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Nov 11, 2016 - transition at TSR = 72 K.5,6 BiMnO3 is the only compound among BiBO3 that shows true ferromagnetic (FM) ordering at. TC = 99−101 K.9,...
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Solid Solutions between BiMnO3 and BiCrO3 Alexei A. Belik* International Center for Materials Nanoarchitectonics (WPI-MANA) and Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305−0044, Japan S Supporting Information *

ABSTRACT: Solid solutions BiMn1−xCrxO3 (0 ≤ x ≤ 1) have been prepared at 6 GPa and 1370−1620 K. Their structural properties have been studied with synchrotron X-ray powder diffraction, and their physical properties have been investigated by dc/ac magnetic, specific heat, dielectric, and differential scanning calorimetry measurements. A magnetic phase diagram of BiMn1−xCrxO3 is established. A phase with orbital ordering observed in BiMnO3 is suppressed at x > 0.1, accompanied by a drop in the ferromagnetic Curie temperature TC from 101 K for x = 0 to 76 K for x = 0.15 and sharp changes in the lattice parameters. The TC value monotonically decreases up to x = 0.3 (with TC = 53 K). For intermediate compositions with x = 0.4, 0.5, spin-glass magnetic properties are found at 28 and 24 K, respectively. The Néel temperature TN linearly increases from 36 K for x = 0.6 to 111 K for x = 1.0. A spin-reorientation transition is observed at 61 K for x = 0.9 and 72 K for x = 1.0. Re-entrant spin-glass transitions are also observed for samples with x = 0.3, 0.6, 0.7 by ac susceptibility at low temperatures. At high temperatures, a structural phase transition from C2/c to Pnma symmetry is observed for all compositions with a monotonic change of the phase transition temperature. The magnetic phase diagram from the BiMnO3-rich side (x ≤ 0.5) resembles a phase diagram of BiMn1−xScxO3 solid solutions, indicating that the nature of substituting cations (magnetic or nonmagnetic) is not crucial for doped BiMnO3. conditions,11 oxygen content,12−14 and iso- and alio-valent substitutions,15−18 and it exhibits interesting behavior at high pressure.19,20 Alloying two or more BiBO3 perovskites is a way to significantly modify their structural and physical properties.21−25 BiMn1−xScxO316,17 and BiMn1−xGaxO325 solid solutions with the dilution of the Mn sublattice by nonmagnetic ions were investigated in the whole compositional range of 0 ≤ x ≤ 1. The C2/c structure was found at all x in BiMn1−xScxO3 and at 0 ≤ x ≤ 0.5 in BiMn1−xGaxO3, but a phase separation into two C2/c phases (one with orbital order and another one without orbital order) takes place near x = 0.1.15−17 It is interesting that two polar Cm phases with pseudosupertetragonality emerge in BiMn1−xGaxO3 at 0.6 < x < 0.9.25 In BiMn1−xCrxO3 solid solutions, a composition with x = 0.5 was investigated, where three magnetic anomalies at 25, 50, and 97 K were found,26 and compositions in a BiMnO3-rich region with 0 ≤ x ≤ 0.15 were studied, where a phase separation into two C2/c phases was also found near x = 0.1.15 Properties of BiCr1−xGaxO3 solid solutions25,27 and BiCr0.5Ni0.5O328 have recently been investigated. A polar R3c phase emerges in BiCr1−xGaxO3, while an incommensurate structural modulation takes place in BiCr0.5Ni0.5O3. In this work, we investigated BiMn1−xCrxO3 solid solutions in the whole compositional range and established a magnetic phase diagram. All members are isostructural with each other and crystallize in space group C2/c, but compositional

1. INTRODUCTION ABO3 perovskite-type compounds with Bi3+ at the A site are interesting materials and show structural and physical properties distinct from those of perovskites with rare-earth cations at the A site.1,2 For example, the majority of RBO3 perovskites (R = rare earths and B = transition metals and group IIIa elements) crystallize in the GdFeO3-type Pnma structure, while BiBO3 perovskites have different structural distortions of the perovskite structure, and none of them have the GdFeO3-type Pnma structure under ambient conditions (except for BiRhO3).2 BiBO3 perovskites are promising as lead-free ferroelectric, piezoelectric, and multiferroic materials. The most studied BiBO3 perovskite is BiFeO3a type I multiferroic.1 It has a ferroelectric Curie temperature, TE, of 1100 K and an antiferromagnetic (AFM) Néel temperature, TN, of 640 K. Bulk BiScO3,3 BiCrO3,4−7 and BiMnO38 are isostructural with each other and crystallize in space group C2/c at room temperature. BiCrO3 is a canted antiferromagnet with TN = 111 K with an additional spin reorientation (SR) transition at TSR = 72 K.5,6 BiMnO3 is the only compound among BiBO3 that shows true ferromagnetic (FM) ordering at TC = 99−101 K.9,10 BiCrO3 and BiMnO3 exhibit a structural phase transition from C2/c symmetry to Pnma symmetry at Tstr ≈ 420 and 770 K, respectively.2 There was a great deal of controversy about the crystal symmetry and ferroelectric properties of bulk BiMnO3 in the past; now it is accepted that bulk BiMnO3 is not ferroelectric.2,8,10 Still, BiMnO3 is a fascinating playground for different studies because it has lattice, spin, and orbital degrees of freedom, it shows high sensitivity to different perturbations, such as synthesis © XXXX American Chemical Society

