Structure, Magnetism, and Valence States of Cobalt and Platinum in Quasi-One-Dimensional Oxides A 3 CoPtO 6 with A = Ca, Sr

Structure, Magnetism, and Valence States of Cobalt and Platinum inQuasi-One-Dimensional Oxides A3CoPtO6 with A = Ca, SrD. 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, Nothnitzer Straße 40, 01187 Dresden, Germany‡National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia§II.Physikalisches Institut, Universitat zu Koln, Zulpicher Straße 77, 50937 Koln, 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 absorptionspectroscopy at the Co-K-, Co-L2,3-, and Pt-L3-edges and X-ray photoelectronspectroscopy establishes unambiguously the divalent state of Co and thetetravalent state of Pt in both compounds, in contrast to the earlier assumptionof the Co3+ and mixed Pt2+/Pt4+ valence states. Magnetization measurementsreveal the paramagnetic behavior down to 2 K with strong evidence for anunquenched orbital moment of the high-spin Co2+. The simple paramagneticbehavior of A3CoPtO6 contrasts with the magnetic transitions observed inCa3CoRhO6, Sr3CoIrO6, and other isostructural materials. This difference isascribed to the nonmagnetic 5d6 state of Pt4+ that prevents magnetic couplingsbetween the Co2+ ions.

■ INTRODUCTIONQuasi-one-dimensional Co oxides are an interesting group ofinorganic materials. Their parent compound, Ca3Co2O6,belongs to the K4CdCl6 structural type.1 Ca3Co2O6 is asemiconductor that shows a sequence of perplexing magnetictransitions, steplike magnetization process,2−4 and an intricate,time-dependent incommensurately modulated magnetic struc-ture.5,6 It is also studied as a thermoelectric material7−9 and amaterial for solid-oxide fuel cells.10 The crystal structure ofCa3Co2O6 entails chains formed by alternating CoO6 trigonalprisms and CoO6 octahedra. A chemical modification of thiscrystal structure is possible via a tuning of the chain structure(e.g., by introducing additional octahedra between the trigonalprisms, as in Sr6Co5O15

11) or via a replacement of theoctahedrally coordinated Co atom with 3d, 4d, and even 5dcations. The latter approach results in a family of A3CoMO6oxides with A = Ca, Sr and M = Co, Mn, Rh, Ru, Ir, Pt (Figure1). Similar to Ca3Co2O6, these compounds reveal largethermopower7 and peculiar magnetic phenomena induced bythe magnetic frustration.12−16

The valence of Co ions in A3CoMO6 has long been underdebate. The magnetic response of Co ions is inexplicablyintertwined with their spin state and may provide ambiguousinformation on the Co valence, especially in systems withseveral magnetic ions. Ca3Co2O6 itself features a uniquecombination of low-spin Co3+ in CoO6 octahedra and high-spin Co3+ in CoO6 trigonal prisms.17,18 The robust Co3+ state

in Ca3Co2O6 led to an assumption on the similar trivalent stateof both Co and Rh in Ca3CoRhO6 (refs 19, 20). While thecombination of Co3+ and Rh3+ is indeed in good agreementwith neutron-scattering data,19,20 a careful spectroscopicstudy21 put forward an alternative Co2+/Rh4+ scenario, inagreement with the electronic structure and the giant orbitalmoment of more than 1 μB on Co ions, which is possible onlyfor Co2+ (ref 22).Here, we focus on Pt-containing members of the A3CoMO6

family and elucidate the valences of Co and Pt ions. WhileCa3CoPtO6 is a novel, hitherto unknown compound, its Sranalogue is known since the 1990s.23,24 Following theCa3CoRhO6 scenario, one expects the Co

2+/Pt4+ valence statesin Sr3CoPtO6. However, earlier photoemission spectra24

indicated a lower valence of Pt, which led the authors of ref24 to postulate the presence of Co3+ together with the 1:1mixture of Pt2+ and Pt4+, because the 3+ state is uncommon forPt.25 Here, we critically revise this scenario and argue for themore conventional Co2+/Pt4+ regime confirmed by an extensivespectroscopic study and crystallographic analysis. BothCa3CoPtO6 and Sr3CoPtO6 are paramagnets down to 2 K, ina sharp contrast to other members of the A3CoMO6 familywhich show magnetic ordering with fairly high ordering

