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Article

Two Perovskite Modifications of BiFe0.6Mn0.4O3 Prepared by High-Pressure and Post-Synthesis Annealing at Ambient Pressure

by
Alexei A. Belik
Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan
Inorganics 2024, 12(8), 226; https://doi.org/10.3390/inorganics12080226
Submission received: 29 July 2024 / Revised: 16 August 2024 / Accepted: 16 August 2024 / Published: 19 August 2024
(This article belongs to the Special Issue The State of the Art of Research on Perovskites Materials)
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Abstract

:
BiFeO3-related perovskite-type materials attract a lot of attention from the viewpoint of applications and fundamental science. In this work, we prepared two modifications of heavily Mn-doped BiFeO3 with the composition of BiFe0.6Mn0.4O3. A high-pressure (HP) modification was prepared at about 6 GPa and 1400 K. An ambient pressure (AP) modification was prepared by heating the HP modification at 780 K in the air at AP (post-synthesis annealing). Crystal structures of both modifications and in situ transformation were investigated with synchrotron powder X-ray diffraction. The transformation started at about 700 K and finished at about 780 K. The HP modification crystallized in space group Pnma with a = 5.57956 Å, b = 15.70576 Å, and c = 11.22557 Å, and the AP modification crystallized in space group Pbam with a = 5.63839 Å, b = 11.2710 Å, and c = 7.75923 Å (all parameters were at room temperature). Post-synthesis annealing of the HP modification (conversion polymorphism) is the only way to prepare the Pbam modification of oxygen stoichiometric BiFe0.6Mn0.4O3. Magnetic properties of both modifications have been reported. The Néel temperatures are TN = 350 K (HP) and TN = 335 K (AP). HP modification shows larger spin canting. Both modifications show negative magnetization phenomena at low temperatures in low magnetic fields.

1. Introduction

Materials with all possible ordered magnetic spins and electric dipoles are called multiferroics nowadays [1,2,3,4], even though the term ‘multiferroics’ was initially introduced only for materials with simultaneous ferroelectric and ferromagnetic (FM) orders [5]. The BiFeO3 perovskite is arguably the most studied multiferroic material [6], which attracted renewed interest after the work on thin-film samples [7]. BiFeO3 belongs to the so-called type-I multiferroics [1,2,3,4]. In type-I multiferroics, a ferroelectric transition takes place at a different temperature in comparison with the temperature of a magnetic transition because magnetism and ferroelectricity have different origins. In BiFeO3, the ferroelectric (FE) transition takes place at TFE = 1100 K and originates from the activity of a lone-electron pair of Bi3+ cations [6]. BiFeO3 crystallizes in the space group R3c below TFE. The pure antiferromagnetic (AFM) transition occurs at the Néel temperature TN = 643 K and originates from superexchange interactions between magnetic Fe3+ cations [6]. Magnetic Fe3+ cations on the three-dimensional perovskite lattice usually give large magnetic transition temperatures above room temperature, as in RFeO3 [8]. Neighboring spins are canted in BiFeO3. However, the absence of any net FM moments in BiFeO3 originates from a long period of incommensurate spin ordering, which averages the total net moment to zero [6].
Different crystallographic phases have close energies and compete with each other in BiFeO3, as can be seen from a cascade of structural phase transitions under high pressure [9,10,11] and the stabilization of different phases with small amounts of doping on both Bi and Fe sublattices [12,13,14,15,16,17]. At ambient pressure (AP), solid solutions can usually be prepared in whole compositional ranges from x = 0 to x = 1 in cases of (1) isovalent doping in the Bi sublattice, for example, Bi1−xRxFeO3 with R3+ = rare-earth elements [13,14,15] and (2) simultaneous aliovalent doping at both Bi and Fe sublattices, for example, (1−x)BiFeO3−xPbTiO3 [18] and (1−x)BiFeO3−xBaTiO3 [19]. On the other hand, doping only at the Fe sublattice, BiFe1−xMxO3 with M = 3d transition metals can be realized in very limited compositional ranges at AP. A maximum doping level of about x = 0.3 is realized in the BiFe1−xMnxO3 system at AP [17,20,21]. The main reason for this seems to be the fact that BiMO3 with other 3d transition metals (except for M = Fe) is not stable at AP [22].
During two decades of intensive research on BiFeO3-related materials, different preparation methods of Bi1−xRxFeO3 and BiFe1−xMxO3 have been developed and used, for example, different modifications of a standard solid-state synthesis (e.g., rapid synthesis) in different atmospheres, high-pressure (HP) high-temperature methods, variable soft-chemistry methods, plasma syntheses, and so on. However, in the case of BiFe1−xMnxO3 solid solutions, only standard solid-state synthesis in different atmospheres and HP high-temperature methods have been described in the literature.
The HP high-temperature method has significant advantages because BiFe1−xMxO3 solid solutions for compositional ranges from x = 0 to x = 1 can be prepared [17,23,24,25]. This method also often gives different modifications of BiFe1−xMxO3 in comparison with AP synthesis. In addition, a “conversion polymorphism” phenomenon [23] is sometimes observed in HP phases of BiFe1−xMxO3 when post-synthesis annealing produces new phases of BiFe1−xMxO3, which cannot be accessible using other methods.
In this work, we describe such a “conversion polymorphism” phenomenon in the high-pressure-synthesized BiFe0.6Mn0.4O3. An HP modification was prepared at about 6 GPa and 1500 K. An AP modification was prepared by heating the HP modification at 773 K in air at AP. The HP modification crystallizes in the space group Pnma and is isostructural with some Bi1−xRxFeO3 compounds, while the AP modification crystallizes in the space group Pbam and is isostructural with an antiferroelectic phase of PbZrO3. The magnetic properties of both modifications were studied.