Received: September 15, 2016

A

DOI: 10.1021/acs.inorgchem.6b02237 Inorg. Chem. XXXX, XXX, XXX−XXX

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between −50 and 50 kOe at 5 K. Frequency-dependent ac susceptibility measurements were performed with a Quantum Design MPMS-1T instrument at different frequencies ( f) and different applied oscillating magnetic fields (Hac). The specific heat, Cp, at magnetic fields of 0 and 70 kOe was recorded between 2 and 300 K on cooling by a pulse relaxation method using a commercial calorimeter (Quantum Design PPMS). Dielectric properties were measured using a NOVOCONTROL Alpha-A High Performance Frequency Analyzer between 5 and 300 K on cooling and heating in the frequency range of 100 Hz and 2 MHz and at 0 and 90 kOe. Differential scanning calorimetry (DSC) curves were recorded on a Mettler Toledo DSC1 STARe system at a heating/cooling rate of 10 K/min under N2 flow between 293 K and maximum 813 K in open Al capsules; each sample was heated up to its Tstr value plus about 50 K. Three DSC runs were performed to check the reproducibility.

dependence of some lattice parameters is not monotonic with minima near x = 0.5. Magnetic properties are changed in systematic ways from both end members of the solid solutions with spin-glass behavior and minimum magnetic transition temperatures for x = 0.4, 0.5. The magnetic phase diagram from the BiMnO3-rich side (x ≤ 0.5) resembles a phase diagram of BiMn1−xScxO3 solid solutions, indicating that the nature of substituting cations (magnetic or nonmagnetic) is not crucial for doped BiMnO3.

2. EXPERIMENTAL SECTION 2.1. Synthesis of BiMn1−xCrxO3. Solid solutions of BiMn1−xCrxO3 were prepared with x = 0, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1. Stoichiometric mixtures of Bi2O3 (99.9999%), Cr2O3 (99.9%), and Mn2O3, with a total weight of about 0.7 g for each sample, were placed in Au capsules and treated in a belt-type highpressure apparatus at 6 GPa and 1370 K (x = 0), 1440 K (x = 0.03− 0.3), 1500 K (x = 0.4−0.8), and 1620 K (x = 0.9 and 1) for 60 min. After heat treatments, the samples were quenched to room temperature, and the pressure was slowly released. The resultant samples were black pellets for x = 0−0.8 or greenish black for x = 0.9, 1; pellets were hard for x = 0.3−1 and loose for x = 0−0.2. X-ray powder diffraction showed that all of the samples contained a small amount of Bi2O2CO3 impurity. Single-phase Mn2O3 was prepared from commercial MnO2 (99.997%) by heating in air at 923 K for 24 h. We note that 1620 K is very close to the melting point of Au at 6 GPa; therefore, one should take care and preferably use Pt capsules. The temperature 1620 K was a “set” temperature based on a calibration table; the real synthesis temperature could be lower. 2.2. X-ray Powder Diffraction Experiments and Structure Refinements. X-ray powder diffraction (XRPD) data were collected at room temperature on a RIGAKU Ultima III diffractometer using Cu Kα radiation (2θ range of 5−100°, step width of 0.02°, and counting time of 2−10 s/step). Room-temperature synchrotron XRPD data were collected on a large Debye−Scherrer camera at the BL02B2 beamline of SPring-8.29 An incident beam from a bending magnet was monochromated to λ = 0.4227 Å. Samples were contained in (boro)glass capillary tubes with an inner diameter of 0.2 mm, and capillary tubes were rotated during measurements. Synchrotron XRPD data were collected using image plates in a 2θ range from 1 to 75° with a step of 0.01° (the data from 2.5 to 52.5° were used in the refinements because reflections above 52.5° were extremely weak). Diffraction data were analyzed by the Rietveld method with RIETAN2000.30 Coefficients for analytical approximation to atomic scattering factors for Bi, Mn, Cr, and O were taken from ref 31. The pseudoVoigt function of Toraya was used as a profile function.32 The background was represented by a composite background function, i.e., 11th-order Legendre polynomial multiplied by a set of numerical values (at each point), which roughly approximates the background. 2.3. Physical Properties. Magnetic measurements were performed on SQUID magnetometers (Quantum Design, MPMS-1T and MPMS-XL) between 2 and 350 (400) K in different applied fields under both zero-field-cooled (ZFC) and field-cooled on cooling (FCC) conditions. In the ZFC regime, two cooling procedures were used. In the first procedure, a sample was rapidly (within 3−5 min) inserted into a magnetometer, which was kept at 10 K; then, the temperature was set to 2 K, and finally a measurement magnetic field was applied. This procedure will be called q-ZFC (or just ZFC), where q stands for quenched. In the second procedure, a sample was inserted into a magnetometer kept at 300 K, and then the temperature was set to 2 K with a rate of 10 K/min, and at 2 K, a measurement magnetic field was applied. This procedure will be called s-ZFC, where s stands for slow (cooled). A trapped field (TF) inside magnetometers was usually not specified in the q-ZFC procedure, but we used a “reset magnet” option of MPMS-1T, which reduces the absolute value of the TF below about 0.01 Oe. In the s-ZFC procedure, the sign of the TF was always checked (and set, if needed, before inserting a sample) to be positive (a positive trapped field: PTF) using a superconducting Nb sample. Isothermal magnetization measurements were performed

