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Article

The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System

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 2025, 13(3), 91; https://doi.org/10.3390/inorganics13030091
Submission received: 25 February 2025 / Revised: 12 March 2025 / Accepted: 14 March 2025 / Published: 18 March 2025
(This article belongs to the Special Issue Photoelectric Research in Advanced Energy Materials)
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Abstract

:
Perovskite-type materials containing Bi3+ cations at A sites are interesting from the viewpoints of applications and fundamental science as the lone pair of Bi3+ cations often stabilizes polar, ferroelectric structures. This can be illustrated by a lot of discoveries of different new functionalities in bulk and thin films of BiFeO3 and its derivatives. In this work, we investigated solid solutions of BiCr1−xFexO3 with 0.1 ≤ x ≤ 0.4 prepared by a high-pressure (HP) method and post-synthesis annealing at ambient pressure (AP). HP-BiCr1−xFexO3 modifications with 0.1 ≤ x ≤ 0.3 were mixtures of two phases with space groups C2/c and Pbam, and the amount of the C2/c phase decreased with increasing x. The amount of the C2/c phase was also significantly decreased in AP-BiCr1−xFexO3 modifications, and the C2/c phase almost disappeared in AP-BiCr1−xFexO3 with 0.2 ≤ x ≤ 0.3. Fundamental, strong reflections of HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 were almost unchanged; on the other hand, weak superstructure reflections were different and showed clear signs of strong anisotropic broadening and incommensurate positions. These structural features prevented us from determining their room-temperature structures. On the other hand, HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 showed high-temperature structural phase transitions to the GdFeO3-type Pnma modification at Tsrt = 450 K (x = 0.1), Tsrt = 480 K (x = 0.2), Tsrt = 510 K (x = 0.3), and Tsrt = 546 K (x = 0.4). Crystal structures of the GdFeO3-type Pnma modifications of all the samples were investigated by synchrotron powder X-ray diffraction. Magnetic properties of HP-BiCr1−xFexO3 and AP-BiCr1−xFexO3 were quite close to each other (HP vs. AP), and the x = 0.2 samples demonstrated negative magnetization phenomena without signs of the exchange bias effect.

Graphical Abstract

1. Introduction

Bi3+ cations have a lone electron pair similar to Pb2+ cations and, therefore, are often considered as a replacement of toxic lead in ferroelectric and piezoelectric materials. BiFeO3 perovskite has been known since the late 1950s [1,2,3,4]. However, it has received tremendous interest in the 2000s [5,6] after the discovery that thin films of BiFeO3 can have good ferroelectric properties and are effective and model multiferroic materials [7,8,9]. Since then, thousands of publications have been devoted to BiFeO3 perovskite and its derivatives [6]. A lot of fundamental science and practical discoveries have recently been made using BiFeO3 perovskite [10,11,12,13,14], for example, related to manipulations of charge domain walls, chiral spin transport, and spin cycloids, the formation of skyrmion lattices, and so on.
