Structural and Magnetic Transitions in CaCo3V4O12 Perovskite at

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Structural and Magnetic Transitions in CaCo3V4O12 Perovskite at Extreme Conditions Sergey V. Ovsyannikov,*,†,∇ Elena Bykova,†,‡ Anna Pakhomova,†,‡ Denis P. Kozlenko,§ Maxim Bykov,† Sergey E. Kichanov,§ Natalia V. Morozova,∥ Igor V. Korobeinikov,∥ Fabrice Wilhelm,⊥ Andrei Rogalev,⊥ Alexander A. Tsirlin,# Alexander V. Kurnosov,† Yury G. Zainulin,∇ Nadezda I. Kadyrova,∇ Alexander P. Tyutyunnik,∇ and Leonid Dubrovinsky† †

Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstrasse 30, Bayreuth D-95447, Germany Deutsches Elektronen-Synchrotron (DESY), D-22603 Hamburg, Germany § Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russia ∥ Institute of Metal Physics of Russian Academy of Sciences, Urals Division, GSP-170, 18 S. Kovalevskaya Str., Yekaterinburg 620990, Russia ⊥ European Synchrotron Radiation Facility, 71, avenue des Martyrs CS 40220, 38043 Grenoble Cedex 9, France # Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany ∇ Institute for Solid State Chemistry of Russian Academy of Sciences, Urals Division, 91 Pervomayskaya Str., Yekaterinburg 620990, Russia ‡

S Supporting Information *

ABSTRACT: We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCo3V4O12 double perovskite with the high-spin (HS) Co2+ ions in a square-planar oxygen coordination at extreme conditions of high pressures and low temperatures. The single-crystal X-ray diffraction and Raman spectroscopy studies up to 60 GPa showed a conservation of its cubic crystal structure but indicated a crossover near 30 GPa. Above 30 GPa, we observed both an abnormally high “compressibility” of the Co−O bonds in the squareplanar oxygen coordination and a huge anisotropic displacement of HSCo2+ ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co2+ ions, it did not result in the anticipated shrinkage of the cell volume because of a certain “stiffing” of the bonds of the Ca and V cations. We verified that the oxidation states of all the cations did not change across this crossover, and hence, no charge-transfer effects were involved. Consequently, we proposed that CaCo3V4O12 could undergo a phase transition at which the large HS-Co2+ ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCo3V4O12 at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co2+ ions were aligned along one of the cubic axes, and the magnetic structure had a 2-fold periodicity along this axis, compared to the crystallographic one.

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INTRODUCTION Perovskites are a very common class of oxide materials, which finds numerous applications in a variety of technologies. Tuning the properties of perovskite materials by doping or chemical substitution as well as creating completely new perovskites can greatly extend the applicability and functionality of these systems and indicate novel directions for their possible applications. For these reasons, the new unusual perovskites are intensively fabricated and investigated.1−11 Generally, conventional perovskites (ABO3) are composed of two types of cations, namely, an octahedrally coordinated cation B and a cation A with a variable coordination number. More © 2017 American Chemical Society

sophisticated double perovskites, e.g., those that are called “A-site ordered” with the general formula AA′3B4O12 comprise one more site (A′) for cations with an unusual two-dimensional square oxygen coordination (Figure 1). The sites A′ and B are closely located in the perovskite structure, thereby enabling an additional mechanism of electronic interactions through the A′−O−B chains along with regular interactions through the B− O−B chains. This effect gives rise to qualitatively novel phenomena and electronic processes in these double perovReceived: February 7, 2017 Published: May 18, 2017 6251

DOI: 10.1021/acs.inorgchem.7b00330 Inorg. Chem. 2017, 56, 6251−6263

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Inorganic Chemistry

Recently, a novel and unique perovskite material, CaCo3V4O12 with the Co2+ ions occupying the crystallographic sites with the square-planar oxygen coordination (Figure 1) and simultaneously adopting the high-spin (HS) state, was synthesized.19 Magnetic susceptibility measurements established that below 100 K CaCo 3 V 4 O 12 undergoes an antiferromagnetic ordering transition and indicated that a sizable orbital moment may be present on Co2+.19 Subsequent band-structure calculations confirmed an orbital moment on the order of 1 μB,45 but remained uncertain about the exact size of this moment and its orientation. CaCo3V4O12 features a combination of several dissimilar and potentially competing electronic effects, including strong Coulomb correlations, spin− orbit coupling, and orbital polarization, thus posing a daunting challenge for ab initio calculations45 and making this compound an interesting system for experimental studies. Other calculations showed that even Co2+ dopants with the squareplanar coordination can essentially modify the electronic band structure of materials.46 In this work, we synthesized high-quality single crystals of CaCo3V4O12 perovskite and examined the evolution of its structural, vibrational, and transport properties upon strong compression to 60 GPa. We found a crossover near 30 GPa, above which the Co−O bonds in the square-planar oxygen planes demonstrated an abnormally high compressibility. Further analysis of the crystal structure ruled out both pressure-driven charge disproportionation and (high spin → low spin) HS → LS spin transition of the Co2+ ions as possible origins of this anomaly. Consequently, we proposed that this perovskite could undergo a phase transition at which the large HS-Co2+ ions are pushed out of the oxygen planes but continue to vibrate near the planes. In addition, we investigated the crystal and magnetic structures of this perovskite in the lowtemperature antiferromagnetic phase below 100 K by means of neutron diffraction at ambient pressure.

Figure 1. Unit cell of the cubic crystal structure of CaCo3V4O12 perovskite at ambient conditions. The anisotropic atomic displacements of the Co2+ ions in the square-planar planes of oxygen are shown by ellipsoids.

skites, e.g., unusual phase transitions linked to charge disproportionation between cations are observed.12 Although, the A and B positions in the double perovskite structure may be routinely filled by different ions, the squareplanar A′ sites (Figure 1) are rather specific and can be occupied by ions, which are prone to Jahn−Teller distortion. For example, the A′ sites in numerous AA′3B4O12 perovskites can be readily filled either by Cu2+ 12 or by Mn3+ ions.13 However, there are only a few cases of perovskites reported to date with the A′ sites occupied by other transition-metal ions, e.g., by Fe2+ in CaFe3Ti4O12 14 and Ca(Cu2Fe)V4O12,15 by Ti4+ ions in Na(Cu2.5Ti4+0.5)Ti4O12 16 and Sr0.946(Cu2.946Ti0.054)Ti4O12,17 by Pd2+ in CaPd3(Ti,V)4O12,18 and by Co2+ ions in CaCo3V4O12.19 These unusual square-planar-coordinated positions for Fe and Co ions that are linked to unconventional electronic configurations of these ions were also observed in other compounds, e.g., in CaFeO2 20 and Sr3Co2O4Cl2.21 Among different families of the above A-site ordered AA′3B4O12 double perovskites, the most studied to date are compounds with B = Fe because of their nontrivial magnetic properties and charge disproportionation reactions, which can lead to spectacular phenomena.12 The compounds with B = Ti and Mn have been studied as well because of their highly perspective optoelectronic properties17,22,23 and high sensibility to magnetic field or large magnetoresistance effects,24 respectively. Members of other double-perovskite families, for instance, those of V-based perovskites, AA′3V4O12, also find various applications in industry, but they are less systematically studied to date.25−35 Meanwhile, the vanadate perovskites could also uncover a number of unique features of perovskite materials, thereby contributing to a deeper understanding of interrelations between their crystal chemistry and properties. Recent advances in investigations of some other vanadium compounds hint that V-based perovskites may reveal rather spectacular features,36−38 especially, if they are additionally subjected to extreme conditions, such as high pressures.39−44

