Temperature-Driven Multiferroic Phase Transitions and Structural

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Temperature-Driven Multiferroic Phase Transitions and Structural Instabilities Evolution in Lanthanum Substituted Bismuth Ferrite Jie Wei, Chunfang Wu, Tiantian Yang, Zhibin Lv, Zhuo Xu, Da-wei Wang, Raphael Haumont, and Zhenxiang Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12502 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry

Temperature-Driven Multiferroic Phase Transitions and Structural Instabilities Evolution in Lanthanum Substituted Bismuth Ferrite Jie Wei, 1, 2,* Chunfang Wu,3 Tiantian Yang,

1

Zhibin Lv,

1

Zhuo Xu,

1

Dawei Wang,

1

Raphael Haumont,2

Zhenxiang Cheng 4

1

Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for

Dielectric Research, School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an 710049, P. R. China 2

Laboratoire de Physico-Chimie de l'Etat Solide, ICMMO, CNRS-UMR 8182, Bâtiment 410, Université

Paris-Sud XI, 15 rue Georges Clémenceau 91405 Orsay Cedex, France 3

School of Photoelectric Engineering, Xi'an Technological University, Xi'an 710021, P. R. China

4

Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, Innovation Campus,

Squires Way, North Wollongong, NSW 2500, Australia

*Corresponding Author: Jie Wei, E-mail address: [email protected]; [email protected] Other authors: Chunfang Wu, E-mail address: [email protected] Tiantian Yang, E-mail address:

[email protected]

Zhibin Lv, E-mail address: [email protected] Zhuo Xu, E-mail address:

[email protected]

Dawei Wang, E-mail address：[email protected] Raphael Haumont, E-mail address:

[email protected]

Zhenxiang Cheng, E-mail address:

[email protected]

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Abstract: Structural phase transition behavior of a material under the temperature plays a key role on understanding its physical properties, instabilities and design of new functions. It is, however, extremely shortage of investigations on the temperature-driven phase transition behaviors of bismuth ferrite (BiFeO3). Herein, magnetic phase transitions within low-temperature regimes, ferroelectric phase transitions under high temperature, structural instabilities evolution and spin-phonon coupling in 5% La-doped BiFeO3 ceramic (BFLa05) were systemically studied by employing X-ray Diffraction, Differential Scanning Calorimetry, Raman spectroscopy and magnetic measurement, as well as the phenomenological theoretical analysis. Thanks to the outstanding stability of BFLa05 ceramic under high temperature, a complete phase transition sequence of α↔β↔γ was observed, simultaneously determining the crystalline symmetries for β and γ phases as orthorhombic Pnma and cubic Pm-3m, respectively. Octahedral tilt as a typical structure instability exhibited an anomalous change around Néel temperature, while a significant discontinuity in the vicinity of Curie temperature, implying strong interplay or coupling between octahedral tilt and magnetic or ferroelectric ordering. Surprisingly, the intense spin-phonon coupling was observed not only around the Néel temperature, but also at two low temperatures of about 140K and 220K. Furthermore, these two low-temperature magnetic phase transitions accompanying with spin-phonon interactions were well illustrated by a phenomenological theoretical model on basis of the nearest-neighbors approximation and molecular field approximation.

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I. INTRODUCTION As a prototypical multiferroic oxide, Bismuth ferrite (BiFeO3) has been intensely studied due to its high ferroelectric Curie temperature (TC ~1093K) and antiferromagnetic Néel temperature (TN ~ 650K).1-6 Taking account of possible applications, investigations on the growth and physical properties of high-quality BiFeO3 thin film, ceramic, or single crystal have been of major interest over past years.7-12 Nevertheless, investigations on the ferroic phase transitions of BiFeO3 remain relatively deficient. In fact, structural phase transition behavior of a material under high pressure or temperature plays a key role on understanding its physical properties, instabilities and design of new functions as known in the case of perovskite BaTiO3 and PbTiO3.13 BiFeO3 is a complex system in which ferroelectric, magnetic and ferroelastic orders are mutually coupled. Furthermore, these unique properties are believed to be strongly impacted by two structural distortions of ferroelectric cation displacements and octahedral FeO6 tilts.13 In fact, the coexistence and mutual coupling of these two structural instabilities in one compound is extremely rare. Thus, BiFeO3 as a rather exceptional case is a model system to investigate the competition between tilts and polar instabilities under the external thermo-dynamical variables, such as temperature, pressure or electric/magnetic field. Recently, high-pressure (over GPa) driven ferroic phase transitions of BiFeO3 has been investigated, and thus a particular phase diagram under high pressure has been profiled.13-19 Room temperature R3c phase can stably exist from ambient pressure to 3 GPa; In the region of 3~11GPa, two or three structural phase transitions occur and these phase structures have been proposed possibly as monoclinic C2/m and different orthorhombic phases, Ima2, I2cm, Imam, etc.14-17 A GdFeO3-type non-polar phase (Pnma) remains relative stable between 11 and 38GPa. Experimental studies in very high pressure region (38~70GPa) have revealed two structural phase transitions. First transition from Pnma into Pnmm occurs at 38GPa; More interestingly, second one from Pnmm to monoclinic Cmcm at around 45GPa has manifested an insulator-to-metal transition.18, 19 Compared to well investigations on high pressure behavior, the structure phase transitions of BiFeO3 as a function of temperature still remain ambiguous and debated in the literature. The crystalline structure of BiFeO3 at high temperature has been investigated by a few studies in the past, 20-28

