Half-Metallicity in Pb2CoReO6 Double Perovskite ... - ACS Publications

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May 27, 2015 - unusual magnetic structures.6,7 Moreover, theory predicted some A2MReO6 to be ... The Cr−K, Co−K, and Re−L3 X-ray absorption near edge.
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Half-Metallicity in Pb2CoReO6 Double Perovskite and High Magnetic Ordering Temperature in Pb2CrReO6 Perovskite Maria Retuerto,†,‡ Man-Rong Li,† Peter W. Stephens,§ Javier Sánchez-Benítez,⊥ Xiaoyu Deng,¶ Gabriel Kotliar,¶ Mark C. Croft,¶ Alexander Ignatov,¶ David Walker,∥ and Martha Greenblatt*,† †

Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States ‡ Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark § Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794, United States ⊥ Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain ¶ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, New Jersey 08854, United States ∥ Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9 West, PO Box 1000, Palisades, New York 10964, United States S Supporting Information *

ABSTRACT: Pb2MReO6 (M = Co, Cr) perovskites were prepared by high pressure−high temperature method. Rietveld refinements of synchrotron powder X-ray diffraction show that the crystal structure of Pb2CoReO6 is trigonal (space group R-3) with almost complete ordering of Co and Re cations, while Pb2CrReO6 (space group Pm-3m) is a cubic perovskite with one single site for Cr and Re atoms. The difference between the symmetry and the degree of order was further clarified by X-ray absorption spectroscopy that establishes formal oxidation states in these phases as Pb2Co2+Re6+O6 and Pb2Cr3+Re5+O6. Pb2CrReO6 is a simple perovskite with a high magnetic ordering temperature of 643 K. Pb2CoReO6 is a double perovskite with −23% high field negative magnetoresistance at 10 K and 9 T. First-principles calculations of Pb2CoReO6 indicate a half metallic electronic structure.



INTRODUCTION Perovskites are very well studied materials due to the wide variety of promising physical properties they can present including superconductivity (Ba1−xKxBiO3),1 multiferroicity (BiFeO3),2 and magnetoresistance (La1−xCaxMnO3).3 In particular, A2BB′O6 double perovskites with two different cations in the B sublattice (A = divalent cations or rare earths; B, B′ = transition metals) have attracted attention since the discovery of room temperature (RT) colossal magnetoresistance (MR) in Sr2FeMoO6.4 Their crystal structure can be represented as corner-sharing BO6 and B′O6 octahedra alternating along the three crystallographic directions of the structure. The larger A cations occupy the cuboctahedral vacancies between the octahedra. Depending on the relative © 2015 American Chemical Society

size between A and B/B′ cations, the crystal structure can adopt different symmetries, and depending on the difference in size and charge between B and B′ cations, they can also present different degrees of order.5 In this work, we extended the family of Re-based perovskites to Pb2CoReO6 and Pb2CrReO6 prepared by high pressure and temperature synthesis. We have determined their crystal structures and characterized their physical properties by experiments and first-principles calculations. Received: April 20, 2015 Revised: May 27, 2015 Published: May 27, 2015 4450

DOI: 10.1021/acs.chemmater.5b01442 Chem. Mater. 2015, 27, 4450−4458

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Chemistry of Materials

approximation21 of exchange-correlation functions were adopted. A 10 × 10 × 4 k-point mesh was used to sample the Brillouin zone. The experimentally determined crystal structure was used in the calculations, and the small mixing between B and B′ site was neglected for simplicity. To explore the magnetic properties, we converged our calculations from a nonmagnetic state (NM) without spin-polarization and a ferromagnetic state with ferromagnetic (FM) coupling between all Co and Re sites. We have also studied the antiferromagnetic (AFM) configuration to understand the possible magnetic structure in the ground state as discussed below.

