PHYSICAL REVIEW MATERIALS 1, 024406 (2017)

Magnetism and the spin state in cubic perovskite CaCoO3 synthesized under high pressure

Hailiang Xia,1,2 Jianhong Dai,1,2 Yuanji Xu,1,2 Yunyu Yin,1,2 Xiao Wang,1,2 Zhehong Liu,1,2 Min Liu,1 Michael A. McGuire,3

Xiang Li,4 Zongyao Li,4 Changqing Jin,1,2,5 Yifeng Yang,1,2,5 Jianshi Zhou,4,* and Youwen Long1,2,5,†1Beijng National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

2School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China3Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

4Materials Science and Engineering Program, Department of Mechanical Engineering, University of Texas, Austin, Texas 78712, USA5Collaborative Innovation Center of Quantum Matter, Beijing 100190, China

(Received 1 April 2017; published 17 July 2017)

Cubic SrCoO3 with an intermediate spin state can only be stabilized by high pressure and high temperature(HPHT) treatment. It is metallic and ferromagnetic with the highest Curie temperature of the transition-metalperovskites. The chemical substitution by Ca on Sr sites would normally lower crystal symmetry from cubicto orthorhombic as seen in the perovskite family of CaMO3 (M = M4+ of transition metals, Ge4+, Sn4+, andZr4+) at room temperature. This structural change narrows the bandwidth, so as to further enhance the Curietemperature as the crossover to the localized electronic state is approached. We report a successful synthesis ofthe perovskite CaCoO3 with a HPHT treatment. Surprisingly, CaCoO3 crystallizes in a simple cubic structurethat remains stable down to 20 K, the lowest temperature in the structural study. The new perovskite has beenthoroughly characterized by a suite of measurements including transport, magnetization, specific heat, thermalconductivity, and thermoelectric power. Metallic CaCoO3 undergoes two successive magnetic transitions at 86 Kand 54 K as temperature decreases. The magnetization at 5 K is compatible with the intermediate spin state t4e1

of Co4+ at the octahedral site. The thermal expansion of the Co-O bond length indicates that the population ofhigh spin state t3e2 increases for T > 100 K. The shortest Co-O bond length in cubic CaCoO3 is responsiblefor delocalizing electrons in the π∗-band and itinerant-electron ferromagnetism at T < 54 K. A comprehensivecomparison between SrCoO3 and CaCoO3 and the justification of their physical properties by first-principlescalculation have also been made in this report. Partially filled π∗ and σ ∗ bands would make CaCoO3 suitable tostudy the Hund’s coupling effect in a metal.

DOI: 10.1103/PhysRevMaterials.1.024406

I. INTRODUCTION

Perovskite cobalt oxides have been well known for aspin-state transition due to the exchange splitting �ex matchingthe crystal-field splitting energy �c. For Co3+ in RCoO3

(R = rare earth), depending on the bandwidth W of the σ ∗band and � = �ex − �c, the perovskites can exhibit thelow-spin (LS) state t6e0 and higher-spin (HS) state t6−xex

[1–5]. The spin state transition can be tuned either bya chemical substitution of the rare earth [6] or varyingtemperature, as in the thermally driven spin-state transitionfound in LaCoO3 [7]. It appears for the perovskites with Co4+that a � = �ex − �c ≈ 0 holds also, which led Potze et al.[8] to propose the intermediate-spin (IS) state t4

2 e1 in orderto explain the x-ray absorption spectra in SrCoO3. The ISstate has been shown to be consistent with the magnetizationmeasurement of a single-crystal sample of SrCoO3 [9]. Thespin state transition is normally associated with anomalies oftransport and magnetic properties [10,11], thermal expansion[12–14], bulk modulus [15,16], and the inelastic property[17]. In addition to the interest of fundament research on thespin-state transition, the capability to manipulate the spin statein transition-metal oxides can also be used to enhance catalyticproperties for clean-energy technology [18].