3. RESULTS AND DISCUSSION 3.1. Structural Properties of BiMn 1 − x Cr x O 3 . BiMn1−xCrxO3 solid solutions are formed in the whole compositional range as expected because the end members are isostructural with each other. Compositional dependence of the lattice parameters is shown in Figure 1, numerical values are summarized in Table S1 in the Supporting Information, and the evolution of laboratory XRPD patterns are given in Figure S1 in the Supporting Information. BiMn1−xCrxO3 with x = 0.05, 0.1 contained two C2/c phases: one phase is with orbital ordering (the so-called C2/c(I) phase), and another one is without

Figure 1. Compositional dependences of the lattice parameters and unit-cell volume (V) in BiMn1−xCrxO3 solid solutions. Filled symbols show results obtained from laboratory XRPD data and gray symbols results obtained from synchrotron XRPD data. Broken lines show a two-phase region where the C2/c(I) and C2/c(II) phases coexist. B

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to the formation of twin domains with the size of about 10 nm.7 We can assume that a similar domain formation takes place in the BiCrO3-rich side of the BiMn1−xCrxO3 solid solutions with 0.6 ≤ x ≤ 1. The aforementioned features could not be taken into account during the Rietveld analysis. Because of these microstructural features, the refined lattice parameters (especially, the a parameter) depend on the resolution of diffraction data (Figure 1 and Table S1 in the Supporting Information). The Mn/Cr−O bond lengths are almost the same for 0.15 ≤ x ≤ 0.8 and are quite different from those of BiMnO3, confirming that BiMn1−xCrxO3 with 0.15 ≤ x ≤ 1 does not have orbital ordering. 3.2. High-Temperature Structural Phase Transitions in BiMn1−xCrxO3. The end members show a structural phase transition from C2/c symmetry to Pnma symmetry at 768 K in BiMnO3 and 420 K in BiCrO3.2 DSC measurements revealed the existence of such a phase transition over the whole compositional range (Figure 3). The structural phase transition temperature changes monotonically (but not linearly) with the composition (Table 2). There was noticeable irreproducibility between the first heating curve and the second and third heating curves (Figure S6 in the Supporting Information), especially for x = 0.2, 0.3. XRPD data of samples after the DSC experiments showed that BiMn0.8Cr0.2O3 and BiMn0.7Cr0.3O3 partially decomposed after heating to 813 and 793 K, respectively (Figure S5 in the Supporting Information). This fact could explain the observed irreproducibility for them. No decomposition was detected in other samples with x = 0.4−1; the small irreproducibility for them could be explained by annealing effects of the high-pressure phases (for example, by a small change in the oxygen content). The phase transition temperature was reported to be 650 K in BiMn0.5Cr0.5O3;26 this temperature is close to Tstr = 639 K (the first heating) and 649 K (the second and third heating) for our sample. 3.3. Magnetic Properties of BiMn1−xCrxO3. Figures 4a and 5 and Figure S5 in the Supporting Information show dc and ac magnetic susceptibilities of BiMn1−xCrxO3 with x = 0.9, 1. Both samples exhibit similar magnetic properties. Therefore, using available information about magnetic structures of BiCrO3,5,6 we can assign the first magnetic anomaly to TN = 111 K for x = 1 and TN = 95 K for x = 0.9, and the second magnetic anomaly to TSR = 72 K for x = 1 and TSR = 61 K for x = 0.9. In our work, we determine/define magnetic transition temperatures from peak (anomaly) positions on the dc FCC dχ/dT vs T curves at 100 Oe (inset of Figure 4a). Magnetic properties of BiMn1−xCrxO3 with x = 0.7, 0.8 were already different from those with x = 0.9, 1 and were typical for canted antiferromagnets without signs of SR transitions (Figure 4b and Figure S6 in the Supporting Information). It is interesting that q-ZFC curves were negative in a certain temperature range even after cooling in a positive trapped field (at sample positions). It is generally believed that negative trapped fields are responsible for negative magnetization values on ZFC curves.34 We observed that a sample insertion procedure is more crucial in BiMn1−xCrxO3. Even though the trapped field was positive at a sample position, a rapid movement of a sample at low temperatures through a magnet with a nonuniform distribution of a magnetic field could cause a preferential formation of domain structures with negative magnetization. When samples were measured using the s-ZFC protocol, the magnetization was positive. The similar effect of the insertion procedure on ZFC curves was observed in YVO3.35

orbital ordering (the so-called C2/c(II) phase). The amount of the C2/c(I) phase was about 90% for x = 0.05 and 10% for x = 0.1.15 There are sudden changes in all lattice parameters during the transition from C2/c(I) to C2/c(II). The compositional dependence of the a lattice parameter and β angle show minima near x = 0.4, 0.5, indicating that BiMn1−xCrxO3 solid solutions do not obey a simple Vegard law. On the other hand, the unit cell volume decreases smoothly and monotonically throughout the whole compositional range (Figure 1) in agreement with a smaller ionic radius of Cr3+ (rVI = 0.615 Å)33 in comparison with that of Mn3+ (rVI = 0.645 Å). Structure parameters of BiMn0.7Cr0.3O3, BiMn0.6Cr0.4O3, and BiMn0.5Cr0.5O3 from synchrotron XRPD data are summarized in Table 1, and bond lengths and angles are given in Table S2 Table 1. Structure Parameters of BiMn0.7Cr0.3O3, BiMn0.6Cr0.4O3, and BiMn0.5Cr0.5O3 at 293 Ka site

