Spin-Glass Behavior and Incommensurate Modulation in High

Article pubs.acs.org/IC

Spin-Glass Behavior and Incommensurate Modulation in HighPressure Perovskite BiCr0.5Ni0.5O3 Á ngel M. Arévalo-López,*,† Antonio J. Dos santos-García,‡ Jessica R. Levin,§ J. Paul Attfield,† and Miguel A. Alario-Franco§ †

Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom Departamento de Química Industrial y Polímeros, ETSIDI, Universidad Politécnica de Madrid, Madrid 28012, Spain § Departamento de Química Inorgánica, Facultad de Químicas, Universidad Complutense de Madrid, Madrid 28040, Spain ⊥ Laboratoire de Physique des Solides, Université Paris-Sud, CNRS-UMR8502, Orsay 91405, France ‡

S Supporting Information *

ABSTRACT: The BiCr0.5Ni0.5O3 perovskite has been obtained at high pressure. Neutron and synchrotron diffraction data show a Pnma orthorhombic structure with a = 5.5947(1) Å, b = 7.7613(1) Å, and c = 5.3882(1) Å at 300 K and random B-site Cr/Ni distribution. Electron diffraction reveals an incommensurate modulation parallel to the b axis. The combination of either Cr−O−Ni (J > 0) or Cr−O−Cr/Ni−O−Ni (J < 0) nearestneighbor spin interactions results in a random-bond spin-glass configuration. Magnetization, neutron diffraction, and muon-spinrelaxation measurements demonstrate that variations in the local bonding and charge states contribute to the magnetic frustration.

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Bi3+Ni3+O3 at 4 GPa (BiNiO3-III).7,14 BiNiO3 also shows a colossal negative thermal expansion that can be tuned by lanthanum substitution.15,16 Moreover, predictions have been made on double perovskites with multiple B site cations, where, according to the Goodenough−Kanamori rules, FM exchange is favorable.17 Following these predictions, Bi2FeCrO6 thin films, with Fe3+ (d5) and Cr3+ (d3), were synthesized and showed FM order and ferroelectric polarization at room temperature.18 Another example is the HPHT-synthesized Bi2NiMnO6 in which Ni2+ (d8) and Mn4+ (d3) cations ordered in a rock-salt manner and developed ferroelectricity at TCE = 485 K and ferromagnetism at TCM = 140 K.19,20 We report the high-pressure synthesis of the complex perovskite BiCr0.5Ni0.5O3, in which the combination of either Cr4+ (d2)−Ni2+ (d8) or Cr3+ (d3)−Ni3+ (d7) might develop interesting interactions. Despite the random average cation distribution over the B sites seen in powder diffraction data, electron diffraction evidences an incommensurate modulation along the b axis.

INTRODUCTION Transition-metal oxides with the perovskite structure are of fundamental and technological importance because of the large variety of functional properties they exhibit. An important source of new oxide perovskites is high pressure−high temperature (HPHT) synthesis, as shown in a recent review with over 60 new compounds prepared by this technique in the last 2 decades.1 Among these new oxides, the bismuth-based perovskites form an important family because of their possible use as lead-free piezoelectrics and multiferroic materials. However, there are only a few examples that can be prepared at ambient pressure. BiFeO3 is by far the most studied room temperature multiferroic. Unlike other reported simple BiMO3 (M = Al, Sc, Cr, Mn, Co, Ni, Rh, In) perovskites that require HPHT for their synthesis,2−9 BiFeO3 is obtained from ambient-pressure synthesis conditions, which makes it easily accessed. Moreover, it has a ferroelectric Curie temperature of TE = 1100 K and an antiferromagnetic (AFM) Néel temperature of TN = 640 K.10,11 In addition, BiCrO3, BiMnO3, and BiNiO3 have also received attention. BiCrO3 crystallizes in a C2/c monoclinic structure with a transition to an orthorhombic Pnma structure at 420 K and a G-type magnetic structure below 109 K.4 BiMnO3 also with a C2/c monoclinic structure presents orbital order up to 474 K and a ferromagnetic (FM) transition at TC = 105 K.12,13 BiNiO3 is an insulator with P1̅ symmetry at ambient conditions with an unusual disproportionation Bi0.53+Bi0.55+Ni2+O3 down to 2 K accompanied by a G-type magnetic structure (BiNiO3-I); it shows a pressure-induced transition to a metallic Pnma © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Polycrystalline specimens of BiCr0.5Ni0.5O3 were synthesized via the same high-pressure “belt”-type press method as that used to prepare the PbM0.5M′0.5O3 (M, M′ = Ti, V, Cr) perovskites.21 A stoichiometric Special Issue: To Honor the Memory of Prof. John D. Corbett Received: August 18, 2014