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EXPERIMENTAL DETAILS

Synthesis and Characterization of Samples. The samples of the CaCo3V4O12 perovskite were synthesized at high-pressure hightemperature (HP−HT) conditions by using a 1200 tonne Multi-Anvil Press at BGI (Bayreuth). The samples were synthesized from stoichiometric mixtures of powders of Ca(VO3)2, Co(VO3)2, and Co with chemical purities better than 99.9% at 15−18 GPa and 1100− 1300 °C over several hours. The HP−HT syntheses at 15−16 GPa and 1100−1200 °C produced bulk polycrystalline samples, whereas the syntheses at higher pressures and temperatures (17−18 GPa and 1200−1300 °C) led to growth of single crystals with sizes of 50−200 μm. The synthesis procedures were similar to those reported in previous works47,48 and included a rhenium sample capsule, a LaCrO3 heater, and other parts adjusted into an octahedral container made of (MgO)0.95(Cr2O3)0.05.49 The samples were examined by scanning electron microscopy, by microprobe analysis, and by conventional structural methods using a LEO-1530 instrument, JEOL-JSM 6390 LA microscope with a JED-2300 attachment, and a high-brilliance Rigaku diffractometer (λ = 0.7108 Å), respectively. Neutron Diffraction Studies. We examined the magnetic structure of CaCo3V4O12 by using a DN-12 neutron diffractometer for microscopic samples,50 located at the IBR-2 high-flux pulsed reactor (FLNP JINR, Dubna, Russia). A sample with a volume of about 1.5 mm3 was placed inside a CCR-based cryostat. We collected neutron powder diffraction patterns at scattering angles of 2θ = 45.5 and 90° from room temperature and down to 10 K, across the antiferromagnetic ordering transition around 100 K.19 We analyzed the neutron diffraction patterns by the Rietveld refinement method using the Fullprof program.51 We considered possible models of the lowtemperature magnetic structure of CaCo3V4O12 taking into account a 6252


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Inorganic Chemistry symmetry analysis performed in the framework of BasIreps program (https://www.ill.eu/sites/fullprof/). XANES Measurements. The X-ray absorption near edge spectra (XANES) were collected at the ID12 beamline of ESRF (Grenoble, France) using total fluorescence yield in a backscattering geometry.52 A source of X-rays was a helical undulator in pure circular mode; i.e., only the fundamental harmonic was emitted on axis. The doublecrystal fixed-exit monochromator equipped with a pair of Si ⟨111⟩ crystals cooled down to −140 °C was exploited. Given the ultralow emittance of the source, we checked that the energy resolution was close to the theoretical limit: 0.58 eV at calcium K-edge; 0.79 eV at vanadium K-edge; 1.14 eV at cobalt K-edge. In any case, the energy resolution was better than the core−hole lifetime broadening at the corresponding edges. We gathered the spectra at the Ca, Co, and V Kedges at ambient pressure above and below the point of the antiferromagnetic transition (100 K).19 Single-Crystal X-ray Diffraction Studies under Pressure. Single-crystal synchrotron X-ray diffraction studies of CaCo3V4O12 under high pressure were performed at the ID09A beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using a wavelength of λ = 0.415 13 Å. Some low-temperature singlecrystal X-ray diffraction experiments were also carried out at ID27 of ESRF with the wavelength of λ = 0.3738 Å. We utilized a high-pressure diamond anvil cell (DAC) of the Le-Toullec-type53 and filled the cell with a neon pressure transmitting medium. Together with a single crystal of CaCo3V4O12 with sizes of about 30 × 30 × 5 μm3, we loaded inside the cell a small Sm-YAG chip for pressure estimation. We collected the diffraction images using a MAR555 flat panel detector. We used silicon powder to calibrate the coordinates of the beam center, tilt angle, and tilt plane rotation angle. The single crystal of CaCo3V4O12 was compressed from about 2 to 55 GPa with a 2−5 GPa step. At each pressure step, we collected the X-ray diffraction images upon continuous rotation of the cell from −20° to +20° Ω. At selected pressure points, we collected the data with a narrow 0.5° scanning step in the range from −32° to +32° Ω. We analyzed the 2D images with Dioptas software54 and integrated the reflection intensities by using CrysAlisPro.55 In the data refinement, we applied an empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm (included in the CrysAlis Pro software). To calibrate an instrumental model in the CrysAlisPro software, i.e., the sample-to-detector distance, detector’s origin, offsets of goniometer angles, and rotation of both X-ray beam and the detector around the instrument axis, we used a single crystal of orthoenstatite ((Mg1.93, Fe0.06)(Si1.93, Al0.06)O6, Pbca space group, a = 8.8117(2), b = 5.18320(10), and c = 18.2391(3) Å). The crystal structure of CaCo3V4O12 under pressure was refined by a full-matrix least-squares method in an anisotropic approximation for all atoms, using the WinGX software.56 Raman Spectroscopy under Pressure. For the Raman spectroscopy studies we selected high-quality single crystals of CaCo3V4O12 and verified their structure by single-crystal X-ray diffraction. A sample of about 10 μm size was loaded in a hole of about 100 μm in diameter drilled inside a rhenium gasket preindented to the thickness of 35 μm, and the gasket was squeezed between two diamond anvils with a culet size of 250 μm. The hole in the rhenium gasket was filled with a neon pressure-transmitting medium, using a BGI’s gas-loading apparatus.57 For the generation of high pressures, we employed a BX90 diamond anvil cell (DAC).58 The pressure values in the high-pressure cell were determined by the shift of the ruby luminescence line.59 The Raman spectra were excited by the red 632.8 nm line of a He−Ne laser. The nonpolarized Raman spectra were recorded by a LabRam spectrometer in the backscattering geometry. The spectra had a rather low intensity and typical times of spectra acquisition varied between 10 and 30 min. We performed two Raman experiments, the first one up to 15 GPa and the second one up to 60 GPa, the pressure value at which the Raman signal vanished. Measurements of Seebeck Effect under Pressure. Pressure dependencies of the thermopower (Seebeck coefficient) up to 20 GPa were measured on two single-crystalline samples for several pressure cycles. These measurements were carried out on an automated