2. Results and Discussion

The as-synthesized HP-BiFe0.6Mn0.4O3, crystallized in a complex PbZrO3-related superstructure √2ap × 4ap × 2√2ap (where ap (≈3.95 Å) is the parameter of the cubic perovskite subcell) with a = 5.57956(3) Å, b = 15.70576(8) Å, c = 11.22557(6) Å, and the space group Pnma (No. 62) [17]. HP-BiFe0.6Mn0.4O3 contained a small amount of Bi2O2CO3 impurity (about 1.1 weight%). HP-BiFe0.6Mn0.4O3 with heavy doping at the Fe sublattice has the same (super)structure as Bi0.82La0.18FeO3 [13] and Bi0.85Nd0.15FeO3 [26] with small doping at the Bi sublattice. This fact shows that the centrosymmetric Pnma structure with √2ap × 4ap × 2√2ap is one of the competing phases with the ferroelectric R3c structure of the parent BiFeO3. Refined structural parameters of HP-BiFe0.6Mn0.4O3 are summarized in Table 1, and Figure 1a shows fragments of experimental, calculated, and difference synchrotron XRPD data after the Rietveld fit.
In situ high-temperature structural studies of HP-BiFe0.6Mn0.4O3 showed that synchrotron patterns (and superstructure reflections) did not change from 297 K to 680 K (Figure 2). However, the temperature dependence of the lattice parameters of HP-BiFe0.6Mn0.4O3 showed some anomalies above 620 K (Figure 3). At 700 K, new reflections started to emerge (Figure 2). With further increases in temperature, intensities of new reflections increased, while intensities of superstructure reflections corresponding to the Pnma model (√2ap × 4ap × 2√2ap) decreased. Finally, at 780 K, all superstructure reflections corresponding to the Pnma model disappeared (Figure 2). All peaks at 780 K could be indexed in a √2ap × 2√2ap × 2ap superstructure with a = 5.64743(13) Å, b = 11.2983(2) Å, c = 7.82606(5) Å, and the space group Pbam (No. 55). During cooling, the new modification remained down to 297 K; this modification can be called AP-BiFe0.6Mn0.4O3. Refined structural parameters of AP-BiFe0.6Mn0.4O3 at 297 K and 780 K are summarized in Table 2 and Table 3, and Figure 1b shows the fragments of experimental, calculated, and difference synchrotron XRPD data after the Rietveld fit at 297 K as an example. The temperature dependence of the lattice parameters on heating and cooling is illustrated in Figure 3 and Figure 4. The crystal structure of AP-BiFe0.6Mn0.4O3 is isostructural with the antiferroelectric phase of PbZrO3 [27,28]. This crystal structure was also reported, for example, for Bi0.8La0.2FeO3 [11], Bi0.875Pr0.125FeO3 [15], and Bi0.89Ca0.11FeO3 [16]. Therefore, this centrosymmetric Pbam structure with √2ap × 2√2ap × 2ap is another competing phase.
We note that the Pnma model with the √2ap × 4ap × 2√2ap superstructure is a direct subgroup of the Pbam model with the √2ap × 2√2ap × 2ap superstructure. In many cases, additional superstructure reflections of the Pnma model are very weak. Therefore, a real Pnma model was sometimes replaced by a simplified Pbam model to obtain reliable, refined structural parameters [29], as the Pnma model has 32 refined fractional coordinates of atoms, while the Pbam model has 16 such parameters. However, in the case of BiFe0.6Mn0.4O3, the two modifications were quite different as the fundamental perovskite lattice parameters (ap) were different (Figure 4), and the reflection splitting of strong fundamental reflections was different at room temperature (Figure 1 and Figure 5). Therefore, the AP-BiFe0.6Mn0.4O3 modification could not be considered as a simplified version of the HP-BiFe0.6Mn0.4O3 modification.