WP

x

y

z

B (Å2)

Bi

8f

M1

4e

M2

4d

O1

8f

O2

8f

O3

8f

0.13380(9) 0.13346(10) 0.13317(12) 0 0 0 0.25 0.25 0.25 0.0836(11) 0.0805(11) 0.0786(13) 0.1613(12) 0.1597(14) 0.1557(16) 0.3566(12) 0.3562(13) 0.3553(15)

0.21737(13) 0.21807(14) 0.21790(15) 0.2256(7) 0.2277(8) 0.2286(9) 0.25 0.25 0.25 0.1903(19) 0.2035(19) 0.1999(23) 0.5553(19) 0.5590(19) 0.5503(24) 0.5343(19) 0.5283(19) 0.5248(25)

0.12842(13) 0.12897(13) 0.12959(14) 0.75 0.75 0.75 0.5 0.5 0.5 0.5825(12) 0.5789(13) 0.5785(13) 0.3774(14) 0.3723(16) 0.3677(17) 0.1628(12) 0.1588(13) 0.1594(15)

0.830(15) 0.845(15) 0.847(17) 0.66(10) 0.35(11) 0.47(13) 0.97(12) 1.30(14) 1.13(16) 0.9(3) 1.1(3) 0.5(3) 0.4(2) 0.9(3) 0.7(3) 0.4(3) 0.2(3) 0.5(3)

a Space group C2/c (No. 15); Z = 8. The first line of each site (x, y, z, and B) is for BiMn0.7Cr0.3O3, the second line is for BiMn0.6Cr0.4O3, and the third line is for BiMn0.5Cr0.5O3. g(Bi) = g(M1) = g(M2) = g(O1) = g(O2) = g(O3) = 1, where g is the occupation factor; Mn and Cr atoms are distributed statistically between the M1 and M2 sites according to the total chemical composition. WP indicates the Wyckoff position. BiMn0.7Cr0.3O3: a = 9.47052(15) Å, b = 5.57833(9) Å, c = 9.68239(15) Å, β = 108.2219(11)°, V = 485.867(13) Å3, Rwp = 4.40%, Rp = 2.99%, RB = 1.72%, and RF = 1.15%. BiMn0.6Cr0.4O3: a = 9.45744(15) Å, b = 5.56785(10) Å, c = 9.65897(16) Å, β = 108.1071(12)°, V = 483.430(14) Å3, Rwp = 3.46%, Rp = 2.28%, RB = 1.67%, and RF = 1.83%. BiMn0.5Cr0.5O3: a = 9.4554(2) Å, b = 5.55359(12) Å, c = 9.6418(2) Å, β = 108.1058(14)°, V = 481.237(17) Å3, Rwp = 4.84%, Rp = 3.04%, RB = 1.58%, and RF = 0.74%.

in the Supporting Information. Structural information on BiMn0.4Cr0.6O3, BiMn0.3Cr0.7O3, and BiMn0.2Cr0.8O3 is reported in Tables S3 and S4 in the Supporting Information. Figure 2 gives a fragment of experimental, calculated, and difference synchrotron XRPD patterns for BiMn0.5Cr0.5O3 (other synchrotron XRPD fitting results are shown in Figures S2− S4 in the Supporting Information). With an increase in x from 0.5 to 1, discrepancies between the observed and calculated synchrotron XRPD patterns are enhanced because of anisotropic peak broadening and strong diffuse scattering between some reflections. These features were observed in BiCrO36,7 and BiCr1−xGaxO3 (0.1 ≤ x ≤ 0.3)27 and attributed C

DOI: 10.1021/acs.inorgchem.6b02237 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Portions of experimental (black crosses), calculated (red line), and difference (blue line) synchrotron X-ray powder diffraction patterns of BiMn0.5Cr0.5O3 at room temperature. Possible Bragg positions are indicated by tick marks for the perovskite phase (black marks) and Bi2O2CO3 impurity (green marks). The inset shows details.

Only one transition at TN = 74 K was detected on the χ′ vs T, χ′′ vs T, and dχ/dT vs T curves of BiMn0.2Cr0.8O3; however, all anomalies were rather broad (Figures S7 and S8 in the Supporting Information). On the other hand, double-peak anomalies at 52 and 56 K were seen on the χ′ vs T, χ′′ vs T, and dχ/dT vs T curves of BiMn0.3Cr0.7O3 (Figures S6, S9, and S10 in the Supporting Information); the origin of these double-peak anomalies is not clear at present. A phase separation could be excluded because no reflection splitting was observed on synchrotron XDPD patterns, and synchrotron XDPD patterns of BiMn0.3Cr0.7O3 and BiMn0.4Cr0.6O3 were very close to each other (Figure S3 in the Supporting Information). Moreover, additional peaks could be seen near 3 K on the χ′′ vs T curves (those peaks were independent of Hac); this feature is a fingerprint of the emergence of a re-entrant spin-glass (RSG) transition. Anomalies that can be assigned to a RSG transition at TRSG = 17 K were much clearer in BiMn0.4Cr0.6O3 on both χ′ vs T and χ′′ vs T curves (Figure 6). While the anomalies at TN = 36 K in BiMn0.4Cr0.6O3 were dependent on Hac because of an interaction between the ac field and a domain structure, there was no dependence on Hac at TRSG.

