of BFO samples before and after heating process (room temperature, 25 ℃). BFO and a little amount of Bi25FeO39phase (weight percentage 3.5%) were refined withandstructural model, respectively. All the criteria R-factors (WP) are less than 6.5%, which suggests that the refinements are successfully performed. The refined structural parameters in the heating process are summarized in Table 1 and Table 2.

Fig. 1 High temperature XRD patterns of BFO at selected temperatures in the heating (a) and cooling (b) process Two main peaks of BFO as index of (104) and (110) are shown in upper insert at 31° to 33°; The main peak of Bi25FeO39 as index of (201) is shown in the upper left insert (b)

Fig. 2 BSE images and EDS results on the surface of BFO powder before (left) and after (right) heating

Fig. 3 Observed (black solid lines), calculated (red solid lines) and difference (blue solid lines) XRD patterns of BFO powder before and after heating process

Table 1 Refined lattice parameters of BFO powder in the heating process

Table 2 Refined structural parameters of BFO powder in the heating process

The temperature dependences of lattice parameters (,) and unit-cell volumes () for BFO in the heating and cooling process are shown in Fig. 4. The variation of the lattice parameters (,) and unit-cell volumes () with temperature could be fitted to a linear function[21-22].It can be observed that the lattice parameters and unit-cell volumes of BFO increase and decrease monotonically in the heating or cooling process, respectively. This linear fitting also suggests that no structural phase transition occurred for BFO.

Fig. 5 shows the relative changes of lattice parameters and cell volumes for BFO in the heating process. The black, blue and red dash lines represent the values calculated from the linear fit of the lattice parameters and cell volumes. The thermal linear and volumetric expansion coefficients of BFO in the heating process (25–800 ℃) are calculated with the following equation[20-21,23]:

Fig. 4 Lattice parameters and cell volume of BFO in the heating process (temperature regime 25–800 ℃, (a)) and cooling process (temperature regime 700–25 ℃, (b))

Fig. 5 Relative changes in lattice parameters of BFO as a function of temperature in the heating process

Wheredenotes,, andat selected high temperatures, and0denotes the value of,, andat 25 ℃. The calculated thermal linear and volumetric expansion coefficients of BFO are listed in Table 3, which show an isotropic and positive behavior. It was suggested that the rhombohedral structure with the space group ofis very stable, which could be the possible mechanism for their non-phase transition and lower thermal expansion behavior during the heating process.

To establish qualitatively the temperature dependent structural evolution, we further analyze the bond length/bond angle. The atomic positional and occupancy parameters of BFO in the heating process are calculated and refined by the least squares method using TOPAS software. The chemical bonding behavior are determined from this Rietveld refinement. For the G-type structure with space groupof BiFeO3, the tilting of FeO6octahedron represents the rhombohedral distortion of the perovskite structure. Fig. 6 exhibits the two types of Fe–O bond lengths and the inter-octahedral O–Fe–O angles in the heating process[24]. Fe–O (I) and Fe–O (II) bond lengths basically maintain invariable and slowly increase in the temperature range of 25–400 ℃, respectively. However, Fe–O (I) bond lengths dramatically increase above 400 ℃, Fe–O (II) bond length drops suddenly around 700 ℃. In addition, We note, the O–Fe–O bond angles exhibit compression in the temperature range of 300–600 ℃. After that, these bond angles show an increase upon heating above 600 ℃, which consequently induces tilts oxygen octahedron and expansion of the unit cell[25]. This oxygen octahedron tilt distortion may result in the decomposition of BiFeO3.

Table 3 The thermal linear and volumetric expansion coefficients of pure BFO

Fig. 6 Fe–O bond lengths and Fe–O–Fe angles of BFO in the heating process

Fig. 7 shows the Raman spectra of BFO at selected temperatures in the heating process. For BFO at 25 ℃, three strong peaks at 138, 172, 219 cm?1and one weak peak around 427 cm?1are assigned as A1-1, A1-2, A1-3 and A1-4 mode, respectively. The peaks at 259, 283, 321, 349, 475, 523, and 553 cm–1belong to E mode[26]. In the heating process, BFO keeps its structure which is in agreement with the HT-XRD results. Increasing temperature results in the shift (toward lower wavenumber) and broaden of Raman bands, which is explained in terms of thermal expansion and oxygen octahedron tilt distortion.

Fig. 7 Raman spectra of BFO at selected temperatures in the heating process

3 Conclusion

In this study, the thermodynamic stability, decomposition and thermal expansion of pure BiFeO3are studied systematically. BFO keeps its rhombohedral structure with the space group ofThe temperature dependences of the relative lattice parameters and cell volume for the BFO are linear, suggesting that, in the heating process (25– 800 ℃), no structural phase transition occurs. However, minor BiFeO3could decompose to Bi2Fe4O9and Bi25FeO39in the cooling process. The oxygen octahedron tilt distortion reflected by the change in bond length may result in the decomposition of BiFeO3. The thermal linear and volumetric expansion coefficients of BFO in the heating process are calculated, which show an isotropic and positive behavior.

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純相BiFeO3的熱穩(wěn)定和熱膨脹性質(zhì)研究

程國峰, 阮音捷, 孫玥, 尹晗迪

(中國科學(xué)院 上海硅酸鹽研究所, 無機(jī)材料分析測試中心, 上海 200050)

本研究利用原位高溫衍射和高溫拉曼技術(shù)對純相BiFeO3粉體的熱穩(wěn)性和熱膨脹性質(zhì)進(jìn)行了系統(tǒng)的解析。在升溫階段BiFeO3始終保持斜方的結(jié)構(gòu), 但是在降溫階段少量BiFeO3會分解成為Bi2Fe4O9和Bi25FeO39, 這種分解可能是由氧八面體的傾斜畸變引起的。此外, 還研究了BiFeO3熱力學(xué)膨脹系數(shù), 發(fā)現(xiàn)它具有各向同性正膨脹性。以上結(jié)果也被拉曼光譜所證實(shí)。本研究的結(jié)果可為制備純相BiFeO3材料提供實(shí)驗(yàn)指導(dǎo)。

鐵酸鉍; 相變; 熱膨脹系數(shù); 高溫X射線衍射; 高溫拉曼光譜

TQ174

A

2019-01-03;

2019-03-27

National Natural Science Foundation of China (51202280); Shanghai Technical Platform for Testing and Characterization on Inorganic Materials (14DZ2292900)

CHENG Guo-Feng (1977–), male, senior engineer. E-mail: gfcheng@mail.sic.ac.cn

1000-324X(2019)10-1128-05

10.15541/jim20190005