Structure, Magnetism, and Valence States of Cobalt and Platinum in

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Structure, Magnetism, and Valence States of Cobalt and Platinum in Quasi-One-Dimensional Oxides A3CoPtO6 with A = Ca, Sr D. Mikhailova,*,† C. Y. Kuo,† P. Reichel,† A. A. Tsirlin,†,‡ A. Efimenko,†,§ M. Rotter,† M. Schmidt,† Z. Hu,† T. W. Pi,∥ L. Y. Jang,∥ Y. L. Soo,⊥ S. Oswald,# and L. H. Tjeng† †

Max-Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia § II.Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany ∥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, R.O.C ⊥ Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan # Institute for Complex Materials, IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany ‡

ABSTRACT: Two quasi-one-dimensional oxides, Ca3CoPtO6 and Sr3CoPtO6, were synthesized and characterized. A combination of X-ray absorption spectroscopy at the Co-K-, Co-L2,3-, and Pt-L3-edges and X-ray photoelectron spectroscopy establishes unambiguously the divalent state of Co and the tetravalent state of Pt in both compounds, in contrast to the earlier assumption of the Co3+ and mixed Pt2+/Pt4+ valence states. Magnetization measurements reveal the paramagnetic behavior down to 2 K with strong evidence for an unquenched orbital moment of the high-spin Co2+. The simple paramagnetic behavior of A3CoPtO6 contrasts with the magnetic transitions observed in Ca3CoRhO6, Sr3CoIrO6, and other isostructural materials. This difference is ascribed to the nonmagnetic 5d6 state of Pt4+ that prevents magnetic couplings between the Co2+ ions.

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INTRODUCTION Quasi-one-dimensional Co oxides are an interesting group of inorganic materials. Their parent compound, Ca3Co2O6, belongs to the K4CdCl6 structural type.1 Ca3Co2O6 is a semiconductor that shows a sequence of perplexing magnetic transitions, steplike magnetization process,2−4 and an intricate, time-dependent incommensurately modulated magnetic structure.5,6 It is also studied as a thermoelectric material7−9 and a material for solid-oxide fuel cells.10 The crystal structure of Ca3Co2O6 entails chains formed by alternating CoO6 trigonal prisms and CoO6 octahedra. A chemical modification of this crystal structure is possible via a tuning of the chain structure (e.g., by introducing additional octahedra between the trigonal prisms, as in Sr6Co5O1511) or via a replacement of the octahedrally coordinated Co atom with 3d, 4d, and even 5d cations. The latter approach results in a family of A3CoMO6 oxides with A = Ca, Sr and M = Co, Mn, Rh, Ru, Ir, Pt (Figure 1). Similar to Ca3Co2O6, these compounds reveal large thermopower7 and peculiar magnetic phenomena induced by the magnetic frustration.12−16 The valence of Co ions in A3CoMO6 has long been under debate. The magnetic response of Co ions is inexplicably intertwined with their spin state and may provide ambiguous information on the Co valence, especially in systems with several magnetic ions. Ca3Co2O6 itself features a unique combination of low-spin Co3+ in CoO6 octahedra and highspin Co3+ in CoO6 trigonal prisms.17,18 The robust Co3+ state © 2014 American Chemical Society

in Ca3Co2O6 led to an assumption on the similar trivalent state of both Co and Rh in Ca3CoRhO6 (refs 19, 20). While the combination of Co3+ and Rh3+ is indeed in good agreement with neutron-scattering data,19,20 a careful spectroscopic study21 put forward an alternative Co2+/Rh4+ scenario, in agreement with the electronic structure and the giant orbital moment of more than 1 μB on Co ions, which is possible only for Co2+ (ref 22). Here, we focus on Pt-containing members of the A3CoMO6 family and elucidate the valences of Co and Pt ions. While Ca3CoPtO6 is a novel, hitherto unknown compound, its Sr analogue is known since the 1990s.23,24 Following the Ca3CoRhO6 scenario, one expects the Co2+/Pt4+ valence states in Sr3CoPtO6. However, earlier photoemission spectra24 indicated a lower valence of Pt, which led the authors of ref 24 to postulate the presence of Co3+ together with the 1:1 mixture of Pt2+ and Pt4+, because the 3+ state is uncommon for Pt.25 Here, we critically revise this scenario and argue for the more conventional Co2+/Pt4+ regime confirmed by an extensive spectroscopic study and crystallographic analysis. Both Ca3CoPtO6 and Sr3CoPtO6 are paramagnets down to 2 K, in a sharp contrast to other members of the A3CoMO6 family which show magnetic ordering with fairly high ordering Received: November 22, 2013 Revised: January 17, 2014 Published: January 24, 2014 5463

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Figure 1. Crystal structure of A3CoMO6 (A = Ca or Sr, M = Co, Rh, Ru, Ir, Pt) presenting one-dimensional chains along the c-direction consisting of alternating face-sharing CoO6 trigonal prisms (brown) and MO6 octahedra (green, left). A projection of one unit cell on the ab-plane is shown (right). Gray spheres are A atoms.

temperatures.12−14,16 We ascribe this unexpected behavior to the nonmagnetic low-spin 5d6 state of Pt4+ that prevents intrachain superexchange interactions between the Co2+ ions.

