Article pubs.acs.org/IC
High-Pressure Synthesis and Ferrimagnetic Ordering of the B‑SiteOrdered Cubic Perovskite Pb2FeOsO6 Qing Zhao,† Min Liu,† Jianhong Dai,† Hongshan Deng,† Yunyu Yin,† Long Zhou,† Junye Yang,† Zhiwei Hu,‡ Stefano Agrestini,‡ Kai Chen,§ Eric Pellegrin,⊥ Manuel Valvidares,⊥ Lucie Nataf,∥ François Baudelet,∥ L. H. Tjeng,‡ Yi-feng Yang,†,# Changqing Jin,†,# and Youwen Long*,†,# †
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ Max-Planck Institute for Chemical Physics of Solids, Nothnitzer Straße 40, 01187 Dresden, Germany § Institute of Physics II, University of Cologne, Zulpicher Straße 77, 50937 Cologne, Germany ⊥ ALBA Synchrotron Light Source, E-08290 Cerdanyola del Valles, Barcelona, Spain ∥ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France # Collaborative Innovation Center of Quantum Matter, Beijing 100190, China ABSTRACT: Pb2FeOsO6 was prepared for the first time by using high-pressure and high-temperature synthesis techniques. This compound crystallizes into a Bsite-ordered double-perovskite structure with cubic symmetry Fm3m ̅ , where the Fe and Os atoms are orderly distributed with a rock-salt-type manner. Structure refinement shows an Fe−Os antisite occupancy of about 16.6%. Structural analysis and X-ray absorption spectroscopy both demonstrate the charge combination to be Pb2Fe3+Os5+O6. A long-range ferrimagnetic transition is found to occur at about 280 K due to antiferromagnetic interactions between the adjacent Fe3+ and Os5+ spins with a straight (180°) Fe−O−Os bond angle, as confirmed by X-ray magnetic circular-dichroism measurements. First-principles theoretical calculations reveal the semiconducting behavior as well as the Fe3+(↑)Os5+(↓) antiferromagnetic coupling originating from the superexchange interactions between the half-filled 3d orbitals of Fe and t2g orbitals of Os.
1. INTRODUCTION
electrical interactions may occur via B−B, B′−B′, and/or B−B′ pathways, giving rise to many interesting physical properties such as ferroelectricity, colossal magnetoresistance, half-metallic behavior, and high-temperature spin ordering.2−6 A well-known example is Sr2FeMoO6, where a high ferromagnetic (FM) Curie temperature (TC = 420 K) and half-metallic transport properties are observed, giving the material high potential for use in spintronic devices above room temperature.7−9 This fascinating finding stimulates other investigations on B-siteordered perovskites with 3d−4d (5d)-hybridized B−B′ charge combinations. Subsequently, a ferrimagnetic (FiM) phase transition is found to occur in Sr2CrOsO6, with the highest TC recorded (725 K) in perovskite oxides discovered to date.4 Recently, the double-perovskite family of A2FeOsO6 (A = Ca, Ca0.5Sr0.5, and Sr) prepared under high pressure received special attention because of the exotic spin interactions. In this family, the B-site Fe3+ and B′-site Os5+ ions are ordered with a rock-salt-type manner. With decreasing A-site size, the crystal symmetry is reduced from tetragonal I4/m (A = Sr) to monoclinic P21/n (A = Ca0.5Sr0.5 and Ca) because of the increasing octahedral distortion, with the average Fe−O−Os
B-site-ordered double-perovskite oxides with the chemical formula A2BB′O6 have been studied widely in the past decade. Compared with the simple ABO3 perovskite, both the B and B′ sites in this ordered one accommodate transition metals, as shown in Figure 1.1 As a result, some peculiar magnetic and
Figure 1. Crystal structure of a B-site-ordered double perovskite with space group Fm3̅m. The B/B′-site transition metals form B/B′O6 octahedra. © XXXX American Chemical Society
Received: July 10, 2016
A
DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
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measurements were performed by using a standard four-probe method on a physical property measurement system (Quantum Design, PPMS-9T). First-principles calculations were carried out using a full-potential linearized augmented plane-wave method, as implemented in the WIEN2K package.19 The used Muffin-tin radii were 2.50 au for Pb, 2.00 au for Fe, 2.00 au for Os, and 1.60 au for O. The maximum modulus for the reciprocal vectors Kmax was chosen such that RMTKmax = 8.0. We took the generalized gradient approximation (GGA) exchange-correlation functional by the Perdew, Burke, and Ernzerhof exchange-correlation energy with an effective Ueff = 4 eV for Fe and 2 eV for Os in the GGA+U calculations.20−22 A total of 1000 k-point meshes in the Brillouin zone were used in the calculations. The lattice parameters and atom positions obtained in the experiment (see Table 1) were used for the calculations.