BiFeO3 crystallizes in space group R3c below its ferroelectric transition temperature TFE = 1100 K, and it shows a number of high-temperature structural transitions [6]. Its first transition at TFE is to a GdFeO3-type Pnma phase [15], not to a centrosymmetric pair (space group R-3c) of space group R3c as one would expect. The ferroelectricity of BiFeO3 originates from the activity of the lone electron pair of Bi3+ cations [6]. The pure antiferromagnetic (AFM) transition occurs at the Néel temperature TN = 643 K and originates from superexchange interactions between magnetic Fe3+ cations [6]. The absence of any net ferromagnetic (FM) moments in BiFeO3 originates from long-period incommensurate spin ordering, which averages the total net moment to zero [6]. BiFeO3 perovskite is the only compound among other simple BiMO3 perovskites that can be prepared in bulk form under ambient pressure (AP) conditions.
BiCrO3 perovskite has been known since the middle of the 1960s [16,17,18], but it received renewed interest in the 2000s [19,20,21,22,23,24,25,26,27,28,29,30,31] as a counterpart to BiFeO3. It crystallizes in space group C2/c at room temperature (RT) and shows a structural phase transition to a GdFeO3-type Pnma modification above Tstr = 420 K [23,24]. BiCrO3 demonstrates an AFM transition at TN = 112 K to a G-type AFM structure with small spin canting (and, therefore, with the appearance of weak FM properties) [23,24,25,26,30,31], and there is a spin-reorientation transition near 72–75 K with an increased weak FM moment. BiCrO3 perovskite can only be stabilized in bulk form under high-pressure (HP) synthesis conditions [32].
Other simple BiMO3 perovskites have been studied less because they need an HP high-temperature method for their preparation in bulk form [32]. It is interesting that there are many structural variations in simple BiMO3 perovskites [27,32]: samples with M = Sc, Cr, and Mn crystallize in space group C2/c; M = Al and Fe samples—in polar space group R3c; the M = Co sample—in polar space group P4mm; the M = Ni sample—in space group P-1; the M = Ga sample—in a pyroxene-type (non-perovskite) Pcca structure; and the M = Rh sample—in space group Pnma [32]. Therefore, different BiM11−xM2xO3 solid solutions can show complex structural variations and transformations as a function of composition x and temperature. Such solid solutions have received a tremendous amount of interest in the literature [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. For example, the appearance of polar R3c phases was observed in the solid solution system between centrosymmetric BiCrO3 and BiGaO3 [47,50]. In the solid solutions between BiFeO3 and BiCrO3, one composition has been investigated a lot, namely Bi2FeCrO6, after the first-principle predictions [52] that this composition (with ordered arrangements of Fe3+ and Cr3+ cations) should show large ferroelectric polarization and large magnetization [53,54,55,56,57,58]. In particular, thin films of Bi2FeCrO6 have attracted a lot of attention from the application point of view as photovoltaic materials [57,58]. The number of investigations focusing on properties of bulk BiCr1−xFexO3 solid solutions is very limited because such solid solutions need an HP method for their preparation [59,60,61,62,63,64], and the majority of such studies are focused on studies of Fe-rich samples [60,61,62].
Therefore, in this work we investigated samples from the Cr-rich side of bulk BiCr1−xFexO3 solid solutions with 0.1 ≤ x ≤ 0.4. We found structural phase transitions to the Pnma modification with systematic changes in the phase transition temperature. We found that the as-synthesized samples do not transform back into their initial state after cooling from high temperatures, therefore realizing the conversion polymorphism phenomenon [41]. Weak incommensurate structural modulations were found in BiCr1−xFexO3 solid solutions with 0.1 ≤ x ≤ 0.4. Detailed magnetic properties are reported.