A

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mixture of Bi2O3, NiO, and CrO2 was reacted inside a gold capsule at the optimum conditions of 1373 K under 53 kbar of pressure for 30 min, followed by a quench to ambient temperature and slow depressurization. The prepared sample was then purified by a 10% HCl acid wash to remove a small amount of bismuth impurities. Samples were initially characterized by powder X-ray diffraction with a Philips X’pert diffractometer using monochromatic Cu Kα1 radiation. Room temperature high-resolution synchrotron data were obtained from the ID31 beamline (λ = 0.3999 Å) at the ESRF. Neutron powder diffraction data were collected on the ILL D20 instrument (λ = 1.8687 Å) at T = 2 and 300 K. Direct-current (dc) and alternating-current (ac) magnetic susceptibilities were characterized using a superconducting quantum interference device magnetometer (Quantum Design). Muon-spinrelaxation (μSR) data were taken on the EMU pulsed muon beamline at the ISIS facility at the Rutherford Appleton Laboratory. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) were performed on a JEOL JEM 3000F field emission gun microscope and on a JEOL JEM FX2000 microscope, respectively. The composition of each investigated crystal was checked by energy-dispersive X-ray spectrometry (EDX; Link Pentafet 5947 model, Oxford Microanalysis Group) by in situ observations. Data analyses were performed with Fullprof and WiMDA for Rietveld refinement and muon data, respectively.22,23

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RESULTS AND DISCUSSION Structural and Microstructural Characterization. BiCr 0.5 Ni 0.5 O 3 , along with the majority of the BiMO 3 perovskites, requires the use of high-pressure synthesis in order to stabilize the covalent Bi3+ into the A site. Trials to synthesize BiCr0.5Ni0.5O3 at room pressure were unsuccessful, in contrast to Bi0.5Sr0.5CrO3 or Bi2Mn4/3Ni2/3O3, which can be synthesized at ambient pressures.24,25 The respective end members BiCrO3, SrCrO3, BiMnO3, and BiNiO3, however, require at least 4 GPa to be formed.4,5,7,26 BiCr0.5Ni0.5O3 can be considered as a solid solution between the end members because its orthorhombic perovskite distortion corresponds to those of the high-temperature phase for BiCrO3 (T > 420 K) and the high-pressure phase for BiNiO3 (P > 4 GPa).4,14 Laboratory X-ray diffraction suggested the orthorhombic Pnma perovskite structure (√2ap × 2ap × √2ap, where ap is the cubic perovskite lattice parameter) for BiCr0.5Ni0.5O3. A combination of Rietveld refinement against room temperature neutron powder (NPD) and synchrotron X-ray (SXRD) diffraction data were used for structural determination, and the high contrast in neutron scattering lengths of Cr and Ni confirmed their random distribution on the B site. Plots are shown in Figure 1, and results are summarized in Tables 1 and 2. The Bi and O occupancies were fixed to their ideal compositions in the final refinement because these were found to be fully occupied within error. A small amount of unreacted NiO (3.9 wt %) was observed and included in the fit. This is equivalent to 17 mol % and possibly the origin of the compositional modulation observed, vide infra. Bond-valencesum (BVS) calculations using an interpolation method27 yield average charges of 2.84+ and 2.6+ for Bi and Cr/Ni sites, respectively, which confirm the Bi3+ and (Cr/Ni)3+ overall charge distribution (Table 2). Neutron diffraction at T = 2 K showed no significant changes in the structure and no longrange magnetic ordering (Figure S1 in the Supporting Information, SI), in accordance with the magnetic properties discussed later. BiCr0.5Ni0.5O3 shows a structure very similar to that of the high-temperature orthorhombic BiCrO3.4 The Pnma cell parameters for BiCrO3 at 490 K are a = 5.546 Å, b = 7.757