A2MReO6 (A= Sr and Ca, M = Fe and Cr) rhenium-based transition metal perovskites were shown to have interesting magnetic properties including high magnetic ordering temperatures (TC), high coercive magnetic fields (hard magnets), and unusual magnetic structures.6,7 Moreover, theory predicted some A2MReO6 to be half-metallic, hence useful for spintronic applications.8 However, both calculations and experiments on A2FeReO6 and A2CrReO6 (A = Sr and Ca) indicate that the half metallic behavior is strongly influenced by spin−orbit coupling associated with anisotropy of the magnetic properties.9−11 A2CrReO6 double perovskites have some of the highest magnetic ordering temperatures among the perovskitelike oxides, for example, Sr2CrReO6 with T = 635 K.12 In addition to the above-discussed fundamental and potentially useful properties of Re-based perovskites, Pb2+ containing perovskites are of interest for their possible multiferroic character. Pb2+ with 6s2 lone electron pair could yield displacement of Pb2+ cations from the center of the oxygen polyhedron, splitting into disordered positions and creating a spontaneous polarization. Some examples of lead double perovskites where Pb breaks the symmetry of its position are Pb2CoWO6, Pb2TmSbO6, and Pb2ScSbO6.13−15





RESULTS AND DISCUSSION Crystal Structure. Well-crystallized Pb2MReO6 (M = Co, Cr) samples were first characterized by PXRD with a perovskite-type structure. To perform accurate crystal structure refinements of such small samples, we collected SXRD data (Figure 1). Pb2CoReO6 shows superstructure peaks corre-

EXPERIMENTAL SECTION

The target Pb2MReO6 (M = Co, Cr) phases were prepared from a stoichiometric mixture of PbO, Cr2O3, or CoO, and Re, ReO3 reagent grade Sigma-Aldrich starting materials. In the case of Co compound, we use ReO3 to have Co2+/Re6+ configuration, but in the case of Cr compound, since we use Cr2O3 (Cr3+) as starting material, then we need both Re and ReO3 for charge balance. The finely ground mixture was inserted into a Pt capsule, which was placed inside a MgO crucible surrounded by a LaCrO3 heater in a Walker-type multianvil press.16 The mixture was reacted at 1423 K (for Co sample) and 1473 K (for Cr sample), both under 8 GPa for 30 min, and then quenched to RT by turning off the voltage supply to the resistance furnace, which reduced the temperature of the furnace to RT in a few seconds. The pressure was maintained during the temperature quenching, and then it was decompressed slowly. This synthesis yielded ∼20 mg of bulk product. Powder X-ray diffraction (PXRD) was used for phase identification and determination of purity with a Bruker-AXS D8 diffractometer (40 kV, 30 mA), controlled by a DIFFRACTplus software, in Bragg− Brentano reflection geometry with Cu radiation (λ = 1.5418 Å). The preferred orientation and small sample amount limited accurate crystal structure refinements by laboratory PXRD, thus high-resolution synchrotron X-ray diffraction (SXRD) data were collected on beamline X-16C (λ = 0.70019 Å) at the Brookhaven National Synchrotron Light Source (NSLS). Diffraction data analysis and Rietveld refinements were performed with the TOPAS package and Fullprof software.17,18 The Cr−K, Co−K, and Re−L3 X-ray absorption near edge spectroscopy (XANES) data were obtained on beamline X-19A at NSLS with a Si-111 double crystal monochromator. The data were collected in both the transmission and fluorescence mode with simultaneous standards. The spectra were fit to linear pre- and postedge backgrounds and normalized to unity absorption edge step across the edge. The magnetic measurements were carried out in a commercial superconducting quantum interference device magnetometer (SQUID). The dc magnetic susceptibility data were collected in the 5 ≤ T ≤ 700 K range under an applied magnetic field of 1000 Oe. Isothermal magnetization curves were obtained for magnetic fields: −5T < H < 5T at T = 5 and 300 K. Magnetotransport measurements were carried out in a Physical Properties Measurement System (PPMS) from 5−400 K and up to 9 T. The electronic structure of Pb2CoReO6 was studied by firstprinciples calculations based on density functional theory (DFT as implemented in VASP code).19 The projector-augmented wave potential20 and the Perdew−Burke−Ernzerhof generalized gradient

Figure 1. Rietveld fit from SXRD data at RT for (a) Pb2CoReO6 with ReO2 as an impurity second phase and (b) Pb2CrReO6 with ReO2 as a second phase. Observed (circles), calculated (full line), and difference (bottom) profiles.

sponding to long-range Co/Re ordering over two different B and B′ positions. Also, the splitting of some reflections indicates that the symmetry is lower than cubic perovskite. The SXRD pattern was successfully indexed assuming a trigonal unit cell with a = √2ao and c = 2√3ao (ao is the unit cell parameter of the primitive perovskite). The R-3 space group was consistent with the observed and calculated SXRD patterns, as shown in Figure 1, panel a. Pb is in the A positions located at 6c (0 0 z), Co at 3b (0 0 1/2), and Re at 3a (0 0 0) in B and B′, and there is only one position for O atoms at 18f (x y z).22 The refined antisite disorder between Co and Re gave 4451