Figure 6 shows the crystal structures of HP-BiFe0.6Mn0.4O3, AP-BiFe0.6Mn0.4O3, and PbZrO3 (at room temperature) for comparison [28]. (FeMn)O6 octahedra are strongly distorted in both modifications; a similar effect is observed for TiO6 octahedra in PbZrO3. These features can be explained by the formation of strong covalent Bi–O and Pb–O bonds originating from the stereochemical activity of the lone pair of Bi3+ and Pb2+ cations. In other words, elongated (FeMn)–O or Zr–O bonds are simultaneously involved in short Bi–O or Pb–O bonds, respectively [17]. The HP synthesis method usually stabilizes a modification with higher density. The density of HP-BiFe0.6Mn0.4O3 (8.439 g/cm3) was indeed slightly higher than that of AP-BiFe0.6Mn0.4O3 (8.418 g/cm3) (Table 1 and Table 2). To achieve higher density and to accommodate the lone pair of Bi3+, an additional octahedral rotation along the b axis was necessary (Figure 6c), resulting in a superstructure and a stressed structure in HP-BiFe0.6Mn0.4O3. Heating at AP results in the release of stress. The transformation of HP-BiFe0.6Mn0.4O3 into AP-BiFe0.6Mn0.4O3 involves small rotations of (FeMn)O6 octahedra (Figure 6b,c) and small shifts of Bi3+ cations.
No clear DSC anomalies were observed during the first round of heating of HP-BiFe0.6Mn0.4O3, suggesting that the thermal effect of the HP-to-AP transformation was very weak. The absence of any DSC anomalies could also be related to the fact that the HP-to-AP transformation occurred in a wide temperature range from 700 K to 780 K (Figure 2), preventing any detectable thermal effect. No clear DSC anomalies were observed during the first cooling of the already-formed AP-BiFe0.6Mn0.4O3, and no DSC anomalies were detected during the second DSC run on the already-formed AP-BiFe0.6Mn0.4O3. This fact suggests that AP-BiFe0.6Mn0.4O3 did not undergo any structural phase transitions between 297 K and 773 K. In the case of BiFe0.6Mn0.4O3, we observed a full transformation of an HP structure into another AP structure. In other members of HP-BiFe1−xMnxO3 solid solutions, partial transformations into different modifications were reported [30,31].
Temperature-dependent magnetic measurements showed that the Néel temperatures were TN = 350 K for HP-BiFe0.6Mn0.4O3 and TN = 335 K for AP-BiFe0.6Mn0.4O3 (Figure 7 and Figure 8). M versus H measurements showed that HP-BiFe0.6Mn0.4O3 had larger spin canting, especially at T = 5 K, as a clear hysteresis opened up (Figure 9 and Figure 10). At T = 100, 200, and 300 K, the M versus H curves of HP-BiFe0.6Mn0.4O3 were nearly linear with narrow cigar-type hysteresis, suggesting that spin canting was quite small. At T = 5, 100, 200, and 300 K, the M versus H curves of AP-BiFe0.6Mn0.4O3 were almost linear, suggesting nearly complete AFM states. Above TN, at T = 400 K, the M versus H curves of both modifications were linear because of the paramagnetic state (Figure 11) and coincided with each other; in other words, the M versus H curves were independent of the crystal structure. Exchange-bias-like effects were observed on the M versus H curves: at T = 5 K (negative exchange-bias-like effect) in AP-BiFe0.6Mn0.4O3 and at T = 100 K (positive exchange-bias-like effect) and 200 K (negative exchange-bias-like effect) in HP-BiFe0.6Mn0.4O3 (Figure 10). Exchange-bias-like effects were observed in other BiFeO3-based solid solutions [24,32,33]. In many cases, the “extrinsic” origins of exchange-bias-like effects were suggested, such as the presence of an antiferromagnetic core and a diluted antiferromagnetic shell [32] or non-uniform structure distortions and magnetic phase separation [33].