CoO, Co-K spectrum of EuCoO3 (ref 26), and Pt-L3 spectrum of Sr4PtO6 were also measured as reference systems for Co2+, Co3+, and Pt4+, respectively. For these measurements the samples were pressed into pellets of about 5 mm in diameter and 2 mm thick and annealed at synthesis temperatures for several hours. Fresh sample surfaces were obtained by fracturing the pellets in situ in UHV chambers with a base pressure of 1 × 10−9 mbar. The Pt-L3- and Co-K-edge spectra were collected at room temperature applying the bulk sensitive total fluorescence yield method using a Lytle detector. The CoL2,3-edge spectra were recorded in the total electron yield mode. The photon energy resolution was about 1.5 eV at the Pt-L3-edge, 1.4 eV at the Co-K-edge, and 0.3 eV at Co-L2,3edge. X-ray Photoelectron Spectroscopy. The photoemission spectra were measured using an XPS setup equipped with a Vacuum Generators twin crystal monochromatized Al Kα source and a Scienta electron energy analyzer R3000. The overall resolution was set to about 0.4 eV. The binding-energy scale was calibrated using the Fermi-level value of metallic silver at 1482.24 eV as 0 eV for the binding energy. To obtain clean surfaces, the samples were fractured in situ in XPS setup with base pressures of 1 × 10−10 mbar. Electronic Structure Calculations. Scalar-relativistic electronic structure calculations for Sr3CoPtO6 were performed in the framework of density functional theory (DFT) using the full-potential FPLO code with the basis set of atomic-like local orbitals.27 The local-density approximation (LDA) exchangecorrelation potential by Perdew and Wang (ref 28) was applied. The first Brillouin zone was sampled with a fine k mesh of 13824 points (1313 points in the symmetry-irreducible part). Additionally, DFT+U+SO (spin−orbit) calculations with a mean-field correction for correlation effects were performed in the VASP5.2 code.29,30 We used the parameters UCo = 5 eV and JCo = 0.9 eV for the on-site Coulomb repulsion and Hund’s exchange, respectively.17,22

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EXPERIMENTAL SECTION Synthesis and Sample Characterization. The Sr3CoPtO6 and Ca3CoPtO6 samples were prepared by solidstate reactions in air at 1273 K (Sr) and 1223 K (Ca) from stoichiometric powder mixtures of CoO (Alfa Aesar, 99.999%) with PtO2 (Alfa Aesar, 99.99%) and SrCO3/CaCO3 (Alfa Aesar, 99.99%), placed into Pt crucibles. Two intermediate grindings were required to obtain phase-pure materials. In order to determine the optimal synthesis conditions and thermal stability of the products, thermal behavior of the synthesized Sr3CoPtO6 and of a mixture of initial compounds SrCO3, CoO, and PtO2 was investigated in a STA 449 (Netzsch, Selb, Germany) in air and under O2 atmosphere. About 25 mg of powder was heated in a Pt/Ir crucible with the scan rate of 10 K/min from room temperature up to 1273 K. The phase analysis and the determination of unit cell parameters were carried out using X-ray powder diffraction (XPD), performed with a laboratory Huber G670 Guinier camera (Cu Kα1 radiation, Ge monochromator, image plate detector, 2θ = 3−100° angle range). The Sr4PtO6 compound, used as a Pt4+ reference material for X-ray absorption spectroscopy (XAS) measurements, was synthesized by a solid-state reaction from stoichiometric powder mixtures of PtO2 with SrCO3 in air at 1423 K during 24 h followed by an annealing at 773 K during 12 h. Magnetic Measurements. The temperature dependence of the magnetization was measured both in zero-field-cooled (ZFC) and in field-cooled (FC) modes between T = 1.8 and 350 K for powdered Ca3CoPtO6 and Sr3CoPtO6 samples at the field strengths of 0.1, 0.5, and 5 T using a SQUID magnetometer (MPMS) from Quantum Design. The field dependence of the magnetization was determined at 2 K up to 5 T after cooling the sample in zero field. Same samples were used for powder diffraction and magnetization measurements. X-ray Absorption Spectroscopy. The Co-K-, Co-L2,3-, and Pt-L3-edge XAS spectra of Sr3CoPtO6 and Ca3CoPtO6 were recorded at the 16 B, 08B, and 07C beamlines, respectively, of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The Co-L2,3-edge spectrum of

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RESULTS AND DISCUSSION 1. Synthesis and Crystal Structure Characterization. The Sr3CoPtO6 compound forms upon heating reactants in air to 1273 K with the heating rate of 10 K/min during the DTATG measurement of the initial SrCO3, CoO, and PtO2 mixture (Figure 2). The reaction is a stepwise process including the 5464

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Figure 2. DTA-TG measurement of the initial mixture of SrCO3, CoO, and PtO2.