Figure 3. Differential scanning calorimetry curves of BiMn1−xCrxO3 solid solutions on heating (10 K/min). The first heating curves are shown (some of them were shifted along the y axis for clarity). See Figures S5 and S6 in the Supporting Information for details.

Table 2. Temperatures of Magnetic Anomalies, Results of the Curie−Weiss Fits, Parameters of Isothermal Magnetization Curves, and Temperature of the Structural Phase Transition in BiMn1−xCrxO3a x

magnetic anom (K)

μeff(calcd) (μB/fu)

μeff(exptl) (μB/fu)

Θ (K)

MS (μB/fu)

Mr (μB/fu)

HC (Oe)

Tstr (K)

0 0.15b 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

102 76 70 53, 9 28 24 36, 17 56, 52, 3 72 95, 61 111, 72

4.899 4.759 4.712 4.615 4.517 4.416 4.313 4.207 4.099 3.987 3.873

5.036 4.840 4.750 4.636 4.345 4.238 3.983 3.885 3.808 3.704 4.173

126 108 103 88 74 53 30 −10 −66 −187 −390

3.92 2.92 2.61 1.99 1.53 1.08 0.82 0.56 0.35 0.16 0.06

0.013 0.568 0.538 0.643 0.218 0.091 0.116 0.073 0.024 0.010 0.016

3 195 464 685 793 776 1140 1452 1791 1221 1767

768 738 752 740 683 639 593 521 499 432 420

a Magnetic anomaly temperatures (TC, TN, TSR, TSG, and TRSG) are defined by peak positions on dc FCC dχ/dT vs T curves or by peak positions on ac χ′′ vs T curves. The effective magnetic moment (μeff) and Curie−Weiss temperature (Θ) are determined by the Curie−Weiss fit of the inverse FCC χ vs T curves measured at 10 kOe between 200 and 340 K. MS is the magnetization at 5 K and 50 kOe, Mr is the remnant magnetization, and HC is the coercive field; all the values are given per formula unit (fu). Tstr is the temperature of the structural phase transition from C2/c to Pnma symmetry; the peak positions on the first heating DSC curves are given. bFrom ref 15.

D

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Figure 6. (a) Real parts (χ′ vs T) and (b) imaginary parts (χ′′ vs T) of the ac susceptibilities of BiMn0.4Cr0.6O3. Measurements were performed on cooling from 150 to 2 K at a zero static magnetic field using ac fields with amplitudes Hac = 0.05, 0.5, and 5 Oe and frequency of 110 Hz. Insets show χ′ vs T and χ′′ vs T curves measured at different frequencies at a zero static magnetic field with Hac = 0.05 Oe.

Figure 4. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility (χ = M/H) curves of BiMn1−xCrxO3 with (a) x = 0.9, 1 and (b) x = 0.8 at 100 Oe. The inset in panel (a) shows the same curves plotted as dχ/dT vs T to emphasize the phase transition temperatures. In panel (b), two protocols were used for ZFC measurements: q-ZFC and s-ZFC, where the trapped field was confirmed to be positive (PTF) in both cases.

located on the BiCrO3-rich side, one can assume that the additional anomaly originates from BiMn0.1Cr0.9O3 impurity. We believe that it is more likely that BiMnO3 is responsible for the additional anomaly, and this additional anomaly in BiMn0.1Cr0.9O3 is just hidden by its intrinsic properties. No additional anomalies were found in BiMn0.5Cr0.5O3 in our work (Figure S11), but anomalies were detected at 97 K in BiMn0.5Cr0.5O3 in ref 26. This fact shows that the additional anomaly near 100 K is extrinsic, and its appearance depends on the synthesis conditions and sample quality. The amount of the impurity is below the detection limit of powder diffraction methods for x = 0.6−0.8, and it could be below the detection limit of magnetic measurements for x = 0.2−0.5. It remains unclear why the BiMnO3 impurity is visible on the BiCrO3-rich side of the BiMn1−xCrxO3 system at x = 0.6−0.8 (and probably x = 0.9). One possibility is that this impurity is segregated at grain surfaces in all BiMn1−xCrxO3 independent of the composition. For example, a segregation/formation of a new perovskite phase (with distinct magnetic properties and TC = 107 K) at grain surfaces was detected in pure BiMnO3.11 BiMn0.5Cr0.5O3 and BiMn0.6Cr0.4O3 behaved as spin glasses (SG) with freezing or SG temperatures, TSG, of 24 and 28 K, respectively. The χ′ vs T and χ′′ vs T curves of BiMn0.5Cr0.5O3 and BiMn0.6Cr0.4O3 were independent of Hac, indicating the absence of interactions with domain structures (Figures S11− S14 in the Supporting Information), while they showed characteristic frequency dependence of the maximum position (inset of Figure 7). The divergence between the dc ZFC and FCC curves at 100 Oe was also typical for spin glasses (Figure 7).