minipress setup enabling a smooth generation of the force applied to a high-pressure cell with a sample, together with a simultaneous automatic recording of all output signals from the cell.60 The force applied to the high-pressure cell was automatically measured in situ by a digital dynamometer. Then, on the basis of a calibration curve, the applied force values were automatically recalculated to GPa units. We utilized an anvil-type high-pressure cell of the modified Bridgman-type with the anvils made of sintered diamonds of a culet size of 600 μm.61 A disk-shaped sample with typical sizes of 200 × 200 × 30 μm3 was loaded in a container made of limestone (soft CaCO3-based material), which served both as a gasket and as a pressure-transmitting medium. The upper anvil was heated up by an electrical heater to generate a temperature difference (ΔT) of a few Kelvins along the sample thickness. This ΔT difference was measured by means of the thermocouples attached near the tips of the diamond anvils. The thermal conductivity of the diamond anvils is several orders higher compared to that of CaCo3V4O12 samples, and hence, the ΔT value that was being measured between the anvils was nearly identical to that along the sample thickness (note that this fact was verified by measurements of semiconductor crystals with precisely known Seebeck coefficients). A thermoelectric voltage, which was generated by this temperature difference, was measured in the same direction, along ΔT by means of electrical probes attached to the anvils’ tips. A relative uncertainty in the determination of the Seebeck coefficient by the method employed was below 5%, and it was related to a minor potential uncertainty in the ΔT value. Other details of the measurements were similar to those reported earlier.10

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RESULTS AND DISCUSSION In this work, we examined the properties of CaCo3V4O12 perovskite at extreme conditions, under high pressures at room temperature and at low temperatures at ambient pressure. We investigated the crystal and magnetic structures of CaCo3V4O12 perovskite across the antiferromagnetic transition around 100 K,19 by means of ambient-pressure neutron powder diffraction down to 10 K. In addition, we examined possible variations in the oxidation states of the cations across this transition by means of X-ray absorption near edge spectra (XANES). In the high-pressure studies, we followed pressure evolution of the perovskite structure of CaCo3V4O12 and its vibrational properties to 60 GPa by means of single-crystal Xray diffraction and Raman spectroscopy, respectively. These studies were supplemented by investigating pressure effect on the thermoelectric power of this perovskite. Ambient-Pressure Crystal and Magnetic Structure at Low Temperatures. Our ambient-pressure studies show that the original crystal structure of CaCo3V4O12 does not change at the antiferromagnmetic transition at 100 K. To determine the magnetic structure of the low-temperature antiferromagnetic phase of CaCo3V4O12 below 100 K,19 we performed a neutron diffraction study across this transition, from 290 K and down to 10 K. We cooled CaCo3V4O12 below TN ≈ 100 K, and several magnetic peaks at dhkl ≈ 7.24, 4.88, 4.22, and at 3.27 Å (Figure 2) appeared, indicating the formation of the long-range antiferromagnetic (AFM) order. The intensities of all the magnetic peaks could not be described by a model with a single propagation vector. However, a model with two magnetic sublattices of the Co2+ ions (Figure 3) could provide a good fit of the experimental patterns (Figure 2, Table 1). We found the propagation vectors for these sublattices as k1 = (0 0 1) and k2 = (0 0 1/2), respectively. The first magnetic sublattice is formed by the Co2+ ions located at the (0 0 1/2) and (1/2 1/2 0) positions in the unit cell, whereas the second one is formed by the Co2+ ions at (1/2 0 0), (1/2 0 1/2), (0 1/2 0), and (0 1/2 1/2) positions (Figure 3). No signs for magnetic ordering of the V4+ 6253


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Figure 3. “Unit cell” of the antiferromagnetic structure of CaCo3V4O12 perovskite below 100 K, consisting of two crystal unit cells along the caxis. The two magnetic sublattices formed by the Co2+ ions are highlighted in different colors. The longer green arrows correspond to the propagation vector k1, and the shorter blue arrows correspond to k2.

Table 1. Parameters of the Magnetic Structure of CaCo3V4O12 a magnetic sublattices first Co positions propagation vector μCo (μB) Rp and Rwp (%)

Figure 2. Neutron diffraction patterns of polycrystalline CaCo3V4O12 perovskite at selected temperatures (given near the curves) measured at scattering angles of 90° (a) and 45° (b). The points are experimental data, the solid lines are fits using the Rietveld refinement method, and the difference curves for the patterns collected at T = 10 K are shown at the bottom of both plots. The ticks at the top of both plots are calculated reflection positions of the nuclear peaks. The most intensive magnetic peaks are labeled by “AFM” (antiferromagnetic) and marked as either k1 or k2 (the propagation vectors of two magnetic sublattices shown in Figure 3).

a

1

(0 0 /2), (1/2 1/2 0) k1 = (0 0 1)

second 1

1

( /2 0 0), ( /2 0 1/2), (0 1/2 0), (0 1/2 1/2) k2 = (0 0 1/2)

μ1 = 3.5(1) μB μ2 = 2.1(1) μB 4.34 and 5.65 for 2θ = 90° 4.75 and 6.47 for 2θ = 45°

At ambient pressure and T = 10 K.

moment of the Co2+ ions forming the second magnetic sublattice, μ2 = 2.1(1) μB, may be the spin-only value compatible with the ab initio values of 1.69−2.05 μB (it is smaller than 3 μB due to the Co−O hybridization).45 The difference μ2 − μ1 = 1.4 μB should be then ascribed to the large orbital moment, which was also inferred from our previous susceptibility studies.19 However, direct measurement of the orbital moment by, e.g., XMCD would be needed to verify this conjecture. We can assume that the distinctive orbital contribution to the magnetic moments of the cobalt ions, located at the first and second sublattices, together with the anomalously strong out-of-plane displacements of the Co2+ ions and the competing nearest-neighbor and the next-nearestneighbor superexchange interactions between these ions, could somehow lead to the complex magnetic structure of CaCo3V4O12 with the 2-fold periodicity of the magnetic order along the c-axis, compared to the crystallographic one (Figure 3). XANES Spectra. The X-ray absorption near edge spectra (XANES) of CaCo3V4O12 measured both in the ambient and in the low-temperature antiferromagnetic phases at the K-edges of all the cations are presented in Figure 4. XANES spectra are sensitive to electronic configuration of ions; for instance, the positions of all the basic features of XANES spectra, such as absorption threshold, pre-edge peaks, and absorption edge, and of other characteristic peaks usually linearly shift upon variation

ions (3t2g1) were found. The net magnetic moment within the doubled unit cell is zero, because different moments fully compensate each other (Figure 3). This two-component magnetic structure is largely different from antiferromagnetic structures reported for Cu-based double perovskites, e.g., CaCu3Ti4O12.62 The ordered magnetic moments at the Co2+ ions in the first and second sublattices at T = 10 K are μ1 = 3.5(1) μB and μ2 = 2.1(1) μB, respectively. This difference can be traced back to the fact that spins of the first sublattice are perpendicular to their oxygen planes, whereas spins of the second sublattice are parallel to their oxygen planes. Large orbital moments may arise when spins are perpendicular to the oxygen planes yielding μ1 = 3.5(1) μB that clearly exceeds the maximum spin-only value of 3 μB (S =3/2 for HS-Co2+). This assumption is consistent with the results of ab initio calculations, which provided evidence of large orbital moment contribution for the cobalt ions located at the first magnetic sublattice.45 However, the ordered magnetic 6254