Figure 3. Powder diffraction patterns of Sr3CoPtO6 and Ca3CoPtO6 together with the calculated profiles (black lines); based on the Rietveld refinement of the structure model from the ref 14, and difference curves (blue lines). The Ca3CoPtO6 sample contains metallic Pt as a second phase (bottom tick marks).

decomposition of PtO2 and SrCO3. The detected mass loss of 19% (evaporation of CO2) was less than the expected value of 21%, thus indicating residual carbonates present in the sample. Although all reflections in the diffraction pattern could be indexed in the R3c̅ space group, a notable broadening of (hkl) reflections in comparison to the (hk0) ones pointed out a high concentration of structural defects (data not shown). Several intermediate grindings were required to complete the formation of A3CoPtO6 and obtain the samples without peak broadening. The TG analysis of the single-phase Sr3CoPtO6 (6 h in the O2 atmosphere at 1073 K and subsequent cooling to room temperature) showed an irreversible mass loss of 0.67% (w/w), which likely corresponds to the elimination of Pt in the form of oxides. Above 1273 K, platinum-group metals may indeed form volatile oxides in an oxygen-containing atmosphere.31 This effect is enhanced with increasing partial pressure of oxygen. In the Pt−O system, the most stable gaseous oxide is PtO2 (refs 31, 32). In order to avoid the loss of Pt from A3CoPtO6, the synthesis procedure in air instead of pure O2 atmosphere was chosen. The oxygen-free atmosphere is also detrimental for A3CoPtO6. For example, the heat treatment of the SrCO3, CoO, and PtO2 mixture in argon led to the formation of Sr,Co oxides and metallic Pt. The XRD patterns of isostructural Ca 3 CoPtO 6 and Sr3CoPtO6 are presented in Figure 3. The structural parameters obtained from the Rietveld refinement are listed in Table 1. The Ca sample contains metallic Pt (about 1% w/w) as a second phase. For the refinement, a structural model derived from that of Sr3CoIrO6 (ref 14) was applied. The resulting Co−O distances are 2.159(2) Å for the Ca compounds and 2.185(2) Å for the Sr compounds. These values are in very good agreement with the Co−O distance of 2.192(1) Å for the Co2+ (HS) in CoO6 prisms of Sr3CoIrO6 (ref 14). The Pt−O distances of 1.982(2) Å and 1.996(2) Å correlate with the sum of Shannon ionic radii of Pt4+ and O2− of 2.025 Å for PtO6 octahedra.33 The Pt2+ ions would lead to a much longer Pt−O bond length of 2.20 Å that is not observed experimentally. No mixed Co/Pt occupancies were found in either of the structures. 2. Magnetization Measurements of A3CoPtO6. Both Ca3CoPtO6 and Sr3CoPtO6 are paramagnetic down to 2 K (Figure 4). No signatures of magnetic ordering were observed, in contrast to Ca3Co2O6, Ca3CoRhO6, and other A3CoMO6

Table 1. Structural Features of Ca3CoPtO6 and Sr3CoPtO6 (R3̅c, Space Group 167) Refined from X-ray Powder Diffraction Dataa parameters a (Å), c (Å) V (Å3) Z calcd density (g/cm3) Sr (x, y, z) B (Å2) Co (x, y, z) B (Å2) Pt (x, y, z) B (Å2) O (x, y, z) B (Å2) Co−O (Å) Pt−O (Å) Bragg R factor, % Rf factor, % a

Ca3CoPtO6, 298 K

Sr3CoPtO6, 298 K

9.21133(4), 10.97337(6) 806.335(6) 6 5.81014

9.60094(2), 11.21656(3) 895.401(4) 6 6.81912

18e (0.36392(9), 0, 0.25) 1.31(3) 6a (0, 0, 0.25) 1.89(4) 6b (0, 0, 0) 2.33(1) 36f (0.1802(2), 0.0280(2), 0.1128(1)) 1.24(5) 2.159(2) 1.982(2) 2.56 2.13