Received: November 22, 2013Revised: January 17, 2014Published: January 24, 2014

Article

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

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

■ EXPERIMENTAL SECTION

Synthesis and Sample Characterization. TheSr3CoPtO6 and Ca3CoPtO6 samples were prepared by solid-state reactions in air at 1273 K (Sr) and 1223 K (Ca) fromstoichiometric powder mixtures of CoO (Alfa Aesar, 99.999%)with PtO2 (Alfa Aesar, 99.99%) and SrCO3/CaCO3 (AlfaAesar, 99.99%), placed into Pt crucibles. Two intermediategrindings were required to obtain phase-pure materials. Inorder to determine the optimal synthesis conditions andthermal stability of the products, thermal behavior of thesynthesized Sr3CoPtO6 and of a mixture of initial compoundsSrCO3, 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 thescan rate of 10 K/min from room temperature up to 1273 K.The phase analysis and the determination of unit cellparameters were carried out using X-ray powder diffraction(XPD), performed with a laboratory Huber G670 Guiniercamera (Cu Kα1 radiation, Ge monochromator, image platedetector, 2θ = 3−100° angle range).The Sr4PtO6 compound, used as a Pt4+ reference material for

X-ray absorption spectroscopy (XAS) measurements, wassynthesized by a solid-state reaction from stoichiometricpowder mixtures of PtO2 with SrCO3 in air at 1423 K during24 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 and350 K for powdered Ca3CoPtO6 and Sr3CoPtO6 samples at thefield strengths of 0.1, 0.5, and 5 T using a SQUIDmagnetometer (MPMS) from Quantum Design. The fielddependence of the magnetization was determined at 2 K up to5 T after cooling the sample in zero field. Same samples wereused 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 Ca3CoPtO6were recorded at the 16 B, 08B, and 07C beamlines,respectively, of the National Synchrotron Radiation ResearchCenter (NSRRC) in Taiwan. The Co-L2,3-edge spectrum of

CoO, Co-K spectrum of EuCoO3 (ref 26), and Pt-L3 spectrumof Sr4PtO6 were also measured as reference systems for Co2+,Co3+, and Pt4+, respectively. For these measurements thesamples were pressed into pellets of about 5 mm in diameterand 2 mm thick and annealed at synthesis temperatures forseveral hours. Fresh sample surfaces were obtained byfracturing the pellets in situ in UHV chambers with a basepressure of 1 × 10−9 mbar. The Pt-L3- and Co-K-edge spectrawere collected at room temperature applying the bulk sensitivetotal fluorescence yield method using a Lytle detector. The Co-L2,3-edge spectra were recorded in the total electron yieldmode. The photon energy resolution was about 1.5 eV at thePt-L3-edge, 1.4 eV at the Co-K-edge, and 0.3 eV at Co-L2,3-edge.

X-ray Photoelectron Spectroscopy. The photoemissionspectra were measured using an XPS setup equipped with aVacuum Generators twin crystal monochromatized Al Kαsource and a Scienta electron energy analyzer R3000. Theoverall resolution was set to about 0.4 eV. The binding-energyscale was calibrated using the Fermi-level value of metallic silverat 1482.24 eV as 0 eV for the binding energy. To obtain cleansurfaces, the samples were fractured in situ in XPS setup withbase pressures of 1 × 10−10 mbar.