bond angle 165.3° for Sr, 158.9° for Sr0.5Ca0.5, and 150.9° for Ca.10−12 Because there exist lattice instability as well as multiple competing magnetic superexchange interactions in the tetragonal Sr2FeOsO6, two successive antiferromagnetic (AFM) phase transitions take place in this compound.13,14 In contrast, only a single FiM ordering arising from the AFM coupling between the Fe3+ and Os5+ spin sublattices is observed in the monoclinic Ca2FeOsO6 and CaSrFeOsO6 with the Curie temperature TC ≈ 320 and 210 K, respectively.14 Moreover, although the Fe−O−Os superexchange pathway in Ca2FeOsO6 is more bent than that in CaSrFeOsO6, the former possesses a higher spin-ordering temperature than the latter. These behaviors deviate from the Goodenough−Kanamori (GK) rules,15,16 which predict FM interactions between the half-filled eg orbitals of Fe3+ and empty eg orbitals of Os5+ electrons via Fe−O−Os superexchange interactions, with the band angle roughly >145°. Furthermore, according to the GK criteria, a straighter interaction pathway is expected to enhance the magnetic phase-transition temperature. However, this is not the case in Ca2FeOsO6 and CaSrFeOsO6. To further understand the complex magnetic properties of the A2FeOsO6 family, it is desirable to obtain a member of the cubic perovskite structure (i.e., no octahedral distortion and a bent Fe−O−Os interaction pathway). However, such a perovskite system has never been successfully prepared yet. We thus adopt a larger-size cation Pb to substitute for the A site, and a new B-site-ordered perovskite, Pb2FeOsO6 (PFOO), with cubic symmetry is obtained for the first time. To stabilize the cubic perovskite structure of PFOO with a large A-site cation, high pressure is necessary for sample synthesis. Similar to the monoclinic A = Ca and Sr0.5Ca0.5 members, the cubic PFOO also shows a single FiM phase transition at about 280 K.
Table 1. Refined Structural Parameters of PFOO and BVS Values for Fe and Os at Room Temperature.a,b
2. EXPERIMENTAL AND CALCULATION SECTION Polycrystalline PFOO was prepared by using a solid-state reaction method under high-pressure and high-temperature conditions implemented on a cubic-anvil-type high-pressure apparatus. The starting materials that we used were high-purity (>99.9%) PbO, PbO2, Fe2O3, and OsO2 powders. The powders with a stoichiometric mole ratio of 3:1:1:2 were thoroughly mixed in an agate mortar within an argon-filled glovebox and then sealed into a platinum capsule with 2.8 mm diameter and 4.0 mm length. The capsule was treated at 8 GPa and 1673 K for 30 min. When the heating process was finished, the pressure was slowly released to ambient pressure. The phase quality and crystal structure of the high-pressure product were characterized by powder X-ray diffraction (XRD) using a Huber diffractometer with Cu Kα1 radiation at 40 kV and 30 mA. The XRD pattern was collected in the angle (2θ) range from 10° to 100° with steps of 0.005° at room temperature. The data were analyzed by Rietveld refinement using the GSAS program.17 X-ray absorption (XAS) and X-ray magnetic circular-dichroism (XMCD) spectroscopies at the Fe-L2,3 edges were measured at the BL29 BOREAS beamline of ALBA synchrotron radiation facility in Barcelona using a total electron yield mode. The Os-L2,3 XAS and XMCD measurements were performed at the ODE beamline of SOLEIL in Paris using transmission mode. The Fe-L2,3 (Os-L2,3) XMCD signal was measured, with the sample kept at a temperature of 35 K (5 K) and in a magnetic field of 6 T (1.2 T). The Os-L3,2 edge-jump intensity ratio L3/L2 was normalized to 2.08.18 This takes into account the difference in the radial matrix elements of the 2p1/2-to-5d (L2) and 2p3/2-to-5d (L3) transitions. Magnetic susceptibility and magnetization were measured using a superconducting quantum interference device magnetometer (Quantum Design, MPMS-VSM). The magnetic susceptibility data were collected between 2 and 400 K at different magnetic fields. The field dependence of magnetization at different temperatures was measured under fields from −7 to +7 T. Electrical resistivity