as schematized in Fig.1. 3 / 31



Figure 1

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Schematic summaries for investigations on the high-temperature phase transitions of BiFeO3

Earlier studies revealed eight anomalies in the physical properties of BiFeO3 as a function of temperature. 20 Two of them were completely confirmed later as the antiferromagnetic TN~650K and ferroelectric TC~1093K, respectively. More recently, a phase diagram of BiFeO3 on basis of the thermal analysis, spectroscopy, diffraction and other measurements

21-28

was proposed and then

displayed three distinct solid phases between room temperature and the melting point (~1223K): a ferroelectric α phase below TC~1093K; a paraelectric β phase between 1103K and 1198K; and a cubic γ phase in the region of 1198K~1206K before its decomposition. Although α phase is clearly identified as a rhombohedral R3c structure (Schematically shown in Fig.1S in “Supplementary materials”), the crystalline structure of the paraelectric β phase is still unclear because of the partial decomposition of BiFeO3 at high temperature. Several different symmetries as cubic Pm-3m, tetragonal I4/mcm, rhombohedral R3m or R-3c, orthorhombic Pbnm or P2mm, and monoclinic P21/m phase have been successively proposed (as shown in Fig.1).22-28 At higher temperature, Palai et al 23 have manifested the existence of a cubic γ phase at 1198K, just below the peritectic decomposition temperature Tper of about 1208K. The β to γ transition has been suggested as a second-order-like transition, accompanied by an insulator-metal transition in agreement with the observation at high pressure.18, 19 Nevertheless, this phase transition at 1198K very close to Tper has hampered further structural studies of the cubic polymorph and thus most other groups just observed decomposition of BiFeO3 before reaching this phase. It is known that BiFeO3 is a metastable phase at high temperature and its partial decomposition above TC may be one of the main reasons to explain the different 4 / 31


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structures sketched for β phase and the difficulty to observe γ phase. What are worse, some second parasitic phases like Bi25FeO39 and Bi2Fe4O9 arose from this high temperature instability severely hamper accurate determining the crystalline structure of the β and γ phase. Nevertheless, substitution of rare earth elements or other metal elements in Bi- or Fe-site has been proposed as a promising approach to suppress the formation of secondary phases and stabilize the crystalline structure of BiFeO3 at high temperature. Gibbs et al

29

investigated the

high-temperature phase transitions in BiFe0.7Mn0.3O3, and completely observed three solid phases of α, β and γ. However, Mn dopants excessively reduced TC from 1093K to 963K, and likely drove the structural symmetries of γ phase deviating from cubic one. Indeed, our previous study also reported on the temperature-driven phase transitions of Bi0.95+δFe0.9Zr0.1O3 powder. 30 In spite of observation on a complete phase transition sequence of α-β-γ, only one point for γ phase observed at 1183K before its decomposition reveals its poor high temperature stability. Obviously, substitution in Fe-site seems to excessively buckle octahedral tilt, resulting in notably change of TC or have an inappreciable improvement on the high temperature stability of BiFeO3. Therefore, it could not reveal the intrinsic phase transition behaviors of parent BiFeO3. Very recently, the low-temperature magnetic ordering transitions and possibly involved spin-phonon coupling have attracted much attention because of their rich physics and possible application at low temperature. However, the related reports remain extremely scarce. Despite few pioneer explores revealed three ambiguous transition regimes of 140-150K, 200-220K and 230-260 K,31-36 it is strange enough that no single group could observed all these low temperature transitions while even using the similar-type BiFeO3 sample. Obviously, more explores on the low-temperature transition behaviors of BiFeO3 are indeed necessary. In this case, we report on the temperature-driven phase transitions of Bi0.95La0.5FeO3 studied by the Variable Temperature X-Ray Powder Diffraction, Differential Scanning Calorimetry, Raman spectroscopy and magnetic measurements. Surprisingly, in contrast to substitution in Fe-site, both the crystalline structure and ferroelectric Curie temperature (TC) present almost no change in Bi0.95La0.5FeO3. Furthermore, its high temperature stability has been greatly improved because cubic γ phase was still observed at a very high temperature of 1233K. Therefore, a complete high-temperature phase transition sequence of α↔β↔γ was observed in Bi0.95La0.5FeO3, and 5 / 31