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Figure 4. Normalized XANES spectra at the K-edge of (a) Ca, (b) Co, and (c) V recorded at 70 and 298 K in CaCo3V4O12 perovskite. The insets in the plots show the selected magnified parts of these spectra. The spectra have not been corrected for reabsorption effects. The upper row of this figure shows the coordination environment of the Ca, Co, and V ions in the perovskite structure.

Figure 5. Examples of X-ray diffraction data collected from a single-crystalline sample of CaCo3V4O12 at room temperature at two pressures of 2.2 and 54.6 GPa. (a) and (b) are sections of X-ray diffraction images in which the reflections of CaCo3V4O12 are indexed, and the nonindexed reflections correspond to either the single-crystal diamond anvils or to the solidified neon pressure-transmitting medium. (c) and (d) are reciprocal space reconstructions which show that the sample quality remained good up to the highest pressure in our experiment, 54.6 GPa.

in the oxidation state.63 The bond valence sum (BVS) analysis for the perovskite structure of CaCo3V4O12 at ambient conditions suggested the oxidation states of about Ca2+, Co2+, and V4+.19 The XANES V K-edge spectra collected at both 298 and 70 K exhibit the 1s → 3d pre-edge peak near 5469.3 eV (inset in Figure 4c) in good agreement with the position of this

peak (5469.5 eV) reported in VO2.63 Thus, this observation is consistent with the average oxidation state of V4+, and taking into account that both the ambient and low-temperature structures of CaCo3V4O12 have the only nonequivalent crystallographic site for the V ions, a superposition of V3+ + V5+ can be ruled out. 6255


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(Figure 5). We analyzed pressure evolution of the oxidation states of all the cations by means of the conventional bond valence sums (BVS) method.68 In this BVS analysis, we took into account that the total cation charge (+24) for the formula unit should be conserved under pressure, and hence, the “nominal” BVS values determined by this method were renormallized at each pressure point. We found that they weakly changed with pressure and essentially did not depart from the nominal values of +2 for the Ca and Co ions and +4 for the V ions (Figure 6a). Thus, no charge disproportionation reactions were observed in this pressure range.

The XANES Ca K-edge spectra collected at the same conditions exhibit a double pre-edge feature (Figure 4a), which differs from a single peak reported for CaO at 4039.26 eV,64 but the energy of the stronger peak in this doublet is about 4039.5 eV (inset in Figure 4a), i.e., similar to that. However, it should be also noted that the peak position and its intensity for an ion may be moderately varied in dependence on its surroundings, as shown for Ca2+ complexes in different compounds,64,65 and hence, in agreement with the above BVS analysis, from these XANES data we can infer the Ca2+ oxidation state in CaCo3V4O12 and its robustness across the magnetic transition.19 Therefore, charge balance suggests the 2+ oxidation state for the Co ions, in agreement with our BVS analysis. Remarkably, our XANES spectra for the Co ions show a minor shift in the absorption threshold across the transition (inset in Figure 4b). Precise determination of the oxidation states from these XANES spectra turned out to be difficult, as the pre-edge peak was strongly smeared (Figure 4b), and the literature data on the absorption edge value found as the maximum of the first derivative of the absorption edge for the spectra of the Co2+ and Co3+ “reference” compounds were rather discrepant.66,67 An experimentally observed shift of the pre-edge feature in the XANES spectra, collected at 70 and 298 K in the vicinity of the K-edge (inset Figure 4b), may hint at a minor reduction in the average oxidation state of the Co2+ ions in the antiferromagnetic phase below 100 K.19 Note that the spectra for the Ca and V ions exhibited opposite but much smaller shifts upon the transition to the low-temperature phase, thereby hinting at a very tiny increase in their oxidation states (insets in Figure 4a,c). However, we should note that the latter minor variations in the spectra are comparable with experimental uncertainties. Detailed analysis of the XANES spectra was beyond the scope of the present work. These spectra for the V ions with two broad humps (Figure 4c) look rather typical for the octahedral coordination. The absorption edge for the Co ions in CaCo3V4O12 exhibits the fine structure with the peaks at 7721, 7726, 7735, and 7740.5 eV (Figure 4b) in contrast to a single broad maximum in the spectra of Co ions in the rocksalt-structured CoO near 7724.7 eV.66 The observation of a very intense doublet peak of the 12-coordinated Ca ions in CaCo3V4O12, at 4047.9 and 4049.5 eV at both temperatures (Figure 4a), suggests a high density of states of these ions that was not observed for six-coordinated Ca ions in CaO.64 Likely, this feature is related to a strong hybridization of the electronic orbitals of the Ca2+ ions in CaCo3V4O12. Crystal Structure Evolution under High Pressure. We investigated the high-pressure behavior of CaCo3V4O12 at room temperature by means of single-crystal X-ray diffraction. Two examples of X-ray diffraction images measured at the starting and maximal pressures are given in Figure 5. The selected single crystal was of excellent quality, and its tiny reflection spots were very narrow and hardly visible by the naked eye (Figure 5a). These studies have established that the cubic crystal structure of this perovskite is very stable and is preserved up to at least 55 GPa (Figure 5b, Table S1 in the Supporting Information). Since pressurization of crystals to high pressures is known to stimulate a propagation of stresses inside the volume of crystal, we analyzed the sample mosaicity parameters after the pattern integration and verified that the sample may be analyzed as a single crystal in the whole pressure range. Visually, the conservation of the single crystallinity of the sample is seen from comparison of the data gathered at 2.6 and 54.6 GPa

Figure 6. Pressure dependencies of the (a) bond valence sums (BVS) of the Ca, Co, and V cations, (b) V−O−V angle between the neighboring octahedra, and (c) relative unit cell volume of the crystal structure of CaCo3V4O12 perovskite at room temperature.