2. Results and Discussion

All BiCr1−xFexO3 samples contained small amounts of Bi2O2CO3 impurity. The x = 0.1 and 0.2 samples additionally contained very small amounts of Cr2O3 impurity; the amount of Cr2O3 impurity was at the background level in the x = 0.3 and 0.4 samples. HP-BiCr1−xFexO3 samples with x = 0.1, 0.2, and 0.3 consisted of two perovskite-type modifications: one modification had the C2/c symmetry as found in undoped BiCrO3 [24], and the second modification had the PbZrO3-type structure (space group Pbam (No. 55) and a √2ap × 2√2ap × 2ap superstructure, where ap (≈3.95 Å) is the parameter of the cubic perovskite subcell [62,65,66]). The amount of the BiCrO3-type C2/c modification rapidly decreased with increasing x from about 75% (x = 0.1) to about 50% (x = 0.2) and about 5% (x = 0.3) in the HP samples (Figure 1 and Figure 2). The amount of the BiCrO3-type C2/c modification also rapidly decreased with increasing x from about 40% (x = 0.1) to trace amounts (x = 0.2) to undetectable amounts (x = 0.3) in the AP samples (Figure 1 and Figure 2). Both HP- and AP-BiCr0.6Fe0.4O3 contained only a perovskite phase with the PbZrO3-type structure (Figure 2b).
Figure 3 shows the results of the DSC measurements. All samples demonstrated a reversible phase transition with sharp peaks on the DSC heating curves at Tstr = 450 K (x = 0.1), 480 K (x = 0.2), 510 K (x = 0.3), and 546 K (x = 0.4), where Tstr stands for the structural phase transition temperature. The first heating curve was slightly different from the second and third heating curves; all cooling curves were almost identical. The small difference between the heating curves can be explained by the fact the HP modification transforms to the AP modification after the first DSC run. The synchrotron XRD data (Figure 1 and Figure 2; blue curves) clearly showed that the DSC anomalies correspond to a structural phase transition to the Pnma modification. With Tstr = 420 K for undoped BiCrO3 [23,24], Tstr almost linearly increases with increasing x in the BiCr1−xFexO3 solid solutions with 0.0 ≤ x ≤ 0.4. All the samples showed comparable enthalpies (about 7.0–7.9 J/g) of the structural phase transition (Figure 3). However, a systematic increase in enthalpy with increasing x can be seen.
The Pnma modification of all the samples had sharp reflections without any anisotropic broadening or asymmetry. Therefore, structure parameters of the Pnma modifications could be readily refined from synchrotron XRD data. Refined structural parameters of the Pnma modifications for all the samples are summarized in Table 1, and Figure 4 shows fragments of experimental, calculated, and difference synchrotron XRPD data after the Rietveld fit at 550 K for the x = 0.1 sample as an example.
Figure 5 shows the compositional dependence of the lattice parameters and unit cell volume of the Pnma modifications at 550 K (Table S1 provides numerical data). Nearly linear increases in all parameters were observed with the increase in the Fe content in agreement with the larger ionic radius of Fe3+ cations (rVI = 0.645 Å) in comparison to Cr3+ cations (rVI = 0.615 Å) [67], confirming the formation of the solid solutions.
On the other hand, superstructure reflections of the PbZrO3-type Pbam modifications in all the samples showed strong anisotropic broadening, and some reflections showed asymmetry from the high-angle side of the reflections (Figure 1, Figure 2 and Figure S1). For example (Figure 2b), the (021) reflection had the same width as the fundamental, main reflections, and it was symmetrical. The (111) reflection was very broad, and it was nearly symmetrical. The (110) reflection had intermediate broadening, and it showed asymmetry. Moreover, some reflections of the AP modification showed noticeable shifts from their expected, commensurate positions (for example, the (110), (130), and (210) reflections). Such shifts suggest the presence of incommensurate modulations. Such shifts were also present in the HP modifications, but the shifts were less pronounced. Therefore, the presence of incommensurate modulations and significant anisotropic broadening prevented us from determining the precise structural parameters of the HP and AP modifications at RT.
BiFeO3 has an incommensurate AFM structure, but BiFeO3 has a well-defined commensurate crystal structure [6]. Incommensurate structural modulations were only observed in the rare-earth-doped samples, Bi1−xRxFeO3, where R3+ is a rare-earth element, and at very limited compositional regions [40,68,69,70,71,72,73]. To the best of our knowledge, incommensurate structural modulations found in BiCr1−xFexO3 samples, especially in the AP modifications, have never been observed before in the only-transition-metal-doped BiFe1−xMxO3, where M is a transition metal element. Electron diffraction will be essential to understand incommensurate structural modulations of BiCr1−xFexO3 samples.