Electronic Structure Calculations. Scalar-relativisticelectronic structure calculations for Sr3CoPtO6 were performedin the framework of density functional theory (DFT) using thefull-potential FPLO code with the basis set of atomic-like localorbitals.27 The local-density approximation (LDA) exchange-correlation potential by Perdew and Wang (ref 28) was applied.The first Brillouin zone was sampled with a fine k mesh of13824 points (1313 points in the symmetry-irreducible part).Additionally, DFT+U+SO (spin−orbit) calculations with amean-field correction for correlation effects were performed inthe VASP5.2 code.29,30 We used the parameters UCo = 5 eV andJCo = 0.9 eV for the on-site Coulomb repulsion and Hund’sexchange, respectively.17,22

■ RESULTS AND DISCUSSION

1. Synthesis and Crystal Structure Characterization.The Sr3CoPtO6 compound forms upon heating reactants in airto 1273 K with the heating rate of 10 K/min during the DTA-TG measurement of the initial SrCO3, CoO, and PtO2 mixture(Figure 2). The reaction is a stepwise process including the

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 ofalternating 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.

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decomposition of PtO2 and SrCO3. The detected mass loss of19% (evaporation of CO2) was less than the expected value of21%, thus indicating residual carbonates present in the sample.Although all reflections in the diffraction pattern could beindexed in the R3 c space group, a notable broadening of (hkl)reflections in comparison to the (hk0) ones pointed out a highconcentration of structural defects (data not shown). Severalintermediate grindings were required to complete theformation of A3CoPtO6 and obtain the samples without peakbroadening.The TG analysis of the single-phase Sr3CoPtO6 (6 h in the

O2 atmosphere at 1073 K and subsequent cooling to roomtemperature) showed an irreversible mass loss of 0.67% (w/w),which likely corresponds to the elimination of Pt in the form ofoxides. Above 1273 K, platinum-group metals may indeed formvolatile oxides in an oxygen-containing atmosphere.31 Thiseffect is enhanced with increasing partial pressure of oxygen. Inthe Pt−O system, the most stable gaseous oxide is PtO2 (refs31, 32). In order to avoid the loss of Pt from A3CoPtO6, thesynthesis procedure in air instead of pure O2 atmosphere waschosen. The oxygen-free atmosphere is also detrimental forA3CoPtO6. For example, the heat treatment of the SrCO3,CoO, and PtO2 mixture in argon led to the formation of Sr,Cooxides and metallic Pt.The XRD patterns of isostructural Ca3CoPtO6 and

Sr3CoPtO6 are presented in Figure 3. The structural parametersobtained from the Rietveld refinement are listed in Table 1.The Ca sample contains metallic Pt (about 1% w/w) as asecond phase. For the refinement, a structural model derivedfrom that of Sr3CoIrO6 (ref 14) was applied. The resultingCo−O distances are 2.159(2) Å for the Ca compounds and2.185(2) Å for the Sr compounds. These values are in verygood agreement with the Co−O distance of 2.192(1) Å for theCo2+ (HS) in CoO6 prisms of Sr3CoIrO6 (ref 14). The Pt−Odistances of 1.982(2) Å and 1.996(2) Å correlate with the sumof Shannon ionic radii of Pt4+ and O2− of 2.025 Å for PtO6octahedra.33 The Pt2+ ions would lead to a much longer Pt−Obond length of 2.20 Å that is not observed experimentally. Nomixed Co/Pt occupancies were found in either of thestructures.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

compounds that show at least one magnetic transition below100 K.2−4,12−16 Inverse susceptibilities follow the Curie−Weisslaw χ = C/(T − θ) with the parameters listed in Table 2, wherewe recalculated the Curie constant C = NAμeff

2/3kB into theparamagnetic effective moment μeff. In this equation, NA is theAvogadro constant, and kB is the Boltzmann constant. Thesmall θ values indicate very weak magnetic couplings inA3CoPtO6.While the effective moments are typically used to distinguish

between different valence states of transition metals, the case ofCo ions is not straightforward. In Table 2, we provide spin-onlyeffective moments calculated for different valence scenarios:

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 theRietveld refinement of the structure model from the ref 14, anddifference curves (blue lines). The Ca3CoPtO6 sample containsmetallic Pt as a second phase (bottom tick marks).