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Table 1. Atomic Parameters and Agreement Factors after the Rietveld Refinement of SXRD Data for Pb2CoReO6 and Pb2CrReO6 at RT Pb2CoReO6 R-3 a/Å b/Å c/Å V/Å3 Pb 6c (0 0 z) z B/Å2 Co 3b (0 0 1/2) Occ Co/Re B/Å2 Re 3a (0 0 0) Occ Re/Co B/Å2 O 18f (x y z) x y z B/Å2 Rietveld Rp (%) Rietveld Rwp (%) Rietveld χ2 Pawley Rwp (%) Pawley χ2

Pb2CrReO6 Pm-3m a/Å b/Å c/Å V/Å3 Pb 1b (1/2 1/2 1/2) B/Å2

5.64159(3) 5.64159(3) 13.74790(9) 378.939(5) 0.2496(1) 2.13(3)

3.93936(1) 3.93936(1) 3.93936(1) 61.133(1) 2.76(4)

0.947(2)/0.053(2) 0.25(3)

Cr/Re1a (000) Occ B/Å2

0.495(3)/0.505(3) 0.24(4)

0.947(2)/0.053(2) 0.25(3)

O 3d (1/2 0 0) B/Å2

0.09(9)

0.193(2) −0.113(2) 0.0807(6) 0.3(3) 5.80 7.60 12.53 7.19 11.33

Rp (%) Rwp (%) Rietveld χ2 Pawley Rwp (%) Pawley χ2

5.60 7.74 17.89 7.51 16.85

Table 2. Main Interatomic Distances (Å), Angles (O), and BVS for Pb2CoReO6 and Pb2CrReO6 at RT Pb2CoReO6 PbO12 Pb−O × 3 Pb−O × 3 Pb−O × 3 Pb−O × 3 CoO6 Oh Co−O ReO6 Oh Re−O Angle Co−O1−Re BVS Pb Co Re

Pb2CrReO6 2.599(9) 2.770(11) 2.876(12) 3.049(9) 2.824(3) 2.135(14) 1.874(14) 166.7(6)

PbO12 Pb−O

2.786(8)

Cr/ReO6 Oh Cr/Re−O

1.970(8)

Angle Cr/Re−O1−Cr/Re BVS Pb Cr/Re

2.072(3) 1.812(7) 5.777(3)

180 2.237(3) 3.777(8)

make the same contribution to both, so if Rwp only slightly increases above the Pawley value, then the crystallographic model is considered correct. This was the case here, with Pawley Rwp = 7.19% and the refinement Rwp = 7.60%. Table 1 lists the lattice parameters, volume, atomic coordinates, and isotropic atomic displacement parameters as well as the reliability factors; and Table 2 includes the mean interatomic distances and selected bond angles at RT for Pb2CoReO6. The structure of Pb2CoReO6 is relatively distorted with the BO6 octahedra tilted to optimize the bond distances. For this compound, PbO12 polyhedron has four different Pb−O distances ( = 2.824(3) Å), similar to the one expected from the ionic radii (i.r.) sum of 2.89 Å for XIIPb2+ (i.r.: 1.49 Å) and VIO2− (i.r.: 1.40 Å).26 Co−O = 2.135(14) Å and Re−O = 1.874(14) Å observed distances are also comparable but somewhat smaller than the calculated values from the ionic

5.3(2)% of Re atoms in Co sites and vice versa. Co and Re occupancies are constrained to maintain the charge neutrality, and we assume no vacancies at these atom sites. The refinement of oxygen site occupancy factors did not reveal any deviation from full occupancy within the standard deviation. The cell parameters obtained after the refinement are a = 5.6416(2) and c = 13.7479(4) Å. Similar Pb perovskites, Pb2CoTeO6, Pb2MgTeO6, and Pb2TmSbO6, also present R-3 space group.23,24,14 The occurrence of some very small SXRD peaks indicates the presence of impurities; however, we could identify only the high-temperature phase of ReO2 (orthorhombic Pbcn: a = 4.804 Å, b = 5.64 Å, c = 4.603 Å).25 The total amount of impurities by weight is ∼4%. In this case, where all the small impurities could not be identified, Rwp from the refinement was compared with Rwp from the Pawley fit, since the impurities 4452