18e (0.36449(4), 0, 0.25) 2.09(2) 6a (0, 0, 0.25) 2.04(3) 6b (0, 0, 0) 2.06(1) 36f (0.1710(2,) 0.0220(2), 0.1124(2)) 2.30(5) 2.185(2) 1.996(2) 1.86 1.71

Figure 3.

compounds that show at least one magnetic transition below 100 K.2−4,12−16 Inverse susceptibilities follow the Curie−Weiss law χ = C/(T − θ) with the parameters listed in Table 2, where we recalculated the Curie constant C = NAμeff2/3kB into the paramagnetic effective moment μeff. In this equation, NA is the Avogadro constant, and kB is the Boltzmann constant. The small θ values indicate very weak magnetic couplings in A3CoPtO6. While the effective moments are typically used to distinguish between different valence states of transition metals, the case of Co ions is not straightforward. In Table 2, we provide spin-only effective moments calculated for different valence scenarios: 5465

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in higher fields, while reaching only 1.5−2.0 μB/f.u. (formula unit) at 5 T. This magnetization is much smaller than expected for either Co2+ and Pt4+ (spin-only moment of 3 μB), or Co3+ and Pt2+/Pt4+ (spin-only moment of 5 μB) valence regime. However, the increase in the magnetization up to at least 5 T implies that the saturation is not reached. Considering the low θ values derived from the Curie−Weiss fit, the lack of saturation at 5 T cannot be ascribed to antiferromagnetic couplings and should be rather understood as an effect of magnetic anisotropy. Altogether, magnetization measurements were unable to elucidate the valence regime of the A3CoPtO6 compounds. Therefore, we directly probed the valence states of Co and Pt with X-ray absorption spectroscopy. 3. Room Temperature X-ray Absorption Spectroscopy. 3.1. Co L2,3- and K-Edge. Co-L2,3 absorption spectra of A3CoPtO6 (A = Ca, Sr) together with CoO as a Co2+ reference are shown in Figure 5a. The “center of gravity” of the L3 white line of Ca3CoPtO6 and Sr3CoPtO6 lies at the same energy as that of CoO and at more than 1 eV lower energy than that of the trivalent Co oxide EuCoO3 demonstrating the divalent state of cobalt in Ca3CoPtO6 and Sr3CoPtO6. The multiplet spectral structure of both Ca3CoPtO6 and Sr3CoPtO6 is the same as that of Ca3CoRhO6, which we have reproduced from ref 21 indicating the trigonal-prismatic local symmetry (D3d) of Co2+ ions. The different line shape in A3CoPtO6 and CoO reflects the different local environment of Co2+ ions in these compounds. For example, the first multiplet feature in the Co-L2,3 spectrum around 778 eV is very pronounced for CoO and nearly invisible for the A3CoPtO6 compounds. As shown in Figure 5a for the Co2+ ion in the D3d symmetry, two minority electrons have the orbital occupation of d0d2 (ref 21), while for the Co2+ in the octahedral Oh symmetry two minority electrons occupy t2g orbitals.34 We also have measured the Co-K-edge with the more bulk sensitive total fluorescence yield (FY). Figure 5b shows the CoK XAS spectra of Ca3CoPtO6 and Sr3CoPtO6, together with EuCoO3 as a Co3+ reference material. All spectra are normalized to unity step in the absorption coefficient μ (200 eV above the absorption edge). It is well-known that the position of the absorption edge is related to the valence state of 3d transition metals. There are several ways to definite the position of the absorption edge for valence determination of the transition metal, for example the main peak (P) around 7712 eV (ref 35), or the value at 0.8 of the edge jump μ (ref 36). For our samples, both methods give very similar results, namely, that we can observe a chemical shift of 3 eV in going from Ca3CoPtO6 and Sr3CoPtO6 to EuCoO3, but nearly no shift with respect to CoO. This resembles the shift from La2CoO4 to LaCoO3 (ref 35), confirming firmly the Co2+ state in bulk Ca3CoPtO6 and Sr3CoPtO6. 3.2. Pt-L3-Edge. After establishing the Co2+ state in Ca3CoPtO6 and Sr3CoPtO6, we turn to the Pt-L3 XAS spectra to verify the Pt4+ state, as required from the charge balance. The white line in the Pt-L3-edge of Ca3CoPtO6 and Sr3CoPtO6 spectra (Figure 6) lies at the same energy as in the tetravalent Pt reference compound Sr4PtO6, which is isostructural to A3CoPtO6. This indeed confirms the Pt4+ state in both Pt compounds.37 Thus, XAS measurements give unambiguous evidence for the presence of Co2+ and Pt4+ in the Ca3CoPtO6 and Sr3CoPtO6 compounds. 4. Room Temperature X-ray Photoelectron Spectroscopy. The Co 2p core level spectra of Sr3CoPtO6 and

Figure 4. Temperature and field dependence of the magnetization of Ca3CoPtO6 and Sr3CoPtO6.