Table 1. Structural Features of Ca3CoPtO6 and Sr3CoPtO6(R3c, Space Group 167) Refined from X-ray PowderDiffraction Dataa

parameters Ca3CoPtO6, 298 K Sr3CoPtO6, 298 K

a (Å), c (Å) 9.21133(4), 10.97337(6) 9.60094(2), 11.21656(3)V (Å3) 806.335(6) 895.401(4)Z 6 6calcd density(g/cm3)

5.81014 6.81912

Sr 18e 18e(x, y, z) (0.36392(9), 0, 0.25) (0.36449(4), 0, 0.25)B (Å2) 1.31(3) 2.09(2)

Co 6a 6a(x, y, z) (0, 0, 0.25) (0, 0, 0.25)B (Å2) 1.89(4) 2.04(3)

Pt 6b 6b(x, y, z) (0, 0, 0) (0, 0, 0)B (Å2) 2.33(1) 2.06(1)

O 36f 36f(x, y, z) (0.1802(2), 0.0280(2),

0.1128(1))(0.1710(2,) 0.0220(2),0.1124(2))

B (Å2) 1.24(5) 2.30(5)Co−O (Å) 2.159(2) 2.185(2)Pt−O (Å) 1.982(2) 1.996(2)Bragg R factor, % 2.56 1.86Rf factor, % 2.13 1.71aFigure 3.



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 hypotheticaloctahedral 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 μBexpected for the Co2+/Pt4+ pair is lower than the experimentalone, but it can be augmented by the orbital component, whichis very typical for Co2+ (ref 17, 22). On the other hand, theCo3+/Pt2+,Pt4+ scenario overestimates the experimental effec-tive moment, which is not unusual considering the poorlydefined contribution of Pt2+ (note that Pt2+ is typically found inthe diamagnetic state in the 4-fold oxygen environment, andnot in the paramagnetic S = 1 state in the octahedralcoordination) and possible nonmagnetic impurities.The field dependence of the magnetization measured on

powder samples shows a bend around 3 T and keeps increasing

in higher fields, while reaching only 1.5−2.0 μB/f.u. (formulaunit) at 5 T. This magnetization is much smaller than expectedfor either Co2+ and Pt4+ (spin-only moment of 3 μB), or Co

3+

and Pt2+/Pt4+ (spin-only moment of 5 μB) valence regime.However, the increase in the magnetization up to at least 5 Timplies that the saturation is not reached. Considering the lowθ values derived from the Curie−Weiss fit, the lack ofsaturation at 5 T cannot be ascribed to antiferromagneticcouplings and should be rather understood as an effect ofmagnetic 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 Ptwith X-ray absorption spectroscopy.

3. Room Temperature X-ray Absorption Spectrosco-py. 3.1. Co L2,3- and K-Edge. Co-L2,3 absorption spectra ofA3CoPtO6 (A = Ca, Sr) together with CoO as a Co2+ referenceare shown in Figure 5a. The “center of gravity” of the L3 whiteline of Ca3CoPtO6 and Sr3CoPtO6 lies at the same energy asthat of CoO and at more than 1 eV lower energy than that ofthe trivalent Co oxide EuCoO3 demonstrating the divalent stateof cobalt in Ca3CoPtO6 and Sr3CoPtO6. The multiplet spectralstructure of both Ca3CoPtO6 and Sr3CoPtO6 is the same asthat of Ca3CoRhO6, which we have reproduced from ref 21indicating the trigonal-prismatic local symmetry (D3d) of Co

2+

ions. The different line shape in A3CoPtO6 and CoO reflectsthe different local environment of Co2+ ions in thesecompounds. For example, the first multiplet feature in theCo-L2,3 spectrum around 778 eV is very pronounced for CoOand nearly invisible for the A3CoPtO6 compounds. As shown inFigure 5a for the Co2+ ion in the D3d symmetry, two minorityelectrons have the orbital occupation of d0d2 (ref 21), while forthe Co2+ in the octahedral Oh symmetry two minority electronsoccupy t2g orbitals.