HP-BiFe0.6Mn0.4O3 showed peculiar magnetic susceptibility (χ = M/H) curves at H = 100 Oe (Figure 7). When a virgin sample (meaning a sample taken directly after the synthesis; in other words, a sample that was not used for any magnetic measurements beforehand) was measured below its TN (between 2 K and 300 K), the χ values were positive, and the FCC and FCW curves matched with each other and coincided on cycling. A sharp upturn was observed on the χ versus T curves below 120 K, suggesting the development of a weak FM moment in agreement with the M versus H curve at 5 K (Figure 9a). However, when the sample was heated above its TN (up to 400 K), the χ values on the FCC and FCW curves were negative below 150 K, demonstrating a strong negative magnetization effect. Absolute M values (at T = 5 K) were about 70 times larger when measured from 400 K in comparison with the measurement from 300 K. It is probably for this reason that no anomalies were observed near 120 K when measurements were performed from 400 K because strong negative magnetization hid a weak upturn. In addition, the FCC and FCW curves of HP-BiFe0.6Mn0.4O3 did not match above TN as would be expected in a paramagnetic state, but their merging gradually took place on approaching 400 K. This observation could suggest that there are some ferromagnetic-like short-range correlations above TN. On the other hand, the FCC and FCW curves of AP-BiFe0.6Mn0.4O3 almost merged above TN.
The magnetic properties of two AP-BiFe0.6Mn0.4O3 samples are shown in Figure 8: one sample was obtained in a furnace; and another sample was obtained in a DSC experiment (see the experimental part). Both samples showed a magnetic transition at the same temperature of TN = 335 K with different magnitudes of upturns below TN. But, for both samples, the upturn below TN was quite small, suggesting the development of small spin canting. The AP-BiFe0.6Mn0.4O3 sample prepared in a DSC experiment showed a negative magnetization effect below 30 K. The absence of a negative magnetization effect in the AP-BiFe0.6Mn0.4O3 sample prepared in a furnace suggests that this effect can be called “extrinsic” [24] when it is observed in some samples of AP-BiFe0.6Mn0.4O3.
The introduction of Mn3+ cations into BiFeO3 changes the crystal structure from a certain concentration of Mn3+ cations due to the existence of several competing structures and monotonically suppresses TN from 643 K in BiFeO3 to about 270 K in BiFe0.5Mn0.5O3 [17,24,25]. The magnetic transition temperatures of BiFe1−xMnxO3 remain relatively high because of large concentrations of Fe3+ cations. Magnetic properties of BiFeO3 [17] and AP-BiFe0.6Mn0.4O3 were qualitatively similar in the sense that they both showed nearly pure AFM behavior despite their different crystal structures. The temperature dependence of magnetic susceptibility exhibited a sharp rise just below TN in both compounds, indicating an initial development of uncompensated moments, but these uncompensated moments were suppressed at lower temperatures.