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Chemistry of Materials radii for six coordination of 2.245 Å (for high-spin Co2+−O) and 1.95 Å (Re6+−O). The bond distances are similar to other analogous double perovskites, for example, Pb2CoTeO6 with the same space group has = 2.839 Å and Co−O = 2.094 Å,23 which are also smaller than Shannon’s i.r. sums; similarly, Co−O = 2.049 Å and Re−O = 1.92 Å in Sr2CoReO6, with a tetragonal structure.27 For Pb2CrReO6, a cubic model is a good agreement between the observed and calculated SXRD patterns. In this case, the absence of ordering superstructure peaks indicates that Cr and Re are completely disordered and in the same position. The difference between the two structures is related to the oxidation state of Co/Re and Cr/Re, determined as Pb2Co2+Re6+O6 and Pb2Cr3+Re5+O6 (see XANES section). The valences obtained could be related to the starting materials used, which are CoO and ReO3 and Re metal for Pb2CoReO6 and Cr2O3 and ReO3 for Pb2CrReO6, and also to the stability of the different oxidation states under pressure. Usually, the order between B cations requires a charge difference ≥ 3, so only Co sample shows B-cation ordering.5 The Pb2CrReO6 structure could be refined with the simple perovskite model, Pm-3m space group, where there is a single B position for Cr and Re atoms. Pm-3m shows no tilting of the octahedra, and the structural model is CaTiO3. The occupancy between Cr and Re is 0.495(3)/ 0.505(3); these very small deviations from stoichiometry could be related to a small deficiency of oxygen not detected by X-ray. The Pb ions are in 1b (1/2 1/2 1/2), Cr/Re are in 1a (0 0 0), and O in 3d (1/2 0 0) positions. Figure 1, panel b shows the final Rietveld plot for Pb2CrReO6. We find ∼2.4% of orthorhombic ReO2 as a small impurity again. The difference between the Pawley fit (Rwp = 7.51%) and the refinement (Rwp = 7.74%) is again small, which indicates that the cubic model is correct. Tables 1 and 2 also show the refined parameters of Pb2CrReO6. Pb−O = 2.786(8) Å and Cr/Re−O = 1.970(8) Å are in both cases slightly smaller than the ones expected from the ionic radii sum, taking into account the mean distance, 1.997 Å, between both Cr3+−O (2.015 Å) and Re5+−O (1.98 Å). The structure is the same as that of the cubic perovskite PbCrO3 with similar Pb−O = 2.83 Å.28 However, PbCrO3 presents a complex microstructure with a compositional modulation that creates a superstructure, and also Cr in this compound is Cr4+; hence, the bond distance Cr4+−O (2.002 Å) in this material is not comparable to that in Pb2CrReO6. Sr2CrReO6 has Cr−O = 1.956 Å and Re−O = 1.951 Å at 5 K, with a partially ordered structure between Cr and Re and Fm3m cubic space group.12 There are other Cr3+ double perovskites with partial B and B′ cationic ordering, such as Sr2CrTaO6 (Cr3+−O = 1.967 Å and Ta5+−O = 1.975 Å)29 and Sr2CrSbO6 (Cr3+−O = 1.973 Å and Sb5+−O= 1.977 Å),30 but in these cases, B−O and B′−O distances are very similar, and the order could be related to the different covalency of the bonds. In Pb2CrReO6, the size and charge differences between Cr3+ and Re5+ are not sufficient to order them (see next section for the study of the oxidation states), very similar to the case of Sr2CrMoO6.31 This compound has been reported in Fm-3m with two positions for Cr and Mo, distances Cr3+−O = 1.984 Å and Mo5+−O = 1.936 Å and B/B′-site order of only 65%, when 50% is complete disorder. The phenomenological Brown’s bond-valence model (BVS)32 also helps to give an estimation of the valences of the cations by means of an empirical relationship between the observed bond lengths and the valence of a bond. We used two different references to calculate