Table 2. Effective Magnetic Moments of Ca3CoPtO6 and Sr3CoPtO6 from the Experiment and Calculated Values Based on a Spin-Only Model for the Combination Co2+(HS)/Pt4+(LS) and Co3+(HS)/(0.5Pt2+(LS) + 0.5Pt4+(LS)) μeff(calc), μB compd

temp range for fit, K

Θ, K

Ca3CoPtO6 Sr3CoPtO6

2−350 2−350

−8.0 −5.3

Co3+(HS)/ 2+ 4+

0.5Pt + 0.5Pt 6.00 6.00

Co2+/ Pt4+

μeff(exp), μB

3.87 3.87

4.71(1) 4.91(1)

μeff2 = 2S(S+1) for each magnetic ion, where S = 3/2 for Co2+ (high-spin 3d7), S = 2 for Co3+ (high-spin 3d6), S = 0 for Pt4+ (low-spin 5d6), and S = 1 for Pt2+ (5d8 state in the hypothetical octahedral environment). The experimental values of μeff = 4.7−4.9 μB are in between the expectations for the Co2+/Pt4+ and Co3+/Pt2+,Pt4+ scenarios. The spin-only moment of 3.87 μB expected for the Co2+/Pt4+ pair is lower than the experimental one, but it can be augmented by the orbital component, which is very typical for Co2+ (ref 17, 22). On the other hand, the Co3+/Pt2+,Pt4+ scenario overestimates the experimental effective moment, which is not unusual considering the poorly defined contribution of Pt2+ (note that Pt2+ is typically found in the diamagnetic state in the 4-fold oxygen environment, and not in the paramagnetic S = 1 state in the octahedral coordination) and possible nonmagnetic impurities. The field dependence of the magnetization measured on powder samples shows a bend around 3 T and keeps increasing 5466

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Figure 5. (a) Co-L3,2 absorption spectra of A3CoPtO6 together with the reference spectra for Ca3CoRhO6 (ref 21), CoO (Co2+ reference), and EuCoO3 (Co3+ reference). Right panel depicts the likely scheme of orbital occupations for the HS-Co2+ ion in the trigonal-prismatic (D3d: A3CoPtO6 and Ca3CoRhO6) and octahedral (Oh: CoO) symmetries, taken from refs 21 and 34, respectively. The five spin-up electrons are not shown. (b) Normalized Co-K edge absorption spectra of A3CoPtO6 and reference materials CoO and EuCoO3.

A3CoPtO6, the 4f7/2 and 4f5/2 lines are very sharp, indicating that the samples contain a single-valent Pt and not a mixture of valences, i.e., Pt4+ and not Pt2+/Pt4+ as it was claimed in ref 24. 5. Band Structure Calculations. Band structure of Sr3CoPtO6 calculated within the local density approximation (LDA) reveals the broad valence band formed by the O 2p states that are strongly hybridized with the 5d t2g states of Pt, whereas the 5d eg states of Pt also mix with oxygen and lie about 1 eV above the Fermi level (Figure 8, top). The Co 3d states form narrow bands in the vicinity of the Fermi level. They show the crystal-field splitting, which is typical for trigonal-prismatic symmetry of the Co2+ ion.21,22 The LDA band structure is metallic, whereas the dark-brown color of Sr3CoPtO6 suggests insulating behavior. This discrepancy is very typical for transition-metal compounds, where LDA does not capture the essential physics of strong electronic correlations opening the gap in the energy spectrum. The realistic gapped spectrum can be reproduced by the socalled LSDA+U+SO method that includes a mean-field correction for correlation effects U and, additionally, the relativistic effects of the spin−orbit coupling (SO) that are required to lift the residual orbital degeneracy.22 The application of LSDA+U+SO indeed opens the gap of 2.2 eV for the on-site Coulomb repulsion parameter UCo = 5 eV and the ferromagnetic spin configuration (Figure 8, bottom).

Figure 6. Pt-L3 absorption spectra of A3CoPtO6 (A = Ca, Sr) and Pt4+ reference material Sr4PtO6. Dashed line corresponds to the center of gravity of the spectra.