Figure 6a shows the temperature dependence of the lattice parameters of BiCr0.6Fe0.4O3 on heating and cooling (Table S2 provides numerical data). Drastic changes in the fundamental perovskite reflections were observed during the transition from the Pbam modification to the Pnma modification. The unit cell volume also drops by −1.4% at 550 K, where the two phases coexist, suggesting the first-order structural phase transition. It is interesting that the a lattice parameter of the Pnma modification decreased with increasing temperature, suggesting anisotropic thermal expansion. All other parameters of the Pnma modification and all parameters of the Pbam modification increased with increasing temperature as expected. The fundamental lattice parameters of the HP and AP modifications were very close to each other at RT (Figure 6). However, the superstructure reflections were slightly different as discussed above. Therefore, we can discuss the different modifications, namely the HP and AP modifications, and the observation of the conversion polymorphism phenomenon [41] in the BiCr1−xFexO3 system similar to the conversion polymorphism phenomenon in the BiFe1−xScxO3 system [41] and the BiFe1−xMnxO3 system [36,49].
Because of the presence of some structural differences between the HP and AP modifications, we investigated the effects of the structural differences on the magnetic properties. Temperature-dependent magnetization curves of the HP and AP modifications are shown in Figure 7, Figure 8, Figure 9 and Figure 10, and the results of M versus H measurements are shown in Figure 11, Figure 12, Figure 13 and Figure 14 (Figures S2–S5 show magnified parts of Figure 11, Figure 12, Figure 13 and Figure 14). The HP and AP modifications of BiCr0.9Fe0.1O3 showed very similar magnetic properties with one detectable magnetic transition at TN = 100 K despite their different phase compositions (a different ratio of the Pbam and C2/c phases). Undoped BiCrO3 has TN = 112 K and shows a spin reorientation transition at TN2 = 72 K [23]. No signs of a spin reorientation transition were found in BiCr0.9Fe0.1O3. A noticeable weak FM moment appeared below TN = 100 K at H = 100 Oe and H = 10 kOe (Figure 7). The transition with the weak FM moment with TN = 100 K could correspond to the behavior of the C2/c phase. The Pbam phase has a much weaker FM moment (see properties of the x = 0.3 and 0.4 samples); therefore, its contribution to magnetic properties could be hidden in the x = 0.1 sample.
HP-BiCr0.8Fe0.2O3 clearly showed one transition at TN = 90 K (Figure 8) because of the relatively large fraction of the C2/c phase. On the other hand, AP-BiCr0.8Fe0.2O3 clearly showed two transitions, the first one at TN = 90 K from traces of the C2/c phase and the second one at TN = 74 K from the majority of the Pbam phase. Traces of the C2/c phase could still be detected as this phase had a much larger FM moment. It is interesting that both HP- and AP-BiCr0.8Fe0.2O3 showed a negative magnetization phenomenon [74] when the FCC curves were measured at H = 100 Oe and 1 kOe. Negative magnetization phenomena are often observed in different RCr1−xFexO3 or RFe1−xCrxO3 solid solutions, where R3+ is a rare-earth element [75,76].
The magnetic properties of both HP- and AP-BiCr0.7Fe0.3O3 were nearly identical when measured at high magnetic fields, such as H = 10 kOe (Figure 9b), with one AFM-like anomaly at TN = 92 K. A very weak FM contribution (from spin canting) started emerging below about 20 K (Figure 9b and Figure 13a). An extremely weak FM contribution also appeared just below TN = 92 K at weak magnetic fields, such as H = 100 Oe (Figure 9a), and HP-BiCr0.7Fe0.3O3 had a larger FM contribution in comparison to AP-BiCr0.7Fe0.3O3. The weak FM contribution in HP- and AP-BiCr0.7Fe0.3O3 at H = 100 Oe was about 10 times smaller than that of HP- and AP-BiCr0.9Fe0.1O3.
Magnetic properties of both HP- and AP-BiCr0.6Fe0.4O3 were nearly identical when measured at high magnetic fields, such as H = 10 kOe (Figure 10b), with one AFM-like anomaly at TN = 122 K. No weak FM contributions appeared at lower temperatures in comparison with HP- and AP-BiCr0.7Fe0.3O3 at H = 10 kOe. At a small magnetic field of H = 100 Oe, HP-BiCr0.6Fe0.4O3 had a larger FM contribution in comparison to AP-BiCr0.6Fe0.4O3. However, the weak FM contributions were extremely small, meaning that they could not be detected on the M versus H curves, which showed linear behavior in the vicinity of the origin between about −20 kOe and 20 kOe (Figure 14).
At high temperatures, inverse magnetic susceptibilities follow the Curie–Weiss law for all the samples (Figure 8b, Figure 9b and Figure 10b), and parameters of the Curie–Weiss fits are summarized in Table 2. The experimental effective magnetic moments were close to the expected, calculated ones. The Weiss temperature varied between −230 K and −280 K, resulting in a moderate frustration index of about 2.3 to 2.9. It is interesting that some M versus H curves showed noticeable upturn deviations from the linear behavior (at high magnetic fields), suggesting the presence of gradual field-induced transitions, for example, at T = 5 K for x = 0.4 (Figure 14a), T = 50 K for x = 0.3 (Figure 13b), T = 5 K and 50 K for x = 0.2 (Figure 12a,b), and T = 5 K for x = 0.1 (Figure 11a).
Figure 15 summarizes the temperature–composition phase diagram of the BiCr1−xFexO3 system using the results of the current study and the literature data for the BiFeO3 [6], BiCrO3 [23,24,31], and BiCr1−xFexO3 systems with high Fe content [63]. On the scale of Figure 15, TN remains nearly the same for 0.0 ≤ x ≤ 0.4 and then monotonically increases from x = 0.4 to x = 1. Tstr increases gradually for 0.0 ≤ x ≤ 0.4 and then more rapidly from x = 0.4 to x = 1.