Pressure evolution of the unit cell volume of CaCo3V4O12 also did not exhibit any distinct discontinuities (Figure 6c). Fitting the whole data set to the third-order Birch−Murnaghan equation of state (EOS),69,70 we estimated the bulk modulus value as of B0 = 198.1(5) GPa at a variable B′0 = 3.9(0) (blue solid curve in Figure 6c). However, as one can see from this plot the volumetric data perfectly follow the above established EOS up to ∼25 GPa only, where a weak bending is observed. It looks like a tiny “swelling” of the lattice compared to its projected equilibrium volume (Figure 6c). Repeating the fitting procedure but only for the data set to 45 GPa, we found that the experimental points perfectly follows the EOS with a 6256


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Figure 7. Pressure dependencies of the (a) anisotropic displacement parameter of the Ca, V, and Co cations (U11, U22, U33), of the (b) relative change in the shortest cation−oxygen distances, and of the (c) variable oxygen coordinates, z/c and y/b, of CaCo3V4O12 perovskite at room temperature. (d) Schematic representation of the pressure effect on the Co2+ ions in the square-planar oxygen coordination. An applied pressure strongly compresses the large HS-Co2+ ions and enhances their displacements from the central position in the planes. The higher applied pressures above 30 GPa lead to extrusion of these ions from the planes.

indeed accompanied by a noticeable volume drop near 10 GPa.73 Also, for comparison, we can refer to the double perovskite CaMn3V4O12 adopting the same cubic crystal structure with the same oxidation states of Ca2+ and V4+ ions.75 Here, the complete chemical substitution of the HSCo2+ ions by the larger HS-Mn2+ ones (their ionic radius is 97 pm) results in an expansion of the lattice parameter from 7.3428 Å19 to 7.4070 Å,75 driving about 1% change in the volume. Taking into account the similar 10 pm difference in the ionic radii of the HS-Co2+ and LS-Co2+ ions, we anticipate about 1% change in the cell volume, which is hardly seen in our data (Figure 6c). The abnormal enhancement in the compressibility of the Co−O bonds above 30 GPa (Figure 7b) may look like a signature of a very sluggish pressure-driven HS → LS transition, which is not complete even at 55 GPa. However, other observations, such as the enhancement of the anisotropic displacements of the Co2+ ions above 30 GPa (Figure 7a) together with the apparent absence of volume shrinkage at these pressures (Figure 6c), do not support this hypothesis of the HS → LS transition. The significant out-of-plane displacements (U33 parameter) of the Co2+ ions are observed already at ambient pressure and indicate that the HS-Co2+ is too large for its square-planar environment (Figure 1). Upon compression, this environment becomes even more cramped, and hence, these out-of-plane displacements continuously increase (Figure 7a). The minor jump in the out-of-plane displacement parameter U33 of the Co2+ ions around 30 GPa may indicate that the HS-Co2+ ions are eventually pushed out of the oxygen planes, into one or the other side, as shown in Figure 7d, but continue to vibrate near the planes, and hence, the total amplitude of the displacements increases (Figure 7a). Additionally, two other parameters of the thermal ellipsoid, U11 and U22 of the Co2+ ions, as well as the

slightly higher B0 = 202.7(0) GPa at a variable B′0 = 3.9(1) (red dashed curve in Figure 6c). However, the two pressure points above 50 GPa apparently drop from this EOS and suggest a minor volume shrinkage by ∼0.5% (Figure 6c). We have analyzed pressure evolution of the basic structural parameters of CaCo3V4O12, which included the anisotropic atomic displacements, the shortest cation−oxygen distances, and the variable oxygen coordinates, y/b and z/c (Figure 7a−c, Table 2). Pressure dependence of the atomic displacement parameter U33 of the Co2+ ions exhibits a moderate jump near 30 GPa (Figure 7a), which correlates with the kinks in the pressure dependencies of the Co−O bond length (Figure 7b) and the oxygen coordinates (Figure 7c). As found earlier, the Co2+ ions in the CaCo3V4O12 perovskite are in the high-spin (HS) state.19 Under compression, the HS ions can turn either to the low-spin (LS) state or to an intermediate-spin (IS) state with a concurrent reduction in the ionic radius. For example, a HS → LS transition in Co3+ ions happens abruptly at 3 GPa in BiCoO371 and very gradually upon pressurization from ambient to 40 GPa in SrCo0.5Ru0.5O3−δ.72 Recently, a pressure-driven HS → LS transition in Co3+ ions in Sr2CoO3F was found near 8−12 GPa. 73 Hence, one could expect that, under strong compression, the HS-Co2+ ions in CaCo3V4O12 could undergo a transition to the LS state, which should be characterized by a smaller ionic radius. This alleged HS → LS transition should be accompanied by noticeable volumetric effects in the crystal lattice because of the sizable difference in the ionic radii of the HS-Co2+ and LS-Co2+ ions (they are about 88.5 vs 79 pm for six-coordinated ions, respectively).74 For instance, volumetric data of the aforementioned Sr2CoO3F layered perovskite demonstrated that the HS → LS transition in Co3+ ions, with the smaller difference in the ionic radii (75 vs 68.5 pm),74 is 6257


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Table 2. Details of the Crystal Structure Refinement of CaCo3V4O12 from the Single-Crystal X-ray Diffraction Studya at selected pressures at 295 K pressure conditions (GPa) crystal system space group lattice parameter, a (Å) unit cell volume, V (Å3) Z calculated density (g/cm3) F(000) θ range for data collection (deg) completeness to d = 0.8 Å, % index ranges

no. of reflections collected no. of independent reflections/Rint refinement method data/restraints/ parameters goodness of fit on F2 final R indices [I >2σ(I)],R1/wR2 R indices (all data), R1/wR2 largest diff peak/ hole (e/Å3) atom Ca Co V O

Ca Co V O a

site 2a 6b 8c 24g

2.2

32.6

54.6

cubic Im3̅ (no. 204) 7.31844(19)

cubic Im3̅ (no. 204) 7.0418(3)

cubic Im3̅ (no. 204) 6.8839(11)

391.97(3)

349.18(4)

326.22(16)

2 5.1903

2 5.8264

2 6.2366

578 2.299−20.040

578 2.389−20.104

578 2.444−19.781

0.8824

0.912

0.831

−8 < h < 7 −11 < k < 11 −9 < l < 9 418

−9 < h < 8 −10 < k < 10 −7 < l < 7 349

−8 < h < 9 −7 < k < 7 −10 < l < 10 340

124/0.1457

106/0.0755

107/0.0856

full matrix least-squares on F2 124/0/14

106/0/14

107/0/14

1.293

1.141

1.291

0.0495/0.1168

0.0585/0.1529

0.0623/0.2114

0.0597/0.1365

0.0766/0.1784

0.0743/0.2253

2.303/−1.257

1.173/−1.127

0.968/−1.037

x/a

y/b

0 0 0.25 0

0 0.5 0.25 0.2997(4)