Figure 1. Combined Rietveld fit to 300 K (a) D20 NPD (λ = 1.8687 Å) and (b) SXRD ID31 (λ = 0.3999 Å) data for BiCr0.5Ni0.5O3. Inset: Crystal structure along [101]o; Bi/O atoms shown as large (purple)/ small (red) and M are inside the octahedra. NiO is considered to be a secondary phase in both patterns and its magnetic structure a third phase in the neutron fit.

Table 1. Crystallographic Parameters for BiCr0.5Ni0.5O3 at 300 Ka a (Å)

b (Å)

5.5947(1) atom site

7.7613(1) x

Bi Cr/Ni O1 O2

4c 4b 4c 8d

0.0510(4) 0 0.4718(6) 0.2985(5)

V (Å3)

c (Å) 5.3882(1) y

z

/4 0 1 /4 0.0403(5)

0.9949(5) 1 /2 0.0880(5) 0.6983(5)

1

233.969(7) Biso (Å2) 0.58(3) 0.44(7) 0.48(7) 0.65(6)

a Refined lattice parameters, atomic coordinates, and thermal displacement parameters for BiCr0.5Ni0.5O3 in orthorhombic space group Pnma at 300 K from combined refinement of SXRD and NPD. The Bi and O occupations were at their ideal values within error, and Cr/Ni ratio = 0.56/0.44(1). Fitting residuals Rp and Rwp were 0.0294/0.0531 and 0.0388/0.0819 at 300 K for NPD/SXRD.

Table 2. Selected Interatomic Distances (Å) and Angles (deg) for BiCr0.5Ni0.5O3 from the Refinements in Table 1 Bi−O1

Bi−O2 (×2)

⟨Bi−O⟩ BVS (Bi)a

3.172(4) 2.407(4) 2.291(4) 2.668(3) 2.643(3) 2.393(3) 2.59(1) 2.84

M−O1 (×2) M−O2 (×2) ⟨M−O⟩ M−O1−M M−O2−M

2.004(1) 2.007(3) 2.003(3) 2.005(6) 151.1(1) 151.2(1)

BVS (Cr/Ni)a

2.79/2.41

Vi = ∑jSiji = exp(r0 − rij/0.37). Values were calculated using rij = 2.094 for Bi3+, 1.654 for Ni2+, 1.686 for Ni3+, 1.724 for Cr3+, and 1.737 for Cr4+. a

B

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Å, and c = 5.429 Å, showing that it has a slightly less distorted structure than that observed for BiCr0.5Ni0.5O3 (Table 1). Hightemperature X-ray diffraction data show normal thermal expansion up to 773 K, where the material decomposes. Further microscopic analysis of BiCr0.5Ni0.5O3 confirmed the adopted symmetry and supercell. Figure S2 in the SI shows typical SAED patterns observed in BiCr0.5Ni0.5O3. Besides the main reflections of the basic perovskite cell, superstructure maxima confirm the formation of a √2ap × 2ap × √2ap supercell with an a+b−b− Glazer tilt system (Pnma).28 However, a large proportion of the crystals present large areas with extended defects (see Figure S3 in the SI). HRTEM along with SAED demonstrates that the defect is a modulation essentially parallel to the b axis. Parts a and d of Figure 2 respectively show HRTEM images of [100]o and