3. Materials and Methods

The as-synthesized HP modifications of BiCr1−xFexO3 solid solutions with x = 0.1, 0.2, 0.3, and 0.4 were prepared from stoichiometric mixtures of Bi2O3 (Rare Metallic Co., Tokyo, Japan, 99.9999%), Fe2O3 (Rare Metallic Co., Tokyo, Japan, 99.999%), and Cr2O3 (Rare Metallic Co., Tokyo, Japan, 99.9%). The synthesis was performed at about 6 GPa and about 1600 K for 1 h in sealed Pt capsules using a belt-type HP instrument. After annealing at 1600 K, the samples were cooled down to room temperature by turning off the heating current, and the pressure was slowly released. The AP modifications of BiCr1−xFexO3 solid solutions with x = 0.1, 0.2, 0.3, and 0.4 were obtained by heating HP-BiCr1−xFexO3 in air at AP at 623 K for 10 min (with a heating/cooling rate of 10 K/min). We note that at x ≥ 0.5, a different modification with space group R3c is formed [62] (that is confirmed by us); therefore, compositions with x ≥ 0.5 were not included in the present work.
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 scan speed of 2 °/min). Synchrotron XRPD data of HP-BiCr1−xFexO3 were collected at 297 K, upon heating to 550 K (x = 0.1 and 0.2) or to 600 K (x = 0.3 and 0.4), and then on cooling to 297 K using the beamline BL02B2 [77,78] of SPring-8, Japan. Intensity data were taken between 2.082° and 78.216° at a 0.006° interval in 2θ using a wavelength of λ = 0.420138 Å; however, data up to 50° were used in the Rietveld analysis as no detectable experimental reflections were observed above 50°. The measurement time was 300 s at 297 K and 550 K (or 600 K) and 60 s at other temperatures. The samples were placed into open Lindemann glass capillary tubes (with an inner diameter of 0.1 mm), which were rotated during the measurements. The Rietveld analysis of all XRPD data was performed using the RIETAN-2000 program [79]. The reported weight fractions of all the phases were calculated by the RIETAN-2000 program [79] from the refined scale factors.
Magnetic measurements were performed on a SQUID magnetometer (Quantum Design MPMS3, San Diego, CA, USA) between 2 and 350 K in different applied fields using zero-field-cooled (ZFC) and field-cooled on cooling (FCC) procedures. Isothermal magnetization measurements, M versus H, were performed from 70 kOe to −70 kOe and from −70 kOe to 70 kOe using both ZFC and FCC procedures. In the ZFC procedure for M versus H measurements, the samples were cooled from 300 K to a measurement temperature under a zero magnetic field; in the FCC procedure, the samples were cooled from 300 K to a measurement temperature under a magnetic field of 70 kOe. The ZFC and FCC procedures for M versus H measurements were used to check the presence or absence of the exchange bias effect; no detectable difference was observed on M versus H curves measured in the ZFC and FCC procedures, suggesting the absence of the exchange bias effect.
Pieces of pellets were used in magnetic measurements. A pellet of each HP-BiCr1−xFexO3 was first used to obtain magnetic properties of the HP modification; the same pellet was then transformed to AP-BiCr1−xFexO3 as described above, and it was used in magnetic measurements to obtain magnetic properties of the AP modification.
Differential scanning calorimetry (DSC) curves of powder samples of HP-BiCr1−xFexO3 were recorded on a Mettler Toledo DSC1 STARe system between 297 K and maximum 573 K in open Al capsules with a heating/cooling rate of 10 K/min. Three DSC runs were performed to check the reproducibility.

4. Conclusions

In conclusion, two modifications of the BiCr1−xFexO3 perovskite solid solutions were prepared. The HP modifications (as-synthesized) were prepared by a high-pressure high-temperature method at 6 GPa. The AP modifications were obtained using a “conversion polymorphism” strategy after heating at AP above structural phase transition temperatures and cooling to room temperature. The HP and AP modifications had subtle structural differences and showed incommensurate structural modulations. Structural phase transitions to the Pnma modification were observed in all the samples and investigated in details. Subtle differences in magnetic properties of the HP and AP modifications were investigated and reported. In particular, the x = 0.2 samples demonstrated negative magnetization phenomena.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13030091/s1, Figure S1: Fragments (between 4.4° and 7.4°) of experimental and calculated synchrotron powder X-ray diffraction patterns of HP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3 at T = 297 K; Figure S2: A magnified part of Figure 11; Figure S3: A magnified part of Figure 12; Figure S4: A magnified part of Figure 13; Figure S5: A magnified part of Figure 14; Table S1: Numerical data used to plot Figure 5; Table S2: Numerical data used to plot Figure 6.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