*Corresponding author: jszhou@mail.utexas.edu†Corresponding author: ywlong@iphy.ac.cn

In a cubic crystal field, the σ ∗ band of transition-metaloxides is broader than the π∗ band. Populating electrons inthe σ ∗ band normally leads an increase in the conductivity.Electrons in the σ ∗ band can also undergo a localizedto itinerant transition. In the RNiO3 family, an evolutionfrom a localized to an itinerant electronic state can betuned through either chemical substitution [19–21] or byapplying hydrostatic pressure [22–24] on a composition on thelocalized-electron side of the transition. At ambient pressure,the bond length mismatch between the R-O and Ni-O bondsprogressively lowers the crystal symmetry from rhombohedralto orthorhombic depending on the ionic radii of R3+. As aresult, the Ni-O-Ni bond angle is reduced from 180◦ by anangle θ in the phases with lower symmetries. The width of theσ ∗ band is related to cos θ [24]. On crossing the insulator-metal transition, the Ni-O bond length changes abruptly[25], indicating a first-order transition. The electron-electroncorrelations facilitate the spin-spin exchange interaction. Forlocalized electrons, the perturbation formula of superexchangeinteraction TN ∼ J ∼ b2/U applies, where U is the on-sitecorrelation energy and b is the orbital overlap integral. Onthe itinerant-electron side, any magnetic transition graduallyfades away as the bandwidth broadens. The magnetic transitiontemperature peaks at the crossover as predicted by solving theMott-Hubbard Hamiltonian numerically [26]. This scenariohas been applied to explain an enormously high TN found inSrTcO3 and CaTcO3 [27–29].

Compared to Co(III) in RCoO3, Co(IV) in ACoO3 (A =alkaline earth) has a much enhanced d-p hybridization because

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HAILIANG XIA et al. PHYSICAL REVIEW MATERIALS 1, 024406 (2017)

the Co3+/4+ redox potential moves to the top of the O 2p band,which leads to a broadening of the σ ∗ band. The metallicSrCoO3 with the IS state is consistent with a broader σ ∗ band[9,30]. The ferromagnetism found in the metallic SrCoO3,however, is likely caused by the direct exchange interactionin the system with itinerant electrons in the σ ∗ band andlocalized electrons on the t2 orbitals. A further reduction ofthe Co-O bond in the cobalt oxide may lead to either the ISto LS transition or delocalizing the t2 electrons. In this paper,we report the successful synthesis of CaCoO3 for the firsttime. In contrast to the normal relationship between the crystalstructure and the chemical substitution in perovskite oxides,the Ca substitution does not lead to a transition from the cubicto the orthorhombic structure. Instead, CaCoO3 remains in thecubic phase. CaCoO3 not only stands out in terms of the crystalstructure among CaMO3 perovskites (M = M4+ of transitionmetals, Ge4+, Sn4+, Zr4+), but also shows physical propertiesrarely seen in transition-metal oxides.

SrCoO3 crystallizes in the cubic perovskite structure withthe Pm-3m space group [9,30]. High oxygen pressure isneeded to stabilize the Co4+ valance state. By using thestructural prediction software SPuDs [31] for perovskitephases, a geometric tolerance factor t ≈ 1 can be obtainedfor CaCoO3, which means that a similar cubic phase shouldbe stabilized in CaCoO3. However, all attempts to synthesizeCaCoO3 failed so far. This difficulty has been overcome in thiswork by oxidizing the brownmillerite Ca2Co2O5 under highoxygen pressure.