Ca3CoPtO6 show the Co 2p3/2 and Co 2p1/2 main peaks at 780.3 and 795.7 eV, respectively, and strong satellite structures at about 6 eV higher in energy (Figure 7). The intensity ratio and the energy separation in the A3CoPtO6 spectra are similar to those of the CoO reference material with the known highspin Co2+ state. This then supports the XAS finding that cobalt in A3CoPtO6 has the 2+ oxidation state. In the Pt 4f spectra of 5467

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0.01 μB) and a large magnetic moment on Co (μCo(spin) = 2.70 μB, μCo(orb) = 1.73 μB). Therefore, we conclude that Sr3CoPtO6 features the nonmagnetic Pt4+ ions and the magnetic Co2+ ions. This result is in agreement with the spectroscopic determination of the Co2+/Pt4+ valence states in Sr3CoPtO6, whereas the presence of the large orbital moment is quite typical for Co2+ in the trigonal-prismatic coordination21,22 and indeed seen in our magnetization data. The nonmagnetic Pt4+ ions strongly reduce the exchange interactions along the Co−Pt−Co chains. This explains naturally why the A3CoPtO6 compounds are paramagnetic to the lowest temperatures measured, in contrast to Ca3CoRhO6 and Ca3CoIrO6 that show clear magnetic ordering with fairly high ordering temperatures.

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CONCLUSIONS A new quasi-one-dimensional Ca3CoPtO6 oxide was successfully synthesized in addition to the already known Sr3CoPtO6. Both compounds feature the Co2+ and Pt4+ valence states, as evidenced by the XAS and XPS measurements thereby resolving the long-standing issue regarding the possible coexistence of Pt2+ and Pt4+ in Sr3CoPtO6. This way, we refute the unlikely scenario of the mixture of Pt2+ and Pt4+ ions occupying the same crystallographic position in Sr3CoPtO6. Remarkably, the Co2+/M4+ valence state was found in all A3CoMO6 compounds, where M is a 4d or 5d metal. The formation of Co3+ in Ca3Co2O6 turns out to be a unique feature related to the coexistence of Co3+ in the octahedral and trigonal-prismatic coordination. We argue that further members of the A3CoMO6 family should also feature the Co2+ and M4+ ions. The LSDA+U+SOC band-structure calculations yield a large orbital moment of Co2+ and the nonmagnetic state of Pt4+ in perfect agreement with the experimental high-temperature magnetization data that cannot be understood without taking into account the orbital moment of Co2+. A verification of this scenario and a more accurate experimental evaluation of the orbital moment require X-ray magnetic circular dichroism (XMCD) experiments, which are presently underway. The large orbital moment is another generic feature of the A3CoMO6 oxides. It is directly related to the trigonal-prismatic coordination of Co2+ and triggered by strong electronic correlations, as explained in ref 21. In contrast to all Ca3Co2O6-type compounds known so far, the A3CoPtO6 oxides are paramagnetic down to 2 K. We ascribe this atypical behavior to the low-spin state of Pt4+. The nonmagnetic Pt4+ ions prevent magnetic interactions between Co2+. Therefore, the M4+ cation controls the magnetism of A3CoMO6. By choosing different 4d and 5d metals, one can deliberately design ferromagnetic or antiferromagnetic interactions along the structural chains and achieve various regimes of quasi-1D magnets based on Co2+.

Figure 7. Room temperature Pt 4f and Co 2p photoelectron spectra of Ca3CoPtO6 and Sr3CoPtO6 together with Co 2p spectrum of CoO reference material.

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AUTHOR INFORMATION

Corresponding Author

Figure 8. Electronic density of states (DOS) calculated for Sr3CoPtO6 within LDA (top panel) and LSDA+U+SO for the ferromagnetic spin configuration (bottom panel). The Fermi level is at zero energy.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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The LSDA+U+SO calculations elucidate the valence states of the transition-metal ions in Sr3CoPtO6. Irrespective of the starting spin configuration, the calculations converged to the ground state with a nearly zero magnetic moment on Pt (μPt =

ACKNOWLEDGMENTS A.E. acknowledges the support and funding from the European Union via the FP7/2007-2013 under Grant Agreement No. 5468