often observed, for instance, in PbCrO3−δ, which shows a compositional−polar modulation resulting in an average cubic structure with a temperature-driven spin reorientation,29,30 in Bi2Mn4/3Ni2/3O6, with frustrated polar Bi displacements and frustrated spin order on the B site,25 and in Bi1−xCaxFeO331 and Bi0.75La0.25FeO3, with antipolar displacements coupled to octahedral tilting.32 The properties of these materials are determined by the local lone-pair distortions. EDX analyses show a ∼1:1 Cr/Ni ratio on modulated-free crystals but 0.67:0.33 in the modulated ones, in agreement with the small amount of NiO observed as the secondary phase. Therefore, a compositional modulation could be the origin of the complex microstructure. However, further studies with aberration-corrected microscopy are required to confirm this. Besides the compositional segregation, displacive Bi−O interactions are likely to play a major role in the microstructure; for instance, B-site-ordered Pb2CoWO6 presents an incommensurate displacive modulation arising from lone-pair interactions and elastic deformation of the lattice. 33 Bi0.75La0.25FeO3 also presents a modulation due to localization of the lone pair on the Bi3+cation.32 Magnetic Properties. Figure 3a shows the temperature dependence of susceptibilities in zero-field- and field-cooled (ZFC and FC) procedures under a field of 1000 Oe for BiCr0.5Ni0.5O3. The ZFC procedure presents a maximum at ∼30 K and diverges from the FC data below this temperature. The inverse ZFC data were fit between 225 and 300 K with the Curie−Weiss law, χ(T) = C/(T − θ), resulting in θ = 93(1) K

Figure 2. High-resolution electron micrographs of modulated areas in BiCr0.5Ni0.5O3. Electron diffraction patterns of modulated crystals along (a) [101]c ≡ [100]o and (d) [100]c ≡ [101]o. (c and f) Corresponding line scans for the shaded areas in parts b and e, respectively. The modulation goes along b* with an incommensurate q = 0.3 value.

[101]o orientations of a modulated area; each Bragg reflection g is decorated with satellite reflections at H = g ± q, where q is the modulation vector. Intensity scans of the shaded areas in the SAED patterns in parts b and e of Figure 2, shown in parts c and f of Figure 2, respectively, reveal an incommensurate q = 0.3 value. Although q is incommensurate, short-range periodicity of 3(2ap) is evidenced in the HRTEM micrographs, marked by arrows in Figure 2a,d. One of the possible origins of the modulation on BiCr0.5Ni0.5O3 is the lone pair of the Bi ion. Incommensurate modulations on complex lone-pair A-site-based perovskites are

Figure 3. (a) ZFC (closed circles) and FC (open circles) dc susceptibility data of BiCr 0.5 Ni0.5 O3 along with the inverse susceptibility and fit to the Curie−Weiss law. (b) Shift with frequency in the real part of the ac susceptibility and fit to the Vogel−Fulcher law. C

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and C = 0.99(1) emu K mol−1, implying a total effective moment of μeff = 2.8(1) μB, values expected for S = 1 ions in accordance with Ni2+/Cr4+ and low-spin Ni3+/Cr3+ combinations. A small hysteresis with a coercive field of 1100 Oe at 5 K is observed in the M(H) data (see Figure S4 in the SI). The ac susceptibility is frequency-dependent and has a nonzero imaginary component. It was successfully modeled by the Vogel−Fulcher law, ω = ω0 exp[−Ea/kB(Tf− T0)], where ω0 is a characteristic frequency, Ea is the activation energy, and T0 is the ideal gas temperature that allows for spin−spin interactions. The fit to the data shown in Figure 3b gives ln(ω0/s−1) = 22.0(2), Ea/kB = 28.4(3) K, and T0 = 29.96(2) K. Zero-field μSR spectra of polycrystalline BiCr0.5Ni0.5O3 measured at 5 K ≤ T ≤ 150 K are shown in Figure 4.