II. DETAILS ABOUT EXPERIMENTS AND THECALCULATION

We have applied the two-step method used for theSrFe1−xCoxO3 solid solution [32] to synthesize CaCoO3. Asthe first step, oxygen-deficient Ca2Co2O5 with the brown-millerite structure was prepared by following the procedurereported elsewhere [33]. Then a Ca2Co2O5 pellet, 3.0 mm indiameter and 2.0 mm in length, was sealed into a Pt capsuletogether with the oxygen releasing agent KClO4 at both endsof the precursor. The assembly, including the capsule, heater,and pressure medium, was treated at 7 GPa and 1500 K for halfan hour in a cubic anvil-type high-pressure apparatus [34]. Atthe end of this high-pressure treatment, the heating power wasshut down before the pressure was released. The high-pressureproducts were first checked by using powder x-ray diffraction(XRD) on a Huber diffractometer (Cu-Kα1 radiation, 40 kV,30 mA). The sample with a single phase of the perovskitestructure was further studied with a PANalytical X’pert diffrac-tometer (Cu-Kα1 radiation) with an Oxford PheniX cryostat atOak Ridge National Laboratory. Crystallographic parameterswere obtained by using Rietveld refinement with the FULL-PROF program [35]. Thermogravimetric (TG) analysis wasperformed on a Setaram TG-DTA system with a temperaturescan up to 1200 K in a gas flow of the mixture H2 (4%)and N2 (96%). The temperature-dependent magnetization wasmeasured by using a superconducting quantum interferencedevice (SQUID) magnetometer from Quantum Design withthe applied fields H = 0.1 and 1 T. The SQUID magnetometerand the VSM in a Physical Property Measurement System(PPMS) were used to perform the isothermal magnetization

measurements. The data of electrical transport and specificheat were collected on the PPMS. Measurements of thermalconductivity and thermoelectric power were carried out withhomemade setups. The structural study under high pressurewas carried out by using a diamond anvil cell. The detailedinformation about the setup can be found in Ref. [16].

First-principles calculations were performed by using thefull-potential augmented plane-wave and local orbital methods[36], as implemented in the WIEN2k code [37]. To includethe strong correlations for electrons in CaCoO3, we used theGGA-PBE [38] exchange-correlation functional with effectiveCoulomb repulsions (Ueff) in the GGA + U calculations. Themuffin-tin radii were set to be 2.43 a.u. for Ca, 1.82 a.u. for Co,and 1.62 a.u. for O atoms; RMTKmax = 8.00 and 3000 k-pointmeshes were used over the whole Brillouin zone. We alsooptimized the lattice parameter of CaCoO3 and the obtainedlattice parameter is in good agreement with the experimentresults.

III. RESULTS

Figure 1(a) shows the powder XRD pattern of CaCoO3

at room temperature. The structural model used to refine theprofile is a simple Pm-3m cubic perovskite with the latticeparameter a = 3.73431(6) A. The fitting goodness comes as asurprise since all existing CaMO3 (M = M4+ of transitionmetals, Ge4+, Sn4+, Zr4+) adopt the Pbnm orthorhombicstructure at room temperature [39]. Refinements with otherstructural models of the perovskite tilting systems such asa0a0c+ (I4/mcm) and b−a+b− (Pbnm) gave slightly worseresults in terms of the reliable factor R. Moreover, the absenceof the low-angle peaks such as the one near 2θ ∼ 25◦ seemsto rule out the possibility of having the Pbnm structure. Thea0a0c+ tilting system does not produce the weak diffractionsat low angles; but all these tilting systems have diffractionpeaks split from that of the cubic phase at higher angles,which is clearly not matching to the narrow diffraction peaksobserved. The TGA analysis in Fig. 1(b) gives a near oxygenstoichiometry (2.98 ± 0.02) of the sample. We have a goodreason to fix the oxygen occupation in the refinement withthe cubic model. The refinement with the cubic structuralmodel is, however, not airtight. Since the atomic displacementparameter B is the only refining parameter in addition to thelattice parameter, a slightly higher intensity of the peak near

60° forces the program to yield an unusually high B(A2) ∼ 13

for the oxygen; a similar B value has been found at 1000 ◦C inCaTiO3 [40,41]. Whereas there is a high uncertainty aboutthe B value from refining the x-ray diffraction pattern, ahigh B value may indicate oxygen fluctuations, which couldbe correlated to an unusually low thermal conductivity ofthe CaCoO3 perovskite, which will be further discussed ina following paragraph. All structural parameters from therefinement are listed in Table I and more detailed informationabout the structural refinement is provided in the SupplementalMaterial [69].