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(18) Burnus, T.; Hu, Z.; Haverkort, M. W.; Cezar, J. C.; Flahaut, D.; Hardy, V.; Maignan, A.; Brookes, N. B.; Tanaka, A.; Hsieh, H. H.; et al. Valence, Spin, and Orbital State of Co Ions in One-Dimensional Ca3Co2O6: An X-Ray Absorption and Magnetic Circular Dichroism Study. Phys. Rev. B 2006, 74, 245111. (19) Niitaka, S.; Yoshimura, K.; Kosuge, K.; Nishi, M.; Kakurai, K. Partially Disordered Antiferromagnetic Phase in Ca3CoRhO6. Phys. Rev. Lett. 2001, 87, 177202. (20) Loewenhaupt, M.; Schäfer, W.; Niazi, A.; Sampathkumaran, E. V. Evidence for the Coexistence of Low-Dimensional Magnetism and Long-Range Order in Ca3CoRhO6. Europhys. Lett. 2003, 63, 374−380. (21) Burnus, T.; Hu, Z.; Wu, Hua; Cezar, J. C.; Niitaka, S.; Takagi, H.; Chang, C. F.; Brookes, N. B.; Lin, H.-J.; Jang, L. Y.; et al. X-Ray Absorption and X-Ray Magnetic Dichroism Study on Ca3CoRhO6 and Ca3FeRhO6. Phys. Rev. B 2008, 77, 205111. (22) Wu, Hua; Hu, Z.; Khomskii, D. I.; Tjeng, L. H. Insulating State and the Importance of the Spin-Orbit Coupling in Ca3CoRhO6 and Ca3FeRhO6. Phys. Rev. B 2007, 75, 245118. (23) Nguyen, T. N.; zur Loye, H.-C. Structures and Magnetic Properties of a Family of One-Dimentional Oxides, Sr3M′M″O6 (M′ = Co, Ni, Zn and M″=Pt, Ir). Neutron Scattering Mater. Sci. 1995, 376, 603. (24) Nguyen, T. N. Ph.D. Thesis, 1994, Massachusetts Institute of Technology. (25) Schwartz, K. B.; Prewitt, C. T. Structural and Electronic Properties of Binary and Ternary Platinum Oxides. J. Phys. Chem. Solids 1984, 45, 1−21. (26) Hu, Z.; Wu, H.; Haverkort, M. W.; Hsieh, H. H.; Lin, H. J.; Lorenz, T.; Baier, J.; Reichel, A.; Bonn, I.; Felser, C.; et al. Different Look at the Spin State of Co3+ Ions in a CoO5 Pyramidal Coordination. Phys. Rev. Lett. 2004, 92, 207402. (27) Koepernik, K.; Eschrig, H. Full-Potential Nonorthogonal LocalOrbital Minimum-Basis Band-Structure Scheme. Phys. Rev. B 1999, 59, 1743−1757. (28) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244−13249. (29) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (30) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (31) Alcock, C. B.; Hooper, G. W. Thermodynamics of the Gaseous Oxides of the Platinum-Group Metals. Proc. R. Soc. London, Ser. A 1960, 254, 551−561. (32) Schäfer, H.; Gerhardt, W. Glühdrahtexperimente mit den Systemen Pt/O2, Ir/O2, Pt/Cl2, Pt/Br and Rh/Br. Z. Anorg. Allg. Chem. 1984, 512, 79−84. (33) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., A 1976, 32, 751−762. (34) Csiszar, S. I.; Haverkort, M. W.; Hu, Z.; Tanaka, A.; Hsieh, H. H.; Lin, H.-J.; Chen, C. T.; Hibma, T.; Tjeng, L. H. Controlling Orbital Moment and Spin Orientation in CoO Layers by Strain. Phys. Rev. Lett. 2005, 95, 187205. (35) Poltavets, V. V.; Croft, M.; Greenblatt, M. Charge Transfer, Hybridization and Local Inhomogeneity Effects in NaxCoO2•yH2O: An X-Ray Absorption Spectroscopy Study. Phys. Rev. B 2006, 74, 125103. (36) Zeng, Z.; Greenblatt, M.; Subramanian, M. A.; Croff, M. Large Low-Field Magnetoresistance in Perovskite-Type CaCu3Mn4O12 Without Double Exchange. Phys. Rev. Lett. 1999, 82, 3164. (37) Hall, M. D.; Dillon, C. T.; Zhang, M.; Beale, P.; Cai, Z.; Lai, B.; Stampfl, A. P. J.; Hambley, T. W. The Cellular Distribution and Oxidation State of Platinum(II) and Platinum(IV) Antitumour Complexes in Cancer Cells. J. Biol. Inorg. Chem. 2003, 8, 726−732.

214040 of the ITN SOPRANO network. A.A.T. was partly supported by the Mobilitas program of the ESF (Grant No. MTT77). The authors are grateful to Susann Scharsach (Max Planck Institute for Chemical Physics of Solids) for performing the thermal analysis.