U11 0.95(7) 0.76(7) 0.72(5) 0.84(16)

U22 0.95(7) 1.02(7) 0.72(5) 1.16(17)

Atomic Coordinates x/a y/b 0 0 0 0.5 0 0.5 0.25 0.25 0.25 0.8132(5) 0 0.3008(5) Anisotropic Displacements, 100U (Å2) U33 U11 U22 0.95(7) 1.18(8) 1.18(8) 4.08(10) 1.15(10) 1.19(9) 0.72(5) 1.03(7) 1.03(7) 1.75(19) 1.24(18) 1.1(2) z/c

z/c 0 0.5 0.25 0.8152(6)

x/a 0 0 0.25 0

y/b 0 0.5 0.25 0.3042(8)

z/c 0 0.5 0.25 0.8179(8)

U33 1.18(8) 6.21(16) 1.03(7) 1.7(2)

U11 1.71(12) 1.87(14) 1.78(10) 1.9(3)

U22 1.71(12) 1.96(13) 1.78(10) 3.0(3)

U33 1.71(12) 5.9(2) 1.78(10) 2.2(3)

This study was carried out at the ID09A beamline of the European Synchrotron Radiation Facility with a radiation wavelength of 0.41513 Å.

displacement parameters of both Ca2+ and V4+ ions, are also enhanced above 30 GPa; i.e., the crystal lattice is destabilized. In the crystallographic software we used, we tried to split the crystallographic position for the Co2+ ions (0 1/2 1/2) (Table 2) into two, (0 1/2 1/2 − δ) and (0 1/2 1/2 + δ), lying on opposite sides of the oxygen planes (Figure 7d), but did not find any other stable sites for the Co2+ ions. Meanwhile, we noted that for a fixed nonzero (and small) value of δ, the X-ray diffraction data may be refined with the same R factors as for δ = 0. Thus, it seems plausible that above 30 GPa the Co2+ ions can occupy some unstable positions above and below the oxygen planes (Figure 7d). Hence, the anomalous compression of the Co−O bond length above 30 GPa (Figure 7b) may be explained by the fact that above 30 GPa, the “crystallographic”

bond length calculated for the Co (0 1/2 1/2) position located in the center of the oxygen planes begins to differ from the “true” Co−O bond length for the Co2+ ions departed from the oxygen planes (Figure 7d). Note that interrelation between “crystallographic” and “true” bond lengths was already illustrated in previous work on the example of CaNbF6 and other materials.76 Thus, the departure of the Co2+ ions from the oxygen planes above 30 GPa led to the strong contraction of the oxygen planes, and simultaneously, it contributed to a “hardening” of the V−O and Ca−O bonds (Figure 7b). Hence, it did not result in the volume collapse (Figure 6c). In contrast, the alleged HS → LS transition would be accompanied by a noticeable volume contraction. This change in the positions of 6258


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Inorganic Chemistry the Co2+ ions in the structure led to the faster reduction in the V−O−V angle (Figure 6b) with further pressurization. Note that there are examples of perovskites crystallizing in less symmetric crystal lattices, in which a cation having a similar nearly square-planar oxygen coordination occupies a crystallographic site located a bit out of the oxygen plane; e.g., this happens to Mn3+ ions in orthorhombic DyMn3O6.77 For our perovskite, CaCo3V4O12, the Co2+ ions and oxygen atoms constituting the square-planar coordination are constrained by the symmetry to be coplanar, whereas the two O−Co−O angles are unconstrained and deviate by ∼4° from the ideal angle of 90°. Previous works devoted to an isostructural double perovskite, CaFe3Ti4O12 with Fe2+ ions adopting the same nearly square-planar oxygen coordination, also documented the large thermal ellipsoids of these Fe2+ ions, which were elongated perpendicular to the oxygen plane.14,78 Although the structure refinement of this perovskite determined the only (0 1/2 1/2) stable position for the Fe2+ ions, a possibility of statistical disorder of the Fe2+ ions perpendicular to the plane was suggested for explanation of the large thermal parameter.14,78 In our work, using a single-crystal X-ray diffraction method, we could not directly detect the pressure-induced changes in the position of the Co2+ ions in the structure of CaCo3V4O12, and our assumption that above 30 GPa the Co2+ ions depart from the oxygen planes was derived mainly from the analysis of the pressure responses of the Co−O bonds (Figure 7b). We have performed the test single-crystal X-ray diffraction experiments of the crystal structure of CaCo3V4O12 at low temperatures at several pressure points up to 20 GPa. These studies demonstrated the persistence of the cubic-perovskite structures at low temperatures and did not detect any signatures of a structural change (Table 3). The bond valence sums (BVS) analysis68 of this structure also did not reveal any deviations in the oxidation states and suggested Ca2.3+, Co1.8+, and V4.1+. Changes in Vibrational Properties under Pressure. At ambient conditions, the nonpolarized Raman spectra of CaCo3V4O12 crystals exhibit only two distinctly observable and rather broad peaks at 396 and 475 cm−1 (Figure 8a). These peaks shift to the higher frequencies under pressure (Figure 8a,c). This behavior is typically linked to a pressure-driven contraction in the chemical bonds. Above 10 GPa, we noticed the appearance of a new broad peak near 550 cm−1. Its intensity increased upon further pressurization (Figure 8a). In the CaCu3Ti4O12 perovskite adopting the same crystal lattice, the similar Raman spectra with three main peaks at 445, 511, and 575 cm−1 at ambient conditions were reported.79 The first two peaks were assigned to the Ag modes linked to rotation-like vibrations of the TiO6 octahedra, whereas the third one was related to the Fg mode linked to the O−Ti−O antistretching.79 The same assignment of the Raman peaks is applicable to CaCo3V4O12 too, and the lowering in the frequencies compared to CaCu3Ti4O12 is related to the fact that the V4+ ions are heavier than the Ti4+ ones. Other A-site ordered double perovskites, such as ACu3Fe4O12 crystallizing in the same cubic structure, were shown to demonstrate qualitatively similar Raman spectra with three basic peaks. The splitting of these peaks into doublets as well as the appearance of additional weak modes depend on polarization.80 Above 25 GPa, we noted a smooth crossover in the spectra, at which the pressure dependencies of the wave numbers exhibited noticeable changes in their slopes; for example, this is

Table 3. Details of the Crystal Structure Refinement of CaCo3V4O12 from the Single-Crystal X-ray Diffraction Studya at 20.8 GPa and 30 K P, T conditions crystal system space group lattice parameter, a (Å) unit cell volume, V (Å3) Z calculated density (g/cm3) F(000) θ range for data collection (deg) completeness to d = 0.8 Å, % index ranges

reflections collected independent reflections/Rint refinement method data/restraints/ parameters goodness of fit on F2 final R indices [I > 2σ(I)], R1/wR2 R indices (all data), R1/wR2 largest diff. peak/ hole (e/Å3) atom Ca Co V O