Figure 5. Real parts of the ac susceptibility (χ′ vs T) of BiCrO3 and BiMn0.1Cr0.9O3. Measurements were performed on cooling from 150 to 2 K at a zero static magnetic field using an ac field with the amplitude Hac = 5 Oe and frequencies of 2 and 500 Hz. Anomalies near 45 K in BiMn0.1Cr0.9O3 could originate from a trace amount of Mn3O4 impurity. TN is the Néel temperature, and TSR is a spinreorientation temperature.

Note that the samples with x = 0.6−0.8 exhibited very weak FM-like anomalies near 98−100 K (Figures S8 and S11−S13 in the Supporting Information). The temperature of this additional anomaly is close to TN = 95 K of BiMn0.1Cr0.9O3 and TC = 100 K of BiMnO3. Because the samples with x = 0.6−0.8 are E

DOI: 10.1021/acs.inorgchem.6b02237 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves of BiMn1−xCrxO3 with x = 0.4, 0.5 at H = 100 Oe. The inset shows χ′ vs T curves of BiMn0.5Cr0.5O3 measured at different frequencies at a zero static magnetic field with Hac = 5 Oe.

Figure 8 shows the dc ZFC and FCC curves of the samples with x = 0.05−0.3 measured at 100 Oe. The ZFC curves of the

Figure 9. Isothermal magnetization curves of BiMn1−xCrxO3 with (a) x = 0.8, 0.9, 1, (b) x = 0.5, 0.6, 0.7, and (c) x = 0.2, 0.3, 0.4 at T = 5 K.

the saturated magnetic moment at 5 K and 50 kOe (MS) are reported in Table 1. The MS values monotonically decrease with increasing x. The MS values for the samples with x = 0.15 (MS = 2.92), 0.2 (MS = 2.61), 0.3 (MS = 1.99) are in agreement with the values expected for a situation when all spins of Cr3+ cations are antiparallel to spins of Mn3+ cations (MS(calcd) = 2.95 for x = 0.15, MS(calcd) = 2.60 for x = 0.2, and MS(calcd) = 1.90 for x = 0.3). This situation can be represented by a ferrimagnetic (FiM) model: that is, when a Cr FM sublattice is coupled antiferromagnetically with an Mn FM sublattice. By this analogy, we will call magnetic phases of the samples with x = 0.15−0.3 as FiM-like phases for simplicity. Of course, there is one magnetic sublattice in BiMn1−xCrxO3 with a statistical distribution of Mn3+ and Cr3+ cations, and there should be FM Mn−O−Mn and AFM Cr−O−Cr and Mn−O−Cr interactions.

Figure 8. ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves of BiMn1−xCrxO3 with (a) x = 0.05, 0.1, 0.1515 and (b) x = 0.2, 0.3 at H = 100 Oe.

samples with x = 0.15, 0.2, 0.3 started from small values, gradually increased and showed a very broad maximum, and finally decreased when approaching TC, while the FCC curves demonstrated basically saturation behavior typical for materials with FM-like properties. The samples with x = 0.03, 0.05 showed saturation-like behavior on both ZFC and FCC curves. Other details of magnetic data are given in Figures S15−S18 in the Supporting Information. Figure 9 and Figures S19 and S20 in the Supporting Information depict the isothermal magnetization curves at 5 K. The coercive field (HC), the remnant magnetization (Mr), and F

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Mn−O−Mn interactions gives positive Curie−Weiss temperatures for the samples with x = 0−0.6: that is, even for samples with SG and canted AFM properties. Specific heat measurements confirmed the primary magnetic phase transition temperatures (Figures S22−S26 in the Supporting Information). Specific heat anomalies at TC from the BiMnO3-rich side were significantly suppressed in comparison with TN from the BiCrO3-rich side. No specific heat anomalies were found in BiMn0.5Cr0.5O3, in agreement with its spin-glass nature. 3.4. Dielectric Properties of BiMn1−xCr x O3. The dielectric constant of BiCrO3 was frequency independent below about 80 K with a value of 48 (Figure S27 in the Supporting Information). Between about 80 and 170 K strong frequency dependence was observed, and the temperature− frequency dependence of the maximum of dielectric loss follows the Arrhenius law with an activation energy of 0.24 eV. No intrinsic dielectric anomalies were found at TN and TSR values of BiCrO3. The dielectric constant at low temperatures was about 70 in BiMn0.3Cr0.7O3, 64 in BiMn0.7Cr0.3O3, and 58 in BiMn0.8Cr0.2O3 (Figures S28−S30 in the Supporting Information). Similar values of dielectric constant were observed in Bi1−xLaxMnO3 solid solutions.18 No dielectric anomalies were observed at the magnetic transition temperatures of all members of BiMn1−xCrxO3, and the magnetodielectric effect was negligible. 3.5. Phase Diagrams of BiMn1−xCrxO3. Using the results of different magnetic measurements, a magnetic phase diagram of the BiMn1−xCrxO3 system can be constructed (Figure 10). A whole phase diagram including the high-temperature region with structural phase transitions is also shown in Figure 10. In the middle of the system (x = 0.4, 0.5), SG states are realized with classical frequency-dependent positions of maxima on the χ′ vs T and χ′′ vs T curves. The formation of the SG states can be understood from strong competitions between FM and AFM interactions, which prevent the appearance of long-range magnetic ordering. SG magnetic properties were also observed in BiCr0.5Ni0.5O3the middle of BiCr1−xNixO3 solid solutions28and in the BiMn1−xScxO3 system with x = 0.4− 0.7.16,17 Because of the presence of the SG region it is, therefore, not surprising that re-entrant SG magnetic properties emerge at lower temperatures from both sides of the SG region. In BiMn0.7Cr0.3O3 and BiMn0.4Cr0.6O3, long-range magnetic ordering first sets in at TC and TN, respectively, with the formation of domain structures, as confirmed by Hac field dependent magnetic properties. With decreasing temperature, competitions between FM and AFM interactions are enhanced, resulting in SG states in the ground state. The magnetic phase diagram of the BiMn1−xCrxO3 system from the BiMnO3-rich site (x ≤ 0.5) is similar to the magnetic phase diagram of the BiMn1−xScxO3 system.16,17 This fact shows that the nature of the substituting cations (magnetic or nonmagnetic) does not have great effects on the properties of doped BiMnO3. This feature could be understood from the fact that BiMnO3 has intrinsic frustration from competing FM and AFM interactions.36 In all cases, the C2/c(I) phase with orbital ordering observed in pure BiMnO3 is suppressed at very small doping levels. The magnetic transition temperature of the C2/c(II) phase without orbital ordering drops in comparison with that of the C2/c(I) phase and continues to decrease with an increase in the substitution level. Effects of doping on the properties of BiCrO3 have been less studied. We found that the introduction of Mn3+ cations