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simultaneously the crystalline symmetries for β and γ phases were suggested as orthorhombic Pnma and cubic Pm-3m, respectively. More interestingly, the intense spin-phonon coupling was observed not only around the Néel temperature, also at two low temperatures of about 140K and 220K. A phenomenological theoretical model was employed to illustrate these two low-temperature phase transitions accompanied with spin-phonon coupling interactions.

II. EXPERIMENTAL SECTION Pure BiFeO3 and Bi0.95La0.05FeO3 ceramics were prepared by a conventional solid-state sintering procedure using high-purity oxides of Bi2O3, Fe2O3, and La2O3 as the starting materials. After a series process of weighing, ball milling and drying, the mixed powders were pressed into small cylindrical pellets with a diameter of ~8 mm and thickness of ~0.5 mm. Then, the samples were sintered at 1093K for 3h under air atmosphere so as to obtain pure phase. X-ray diffraction (XRD) patterns from 2=100 to 900 with a step of 0.040 were collected with a Philips X-celerator Bragg-Brentano diffractometer using Cu Kα radiation ( =1.54056 Å). The variable temperature XRD (VT-XRD) patterns were recorded from room temperature to 1253K in air by using a furnace with accuracy better than 2K. XRD data were refined using XND analysis program on basis of the Rietveld method. Thermal analysis (Differential Scanning Calorimetry, DSC) was implemented to follow the phase transformations involved up to 1273K with a heating rate of 2K/min. Any appearance of endothermic/exothermic peaks should be related to either structural or phase transition. The polarized Raman spectra were recorded in a back-scattering geometry using a LABRAM Jobin–Yvon spectrometer (He–Ne laser, 632nm). High- or low-temperature Raman measurements were carried out by using a commercial LINKAM heating/cooling stage placed under the Raman microscope. The superconducting quantum interference device magnetometer (SQUID, Quantum Design) was used in the magnetic measurements.

III. RESULTS AND DISCUSSION A. High Temperature ferroelectric phase transitions The high-temperature phase evolutions of Bi0.95La0.05FeO3 (BFLa05) powder were well studied using thermal analysis in combination with the Variable Temperature X-Ray Powder Diffraction. Figure 2 shows DSC data of BFLa05 powder recorded from 973K to 1273K during the heating 6 / 31


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process. Three distinct endothermic peaks were observed in the DSC curve, which could be identified at 1098K, 1193K and 1233K, respectively. Undoubtedly, the peak at 1098K corresponds to the ferroelectric Curie temperature (TC), which is almost same as that of parent BiFeO3. It suggests that BFLa05 could completely reveal the intrinsic phase transitions of BiFeO3. Uncertainly, the two peaks at 1193K and 1233K are initially supposed as the first decomposition temperature and the peritectic plateau respectively, looking like the observation of Zr-doped BiFeO3 ceramics in our previous study.30 Nonetheless, a careful inspection on the DSC curve shows that the relative intensity of the peak at 1193K is very weak. It is unreliable to consider it as a decomposition temperature.

Figure 2

DSC curve for Bi0.95La0.05FeO3 ceramic measured from 973K to 1273K.

In order to well understand the transformations stressed by DSC measurement, it is better to exactly determine the crystalline structure of these high-temperature phases by the variable temperature XRD patterns (VT-XRD). Figure 3 shows the VT-XRD patterns of BFLa05 powder recorded under different temperatures (from 300K to 1253K). Firstly, a Rietveld refinement for Room-temperature XRD pattern (300K) of BFLa05 powder shows that all the diffraction peaks can be indexed as a rhombohedral perovskite structure with R3c space group (see Fig. 3&4 and Table 1), which clearly indicates a pure phase for as-prepared sample without any secondary phase. Note that the substitution of La in Bi-site does not alter the crystalline structure of parent BiFeO3, which means that BFLa05 should be a good candidate to reflect the intrinsic phase transition behaviors of parent compound. As shown in Fig.2&3, this rhombohedral R3c structure, namely the ferroelectric α phase can stably exist from 300K to 1098K. However, XRD pattern recorded at 1103K above TC presented quite difference profiles, as seen 7 / 31