3. Materials and Methods

The HP modification of BiFe0.6Mn0.4O3 was prepared from a stoichiometric mixture of Bi2O3 (Rare Metallic Co., Tokyo, Japan, 99.9999%), Fe2O3 (Rare Metallic Co., Tokyo, Japan, 99.999%), and Mn2O3. Single-phase Mn2O3 was prepared from a commercial MnO2 chemical (Rare Metallic Co., Tokyo, Japan, 99.99%) by annealing in the air at 923 K for 24 h. The synthesis was performed at about 6 GPa and about 1400 K for 1.5 h in a sealed Au capsule using a belt-type HP instrument. After annealing at 1400 K, the sample was cooled down to room temperature by turning off the heating current, and the pressure was slowly released. As the used synthesis conditions for HP-BiFe0.6Mn0.4O3 gave high-quality samples, we did not investigate the effects of synthesis conditions on the quality of HP-BiFe0.6Mn0.4O3 and pressure–temperature stability ranges of HP-BiFe0.6Mn0.4O3 (it was also out of the scope of the present work). But we did note that the pressure–temperature stability ranges of BiFe1−xMnxO3 solid solutions can be relatively large as a lower pressure of 5 GPa and lower temperature of 1073 K have been used in the literature [25]. The AP modification of BiFe0.6Mn0.4O3 was prepared by heating HP-BiFe0.6Mn0.4O3 in the air at AP at 773 K for 10 min (with a heating–cooling rate of 10 K/min).
X-ray powder diffraction (XRPD) data were collected at room temperature on a MiniFlex600 diffractometer (Rigaku, Tokyo, Japan) using CuKα radiation (2θ range of 8–100°, a step width of 0.02°, and a scan speed of 2°/min). Synchrotron XRPD data of HP-BiFe0.6Mn0.4O3 were collected at 297 K upon heating to 780 K and cooling to 297 K using the beamline BL02B2 [34] of SPring-8, Japan. Intensity data were taken between 2.08° and 78.21° at a 0.006° interval in 2θ using a wavelength of λ = 0.420138 Å; however, data up to 60° (at 297 K) were used in the Rietveld analysis as no experimental reflections were observed above 60°. The measurement times were 300 s at 297 K and 780 K and 60 s for other temperatures. The sample was placed into an open Lindemann glass capillary tube (inner diameter: 0.1 mm), which was rotated during measurements. The Rietveld analysis of all XRPD data was performed using the RIETAN-2000 program [35].
Magnetic measurements were performed on a SQUID magnetometer (Quantum Design MPMS3, San Diego, CA, USA) between 2 and 300 K (or 400 K) in an applied field of 100 Oe on cooling (FCC: field-cooled on cooling) and warming (FCW: field-cooled on warming). Isothermal magnetization measurements for M versus H were performed from 70 kOe to −70 kOe and from −70 kOe to 70 kOe starting from 300 K, then at 200 K, 100 K, 5 K, and 400 K; the temperature was changed under the applied field of 70 kOe. In other words, after finishing M versus H measurements at, for example, 300 K, the field was kept at 70 kOe, and the temperature was changed to 200 K. A piece of an HP-BiFe0.6Mn0.4O3 pellet (41.12 mg) was used in the magnetic measurements. This pellet was then transformed to AP-BiFe0.6Mn0.4O3 (by annealing in a furnace in the air at AP at 773 K for 10 min as described above), and the same pellet was used in magnetic measurements.
Differential scanning calorimetry (DSC) curves of a powder sample of HP-BiFe0.6Mn0.4O3 were recorded on a Mettler Toledo DSC1 STARe system between 297 K and 773 K in an open Al capsule with a heating/cooling rate of 10 K/min. Two DSC runs were performed to check the reproducibility. No DSC anomalies were observed. Laboratory and synchrotron XRPD data were taken, and magnetic properties were measured for the sample after this DSC experiment, and the transformation to AP-BiFe0.6Mn0.4O3 was confirmed.

4. Conclusions

In conclusion, two modifications of the BiFe0.6Mn0.4O3 perovskite were prepared: the HP modification was prepared by the direct high-pressure high-temperature method at 6 GPa, and the AP modification was prepared with a “conversion polymorphism” strategy. The transformation of the HP modification to the AP modification was studied in situ, and crystal structures of both modifications were investigated with synchrotron powder X-ray diffraction. The peculiar magnetic properties of HP-BiFe0.6Mn0.4O3 and AP-BiFe0.6Mn0.4O3 were investigated and have been reported herein.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the author on request.