34

We also have measured the Co-K-edge with the more bulksensitive total fluorescence yield (FY). Figure 5b shows the Co-K XAS spectra of Ca3CoPtO6 and Sr3CoPtO6, together withEuCoO3 as a Co3+ reference material. All spectra arenormalized to unity step in the absorption coefficient μ (200eV above the absorption edge). It is well-known that theposition of the absorption edge is related to the valence state of3d transition metals. There are several ways to definite theposition of the absorption edge for valence determination ofthe transition metal, for example the main peak (P) around7712 eV (ref 35), or the value at 0.8 of the edge jump μ (ref36). For our samples, both methods give very similar results,namely, that we can observe a chemical shift of 3 eV in goingfrom Ca3CoPtO6 and Sr3CoPtO6 to EuCoO3, but nearly noshift with respect to CoO. This resembles the shift fromLa2CoO4 to LaCoO3 (ref 35), confirming firmly the Co

2+ statein bulk Ca3CoPtO6 and Sr3CoPtO6.

3.2. Pt-L3-Edge. After establishing the Co2+ state inCa3CoPtO6 and Sr3CoPtO6, we turn to the Pt-L3 XAS spectrato verify the Pt4+ state, as required from the charge balance.The white line in the Pt-L3-edge of Ca3CoPtO6 and Sr3CoPtO6spectra (Figure 6) lies at the same energy as in the tetravalentPt reference compound Sr4PtO6, which is isostructural toA3CoPtO6. This indeed confirms the Pt4+ state in both Ptcompounds.37 Thus, XAS measurements give unambiguousevidence for the presence of Co2+ and Pt4+ in the Ca3CoPtO6and Sr3CoPtO6 compounds.

4. Room Temperature X-ray Photoelectron Spectros-copy. The Co 2p core level spectra of Sr3CoPtO6 and

Figure 4. Temperature and field dependence of the magnetization ofCa3CoPtO6 and Sr3CoPtO6.

Table 2. Effective Magnetic Moments of Ca3CoPtO6 andSr3CoPtO6 from the Experiment and Calculated ValuesBased on a Spin-Only Model for the CombinationCo2+(HS)/Pt4+(LS) and Co3+(HS)/(0.5Pt2+(LS) +0.5Pt4+(LS))

μeff(calc), μB

compdtemp rangefor fit, K Θ, K

Co3+(HS)/0.5Pt2+ + 0.5Pt4+

Co2+/Pt4+

μeff(exp),μB

Ca3CoPtO6 2−350 −8.0 6.00 3.87 4.71(1)Sr3CoPtO6 2−350 −5.3 6.00 3.87 4.91(1)



Ca3CoPtO6 show the Co 2p3/2 and Co 2p1/2 main peaks at780.3 and 795.7 eV, respectively, and strong satellite structuresat about 6 eV higher in energy (Figure 7). The intensity ratioand the energy separation in the A3CoPtO6 spectra are similarto those of the CoO reference material with the known high-spin Co2+ state. This then supports the XAS finding that cobaltin A3CoPtO6 has the 2+ oxidation state. In the Pt 4f spectra of

A3CoPtO6, the 4f7/2 and 4f5/2 lines are very sharp, indicatingthat the samples contain a single-valent Pt and not a mixture ofvalences, i.e., Pt4+ and not Pt2+/Pt4+ as it was claimed in ref 24.

5. Band Structure Calculations. Band structure ofSr3CoPtO6 calculated within the local density approximation(LDA) reveals the broad valence band formed by the O 2pstates that are strongly hybridized with the 5d t2g states of Pt,whereas the 5d eg states of Pt also mix with oxygen and lieabout 1 eV above the Fermi level (Figure 8, top). The Co 3dstates form narrow bands in the vicinity of the Fermi level.They show the crystal-field splitting, which is typical fortrigonal-prismatic symmetry of the Co2+ ion.21,22

The LDA band structure is metallic, whereas the dark-browncolor of Sr3CoPtO6 suggests insulating behavior. Thisdiscrepancy is very typical for transition-metal compounds,where LDA does not capture the essential physics of strongelectronic correlations opening the gap in the energy spectrum.The realistic gapped spectrum can be reproduced by the so-called LSDA+U+SO method that includes a mean-fieldcorrection for correlation effects U and, additionally, therelativistic effects of the spin−orbit coupling (SO) that arerequired to lift the residual orbital degeneracy.22 Theapplication of LSDA+U+SO indeed opens the gap of 2.2 eVfor the on-site Coulomb repulsion parameter UCo = 5 eV andthe ferromagnetic spin configuration (Figure 8, bottom).