the bond valence parameters.33,34 Table 2 shows the values of the bond valences of Pb, Co, Re, and Cr/Re that are close to the expected values. In both Pb2CoReO6 and Pb2CrReO6 refinements, Pb2+ has a larger displacement factor than the other atoms. This is usual for Pb2+ 6s2 lone pair cations in the A site of the perovskites, and it was observed in many other phases due to disorder of the Pb2+ over the A site position, as reported for Pb2YSbO6 Pb2MnWO6 and Pb2MnReO6.35−37 X-ray Absorption near Edge Spectroscopy. The signatures of the 3d transition metal valence state variations can be discerned in both the main- and pre-edge regions of their K-edge X-ray absorption spectra. The main-edge features are dominated by 1s to 4p transitions, riding on a step-feature continuum onset. Despite substantial variations/energy-splitting in the main edge features, the chemical shift (to higher energy with increasing valence) of the main edge has been widely used to chronicle the evolution of the transition metal valence state in oxide-based materials. The pre-edge features are due to quadrupole allowed 1s/3d or dipole allowed 1s/3d-p-hybridized transitions. It is the Coulomb interaction between core 1s-hole and d-electron states that shifts these transitions down into the pre-edge region. The structure and energy shift of the pre-edge features can offer a second method of identifying the valence. In Figure 2, panel a, the Co−K main edge for Pb2CoReO6 is compared to those for a series of formally Co2+, Co3+, and ∼Co4+ standard compounds. The low energy of the chemical shift of Pb2CoReO6 spectrum places it clearly in the Co2+ range. The Co−K pre-edge parts of the same spectra are shown in the lower part of Figure 2, panel a. Several points regarding the Pb2CoReO6 pre-edge spectrum reinforce the Co2+ identification. Specifically, the single peak, low intensity, and low chemical shift of the Pb2CoReO6 pre-edge set it apart from the higher valence spectra and in line with the Co2+ standards.38 In Figure 2, panel b, the Cr−K main edge for Pb2CrReO6 is compared to those for a series of formally Cr3+, Cr4+, and Cr6+ standard compounds.39 The energy of the chemical shift of the Pb2CrReO6 spectrum places it clearly in the Cr3+ range. The Cr−K pre-edge portions of the same spectra are shown in the lower part of Figure 2, panel b. The three a1-a2-a3 features of the Pb2CrReO6 pre-edge are characteristic of Cr3+. The L3 edges of Re, and other 4d/5d transition metals, manifest intense “white line” (WL) features due to dipole allowed transitions into unoccupied final d states. The Re7+-5d0 compound standard spectra (Sr4Fe3ReO12)40 in Figure 3 nicely illustrates the octahedral-ligand-field splitting of the WL feature into a lower (higher) energy t2g(eg) 5d-hole state features. Both the relative t2g versus eg feature intensities and the chemical shift of the centrum of the WL feature are useful in chronicling the 5d-configuration/valence changes in perovskite oxide compounds.41,42 In Figure 3 is observed a systematic downward sequence of chemical shift from Sr4Fe3ReO12 (Re7+), to Pb2CoReO6, to Pb2CrReO6. Moreover, the t2g feature is also much reduced in Pb 2 MReO 6 compounds relative to Sr4Fe3ReO12. In the lower part of Figure 3, Pb2MReO6 (M = Co, Cr) spectra are displaced in energy so that their eg features overlap; the t2g feature of M = Co is clearly enhanced relative to that of M = Cr. Thus, both the chemical shift and the t2g feature intensities at the Re-L3 edges support the assignment of Re6+ in M = Co and Re5+ in M = Cr, consistent with the Co2+ (Pb2Co2+Re6+O6) and Cr3+ (Pb2Cr3+Re5+O6) states determined from the K-edges. This result is in agreement with the 4453

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Figure 4. (a) Thermal evolution of the magnetic susceptibility and the reciprocal susceptibility (inset) measured with H = 1000 Oe of Pb2CoReO6. (b) Isothermal magnetization curves at 5 and 300 K of Pb2CoReO6. (c) Thermal evolution of the magnetic susceptibility measured with H = 1000 Oe of Pb2CrReO6. (d) Isothermal magnetization curves at 5 and 300 K of Pb2CrReO6.