Acknowledgments

Synchrotron radiation was used at the powder diffraction beamline BL02B2 at SPring-8 with permission from the Japan Synchrotron Radiation Research Institute (Proposal Number: 2021A1334). We thank S. Kobayashi for his help at BL02B2 of SPring-8. MANA was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. The experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of (a) HP-BiFe0.6Mn0.4O3 and (b) AP-BiFe0.6Mn0.4O3 at T = 297 K between 2° and 20°. The tick marks show possible Bragg reflection positions for the main phase (brown) and Bi2O2CO3 impurity (blue) from top to bottom.
Figure 1. The experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of (a) HP-BiFe0.6Mn0.4O3 and (b) AP-BiFe0.6Mn0.4O3 at T = 297 K between 2° and 20°. The tick marks show possible Bragg reflection positions for the main phase (brown) and Bi2O2CO3 impurity (blue) from top to bottom.
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Figure 2. Temperature evolution of synchrotron powder X-ray diffraction patterns of HP-BiFe0.6Mn0.4O3 at selected temperatures. The zoomed parts emphasizing superstructure reflections are shown. Stars mark reflections from Bi2O2CO3 impurity. Black circles show characteristic reflections from HP-BiFe0.6Mn0.4O3, and crosses show a characteristic reflection from AP-BiFe0.6Mn0.4O3.
Figure 2. Temperature evolution of synchrotron powder X-ray diffraction patterns of HP-BiFe0.6Mn0.4O3 at selected temperatures. The zoomed parts emphasizing superstructure reflections are shown. Stars mark reflections from Bi2O2CO3 impurity. Black circles show characteristic reflections from HP-BiFe0.6Mn0.4O3, and crosses show a characteristic reflection from AP-BiFe0.6Mn0.4O3.
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Figure 3. (Up) Temperature dependence of the lattice parameters of HP-BiFe0.6Mn0.4O3 on heating. (Down) Temperature dependence of the lattice parameters of AP-BiFe0.6Mn0.4O3 on heating (black symbols) and cooling (blue symbols). Numbers show weight fractions (in%) of the corresponding phases.
Figure 3. (Up) Temperature dependence of the lattice parameters of HP-BiFe0.6Mn0.4O3 on heating. (Down) Temperature dependence of the lattice parameters of AP-BiFe0.6Mn0.4O3 on heating (black symbols) and cooling (blue symbols). Numbers show weight fractions (in%) of the corresponding phases.
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Figure 4. Temperature dependence of the normalized lattice parameters of HP-BiFe0.6Mn0.4O3 (Pnma) on heating and AP-BiFe0.6Mn0.4O3 (Pbam) on cooling.
Figure 4. Temperature dependence of the normalized lattice parameters of HP-BiFe0.6Mn0.4O3 (Pnma) on heating and AP-BiFe0.6Mn0.4O3 (Pbam) on cooling.
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Figure 5. Zoomed parts (to emphasize superstructure reflections) of room-temperature synchrotron powder X-ray diffraction patterns of HP-BiFe0.6Mn0.4O3 and AP-BiFe0.6Mn0.4O3. Stars mark reflections from Bi2O2CO3 impurity. The (hkl) indexes of main superstructure reflections are given. Intensities were normalized to 1, and data for the AP modification were shifted by +0.03 (or 3%).
Figure 5. Zoomed parts (to emphasize superstructure reflections) of room-temperature synchrotron powder X-ray diffraction patterns of HP-BiFe0.6Mn0.4O3 and AP-BiFe0.6Mn0.4O3. Stars mark reflections from Bi2O2CO3 impurity. The (hkl) indexes of main superstructure reflections are given. Intensities were normalized to 1, and data for the AP modification were shifted by +0.03 (or 3%).
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Figure 6. Fragments of crystal structures of (a) PbZrO3 (at room temperature [28]), (b) AP-BiFe0.6Mn0.4O3, and (c) HP-BiFe0.6Mn0.4O3 (Bi2 atoms are hidden by Bi1 and Bi3). Arrows in panel (a) show the displacements of Pb2+ inside cavities. For simplicity, octahedral sites are marked as Fe, Fe1, and Fe2.