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). The author thanks S. Kobayashi for his help at BL02B2 of SPring-8 and I.S. Soboleva for her preliminary studies of the BiCr1−xFexO3 system. MANA was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.9Fe0.1O3 and (b) BiCr0.8Fe0.2O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 550 K—by blue lines, and the AP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (C2/c, Pnma, and Pbam) and Bi2O2CO3 and Cr2O3 impurities. The characteristic reflections of the C2/c modification are additionally marked by black/red octothorps. The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars.
Figure 1. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.9Fe0.1O3 and (b) BiCr0.8Fe0.2O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 550 K—by blue lines, and the AP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (C2/c, Pnma, and Pbam) and Bi2O2CO3 and Cr2O3 impurities. The characteristic reflections of the C2/c modification are additionally marked by black/red octothorps. The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars.
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Figure 2. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.7Fe0.3O3 and (b) BiCr0.6Fe0.4O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 600 K—by blue lines, and the AP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (Pnma and Pbam) and Bi2O2CO3 impurity. The characteristic reflections of the C2/c modification are marked by black octothorps. The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars. (hkl) indices of some superstructure reflections of the Pbam-related modification are given.
Figure 2. Magnified fragments of experimental synchrotron powder X-ray diffraction patterns of (a) BiCr0.7Fe0.3O3 and (b) BiCr0.6Fe0.4O3. Patterns for the as-synthesized HP modifications at T = 297 K are shown by black lines, for the Pnma modifications at T = 600 K—by blue lines, and the AP modifications at T = 297 K—by red lines. The tick marks show possible Bragg reflection positions for the main perovskite phases (Pnma and Pbam) and Bi2O2CO3 impurity. The characteristic reflections of the C2/c modification are marked by black octothorps. The characteristic reflections of the Bi2O2CO3 impurity are additionally marked by green stars. (hkl) indices of some superstructure reflections of the Pbam-related modification are given.
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Figure 3. Differential scanning calorimetry (DSC) curves of (a) BiCr0.9Fe0.1O3, (b) BiCr0.8Fe0.2O3, (c) BiCr0.7Fe0.3O3, and (d) BiCr0.6Fe0.4O3 on heating (the left-hand axes) and cooling (the right-hand axes). Three DSC runs are given for each sample. Temperatures of peak positions on the heating curves are given. The peak areas (in J/g) are also given for the first heating curve, for the second and third heating curve (an average value), and for the first, second, and third cooling curves (an average value).
Figure 3. Differential scanning calorimetry (DSC) curves of (a) BiCr0.9Fe0.1O3, (b) BiCr0.8Fe0.2O3, (c) BiCr0.7Fe0.3O3, and (d) BiCr0.6Fe0.4O3 on heating (the left-hand axes) and cooling (the right-hand axes). Three DSC runs are given for each sample. Temperatures of peak positions on the heating curves are given. The peak areas (in J/g) are also given for the first heating curve, for the second and third heating curve (an average value), and for the first, second, and third cooling curves (an average value).
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Figure 4. Fragments (between 2° and 25°) of experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of BiCr0.9Fe0.1O3 at T = 550 K in the Pnma modification. The tick marks show possible Bragg reflection positions for the main phase (black) and Bi2O2CO3 (blue) and Cr2O3 (green) impurities from top to bottom. The inset shows a magnified fragment.
Figure 4. Fragments (between 2° and 25°) of experimental (black crosses), calculated (red line), and difference (blue line at the bottom) synchrotron powder X-ray diffraction patterns of BiCr0.9Fe0.1O3 at T = 550 K in the Pnma modification. The tick marks show possible Bragg reflection positions for the main phase (black) and Bi2O2CO3 (blue) and Cr2O3 (green) impurities from top to bottom. The inset shows a magnified fragment.
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Figure 5. Compositional dependence of the lattice parameters of the Pnma modifications of the BiCr1−xFexO3 solid solutions at T = 550 K. (a) The a and b lattice parameters, (b) the c lattice parameter and unit cell volume. Data on heating are used for x = 0.1, 0.2, and 0.3 and on cooling—for x = 0.4.