indicated by the derivative in the figure. Below this temperature, the zero field-cooling (ZFC) and the field cooling (FC) curves diverge, which indicate spin frustration or competition between different magnetic interactions. At higher temperatures, the reciprocal susceptibility (inset of Figure 4a) has two clearly different linear temperature ranges, above and below 180 K. A Curie−Weiss fit of the inverse of the susceptibility for the higher paramagnetic interval (180−300 K) yields a paramagnetic temperature of θP = −50.3(3) K, which indicates that the predominant magnetic interactions are AFM and an effective paramagnetic moment of μeff = 5.09(1) μB/f.u. However, the Curie−Weiss fit for the lower range, 25−170 K, results in an effective magnetic moment of 4.53(2) μB/f.u. and a positive paramagnetic θP of 10.8(4) K, probably indicating that there are also FM interactions in the system. For the electronic configuration of high-spin Co2+(t2g5eg2), S = 3/2, and Re6+(t2g1), S = 1/2, the theoretical effective magnetic moment is 4.24 μB/f.u. The obtained larger effective moment could be explained with the unquenched spin−orbit coupling of highspin Co2+ configuration with a 4T1g ground state (expected magnetic moment of 5.20 μB/f.u.). This effect has been proposed for other double perovskites including Sr2CoWO6 and Sr2CoReO6.43,27 Alternatively, the difference in the observed and expected effective magnetic moments also may be attributed to the spin−orbit coupling of Re.9 In Figure 4, panel b, the evolution of the magnetization with the applied field at 5 K indicates a general AFM behavior but a weak hysteresis at low fields, which is indicative of the presence of some FM interactions. Thus, although the global behavior of the system is AFM, the sample clearly presents ferrimagnetic (FiM) or FM clusters, which order below 180 K. This scenario is also supported by the first principles calculations where AFM and FiM states are shown to be very close in energy (see the following section). Thus, the magnetism is frustrated and gives rise to a possible spin glass state at low temperature, in accordance with the divergence of ZFC and FC curves below TN (Figure 4a), which is consistent the competition between

Figure 2. (a) Co−K main edge spectra of Pb2CoReO6 along with a series of octahedral O-coordinated Co compounds with varying formal valence s: CoO and La 2 CoVO 6 (Co 2 + ); LaCoO 3 (Co 3 + ); SrCoO3‑δ(∼Co4+). Here, and subsequently, the formal valence is used ignoring more subtle hybridization/covalency effects. Inset: Co− K pre-edge spectra for the same compounds. Note: the pre-edge spectra are displaced vertically for clarity. (b) Cr−K main-edge spectra for Pb2CrReO6 along with those for LaCrO3(Cr3+), CrO2(Cr4+), and K2Co2O7(Cr6+). Here, the Cr−O coordination is octahedral except for the Cr6+ standard where it is tetrahedral (which induces the intense hybridization of the pre-edge feature). Inset: Cr−K pre-edge spectra for the same compounds.

Figure 3. Comparison of the Re-L3 edges of Pb2MReO6, M= Co and Cr, along with the Re7+ Sr4Fe3ReO12 standard.

observation that charge ordering in double perovskites usually requires a charge difference ≥ 3 between the different B cations.5 Magnetic Properties. The thermal evolution of the susceptibility for Pb2CoReO6 is shown in Figure 4, panel a. The susceptibility exhibits an enhancement when the temperature decreases, and it starts saturating around 16 K, as 4454