Page 8 of 31

in Fig.3. For example, the doublet peak near 320 gradually merged to a singlet peak, some characteristic indexes of (006) and (018) for R3c disappeared, and especially some weak peaks (labeled as “#” in the insert) appeared. These remarkable changes in the pattern clearly illuminate the occurrence of a phase transition from α to β at TC and the presence of a new intermediate β phase, which is well consistent with the observation in DSC curve. Concerning the anomaly at 1193K observed in DSC curve, it was initially supposed to be the first decomposition temperature. However, we did not find any trace for decomposition in the XRD patterns recorded at 1213K and 1233K. Obviously, the possibility for a decomposition temperature can be ruled out. More interestingly, a closer inspection of these patterns shows that most doublet peaks continually merged into singlet peaks and those characteristic weak peaks for β phase completely disappeared, implying a new phase transition from β to γ occurred at 1193K. Undoubtedly, the anomaly at 1233K should be corresponding to a decomposition temperature, since we found apparent decomposition and melting due to the existence of the liquid phase and impurities in the pattern recorded at 1253K (insert showing the existence of the liquid phase). The elaborated analysis focusing on the XRD patterns of BFLa05 as a function of temperature in the vicinity of α←→β←→γ phase transitions was also carried out and presented in supplementary materials (as shown in Fig.2S), which could more clearly display the complete process of these two phase transitions.

Figure 3

High-temperature XRD patterns for Bi0.95La0.05FeO3 ceramic

8 / 31


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As above mentioned, most controversies focus on the exact crystalline structure of the paraelectric phases above TC. Therefore, a Rietveld analysis for each diffraction pattern recorded above TC and below the decomposition temperature was performed by using the XND software. As shown in Fig.4, since some weak peaks presented in the XRD pattern at 1103K (see asterisks in the insert), it is impossible to well describe the whole diffraction pattern simply using the classical perovskite symmetries (including cubic Pm-3m, orthorhombic Bmm2, tetragonal P4mm, rhombohedral R3m, and monoclinic Cm or Pm phases). It is known that the intermediate β phase should be paraelectric, some centrosymmetric groups (Cmcm, Pnma, P21/m, and P42/nmc) in agreement with the tilt system of BiFeO3 were tested by Rietveld refinements. Consequently, the best agreement matching between the observed and calculated profiles was obtained with the orthorhombic Pnma space group (Cell parameters were also summarized in Table 1). Moreover, this space group is centrosymmetric, implying a paraelectric phase at this temperature. In fact, Pbnm for β phase of pristine BiFeO3 reported by Arnold et al

26, 27

is just a nonstandard setting of Pnma. In

addition, the non-polar Pnma structure for the intermediate phase of BiFeO3 has previously reported as a high-pressure phase stably existing between 11GPa and 38GPa.13

Figure 4

Refinements of XRD pattern for Bi0.95La0.05FeO3 ceramic recorded at 300K, 1103K and 1223K;

Right side: Schematic to describe the corresponding crystalline structure of R3c, Pnma, and Pm-3m 9 / 31



Table 1

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Cell parameters of Bi0.95La0.05FeO3 at 300K, 1103K and 1223K Cell parameters

Samples

Bi

La

0.95

Lattice parameters (Å)

FeO

0.05

3

(R3c, 300K)

Bi

La

0.95

FeO

0.05

3

(Pnma, 1103K)

a =

c =

5.5796(3)

13.8679(1)

h

a= o 5.6444(1)/ b = o

h

V

cell

(Å3)

373.89(2)

average Fe-O bond length (Å)

Fe-O-Fe angle, ɵ (deg.)

FeO6 tilt angle (deg.)