The inverse FCC magnetic susceptibilities at 10 kOe (Figure S21 in the Supporting Information) between 200 and 340 K are fit by the Curie−Weiss equation χ (T ) = μeff 2 N (3kB(T − Θ))−1

(1)

where μeff is the effective magnetic moment, N is Avogadro’s number, kB is Boltzmann’s constant, and Θ is the Curie−Weiss temperature. The fitting parameters are summarized in Table 1 and in the inset of Figure 10. In the samples with FM and FiM

Figure 10. (top) Low-temperature magnetic phase diagram of the BiMn1−xCrxO3 system. Abbreviations and definitions: PM, paramagnetic; FM, ferromagnetic; FiM, ferrimagnetic; c-AFM, canted antiferromagnetic; SG, spin-glass; RSG, re-entrant spin-glass; TC, ferromagnetic or ferrimagnetic Curie temperature; TN, antiferromagnetic Néel temperature; TSG, spin-glass or freezing temperature; TRSG, temperature of a re-entrant spin-glass transition; TSR, temperature of a spin-reorientation transition. (bottom) High-temperature phase diagram of the BiMn1−xCrxO3 system. Definitions: Tstr, temperature of a structural C2/c to Pnma transition; TOO, temperature of an orbitalordering transition.15 The inset shows the compositional dependence of the effective magnetic moment (in μB/fu; black circles), calculated magnetic moments (in μB/fu; white circles with line), and Curie− Weiss temperatures (red squares).

orders (x = 0−0.3), the μeff values were slightly larger than theoretical values. The same tendency was found in other solid solutions on the BiMnO 3 -rich side: for example, in Bi1−xLaxMnO318 and BiMn1−xMxO3.15 The μeff values were smaller than theoretical values in the samples with SG and canted AFM orders (x = 0.4−0.9). The μeff value of BiCrO3 jumps because BiCrO3 exhibits a significant deviation from the Curie−Weiss behavior even far above TN (Figure S21). Ferromagnetism is intrinsic for the monoclinic C2/c structure of BiMnO3; therefore, some Mn−O−Mn couplings should be strongly FM. On the other hand, Cr−O−Cr and Mn−O−Cr couplings are expected to be AFM. The presence of strong FM G