Figure 5. Compositional dependence of the lattice parameters of the Pnma modifications of the BiCr1−xFexO3 solid solutions at T = 550 K. (a) The a and b lattice parameters, (b) the c lattice parameter and unit cell volume. Data on heating are used for x = 0.1, 0.2, and 0.3 and on cooling—for x = 0.4.
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Figure 6. (a) The temperature dependence of the normalized lattice parameters of BiCr0.6Fe0.4O3 on heating (full symbols) and cooling (empty symbols). (b) The temperature dependence of the normalized unit cell volumes (V/Z). αV is the volumetric coefficient of thermal expansion.
Figure 6. (a) The temperature dependence of the normalized lattice parameters of BiCr0.6Fe0.4O3 on heating (full symbols) and cooling (empty symbols). (b) The temperature dependence of the normalized unit cell volumes (V/Z). αV is the volumetric coefficient of thermal expansion.
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Figure 7. Magnetic properties of HP-BiCr0.9Fe0.1O3 (circles) and AP-BiCr0.9Fe0.1O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe.
Figure 7. Magnetic properties of HP-BiCr0.9Fe0.1O3 (circles) and AP-BiCr0.9Fe0.1O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe.
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Figure 8. Magnetic properties of HP-BiCr0.8Fe0.2O3 (circles) and AP-BiCr0.8Fe0.2O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The main inset on panel (a) shows the FCC curves H = 1 kOe. The secondary inset shows the ZFC curves H = 100 Oe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
Figure 8. Magnetic properties of HP-BiCr0.8Fe0.2O3 (circles) and AP-BiCr0.8Fe0.2O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The main inset on panel (a) shows the FCC curves H = 1 kOe. The secondary inset shows the ZFC curves H = 100 Oe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
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Figure 9. Magnetic properties of HP-BiCr0.7Fe0.3O3 (circles) and AP-BiCr0.7Fe0.3O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
Figure 9. Magnetic properties of HP-BiCr0.7Fe0.3O3 (circles) and AP-BiCr0.7Fe0.3O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
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Figure 10. Magnetic properties of HP-BiCr0.6Fe0.4O3 (circles) and AP-BiCr0.6Fe0.4O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
Figure 10. Magnetic properties of HP-BiCr0.6Fe0.4O3 (circles) and AP-BiCr0.6Fe0.4O3 (triangles). Zero-field-cooled (ZFC: filled curves) and field-cooled on cooling (FCC: empty curves) curves are shown at (a) H = 100 Oe and (b) H = 10 kOe. The right-hand axis on panel (b) gives the inverse FCC χ−1 versus T curves at H = 10 kOe with Curie–Weiss fits (red lines).
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Inorganics 13 00091 g011
Figure 11. Comparison of magnetic properties of HP-BiCr0.9Fe0.1O3 and AP-BiCr0.9Fe0.1O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
Figure 11. Comparison of magnetic properties of HP-BiCr0.9Fe0.1O3 and AP-BiCr0.9Fe0.1O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
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Inorganics 13 00091 g012
Figure 12. Comparison of magnetic properties of HP-BiCr0.8Fe0.2O3 and AP-BiCr0.8Fe0.2O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves were measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
Figure 12. Comparison of magnetic properties of HP-BiCr0.8Fe0.2O3 and AP-BiCr0.8Fe0.2O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves were measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
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Inorganics 13 00091 g013
Figure 13. Comparison of magnetic properties of HP-BiCr0.7Fe0.3O3 and AP-BiCr0.7Fe0.3O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
Figure 13. Comparison of magnetic properties of HP-BiCr0.7Fe0.3O3 and AP-BiCr0.7Fe0.3O3: M versus H curves at (a) T = 5 K, (b) T = 50 K, (c) T = 80 K, and (d) T = 120 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
Inorganics 13 00091 g013
Inorganics 13 00091 g014
Figure 14. Comparison of magnetic properties of HP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 150 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
Figure 14. Comparison of magnetic properties of HP-BiCr0.6Fe0.4O3 and AP-BiCr0.6Fe0.4O3: M versus H curves at (a) T = 5 K, (b) T = 60 K, (c) T = 90 K, and (d) T = 150 K. M versus H curves are measured under the ZFC procedure, when samples are cooled from 300 K to measurement temperatures under H = 0 Oe, and under the FCC procedure (marked as FCC–7 T), when samples are cooled from 300 K to measurement temperatures under H = 70 kOe.