oscillations on the spectra are observed below Tf, implying a highly random distribution of local fields around the muon site, in agreement with the absence of long-range magnetic neutron scattering at low temperatures. As mentioned before, NPD data did not show any long-range magnetic ordering at temperatures as low as 2 K despite the observed maximum in the ZFC magnetic susceptibility and the coercitivity observed in the magnetic hysteresis loop at 5 K. However, ac susceptibility and μSR data demonstrate the spinglass behavior and explain the lack of magnetic intensity on the low-temperature NPD. The competing interactions cause the frustration in BiCr0.5Ni0.5O3. For Cr3+ and Ni3+ cations, Cr−O−Cr and Ni−O−Ni couplings are expected to be AFM because of dominant π- and σ-superexchange interactions, respectively, whereas Cr−O−Ni interactions are FM. Equal numbers of AFM and FM interactions are present for a random 50:50 cation distribution, resulting in a classical random-bond spinglass configuration.34 The modulation may contribute further by introducing strained areas with different exchange constants, as in Pb2NiReO6, where compositional microdomains prevent long-range order and induce spin-glass behavior.35 In conclusion, BiCr0.5Ni0.5O3 requires high pressure to be synthesized and can be considered a solid solution between the high-pressure phase of BiNiO3-III and the high-temperature phase of BiCrO3. Bulk diffraction measurements show a random B-site distribution and an orthorhombically distorted perovskite structure for BiCr0.5Ni0.5O3. However, local probes demonstrate an incommensurate modulation with a modulation vector q = 0.3 parallel to the b axis. Magnetization and μSR measurements demonstrate spin-glass behavior explained by the disorder on the B lattice and the change in the local bonding. The resemblance to the Bi2Mn4/3Ni2/3O6 compound,25 where its incommensurate modulation prevents the magnetic and ferroelectric long-range orderings, makes BiCr0.5Ni0.5O3 a candidate to further test local magnetoelectric couplings.

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ASSOCIATED CONTENT

S Supporting Information *

Low-temperature NPD, SAED, low-magnification microscopy, and magnetic hysteresis loops. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author Figure 4. (a) Zero-field μSR spectra for BiCr0.5Ni0.5O3 at selected temperatures. (b) Fit parameters to Pz(t) = a0 exp[−(λt)β] + bk to the zero-field data as a function of the temperature. The vertical line is placed at 35 K.

*E-mail: aalopez@staffmail.ed.ac.uk.

Thermal evolution of the fitted parameters to a stretched exponential Pz(t) = a0 exp[−(λt)β] + bk, with a0 the initial asymmetry, β the shape parameter, λ the muon depolarization rate, and bk the background asymmetry, is shown in Figure 4b. As expected, λ(T) rises and β(T) decreases with T → Tf+, which is the signal of the slowing down of spin fluctuations as the transition is approached. The observed drop in a0 also reflects the development of static internal fields on cooling, in accordance with M(H) measurements in Figure S4 in the SI, related to the development of a distribution of cluster sizes and associated relaxation rates below the transition. However, no

Notes

Author Contributions

The manuscript was written through contributions of all authors The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. T. Hansen (ILL), A. Fitch (ESRF), and S. Giblin (ISIS) for assistance with diffraction and spectroscopy measurements. We acknowledge support from EPSRC and the Royal Society, U.K. Also support from the Ministerio de Ciencia e Inovacion and the Comunidad de Madrid, Spain, through Grants MAT2010-19460 and S2009/PPQ-1626, is acknowledged. A.J.D.-G. is grateful to MICINN and UCM for a D

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“Juan de la Cierva” postdoctoral contract (Contract JCI-201008229). J.R.L. is thankful for a Fulbright fellowship for funding.

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