Ca Co V O

20.8 GPa, 30 K cubic Im3̅ (no. 204) 7.1350(11) 363.23(17) 2 5.6010 578 2.123−18.713 0.848 −5 < h < 6 −10 < k < 7 −7 < l < 8 199 115/0.0634 full matrix least-squares on F2 115/0/14 1.193 0.0895/0.2673 0.0994/0.2927 1.421/−1.641

Atomic Coordinates site x/a y/b 2a 0 0 6b 0 0.5 8c 0.25 0.25 24g 0 0.3002(6) Anisotropic Displacements, 100U (Å2) U11 U22 1.35(15) 1.35(15) 1.31(14) 1.53(15) 1.40(13) 1.40(13) 1.5(3) 1.6(3)

z/c 0 0.5 0.25 0.8145(8) U33 1.35(15) 2.30(17) 1.40(13) 1.4(3)

a

This study was carried out at the ID27 beamline of the European Synchrotron Radiation Facility with a radiation wavelength of 0.3738 Å.

well seen in the dependence of the 475 cm−1 mode (Figure 8c). Above 25 GPa, all the peaks demonstrate both a progressive decrease in their intensity and a pronounced broadening with pressure (Figure 8a). This crossover well corresponds to the aforementioned structural modification observed in the singlecrystal X-ray diffraction experiments (Figure 7) and may be an indicator of a moderate structural destabilization. In visual examinations, we observed no changes in the black color of the single-crystalline sample of CaCo3V4O12 up to 60 GPa (Figure 8b). For comparison, we compressed in the same cell a couple of semiconductors, which were transformed to metals at moderate pressures and demonstrated an apparent metallic reflectivity (bright shiny color seen, e.g., at 59.1 GPa in Figure 8b). This observation indicates that CaCo3V4O12 is not metallic 6259


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Figure 8. Pressure evolution of (a) the Raman spectra and (c) wave numbers of CaCo3V4O12 at room temperature. In (a), the pressure values are given near the curves. The photographs of the single crystal of CaCo3V4O12 (pointed by the dashed circle and the blue bulk arrow) inside a diamond anvil cell are shown in (b). For comparison, we loaded in the same cell a couple of semiconductors which undergo a pressure-induced metallization and show a bright metallic reflectivity (e.g., seen at 59.1 GPa).

up to 60 GPa. In line with that, the evaluation of the V−O−V angle between the octahedra showed that this angle changes from ∼144.5° to 141.5° under pressure, i.e., departs even further from 180°, which is the optimal angle for polaron hopping conductivity of the delocalized electrons of the V ions.75 Interesting to note here that investigations of perovskite series indeed demonstrated an increase in the electrical conductivity if the V−O−V angle changes toward 180°, e.g., in AMn3V4O12.75 Pressure Effect on the Thermopower. Earlier studies of electrical resistivity and the Hall effect on a bulk polycrystalline sample of CaCo3V4O12 at ambient pressure revealed that CaCo3V4O12 behaves as an n-type semimetal with a high carrier concentration exceeding 1019 cm−3.19 The low intensity of the Raman spectra we observed in CaCo3V4O12 (Figure 8a) is consistent with this conclusion. To explore pressure evolution of the electronic properties of the CaCo3V4O12 perovskite, we measured its Seebeck coefficient on two single crystals to the maximal pressure of about 20 GPa generated in our press. These studies showed that the thermopower of CaCo3V4O12 is surprisingly low, S ≈ −2 μV/K (Figure 9) at ambient conditions. Under pressure, it changes the sign and reaches S ≈ +3 μV/K at 20 GPa (Figure 9). The thermopower curves of single-crystalline CaCo3V4O12 would rather indicate a metallic nature of its conduction in the whole pressure range investigated (Figure 9). Note that even many elemental metals demonstrate higher Seebeck coefficients that more strongly change under pressure.81,82 In an assumption of semimetallicity (or half-metallicity) of this perovskite, the pressure dependencies of its Seebeck coefficient might be explained by a strongly compensated electrical conduction (nearly equal contributions of holes and electrons) with a slight preference to the electron conduction at ambient conditions and a minor shift to the hole conduction under applied stress. However, it should be noted that such a compensated conduction mechanism persisting in the wide pressure range is a rather uncommon phenomenon. The thermopower dependencies (Figure 9) did not exhibit any characteristic features at high

Figure 9. Pressure dependencies of the thermoelectric power measured on two single-crystalline samples of CaCo3V4O12 perovskite for several pressure cycles at room temperature. The arrows point to the directions of pressure variation. The left inset is a photograph of a typical single crystal of CaCo3V4O12. The right inset shows the determination of the Seebeck coefficient (S) at ambient conditions from a linear slope of a thermoelectric voltage (U) on a temperature difference (ΔT) as S = −U/ΔT.

pressures up to 20 GPa in line with the aforementioned data on structural and vibrational properties, which detected the crossover at a higher pressure of 25−30 GPa (Figures 6−8). A minor difference of about 1 μV/K between the thermopower values on the pressurization curves of samples #1 and #2 may be related either to individual features of the crystals or to contributions of small nonhydrostatic pressure components which could slighty tune the n/p charge carriers balance in the samples. 6260