parameter
PFOO
a (Å) Ox Uiso(Pb) (100 Å2) Uiso(Fe) (100 Å2) Uiso(Os) (100 Å2) Uiso(O) (100 Å2) occupancy(Pb) occupancy(Fe/Os) occupancy(Os/Fe) occupancy(O) Pb−O (×12) (Å) Fe−O (×6) (Å) Os−O (×6) (Å) BVS(Fe) BVS(Os) Rwp (%) Rp (%)
7.9187(2) 0.2518(6) 1.83(2) 1.21(5) 0.69(2) 2.5 1.02(2) 0.834(1)/0.166(1) 0.834(1)/0.166(1) 1.0 2.7997(3) 1.994(5) 1.965(5) 3.17 4.61 5.66 3.71
a
Space group: Fm3̅m. bThe BVS values (Vi) were calculated using the formula Vi = ∑jSij and Sij = exp[(r0 − rij)/b)]. The parameters of r0(Fe3+) = 1.759 Å, r0(Os5+) = 1.868 Å, and b = 0.37 were used in the BVS calculation. For the B-site Fe and B′-site Os, six coordinated O atoms were used.
3. RESULTS AND DISCUSSION Figure 2 shows the XRD pattern of PFOO measured at room temperature as well as the related structure refinement results. The diffraction peaks can be indexed on the basis of a cubic perovskite cell with lattice parameter a ≈ 2ap. Here ap indicates the lattice constant for a simple cubic ABO3 perovskite. The
Figure 2. XRD pattern and structure refinement results of PFOO. The observed (circles), calculated (line), and difference (bottom line) are shown. The ticks indicate the Bragg reflections allowed with space group Fm3̅m. A small amount of uncertain impurities (∼5 wt %) was excluded from the pattern. B
DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
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indicating a high-spin state for Fe3+. Having determined an Fe3+ state in PFOO, we turn to the Os-L3 XAS to further confirm the Os5+ state, as required for charge balance. The Os-L3 XAS of PFOO along with Sr2FeOsO6 and La2MgOsO6 is shown in Figure 3b. By comparison with the Os4+ reference La2MgOsO6, the energy position of PFOO apparently shifts toward higher energy and is almost located at the same position as that of the Os5+ reference Sr2FeOsO6. Therefore, like other A2FeOsO6 members, the present PFOO also possesses the Fe3+/Os5+ charge combination at the perovskite B/B′ sites. Figure 4a shows the temperature dependence of magnetic susceptibility measured at different fields using zero-field-
absence of a diffraction peak with h + k, k + l, and h + l = odd suggests face-centering of the unit cell.23 The Rietveld analysis demonstrates that PFOO crystallizes into a B-site-ordered perovskite structure with space group Fm3̅m. In this symmetry, all of the cations occupy fixed atomic positions at 8c (0.25, 0.25, 0.25) for Pb, 4a (0, 0, 0) for Fe, and 4b (0.5, 0.5, 0.5) for Os, and the O is also located at a special site 24e (x, 0, 0).24 This means that the only variable position parameter is Ox, which determines the Fe−O and Os−O distances. The sharp diffraction peaks with h + k + l = odd such as the (111) and (311) peaks reveal that the B-site Fe and B′-site Os are orderly distributed in a rock-salt-type manner, as observed in other A2FeOsO6 family members. The refined structural parameters are listed in Table 1. The lattice constant of PFOO (7.9187 Å) is slightly larger than the c axis (7.8867 Å) of the tetragonal Sr2FeOsO6 because of the introduction of larger-size Pb ions at the A site.10 Because X-ray is enough sensitive to distinguish the transition metals Fe and Os, the degree of order for these two cations was characterized by refining the occupancy parameters. We find that there exists an Fe−Os antisite occupancy of about 16.6%, while the occupancy for Pb is almost unified. Note that the Fe−Os order degree of PFOO is similar to that observed in Ca2FeOsO6 and Sr2FeOsO6.11,12,25 On the basis of the refined Fe−O and Os−O distances, bond-valence-sum (BVS) calculations give the valence state as 3.17+ for Fe and 4.61+ for Os, suggesting an Fe3+/Os5+ charge combination. Because XAS at the transition-metal L2,3 edges is element-specific and highly sensitive to the valence state (an increase of the valence state of the metal ion causes a shift of the XAS L2,3 edges toward higher energies),26−28 this technique was used to confirm the valence states of Fe and Os. Figure 3a