Figure 5. (a) Co-L3,2 absorption spectra of A3CoPtO6 together with the reference spectra for Ca3CoRhO6 (ref 21), CoO (Co2+ reference), andEuCoO3 (Co

3+ reference). Right panel depicts the likely scheme of orbital occupations for the HS-Co2+ ion in the trigonal-prismatic (D3d: A3CoPtO6and 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.

Figure 6. Pt-L3 absorption spectra of A3CoPtO6 (A = Ca, Sr) and Pt4+

reference material Sr4PtO6. Dashed line corresponds to the center ofgravity of the spectra.



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

0.01 μB) and a large magnetic moment on Co (μCo(spin) =2.70 μB, μCo(orb) = 1.73 μB). Therefore, we conclude thatSr3CoPtO6 features the nonmagnetic Pt4+ ions and themagnetic Co2+ ions. This result is in agreement with thespectroscopic determination of the Co2+/Pt4+ valence states inSr3CoPtO6, whereas the presence of the large orbital moment isquite typical for Co2+ in the trigonal-prismatic coordination21,22

and indeed seen in our magnetization data. The nonmagneticPt4+ ions strongly reduce the exchange interactions along theCo−Pt−Co chains. This explains naturally why the A3CoPtO6compounds are paramagnetic to the lowest temperaturesmeasured, in contrast to Ca3CoRhO6 and Ca3CoIrO6 thatshow clear magnetic ordering with fairly high orderingtemperatures.

■ CONCLUSIONSA new quasi-one-dimensional Ca3CoPtO6 oxide was success-fully synthesized in addition to the already known Sr3CoPtO6.Both compounds feature the Co2+ and Pt4+ valence states, asevidenced by the XAS and XPS measurements therebyresolving the long-standing issue regarding the possiblecoexistence of Pt2+ and Pt4+ in Sr3CoPtO6. This way, we refutethe unlikely scenario of the mixture of Pt2+ and Pt4+ ionsoccupying 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. Theformation of Co3+ in Ca3Co2O6 turns out to be a unique featurerelated to the coexistence of Co3+ in the octahedral andtrigonal-prismatic coordination. We argue that further membersof 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+ inperfect agreement with the experimental high-temperaturemagnetization data that cannot be understood without takinginto account the orbital moment of Co2+. A verification of thisscenario and a more accurate experimental evaluation of theorbital moment require X-ray magnetic circular dichroism(XMCD) experiments, which are presently underway. Thelarge orbital moment is another generic feature of theA3CoMO6 oxides. It is directly related to the trigonal-prismaticcoordination of Co2+ and triggered by strong electroniccorrelations, as explained in ref 21.In contrast to all Ca3Co2O6-type compounds known so far,

the A3CoPtO6 oxides are paramagnetic down to 2 K. Weascribe this atypical behavior to the low-spin state of Pt4+. Thenonmagnetic Pt4+ ions prevent magnetic interactions betweenCo2+. Therefore, the M4+ cation controls the magnetism ofA3CoMO6. By choosing different 4d and 5d metals, one candeliberately design ferromagnetic or antiferromagnetic inter-actions along the structural chains and achieve various regimesof quasi-1D magnets based on Co2+.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSA.E. acknowledges the support and funding from the EuropeanUnion via the FP7/2007-2013 under Grant Agreement No.

Figure 7. Room temperature Pt 4f and Co 2p photoelectron spectra ofCa3CoPtO6 and Sr3CoPtO6 together with Co 2p spectrum of CoOreference material.

Figure 8. Electronic density of states (DOS) calculated for Sr3CoPtO6within LDA (top panel) and LSDA+U+SO for the ferromagnetic spinconfiguration (bottom panel). The Fermi level is at zero energy.



mailto:[email protected]

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

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