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Takayama-Muromachi, E. Origin of the Monoclinic-to-Monoclinic Phase Transition and Evidence for the Centrosymmetric Crystal Structure of BiMnO3. J. Am. Chem. Soc. 2007, 129, 971−977. (9) Sugawara, F.; Iiida, S.; Syono, Y.; Akimoto, S. Magnetic Properties and Crystal Distortions of BiMnO3 and BiCrO3. J. Phys. Soc. Jpn. 1968, 25, 1553−1558. (10) Montanari, E.; Calestani, G.; Righi, L.; Gilioli, E.; Bolzoni, F.; Knight, K. S.; Radaelli, P. G. Structural Anomalies at the Magnetic Transition in Centrosymmetric BiMnO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 220101. (11) Montanari, E.; Righi, L.; Calestani, G.; Migliori, A.; Gilioli, E.; Bolzoni, F. Room Temperature Polymorphism in Metastable BiMnO3 Prepared by High-Pressure Synthesis. Chem. Mater. 2005, 17, 1765− 1773. (12) Sundaresan, A.; Mangalam, R. V. K.; Iyo, A.; Tanaka, Y.; Rao, C. N. R. Crucial Role of Oxygen Stoichiometry in Determining the Structure and Properties of BiMnO3. J. Mater. Chem. 2008, 18, 2191− 2193. (13) Belik, A. A.; Kodama, K.; Igawa, N.; Shamoto, S.; Kosuda, K.; Takayama-Muromachi, E. Crystal and Magnetic Structures and Properties of BiMnO3+δ. J. Am. Chem. Soc. 2010, 132, 8137−8144. (14) Belik, A. A.; Matsushita, Y.; Tanaka, M.; Takayama-Muromachi, E. Low-Temperature Vacuum Reduction of BiMnO3. Inorg. Chem. 2011, 50, 7685−7689. (15) Belik, A. A.; Takayama-Muromachi, E. Effects of Isovalent Substitution in the Manganese Sublattice on Magnetic, Thermal, and Structural Properties of BiMnO3: BiMn1‑xMxO3 (M = Al, Sc, Cr, Fe, Ga; 0 ≤ x ≤ 0.2). Inorg. Chem. 2007, 46, 5585−5590. (16) Belik, A. A.; Yokosawa, T.; Kimoto, K.; Matsui, Y.; TakayamaMuromachi, E. High-Pressure Synthesis and Properties of Solid Solutions between BiMnO3 and BiScO3. Chem. Mater. 2007, 19, 1679−1689. (17) Belik, A. A.; Takayama-Muromachi, E. Ac Susceptibility Studies of Multiferroic BiMnO3 and Solid Solutions Between BiMnO3 and BiScO3. J. Phys.: Condens. Matter 2008, 20, 025211. (18) Chen, W.-T.; Sher, F.; Mathur, N. D.; Kavanagh, C. M.; Morrison, F. D.; Attfield, J. P. Structural, Magnetic, and Electrical Properties of Bi1−xLaxMnO3 (x = 0.0, 0.1, and 0.2) Solid Solutions. Chem. Mater. 2012, 24, 199−208. (19) Belik, A. A.; Yusa, H.; Hirao, N.; Ohishi, Y.; TakayamaMuromachi, E. Peculiar High-Pressure Behavior of BiMnO3. Inorg. Chem. 2009, 48, 1000−1004. (20) Calestani, G.; Orlandi, F.; Mezzadri, F.; Righi, L.; Merlini, M.; Gilioli, E. Structural Evolution under Pressure of BiMnO3. Inorg. Chem. 2014, 53, 8749−8754. (21) Yu, R. Z.; Matsuda, N.; Tominaga, K.; Shimizu, K.; Hojo, H.; Sakai, Y.; Yamamoto, H.; Oka, K.; Azuma, M. High-Temperature Monoclinic Cc Phase with Reduced c/a Ratio in Bi-based Perovskite Compound Bi2ZnTi1−xMnxO6. Inorg. Chem. 2016, 55, 6124−6129. (22) Shimizu, K.; Hojo, H.; Ikuhara, Y.; Azuma, M. Enhanced Piezoelectric Response due to Polarization Rotation in CobaltSubstituted BiFeO3 Epitaxial Thin Films. Adv. Mater. 2016, 28, 8639−8644. (23) Bridges, C. A.; Allix, M.; Suchomel, M. R.; Kuang, X. J.; Sterianou, I.; Sinclair, D. C.; Rosseinsky, M. J. A Pure Bismuth A Site Polar Perovskite Synthesized at Ambient Pressure. Angew. Chem., Int. Ed. 2007, 46, 8785−8789. (24) Chong, S. Y.; Szczecinski, R. J.; Bridges, C. A.; Tucker, M. G.; Claridge, J. B.; Rosseinsky, M. J. Local Structure of a Pure Bi A Site Polar Perovskite Revealed by Pair Distribution Function Analysis and Reverse Monte Carlo Modeling: Correlated Off-Axis Displacements in a Rhombohedral Material. J. Am. Chem. Soc. 2012, 134, 5836−5849. (25) Belik, A. A.; Rusakov, D. A.; Furubayashi, T.; TakayamaMuromachi, E. BiGaO3-Based Perovskites: A Large Family of Polar Materials. Chem. Mater. 2012, 24, 3056−3064. (26) Mandal, P.; Iyo, A.; Tanaka, Y.; Sundaresan, A.; Rao, C. N. R. Structure, Magnetism and Giant Dielectric Constant of BiCr0.5Mn0.5O3 Synthesized at High Pressures. J. Mater. Chem. 2010, 20, 1646−1650.

suppresses both TN and TSR, and TSR completely disappears for x ≤ 0.8. The strong suppression of TN and TC from both sides of the phase diagram is also caused by competing FM and AFM interactions. TN usually changes monotonically (that is, without a deep minimum) when all interactions are AFM in a solid solution: for example, in YFe1−xCrxO3.37

4. CONCLUSION We prepared BiMn1−xCrxO3 solid solutions over the whole compositional range and investigated their structural and magnetic properties. We found that magnetic properties are changed in systematic ways from both end members of the solid solutions with noticeable suppression of magnetic transition temperatures and spin-glass behavior in the middle of the phase diagram.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02237. Details of laboratory and synchrotron XRPD patterns and details of magnetic and dielectric properties (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.A.B.: [email protected]. Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the World Premier International Research Center Initiative (WPI Initiative, MEXT, Japan). The synchrotron radiation experiments were performed at the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal Numbers 2007A2087 and 2009A1136). We thank Dr. J. Kim and Dr. N. Tsuji for their assistance at SPring-8.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02237 Inorg. Chem. XXXX, XXX, XXX−XXX