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ASSOCIATED CONTENT

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REFERENCES

(1) Yi, W.; Princep, A. J.; Guo, Y.; Johnson, R. D.; Khalyavin, D.; Manuel, P.; Senyshyn, A.; Presniakov, I. A.; Sobolev, A. V.; Matsushita, Y.; Tanaka, M.; Belik, A. A.; Boothroyd, A. T. Sc2NiMnO6: A DoublePerovskite with a Magnetodielectric Response Driven by Multiple Magnetic Orders. Inorg. Chem. 2015, 54, 8012−8021. (2) Yuan, Y.; Feng, H. L.; Ghimire, M. P.; Matsushita, Y.; Tsujimoto, Y.; He, J.; Tanaka, M.; Katsuya, Y.; Yamaura, K. High-Pressure Synthesis, Crystal Structures, and Magnetic Properties of 5d DoublePerovskite Oxides Ca2MgOsO6 and Sr2MgOsO6. Inorg. Chem. 2015, 54, 3422−3431. (3) Li, M.-R.; Hodges, J. P.; Retuerto, M.; Deng, Z.; Stephens, P. W.; Croft, M. C.; Deng, X.; Kotliar, G.; Sánchez-Benítez, J.; Walker, D.; Greenblatt, M. Mn2MnReO6: Synthesis and Magnetic Structure Determination of a New Transition-Metal-Only Double Perovskite Canted Antiferromagnet. Chem. Mater. 2016, 28, 3148−3158. (4) Solana-Madruga, E.; Arevalo-Lopez, A. M.; Dos Santos-Garcia, A. J.; Urones-Garrote, E.; Avila-Brande, D.; Saez-Puche, R.; Attfield, J. P. Double Double Cation Order in the High-Pressure Perovskites MnRMnSbO6. Angew. Chem., Int. Ed. 2016, 55, 9340−9344. (5) Rusakov, D. A.; Abakumov, A. M.; Yamaura, K.; Belik, A. A.; Van Tendeloo, G.; Takayama-Muromachi, E. Structural Evolution of the BiFeO3−LaFeO3 System. Chem. Mater. 2011, 23, 285−292. (6) Abakumov, A. M.; Batuk, D.; Hadermann, J.; Rozova, M. G.; Sheptyakov, D. V.; Tsirlin, A. A.; Niermann, D.; Waschkowski, F.; Hemberger, J.; Van Tendeloo, G.; Antipov, E. V. Antiferroelectric (Pb,Bi)1‑xFe1+xO3‑y Perovskites Modulated by Crystallographic Shear Planes. Chem. Mater. 2011, 23, 255−265. (7) García-Martín, S.; King, G.; Urones-Garrote, E.; Nénert, G.; Woodward, P. M. Spontaneous Superlattice Formation in the Doubly Ordered Perovskite KLaMnWO6. Chem. Mater. 2011, 23, 163−170. (8) García-Martín, S.; King, G.; Nénert, G.; Ritter, C.; Woodward, P. M. The Incommensurately Modulated Structures of the Perovskites NaCeMnWO6 and NaPrMnWO6. Inorg. Chem. 2012, 51, 4007−4014. (9) Ovsyannikov, S. V.; Abakumov, A. M.; Tsirlin, A. A.; Schnelle, W.; Egoavil, R.; Verbeeck, J.; Van Tendeloo, G.; Glazyrin, K.; Hanfland, M.; Dubrovinsky, L. Perovskite-like Mn2O3: a Path to New Manganites. Angew. Chem., Int. Ed. 2013, 52, 1494−1498. (10) Ovsyannikov, S. V.; Karkin, A. E.; Morozova, N. V.; Shchennikov, V. V.; Bykova, E.; Abakumov, A. M.; Tsirlin, A. A.; Glazyrin, K. V.; Dubrovinsky, L. A Hard Oxide Semiconductor with A Direct and Narrow Bandgap and Switchable p−n Electrical Conduction. Adv. Mater. 2014, 26, 8185−8191. (11) Zhao, Q.; Liu, M.; Dai, J. H.; Yin, Y. Y.; Zhou, L.; Yang, J. Y.; Hu, Z. W.; Agrestini, S.; Chen, K.; Pellegrin, E.; Valvidares, M.; Nataf, L.; Baudelet, F.; Tjeng, L. H.; Yang, Y. F.; Jin, C. Q.; Long, Y. W. HighPressure Synthesis and Ferrimagnetic Ordering of the B-Site-Ordered Cubic Perovskite Pb2FeOsO6. Inorg. Chem. 2016, 55, 9816−9821. (12) Long, Y. W.; Hayashi, N.; Saito, T.; Azuma, M.; Muranaka, S.; Shimakawa, Y. Temperature-Induced A−B Intersite ChargeTransfer in an A-site-ordered LaCu3Fe4O12 Perovskite. Nature 2009, 458, 60−63. (13) Prodi, A.; Gilioli, E.; Gauzzi, A.; Licci, F.; Marezio, M.; Bolzoni, F.; Huang, Q.; Santoro, A.; Lynn, J. W. Charge, Orbital and Spin Ordering Phenomena in the Mixed Valence Manganite (NaMn3+3) (Mn3+2Mn4+2)O12. Nat. Mater. 2004, 3, 48−52. (14) Leinenweber, K.; Linton, J.; Navrotsky, A.; Fei, Y.; Parise, J. B. High-Pressure Perovskites on the Join CaTiO3-FeTiO3. Phys. Chem. Miner. 1995, 22, 251−258. (15) Kadyrova, N. I.; Zainulin, Yu. G.; Tyutyunnik, A. P.; Mel’nikova, N. V.; Ustinova, I. S. High-Pressure/High-Temperature Synthesis, Crystal Structure, and Electrical Properties of CaCu3−xFexV4O12. Inorg. Mater. 2011, 47, 1396−1401. (16) Avdeev, M.; Nalbandyan, V. B. New Cubic Perovskite Na(Cu2.5Ti0.5)Ti4O12 with Square Planar Coordination of Ti4+. Inorg. Chem. 2006, 45, 2217−2220. (17) Li, J.; Subramanian, M. A.; Rosenfeld, H. D.; Jones, C. Y.; Toby, B. H.; Sleight, A. W. Clues to the Giant Dielectric Constant of CaCu3Ti4O12 in the Defect Structure of “SrCu3Ti4O12. Chem. Mater. 2004, 16, 5223−5225.

In this work, we have investigated the structural, vibrational, magnetic, and electronic properties of CaCo3V4O12 double perovskite at the extreme conditions of high pressures and low temperatures. We have proposed that this perovskite, when compressed to about 30 GPa, is subjected to an unusual phase transition, at which the large HS-Co2+ ions do not turn into the LS-Co2+ ones with the smaller radius but are pushed out of the oxygen planes. We found no apparent signatures of metallization of this perovskite up to 60 GPa. From the highpressure thermoelectric power measurements its electrical conduction may be described as semimetallic and strongly compensated. By means of ambient-pressure neutron powder diffraction, we established that below 100 K CaCo3V4O12 transforms into an antiferromagnetic phase in which all the magnetic moments of the Co2+ ions are aligned along the c-axis, and the magnetic structure has a 2-fold periodicity along this axis, compared to the crystallographic one.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00330. Details of the crystal structure determination at different pressures (PDF) Accession Codes

CCDC 1545727−1545730 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], sergey2503@ gmail.com. (S. Ovsyannikov) ORCID

Sergey V. Ovsyannikov: 0000-0003-1027-0998 Funding

This work was partly supported by the Russian Foundation for Basic Research (project no. 15-03-00868-a) and by Deutsche Forschungsgemeinschaft (DFG, project no. OV-110/1-3). AAT was funded by the Federal Ministry for Education and Research through the Sofja Kovalevskaya Award of the Alexander von Humboldt Foundation. The thermopower measurements were carried out within the state assignment of FASO of Russia (theme “Electron” No. 01201463326). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. M. Hanfland and Dr. M. Mezouar (ESRF, Grenoble, France) for the assistance in the collection of the X-ray diffraction data and to D. Vasiukov (BGI) for the assistance in the collection of the XANES spectra. 6261


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Inorganic Chemistry

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