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Inorganics 13 00091 g015
Figure 15. The temperature–composition phase diagram of the BiCr1−xFexO3 system. Tstr is the temperature of the structural transition to the Pnma modification. TN is the Néel temperature. TSR is the temperature of the spin reorientation transition.
Figure 15. The temperature–composition phase diagram of the BiCr1−xFexO3 system. Tstr is the temperature of the structural transition to the Pnma modification. TN is the Néel temperature. TSR is the temperature of the spin reorientation transition.
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Table 1. Structure parameters of BiCr1−xFexO3 at high temperatures from synchrotron powder X-ray diffraction data.
Table 1. Structure parameters of BiCr1−xFexO3 at high temperatures from synchrotron powder X-ray diffraction data.
x0.10.20.30.4
T (K)550550600600
a (Å)5.55595 (3)5.56458 (3)5.57383 (2)5.58329 (2)
b (Å)7.77499 (6)7.78492 (6)7.80159 (3)7.81360 (3)
c (Å)5.44100 (3)5.44552 (3)5.45751 (2)5.46389 (2)
V (Å3)235.037 (3)235.899 (3)237.318 (2)238.366 (2)
ρcal (g/cm3)8.7438.7218.6808.653
x (Bi)0.04309 (6)0.04275 (8)0.04304 (7)0.04281 (8)
z (Bi)0.99574 (14)0.99616 (23)0.99618 (18)0.99613 (21)
Biso (Bi) (Å2)1.173 (7)1.364 (11)1.528 (10)1.643 (11)
Biso (Cr/Fe) (Å2)0.55 (2)0.72 (3)0.76 (2)0.79 (3)
x (O1)0.4800 (9)0.4871 (12)0.4852 (11)0.4860 (12)
z (O1)0.0823 (10)0.0747 (13)0.0847 (12)0.0824 (13)
Biso (O1) (Å2)0.62 (13)0.10 (17)0.88 (16)0.55 (18)
x (O2)0.2915 (9)0.2921 (15)0.2945 (11)0.2994 (13)
y (O2)0.0385 (6)0.0413 (11)0.0374 (8)0.0381 (10)
z (O2)0.7068 (9)0.7071 (15)0.7030 (11)0.7041 (13)
Biso (O2) (Å2)1.06 (10)2.4 (2)1.51 (13)1.97 (17)
Rwp (%)5.487.186.397.09
Rp (%)4.075.284.815.18
RI (%)3.184.174.264.16
RF (%)2.513.814.354.56
Impurities:
Bi2O2CO31.1 wt. %0.8 wt. %1.6 wt. %1.6 wt. %
Cr2O30.6 wt. %0.9 wt. %––
Crystal system: orthorhombic. Space group: Pnma (No. 62); Z = 4. Source: synchrotron powder X-ray diffraction (λ = 0.420138 Å). d-space range used in the refinements: 0.497–11.563 Å. Fractional coordinates: Bi: 4c (x, 0.25, z), Cr/Fe: 4b (0, 0, 0.5), O1: 4c (x, 0.25, z), and O2: 8d (x, y, z). Occupation factors, g, of the Bi and O sites are 1. The occupation factor of the Cr/Fe site is mixed based on the nominal compositions.
Table 2. Temperatures of structural transitions (Tstr) and magnetic anomalies (TN) and parameters of the Curie–Weiss fits and M versus H curves at T = 5 K for BiCr1−xFexO3.
Table 2. Temperatures of structural transitions (Tstr) and magnetic anomalies (TN) and parameters of the Curie–Weiss fits and M versus H curves at T = 5 K for BiCr1−xFexO3.
xTstr (K)TN (K)μeff (μB/f.u.)μcalc (μB/f.u.)θ (K)MS (μB/f.u.)
0.1 (HP)4501003.9954.123−2470.075
0.1 (AP)450984.0614.123−2590.073
0.2 (HP)480904.0684.359−2400.086
0.2 (AP)47890, 743.9344.359−2270.087
0.3 (HP)511924.2694.583−2640.099
0.3 (AP)510924.3054.583−2730.099
0.4 (HP)5461224.2934.796−2790.083
0.4 (AP)5461224.3214.796−2790.083
The Curie–Weiss fits are performed between 250 and 350 K using the FCC χ−1 versus T data at 10 kOe. MS is the magnetization value at T = 5 K and H = 70 kOe. μcalc is calculated using 5.916μB for Fe3+ and 3.873μB for Cr3+. TN values are determined from peaks on the 100 Oe FCC d(χT)/dT versus T curves. Tstr values are determined from peak positions on the heating DSC curves, where the anomalies on the first heating curves are assigned to the HP modification and the anomalies on the second and third heating curves are assigned to the AP modification. Tstr corresponds to a transition to the GdFeO3-type Pnma modification.
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Belik, A.A. The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System. Inorganics 2025, 13, 91. https://doi.org/10.3390/inorganics13030091

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Belik AA. The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System. Inorganics. 2025; 13(3):91. https://doi.org/10.3390/inorganics13030091

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Belik, Alexei A. 2025. "The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System" Inorganics 13, no. 3: 91. https://doi.org/10.3390/inorganics13030091

APA Style

Belik, A. A. (2025). The Conversion Polymorphism of Perovskite Phases in the BiCrO3–BiFeO3 System. Inorganics, 13(3), 91. https://doi.org/10.3390/inorganics13030091

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