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REFERENCES

(1) Fjellvåg, H.; Gulbrandsen, E.; Aasland, S.; Olsen, A.; Hauback, B. C. Crystal Structure and Possible Charge Ordering in OneDimensional Ca3Co2O6. J. Solid State Chem. 1996, 124, 190−194. (2) Aasland, S.; Fjellvåg, H.; Hauback, B. Magnetic Properties of the One-Dimensional Ca3Co2O6. Solid State Commun. 1997, 101, 187− 192. (3) Kageyama, H.; Yoshimura, K.; Kosuge, K.; Mitamura, H.; Goto, T. Field-Induced Magnetic Transitions in the One-Dimensional Compound Ca3Co2O6. J. Phys. Soc. Jpn. 1997, 66, 1607−1610. (4) Fleck, C. L.; Lees, M. R.; Agrestini, S.; McIntyre, G. J.; Petrenko, O. A. Field-Driven Magnetisation Steps in Ca3Co2O6: A Single-Crystal Neutron-Diffraction Study. EPL 2010, 90, 67006. (5) Moyosh, T.; Motoya, K. Incommensurate Magnetic Structure and Its Long-Time Variation in a Geometrically Frustrated Magnet Ca3Co2O6. J. Phys. Soc. Jpn. 2011, 80, 034701. (6) Agrestini, S.; Fleck, C. L.; Chapon, L. C.; Mazzoli, C.; Bombardi, A.; Lees, M. R.; Petrenko, O. A. Slow Magnetic Order-Order Transition in the Spin Chain Antiferromagnet Ca3Co2O6. Phys. Rev. Lett. 2011, 106, 197204. (7) Maignan, A.; Hebert, S.; Martin, C.; Flahaut, D. One Dimensional Compounds with Large Thermoelectric Power: Ca3Co2O6 and Ca3CoMO6 with M=Ir4+ and Rh4+. Mater. Sci. Eng., B 2003, 104, 121−125. (8) Mikami, M.; Funahashi, R.; Yoshimura, M.; Mori, Y.; Sasaki, T. High-Temperature Thermoelectric Properties of Single-Crystal Ca3Co2O6. J. Appl. Phys. 2003, 94, 6579−6582. (9) Takami, T.; Horibe, M.; Itoh, M.; Cheng, J. Controlling Independently the Electric and Thermal Properties by Shrinking the Particle Size Down to Nanosize in Quasi-One-Dimensional Ca3Co2O6 and Sr6Co5O15. Phys. Rev. B 2010, 82, 085110. (10) Wei, T.; Huang, Y.-H.; Zeng, R.; Yuan, L.-X.; Hu, X.-L.; Zhang, W.-X.; Jiang, L.; Yang, J.-Y.; Zhang, Z.-L. Evaluation of Ca3Co2O6 as Cathode Material for High-Performance Solid-Oxide Fuel Cell. Sci. Rep. 2013, 3, 1125. (11) Botana, A. S.; Botta, P. M.; de la Calle, C.; Pineiro, A.; Pardo, V.; Baldomir, D.; Alonso, J. A. Non-One-Dimensional Behavior in Charge-Ordered Structurally Quasi-One-Dimensional Sr6Co5O15. Phys. Rev. B 2011, 83, 184420. (12) Niitaka, S.; Kageyama, H.; Kato, M.; Yoshimura, K.; Kosuge, K. Synthesis, Crystal Structure, and Magnetic Properties of New OneDimensional Oxides Ca3CoRhO6 and Ca3FeRhO6. J. Solid State Chem. 1999, 146, 137−143. (13) Niitaka, S.; Kageyama, H.; Yoshimura, K.; Kosuge, K.; Kawano, S.; Aso, N.; Mitsuda, A.; Mitamura, H.; Goto, T. High-Field Magnetization and Neutron Diffraction Studies of One-Dimensional Compound Ca3CoRhO6. J. Phys. Soc. Jpn. 2001, 70, 1222−1225. (14) Mikhailova, D.; Schwarz, B.; Senyshyn, A.; Bell, A. M. T.; Skourski, Y.; Ehrenberg, H.; Tsirlin, A. A.; Agrestini, S.; Rotter, M.; Reichel, P.; et al. Magnetic Properties and Crystal Structure of Sr3CoIrO6 and Sr3NiIrO6. Phys. Rev. B 2012, 86, 134409. (15) Wang, X. X.; Li, J. J.; Shi, Y. G.; Tsujimoto, Y.; Guo, Y. F.; Zhang, S. B.; Matsushita, Y.; Tanaka, M.; Katsuya, Y.; Kobayashi, K.; et al. Structure and Magnetism of the Postlayered Perovskite Sr3Co2O6: A Possible Frustrated Spin-Chain Material. Phys. Rev. B 2011, 83, 100410. (16) Rayaprol, S.; Sengupta, K.; Sampathkumaran, E. V. Magnetic Frustration in the Stoichiometric Spin-Chain Compound Ca3CoIrO6. Phys. Rev. B 2003, 67, 180404(R). (17) Wu, H.; Haverkort, M. W.; Hu, Z.; Khomskii, D. I.; Tjeng, L. H. Nature of Magnetism in Ca3Co2O6. Phys. Rev. Lett. 2005, 95, 186401. 5469

dx.doi.org/10.1021/jp411503s | J. Phys. Chem. C 2014, 118, 5463−5469