2.0352(1)

155.72(2)

ɷ= 13.0(3)

Ψ= 12.2(1)

ɷ= 0

c= o 5.6128(2)

252.44(3)

1.9668(1)

Fe-O1-Fe: 160.70 (3) Fe-O2-Fe: 152.92 (1)

c= 3.9962 (2)

63.82 (3)

1.9985(1)

180

7.9684(2) Bi

La

0.95

FeO

0.05

3

(Pm-3m, 1223K)

Nevertheless, the studies on the phase transition from β to γ for pristine BiFeO3 were extremely scarce, primarily due to the onset of decomposition and even melting before reaching this transition. As above mentioned, Palai et al 23 observed cubic γ phase only at 1198K. Arnold et al 26, 27, however, suggested the γ phase having the similar structure type as orthorhombic β-phase, despite little decomposition of BiFeO3 in their observation. In our case, we did not find any decomposition of BFLa05 from 1193K to 1233K (see Fig.3&4), implying that La-dopants greatly improved the high-temperature structure stability. As discussed in the analysis for DSC and VT-XRD results, the anomaly at 1193K should correspond to the phase transition from β to γ, rather than a decomposition temperature. A Rietveld analysis for the diffraction pattern recorded at 1223K shows that it could be well described by the classical perovskite symmetries—cubic Pm-3m (as shown in Fig.4 and Table 1). This refinement is well consistent with the results reported by Palai et al

23,

implying that the

γ-phase of BiFeO3 indeed adopts cubic symmetry. By well establishing the space group symmetry for α, β and γ phase, the cell parameters could be precisely extracted from the Rietveld refinement results. Figure 5 shows the temperaturedependence evolution of the rhombohedral R3c, orthorhombic Pnma and cubic Pm-3m cell parameters. In accordance with the earlier papers, 24, 30 an obvious anomaly of the lattice parameters 10 / 31


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(ah-pc ,ch-pc) and the cell volume observed at approximately 640K should be ascribed to TN for BFLa05 ceramic, corresponding to its magnetic transition from antiferromagnetism (AFM) to paramagnetism (PM). Almost negligible contraction in cell volume suggests a second-order nature for this magnetic phase transition. Increasing the temperature to TC, the cell parameters of ah-pc and ch-pc present abrupt change or leap, and a significant discontinuity in the unit cell volume was also observed. Previous study

23

reported that the phase transition at TC for pure BiFeO3 was generally

accompanied by an intense decrease in the unit cell volume with a maximum variation ∆V/V≈-1.4%. Such an important volume change entirely confirms the first-order nature of the ferroelectric phase transition. In our case, the volume change for BFLa05 around TC was estimated as ∆V/V≈-1.3%, very close to the maximum value in the earlier report. Undoubtedly, the ferroelectric phase transition of BFLa05 exhibits the first-order behavior. Nevertheless, concerning the new phase transition from β to γ phase, the cell volume presents a smooth change from 63.45Å3 at 1183K to 63.53Å3 at 1193K, thereby implying a second-order-like phase transition agreed well with DSC result.

Figure 5

(a) Lattice parameters and (b) cell volume as a function of temperature for Bi0.95La0.05FeO3;

All Lattice parameters described in a pseudo-cubic cell ( aℎ ― 𝑝𝑐 = aℎ/ 2 , cℎ ― 𝑝𝑐 = cℎ/ 12 and a𝑜 ― 𝑝𝑐 = a𝑜/ 2 , b𝑜 ― 𝑝𝑐 = b𝑜/2 , c𝑜 ― 𝑝𝑐 = c𝑜/ 2 )

B. Structural instabilities and its evolution Despite much attention to investigate on the universal rules for foretelling phase transitions in perovskites under high-pressure or high-temperature, very recently it is of particular interest to emphasize the two structure instabilities (including octahedral tilts and polar instabilities), and their evolutions as a function of external factors (pressure and temperature) in the model multiferroic 11 / 31



Page 12 of 31

BiFeO3 system. Generally, one type of structural instability can easily prevail in the majority of perovskite compounds at a specific pressure or temperature. For example, CaTiO3 just possesses octahedral tilts, whereas only ferroelectric cation displacements presenting in BaTiO3.37 Surprisingly, these two structural instabilities of both octahedral tilts and ferroelectric cation displacements would be presented in BiFeO3 at a given temperature or pressure.13 In fact, structural instabilities in the perovskites can be semi-quantitatively explained on basis of the Goldschmidt tolerance factor, 38 which is defined as,

𝑡=

𝑅𝐴 + 𝑅𝑂

(1)

2(𝑅𝐵 + 𝑅𝑂)

where RA, RB, and RO are the ionic radii of cation A, B and oxygen (O) for the perovskite structure (ABO3), respectively. The value of tolerance factor t can clearly reflect how much the atoms can be displaced from the ideal packing positions within the perovskite cell. Hence it is well linked to structural modifications such as distortion, rotation, and tilting of the octahedra. Reaney et al

38

found that structural modifications in the perovskites could be simply classified using tolerance factor t. Perovskites with a tolerance factor value of 0.985

Temperature-Driven Multiferroic Phase Transitions and Structural

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