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Chemistry of Materials AFM and FiM interactions. At 300 K, the compound is already in the paramagnetic state (Figure 4b). The dc susceptibility versus temperature data of Pb2CrReO6 are shown in Figure 4, panel c. The magnetic susceptibility shows a maximum around 220 K with a peak characteristic of an AFM transition, and then it decreases slowly with increasing temperature until 643 K, where another magnetic transition is indicated. Such high magnetic ordering temperature has been previously observed in similar Cr−Re perovskites; for example, Sr2CrReO6 has a similar TC of ∼635 K and Ca2CrReO6 a TC of ∼360 K.12 Pb2CrReO6 is a disordered perovskite where Cr− O−Cr and superexchange Cr−O−Re−O−Cr interactions compete, and therefore, a lower TC or two different magnetic transitions could be expected. For example, LaCrO3 orders AFM with TN = 288 K;44 thus in Pb2CrReO6, the peak observed around 220 K could be related to the magnetism of the disordered regions, rich in Cr, and the transition at 643 K can be ascribed to the ordered regions similar to the double perovskites with high TC values. The high TC values found in Cr-based perovskites are controlled by the electronic bandwidth, associated with the orbital hybridization, which is determined by the structure. Usually, Cr3+ perovskites have shorter M−O distances and are less distorted than perovskites with other 3d transition metal cations because the size of Cr3+ is more similar to the four- and five-row ions (Re, Mo, and W in high oxidation states). A2CrMO6 tends to form cubic structures with large overlap between O-2p and metal-3d orbitals, which results in broad bandwidth, large metal−oxygen hopping of electrons, and effective reinforcement of the magnetic interactions. The magnetic transitions observed in Pb2CrReO6 cannot be attributed to orthorhombic ReO2, which displays Pauli paramagnetism.45 The susceptibility data could not be fit to the Curie−Weiss law since the measured paramagnetic region in this compound is too small. The isothermal magnetization curve of Pb2CrReO6 in Figure 4, panel d is characteristic of an AFM behavior with a small curvature in the low field region, which indicates the presence of negligibly weak FM/FiM interactions. Transport and Magnetotransport Properties. The thermal evolution of the resistivity for both Pb2CoReO6 and Pb2CrReO6 at H = 0 T, shown in Figure 5, panel a, is characteristic of semiconducting behavior over the whole measured temperature range. At RT, the resistivity values are 295 and 80 Ω cm for M = Co and Cr, respectively. Although these values at RT are of the same order of magnitude, when the temperature decreases below 100 K, the resistivity of Co sample is much higher than that of Cr sample and becomes too high to be measured below 10 K. There is no feature found around 180 K in the resistivity plot of Pb2CoReO6 related to the slope change of its inverse of the susceptibility at that temperature (inset Figure 4a). The possible FM clusters do not affect the transport behavior at this temperature since the compound is mainly AFM. Neither of the samples follow a thermally activated mechanism, but the M = Co sample follows a Mott’s Variable Range Hopping (VRH) conduction mechanism, where ρ = ρo exp[(T0/T)1/4], for temperatures below 100 K, as shown in the inset of Figure 5, panel a. The fitting allowed us to extract the parameters T0 and ρ0 as 2.04 × 106 K and 4.04 × 10−3 Ω cm, respectively, of similar magnitude as observed in other transition metal oxides.46,47 The VRH behavior is typically found when conduction takes place by hopping between localized states.

Figure 5. (a) Resistivity versus temperature plot at zero field showing semiconductor behavior for both Pb2MReO6 samples. The inset shows the linear fit for M = Co to a VRH conduction mechanism. (b) Magnetoresistance isotherms for Pb2MReO6 at 10 K.

Concerning the changes in the resistivity under a magnetic field, magnetoresistance can be defined as MR(H) = 100 × [R(H) − R(0)/R(0)]. Figure 5, panel b displays the MR isotherms for Pb2MReO6 at 10 K and a magnetic field of H = −9 to 9 T. The changes in resistivity for the M = Cr sample with magnetic field are almost negligible even at the maximum applied field (9 T). By contrast, Pb2CoReO6 shows negative MR increasing (in absolute value) with the field, reaching a maximum value of −23% at 10 K and 9 T. There is no low-field MR, defined for magnetic fields lower than 1 T, usually found in similar oxides. This high field negative MR could be related to the presence of Co−O−Re−O−Co ferrimagnetic clusters (as described above) similar to those mentioned for Sr2CoMoO6−δ,48 which would produce half metallicity and therefore negative MR. The half metallicity would be related to some degree of electronic or holes transfer between Co2+ and Re6+. The low-field MR, responsible for the intergrain MR effect, is negligible, although usually it is the larger effect in half metals. We believe that the absence of intergrain MR could be related to the need of very high applied magnetic fields to flip enough of the spins into the ferrimagnetic state, align the magnetic moments, favor the transport, and progressively enhance the magnetoresistance. The absence of MR in Pb2CrReO6 could be ascribed to the total disorder between Cr and Re that hinders possible half metallic character, as observed for many other double perovskites, and also the absence of charge transfer between Cr and Re.49 First-Principle Calculations. Our first-principle calculations confirm that Pb2CoReO6 energetically favors a spinpolarized state since the converged NM state has a total energy around 1 eV/f.u. higher than the spin-polarized FM states. Interestingly, the converged magnetic structure of the FM state 4455