Figure 4. (a) Temperature dependence of magnetic susceptibility (χ) measured at 0.1 and 2 T. The solid line in the inset shows the Curie− Weiss fitting above 300 K. (b) Field-dependent magnetization measured at different temperatures for PFOO.
cooling (ZFC) and field-cooling (FC) modes. The susceptibility experiences a remarkable increase at TC ≈ 280 K, indicating a FM or FiM phase transition. At a lower field like 0.1 T, the ZFC and FC curves considerably separate from each other. However, when a higher field (e.g., 2 T) is applied to overcome the domain-wall energy, these two susceptibility curves become nearly overlapped. Above 300 K, the inverse magnetic susceptibility measured at 0.1 T can be well fitted on the basis of the Curie−Weiss law, giving a positive Weiss temperature (θ = 273 K) that is comparable to the TC value, in coherence with the FM or FiM spin interactions.30 According to the fitted Curie constant, the effective moment is calculated to be μeff = 3.22 μB/fu. This value is much smaller than the expected one (7.07 μB/fu) considering the spin-only contribution for the high-spin Fe3+ and Os5+. Because a high-spin Fe3+ ion itself can generate a 5.92 μB/fu effective moment, the significantly reduced μeff value in the fitting probably is indicative of the presence of some short-range spin ordering above TC, as mentioned in analogous perovskites.31,32 The FM/FiM behavior of PFOO can be further characterized by magnetization measurements. As presented in Figure 4b, the canonical magnetic hysteresis curves observed below TC reveal the long-range FM or FiM ordering, as demonstrated from the magnetic susceptibility results mentioned above. At 2 K, the saturated spin moment that we measured is about 1.6 μB/fu. Because the spin−orbit-coupling (SOC) effect of Os5+ is not
Figure 3. XAS of PFOO measured at room temperature for (a) the Fe-L2, 3 and (b) Os-L3 edges. The XAS spectra of related references are also shown for comparison.
shows the Fe-L2,3 XAS of PFOO together with the related references Fe0.04Mg0.096O and Fe2O3, with FeO6 octahedral coordination similar to that of the Fe2+ and Fe3+ references, respectively.29 Obviously, PFOO and the Fe2+ reference Fe0.04Mg0.96O display essentially different spectral features. However, the Fe-L2,3 edges of PFOO are at the same energy positions as those of the Fe3+ standard Fe2O3, confirming the presence of the Fe3+ state in PFOO. Furthermore, the multiple spectral feature of PFOO is very similar to that of Fe2O3, C
DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry significant due to the d3-electron configuration,22 the collinear Fe3+(↑)Os5+(↑) FM coupling will produce an 8.0 μB saturation moment in each formula unit, in the assumption of a localelectron model without considering the SOC. Clearly, this value is unreasonably larger than the experimental observation. On the other hand, the collinear Fe3+(↑)Os5+(↓) FiM coupling gives a saturated moment (2.0 μB/fu) that is comparable with the measurement result at 2 K. The Fe−Os antisite effect identified by XRD refinement should be responsible for the reduced spin moment observed in the experiment. We therefore conclude that PFOO experiences a FiM phase transition around 280 K. In addition, the Fe3+(↑)Os5+(↓) FiM coupling can be further confirmed by XMCD measurements at the Fe- and Os-L2,3 edges. Figure 5 shows the XAS (μ+ and μ−) as well as the
(−0.099), implying a stronger orbital moment in the cubic PFOO than that of the distorted analogues. The temperature-dependent resistivity of PFOO is shown in Figure 6. With decreasing temperature, the resistivity of PFOO
Figure 6. Temperature dependence of the resistivity (ρ) of PFOO. The solid line in the inset shows the fitting result using the thermal excitation model between 250 and 300 K.