Figure 6. Fragments of crystal structures of (a) PbZrO3 (at room temperature [28]), (b) AP-BiFe0.6Mn0.4O3, and (c) HP-BiFe0.6Mn0.4O3 (Bi2 atoms are hidden by Bi1 and Bi3). Arrows in panel (a) show the displacements of Pb2+ inside cavities. For simplicity, octahedral sites are marked as Fe, Fe1, and Fe2.
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Figure 7. Magnetic properties of virgin HP-BiFe0.6Mn0.4O3 (Pnma) (a pellet of 41.12 mg). Field-cooled on cooling (FCC) and field-cooled on warming (FCW) χ versus T curves at H = 100 Oe are shown. The first run was measured from 300 K to 2 K and from 2 K to 300 K. The second run was measured from 300 K to 2 K and from 2 K to 400 K. The third run was measured from 400 K to 2 K and from 2 K to 400 K. Insets show the zoomed parts. The magnetic field remained the same (unchanged) through all three runs.
Figure 7. Magnetic properties of virgin HP-BiFe0.6Mn0.4O3 (Pnma) (a pellet of 41.12 mg). Field-cooled on cooling (FCC) and field-cooled on warming (FCW) χ versus T curves at H = 100 Oe are shown. The first run was measured from 300 K to 2 K and from 2 K to 300 K. The second run was measured from 300 K to 2 K and from 2 K to 400 K. The third run was measured from 400 K to 2 K and from 2 K to 400 K. Insets show the zoomed parts. The magnetic field remained the same (unchanged) through all three runs.
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Figure 8. Magnetic properties of AP-BiFe0.6Mn0.4O3 (Pbam). Field-cooled on cooling (FCC) and field-cooled on warming (FCW) χ versus T curves at H = 100 Oe are presented. The data for two samples are shown: an AP-BiFe0.6Mn0.4O3 sample prepared in a furnace (a pellet of 41.12 mg) is shown by blue triangles; an AP-BiFe0.6Mn0.4O3 sample prepared in a DSC experiment is shown by black circles (powder of 9.00 mg). The right-hand axis gives the same FCC curve for an AP-BiFe0.6Mn0.4O3 sample prepared in a furnace.
Figure 8. Magnetic properties of AP-BiFe0.6Mn0.4O3 (Pbam). Field-cooled on cooling (FCC) and field-cooled on warming (FCW) χ versus T curves at H = 100 Oe are presented. The data for two samples are shown: an AP-BiFe0.6Mn0.4O3 sample prepared in a furnace (a pellet of 41.12 mg) is shown by blue triangles; an AP-BiFe0.6Mn0.4O3 sample prepared in a DSC experiment is shown by black circles (powder of 9.00 mg). The right-hand axis gives the same FCC curve for an AP-BiFe0.6Mn0.4O3 sample prepared in a furnace.
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Figure 9. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): M versus H curves at (a) T = 5 K, (b) T = 100 K, (c) T = 200 K, and (d) T = 300 K.
Figure 9. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): M versus H curves at (a) T = 5 K, (b) T = 100 K, (c) T = 200 K, and (d) T = 300 K.
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Figure 10. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): the zoomed parts of the M versus H curves are shown between −8 kOe and 8 kOe at (a) T = 5 K, (b) T = 100 K, (c) T = 200 K, and (d) T = 300 K.
Figure 10. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): the zoomed parts of the M versus H curves are shown between −8 kOe and 8 kOe at (a) T = 5 K, (b) T = 100 K, (c) T = 200 K, and (d) T = 300 K.
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Figure 11. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): (a) M versus H curves between −70 kOe and 70 kOe at T = 400 K and (b) zoomed parts of the M versus H curves between −8 kOe and 8 kOe at T = 400 K.
Figure 11. Comparison of magnetic properties of HP-BiFe0.6Mn0.4O3 (Pnma) and AP-BiFe0.6Mn0.4O3 (Pbam) (pellets of 41.12 mg): (a) M versus H curves between −70 kOe and 70 kOe at T = 400 K and (b) zoomed parts of the M versus H curves between −8 kOe and 8 kOe at T = 400 K.
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Table 1. Structure parameters of HP-BiFe0.6Mn0.4O3 at 297 K refined from synchrotron XRPD data.