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Chemistry of Materials

scope of this work. Our calculations suggest that the realistic ground state of Pb2CoReO6 is likely dominated by the intensive competition between the FiM and the AFM states. The competition may lead to strong frustration of the magnetic moments, which are responsible for the observed low Neel temperature. The experimental magnetization studies discussed earlier support this scenario: at low temperature, the magnetization has a large value (Figure 4b), which is unlikely in a pure AFM ground state; however, it is still smaller than that predicted by the FiM state. Also the calculations support the finding of MR at low temperature by the appearance of halfmetallic FiM interactions in certain regions of the compound.

is actually FiM, in which the Co sites are ferromagnetically coupled to each other and couple antiferromagnetically to the Re site moments (Figure 6a). The Co atoms contribute most of



CONCLUSIONS We have prepared, by a high-pressure and high-temperature method, Pb2CoReO6 and Pb2CrReO6 perovskites. Pb2CrReO6 is a simple perovskite with Cr3+ and Re5+ completely disordered but with a high magnetic ordering temperature of 643 K. Pb2CoReO6 has a close to complete ordering of the B cations, with Co2+ and Re6+ oxidation states, antiferromagnetic ordering with a TN of 16 K, and a high-field magnetoresistance of −23% at 10 K and 9 T. First-principles calculations suggest that the realistic ground state of Pb2CoReO6 is dominated by the competition of different Co−O−Re−O−Co interactions: an antiferromagnetic state between next Co near-neighbors and a ferrimagnetic state between Co and Re moments. This competition can lead to a strong frustration of the magnetic interactions responsible for the observed low Neel temperature, but the ferrimagnetic state could produce negative magnetoresistance due to a half metallic state with a negligible gap in the Fermi level only for the down-spin band, observed from the calculated DOS.

Figure 6. (a) Magnetic structure of the FiM state of Pb2CoReO6. (b) Calculated DOS per formula of Pb2CoReO6 in the FiM state. The positive and negative parts are for the spin-up (red solid line) and -down (blue solid line) components of DOS, respectively. The spin-up component has an energy gap around 1.0 eV, while the spin-down component has a dip near the Fermi level, which does not form a gap. Inset shows the zoom-in of the spin-down component near the Fermi level. The Fermi level is denoted by the dashed line.



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AUTHOR INFORMATION

* Supporting Information S

Crystallographic information files (CIF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b01442.

the magnetization with a magnetic moment around 2.3 μB per site, which is slightly smaller than 3.0 μB, expected from the (t2g5eg2) octahedral configurations, and the Re site has a small magnetic moment of around 0.4 μB. The total magnetic moment per formula is as large as 2.0 μB. This FiM state is half metallic, as shown by its density of states (DOS) depicted in Figure 6, panel b. The up-spin component of the DOS has a gap of about 1.0 eV, which is between the fully filled Co-d states and the Re-t2g states, and the spin-down component is a semimetal with a very small value near the Fermi level due to the hybridization of the Co-t2g states and Re-t2g states. In this case, there is an itinerant electron (or hole) that can jump from Co to Re, but only in the down-spin direction. This process is called half metallicity and produces a decrease of the resistivity when an magnetic field is applied.50 It is also possible that the Co−Co near neighbors (through Co−O−Re−O−Co paths) are AFM. In this case, the so-called type-II AFM structure (AFM-II) may occur on the facecentered sublattice formed by Co sites. In the AFM-II state, the Co sites are coupled FM in the ab plane and AFM along c-axis in the trigonal coordinates. Our calculation shows that the AFM-II state can be stabilized with a total energy slightly lower (∼60 meV/f.u.) than the FiM state. In this AFM-II state, the magnetic moment of Co site is almost the same as in the FiM state, while the moment on the Re site disappears. Note that we have only studied this AFM-II configuration to demonstrate the existence of strong AFM coupling in this material; however, a study of all other possible magnetic structures is beyond the

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the NSF-DMR-0966829 and DOD-VV911NF-12-1-0172 Grants. Use of the National Synchrotron Light Source, Brookhaven National Laboratory was supported by the DOE BES (DE-AC02-98CH10886). M.R. is supported by the Danish Research Councils for Independent Research, Grant No. 12-125226. G.K. and X.D. are supported by NSF-DMR-1435918. J.S.-B. is supported by the Spanish Projects MAT2013-41099-R and RyC-2010-06276.



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