gradually increases from 300 to 2 K, indicating a semiconducting conductivity behavior. The temperature dependence of the resistivity between 250 and 300 K can be fitted using the thermal activation model with the formula ρ = ρ0 exp(EA/kBT), as shown in the inset of Figure 6. Here EA and kB stand for the thermal activation energy and Boltzmann constant, respectively. The fitting yielded EA = 14 meV, implying an energy band gap of about 28 meV. However, because of the Fe−Os antisite (16.6%) effect, this small activation energy cannot reasonably reflect the real energy band gap for a completely B-site-ordered PFOO. As shown later, theoretical calculations indicate a 170 meV band gap. First-principles theoretical calculations were performed to gain deeper insight into the magnetic and electrical properties of PFOO. First, we calculated different magnetic configurations between Fe and Os ions, as observed in the reported A2FeOsO6 family. It was found that both C- and G-type AFM structures have comparable and most favorable system energies. Because the C-type AFM structure does not generate any net spin moment, which deviates from experimental observation, the Gtype AFM structure is used to calculate the electronic properties. In our GGA+U calculations, the magnetic moment was calculated to be 4.12 μB for each Fe, −1.60 μB for each Os, −0.027 μB for each O, and −0.358 μB in the interstitial. The total spin moment is thus 2.00 μB/fu, which is well in agreement with the experimental result observed at 2 K by considering the Fe−Os antisite. Figure 7 shows the calculated electronic band structures and the partial densities of states (DOSs) of PFOO. The calculations show an indirect energy band gap of 170 meV, revealing the semiconducting nature. Note that the gap magnitude is overestimated, as is often expected for GGA+U calculations. Because all of the Fe−O−Os bondings are straight (180°) in the cubic perovskite PFOO, this compound possesses the smallest energy gap in the A2FeOsO6 family (∼2.4 eV for Ca2FeOsO6 and ∼0.25 eV for Sr2FeOsO6).10,11 As seen from the orbital-resolved DOSs in Figure 7, near the Fermi level, the electronic states are dominated by the 3d electrons of Fe as well as the Os t2g electrons, whereas the empty Os eg orbitals are located far away from the Fermi level. Therefore, the spin interactions will mainly take place between the half-filled 3d orbitals of Fe and t2g orbitals of Os, giving rise to an AFM coupling between the Fe3+ and Os5+ magnetic sublattices via straight Fe−O−Os superexchange interaction pathways. As a result, a FiM instead of FM ordering occurs in the present
Figure 5. XAS with helicity parallel (μ+ black line) and antiparallel (μ− red line) to the applied magnetic field and the corresponding XMCD signal μ+ − μ− (blue line) for PFOO at (a) Fe- and (b) Os-L2,3 edges. For clarity, the XMCD signal of Os in part b was enlarged by 40 times.