Table 1. Structure parameters of HP-BiFe0.6Mn0.4O3 at 297 K refined from synchrotron XRPD data.
AtomWPx/ay/bz/cBiso (Å2)
Bi18d0.2169(5)0.00935(9)0.62476(16)1.18(5)
Bi24c0.2135(7)0.250.6251(2)1.37(7)
Bi34c0.7729(7)0.250.3635(2)1.55(7)
FeMn18d0.2397(12)0.1227(6)0.8779(7)1.06(11)
FeMn28d0.2615(11)0.6249(5)0.8770(6)0.54(10)
O18d0.809(4)−0.0026(12)0.6556(18)0.24(16)
O24c0.641(6)0.250.653(3)=B(O1)
O34c0.177(7)0.250.425(3)=B(O1)
O48d0.099(4)0.1267(17)0.7217(19)=B(O1)
O58d−0.003(4)0.3942(15)−0.0260(21)=B(O1)
O68d0.041(4)0.6458(14)0.7424(23)=B(O1)
O78d0.019(5)0.3989(13)0.516(3)=B(O1)
Source: Synchrotron powder X-ray diffraction (λ = 0.42014 Å); d-space range used in the refinement: 0.420–11.563 Å. Crystal system: orthorhombic; space group Pnma (No 62); Z = 16. Molecular weight: 312.458 g/mol. The occupation factors, g, of all Bi and O sites, are unity; g = 0.6Fe + 0.4Mn for FeMn1 and FeMn2. WP: Wyckoff position. Impurity: Bi2O2CO3 (1.1 wt.%). a = 5.57956(3) Å, b =15.70576(8) Å, c = 11.22557(6) Å, and V = 983.711 (9) Å3; Rwp = 9.02%, Rp = 6.43%, RB = 5.14%, and RF = 6.78%; ρcal = 8.439 g/cm3.
Table 2. Structure parameters of AP-BiFe0.6Mn0.4O3 at 297 K refined from synchrotron XRPD data.
Table 2. Structure parameters of AP-BiFe0.6Mn0.4O3 at 297 K refined from synchrotron XRPD data.
AtomWPx/ay/bz/cBiso (Å2)
Bi14g0.2431(3)0.3807(3)02.15(6)
Bi24h0.2061(3)0.3676(2)0.50.32(3)
FeMn8i0.2552(6)0.1239(14)0.2487(13)0.80(5)
O14g0.241(4)0.162(3)01.8(9)
O24h0.338(5)0.093(3)0.51.2(7)
O38i0.078(4)0.2649(16)0.275(3)1.7(5)
O44f00.50.188(3)1.3(7)
O54e000.202(4)2.4(8)
Source: Synchrotron powder X-ray diffraction (λ = 0.42014 Å); d-space range used in the refinement: 0.420–11.563 Å. Crystal system: orthorhombic; space group Pbam (No. 55); Z = 8. Molecular weight: 312.458 g/mol. The occupation factors, g, of all Bi and O sites, are unity; g = 0.6Fe + 0.4Mn for FeMn. WP: Wyckoff position. Impurity: Bi2O2CO3 (1.1 wt.%). a = 5.63839(12) Å, b = 11.2710(2) Å, c = 7.75923(8) Å, and V = 493.103(15) Å3; Rwp = 11.93%, Rp = 8.40%, RB = 4.92%, and RF = 2.66%; ρcal = 8.418 g/cm3.
Table 3. Structure parameters of AP-BiFe0.6Mn0.4O3 at 780 K refined from synchrotron XRPD data.
Table 3. Structure parameters of AP-BiFe0.6Mn0.4O3 at 780 K refined from synchrotron XRPD data.
AtomWPx/ay/bz/cBiso (Å2)
Bi14g0.2383(4)0.3786(5)03.31(13)
Bi24h0.2141(5)0.3700(4)0.51.91(9)
FeMn8i0.2524(9)0.1246(25)0.2499(17)1.56(6)
O14g0.257(5)0.170(5)02.3 (3)
O24h0.304(6)0.091(5)0.5=B(O1)
O38i0.075(4)0.2678(19)0.271(4)=B(O1)
O44f00.50.185(4)=B(O1)
O54e000.206(4)=B(O1)
Source: Synchrotron powder X-ray diffraction (λ = 0.42014 Å); d-space range used in the refinement: 0.549–11.563 Å. Crystal system: orthorhombic; space group Pbam (No. 55); Z = 8. Molecular weight: 312.458 g/mol. The occupation factors, g, of all Bi and O sites, are unity; g = 0.6Fe + 0.4Mn for FeMn. WP: Wyckoff position. Impurity: Bi2O2CO3 (1.1 wt.%). a = 5.64743(13) Å, b = 11.2983(2) Å, c = 7.82606(5) Å, and V = 499.353(16) Å3; Rwp = 10.94%, Rp = 7.43%, RB = 5.81%, and RF = 4.97%; ρcal = 8.312 g/cm3.
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Belik, A.A. Two Perovskite Modifications of BiFe0.6Mn0.4O3 Prepared by High-Pressure and Post-Synthesis Annealing at Ambient Pressure. Inorganics 2024, 12, 226. https://doi.org/10.3390/inorganics12080226

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Belik AA. Two Perovskite Modifications of BiFe0.6Mn0.4O3 Prepared by High-Pressure and Post-Synthesis Annealing at Ambient Pressure. Inorganics. 2024; 12(8):226. https://doi.org/10.3390/inorganics12080226

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Belik, Alexei A. 2024. "Two Perovskite Modifications of BiFe0.6Mn0.4O3 Prepared by High-Pressure and Post-Synthesis Annealing at Ambient Pressure" Inorganics 12, no. 8: 226. https://doi.org/10.3390/inorganics12080226

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