resulting XMCD (Δμ = μ+ − μ−) spectra for Fe3+ and Os5+. Here μ+ and μ− represent different photon helicities parallel and antiparallel to the applied magnetic field, respectively. One can find that the XMCD signal is negative for the Fe-L3 edge but positive for the Os-L3 edge. On the contrary, it becomes positive for the Fe-L2 edge but negative for the Os-L2 edge. The opposite signs of the XMCD spectra at the Fe- and Os-L2,3 edges reveal that AFM coupling occurs between the Fe3+ and Os5+ magnetic sublattices, leading to the long-range Fe3+(↑)Os5+(↓) FiM spin alignment. The observed line shapes of Feand Os-L2,3 XMCD spectra of PFOO are very similar to those of Ca2FeOsO6 and Sr2FeOsO6.33 It is very interesting that the much smaller dichroic signal of the Os-L3 edge compared to that of the Os-L2 edge provides direct evidence for the presence of a substantial orbital moment that is oriented antiparallel to the spin moment. At the XMCD measurement conditions we used, PFOO is not magnetically saturated; we thus cannot use the orbital and spin sum rules to obtain the orbital and spin quantum numbers ⟨Lz⟩ and ⟨Sz⟩. However, the sum rule ⟨Lz⟩/ 2⟨Sz⟩(1 + 7⟨Tz⟩/2⟨Sz⟩) = 2∑Δμ(L3 + L2)/3[∑Δμ(L3) − 2ΣΔμ(L2)] does not depend on the experimental conditions. Using the theoretical value of 7⟨Tz⟩/2⟨Sz⟩ = −0.23 obtained in A2FeOsO6,33 we get ⟨Lz⟩/2⟨Sz⟩ = −0.16 for PFOO. This value is somewhat larger than that of Ca2FeOsO6 and Sr2FeOsO6 D
DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
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benefited from the support of the German funding agency DFG (Project 615811).
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(1) King, G.; Woodward, P. M. Cation ordering in perovskites. J. Mater. Chem. 2010, 20, 5785−5796. (2) Kimura, T.; Goto, T.; Shintani, H.; Ishizaka, K.; Arima, T.; Tokura, Y. Magnetic control of ferroelectric polarization. Nature 2003, 426, 55−58. (3) Salamon, M. B.; Jaime, M. The physics of manganites: Structure and transport. Rev. Mod. Phys. 2001, 73, 583−628. (4) Krockenberger, Y.; Mogare, K.; Reehuis, M.; Tovar, M.; Jansen, M.; Vaitheeswaran, G.; Kanchana, V.; Bultmark, F.; Delin, A.; Wilhelm, F.; Rogalev, A.; Winkler, A.; Alff, L. Sr2CrOsO6: End point of a spinpolarized metal-insulator transition by 5 d band filling. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 020404. (5) Alamelu, T.; Varadaraju, U. V.; Venkatesan, M.; Douvalis, A. P.; Coey, J. M. D. Structural and magnetic properties of (Sr2‑xCax) FeReO6. J. Appl. Phys. 2002, 91, 8909−8911. (6) Kato, H.; Okuda, T.; Okimoto, Y.; Tomioka, Y.; Takenoya, Y.; Ohkubo, A.; Kawasaki, M.; Tokura, Y. Metallic ordered doubleperovskite Sr2CrReO6 with maximal Curie temperature of 635 K. Appl. Phys. Lett. 2002, 81, 328−330. (7) Tokura, Y.; Kobayashi, K.-I.; Kimura, T.; Sawada, H.; Terakura, K. Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure. Nature 1998, 395, 677−680. (8) Tomioka, Y.; Okuda, T.; Okimoto, Y.; Kumai, R.; Kobayashi, K.I.; Tokura, Y. Magnetic and electronic properties of a single crystal of ordered double perovskite Sr2FeMoO6. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 422−427. (9) Serrate, D.; Teresa, J. M. De; Ibarra, M. R. Double perovskites with ferromagnetism above room temperature. J. Phys.: Condens. Matter 2007, 19, 023201. (10) Paul, A. K.; Jansen, M.; Yan, B.; Felser, C.; Reehuis, M.; Abdala, P. M. Synthesis, crystal structure, and physical properties of Sr2FeOsO6. Inorg. Chem. 2013, 52, 6713−6719. (11) Feng, H. L.; Arai, M.; Matsushita, Y.; Tsujimoto, Y.; Guo, Y.; Sathish, C. I.; Wang, X.; Yuan, Y.; Tanaka, M.; Yamaura, K. Hightemperature ferrimagnetism driven by lattice distortion in double perovskite Ca2FeOsO6. J. Am. Chem. Soc. 2014, 136, 3326−3329. (12) Morrow, R.; Freeland, J. W.; Woodward, P. M. Probing the Links between Structure and Magnetism in Sr2−xCaxFeOsO6 Double Perovskites. Inorg. Chem. 2014, 53, 7983−7992. (13) Paul, A. K.; Reehuis, M.; Ksenofontov, V.; Yan, B.; Hoser, A.; Tobbens, D. M.; Abdala, P. M.; Adler, P.; Jansen, M.; Felser, C. Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6. Phys. Rev. Lett. 2013, 111, 167205. (14) Hou, Y. S.; Xiang, H. J.; Gong, X. G. Lattice-distortion Induced Magnetic Transition from Low-temperature Antiferromagnetism to High-temperature Ferrimagnetism in Double Perovskites A2FeOsO6(A = Ca, Sr). Sci. Rep. 2015, 5, 13159. (15) Goodenough, J. B. Theory of the Role of Covalence in the Perovskite-Type Manganites [La, M (II)]MnO3. Phys. Rev. 1955, 100, 564−573. (16) Kanamori, J. Superexchange interaction and symmetry properties of electron orbitals. J. Phys. Chem. Solids 1959, 10, 87−98. (17) Larson, A. C.; Dreele, R. B. V. Los Alamos National Laboratory Report No. LAUR 86-748, 2004. (18) Henke, B. L.; Gullikson, E. M.; Davis, J. C. X-Ray Interactions: Photoabsorption, Scattering, Transmission, and Reflection at E = 50− 30,000 eV, Z = 1−92. At. Data Nucl. Data Tables 1993, 54, 181. (19) Blaha, P.; Schwarz, K.; Madsen, G.; Kvasnicka, D.; Luitz, J. J. Endocrinol. 2001, 196, 123−30. (20) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (21) Kanungo, S.; Yan, B.; Jansen, M.; Felser, C. Ab initio study of low-temperature magnetic properties of double perovskite Sr2FeOsO6. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 214414.
Figure 7. First-principles numerical results for the band structures and partial DOSs of PFOO.
PFOO. Note that Fe−Os antiferromagnetic interactions are also observed in a both A- and B-site-ordered cubic quadruple perovskite CaCu3Fe2Os2O12.34
4. CONCLUSION In summary, a novel oxide PFOO was synthesized at 8 GPa and 1673 K. The compound crystallizes into a B-site-ordered perovskite structure with space group Fm3̅m. This is the first example of a cubic crystal system in the B-site-ordered perovskite family A2FeOsO6. Both BVS calculations and XAS measurements indicate a Pb2Fe3+Os5+O6 charge combination with rock-salt-type ordered distribution between the Fe3+ (t2g3eg2; S = 5/2) and Os5+ (t2g3eg0; S = 3/2) ions. A FiM phase transition caused by the antiferromagnetically coupled Fe3+ and Os5+ spin sublattices occurs near room temperature (TC ≈ 280 K). At 2 K, the saturation spin moment observed in the experiment is 1.6 μB/fu, in accordance with the Fe3+(↑)Os5+(↓) FiM coupling, by considering the Fe−Os antisite occupancy (∼16.6%). The XMCD spectroscopy provides further evidence for this FiM ordering. On the basis of the well-known GK rules, a long-range FM ordering is expected to occur between the half-filled eg orbitals of Fe3+ and the empty eg orbitals of Os5+ via straight (180°) Fe−O−Os interaction pathways in the cubic PFOO. Our theoretical calculations, however, reveal that the empty eg orbitals of Os5+ are far away from the Fermi level. As a result, the half-filled t2g orbitals of Os5+ instead of the empty eg take part in the spin superexchange interactions with the half-filled 3d orbitals of Fe, leading to AFM interactions between Fe3+ and Os5+ spins.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the 973 Project of the Ministry of Science and Technology of China (Grant 2014CB921500), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB07030300), and the National Natural Science Foundation of China (Grant 11574378). K.C. E
DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on September 13, 2016, with errors in Figure 5. The corrected version was reposted on September 15, 2016.
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DOI: 10.1021/acs.inorgchem.6b01649 Inorg. Chem. XXXX, XXX, XXX−XXX