Figure 2(a) shows the temperature dependence of themagnetization. Fitting the temperature dependence of para-magnetic susceptibility above 200 K to a Curie-Weiss lawgives a μeff = 3.58 μB, which is close to the spin-only valueof the IS state S = 3/2 (μeff = 3.87 μB). With decreasing

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FIG. 1. (a) X-ray diffraction pattern and the result of Rietveldrefinement with the cubic model for CaCoO3; observed (circles),calculated (black line), and the difference (blue line) are shown in thepatterns. The detailed information about the refinement is given inthe Supplemental Material [69]. The inset is the structural model of acubic perovskite. (b) Temperature dependence of thermogravimetric(TG) loss for CaCoO3. The weight of CaCoO3 slightly decreases forT > 300 K. This weight change can usually be attributed to the lossof the surface absorption, especially in the grain boundaries due to theusage of oxidizing agent added during the synthesis. Above 400 K,the TG loss starts to decrease quickly, suggesting the occurrence ofdecomposition reaction. According to the TG loss between 400 and1200 K as well as the final products (CaO and Co), the oxygen contentof CaCoO3 can be determined to be 2.98 ± 0.02. This value is veryclose to the ideal chemical composition, confirming the formation ofthe Co4+ valance state in CaCoO3.

TABLE I. Refinement structure parameters of CaCoO3 obtainedat room temperature. Space group: Pm-3m; Atomic positions: Ca(0.5, 0.5, 0.5), Co (0, 0, 0), O (0.5, 0, 0).

Parameter

a (A) 3.73431(6), 97% volume3.7391(2), 3% volume

Rwp (%) 3.59Rp (%) 2.8χ 2 1.4

temperature, an anomaly of M(T ) occurs at 86 K; it seemsan AFM transition as seen from a zoomed-in plot of M(T ) inthe inset. The AFM transition is followed by another magnetictransition at ∼54 K as temperature further decreases. Thesemagnetic transitions can also be confirmed by anomalies in thespecific heat at the corresponding temperatures in Fig. 2(b).A dramatic increase of the magnetization on cooling through55 K identifies this as a ferromagnetic transition. The downturnobserved in the M(T ) data upon cooling below 25 K and theseparation between the field-cooled and zero-field-cooled datamay be attributed to an increase in magnetic anisotropy uponcooling, or it may indicate that the low-temperature magneticstructure is more complex than a simple collinear ferromagnet.Ferromagnetism below 54 K is further supported by thewell-defined magnetic saturation and a small coercive fieldobserved at T < 54 K in Fig. 2(c). A magnetic-field-inducedtransition is observed at temperatures below TN as shownin Fig. 2(d) for T = 70 K. The onset field Ht to trigger thetransition at 70 K is about 5.5 T, which increases slightlyat lower temperatures. The saturation moment Ms shows apronounced increase at Ht . While Ms continues to increasefor H > Ht , it is still less than 1.0 μB at 14 T. In comparison,Ms is about 2.5 μB for SrCoO3 [9].

The resistivity of CaCoO3 in Fig. 2(e) is smaller than thatof SrCoO3 by one order in magnitude at room temperature andbehaves metallic down to 2 K. An obvious drop of resistivityon cooling through Tc observed in CaCoO3 is absent in SrCoO3

[30], but is similar to the behavior of ρ(T ) on cooling througha ferromagnetic transition temperature Tc in metallic SrRuO3

[42–44]. Whereas SrCoO3 exhibits a small magnetoresistance(MR) at low temperatures [9], the MR effect is stronglyenhanced and peaks around Tc in CaCoO3 as shown in Fig. 2(f).

Figure 3 shows the detailed change of magnetizationcurves at different temperatures. Compared to the isothermof M(H ) at T > TN , the magnetic order induces a strongnonlinear behavior at low field and a metamagnetic transitionat Ht which increases progressively as temperature decreases.The correlation between the field-induced change of themagnetization and the MR effect of resistivity at 40 K is shownin Fig. 4. At the magnetic field where the inflection point of themagnetization curve occurs, a decrease of resistivity is clearlyvisible.

In order to confirm an AF transition in CaCoO3 on coolingthrough 86 K, we have measured the Cp(T ) under differentmagnetic fields. Figure 5 shows the Cp(T ) of CaCoO3 in anarrow temperature range near 86 K. The λ-shape anomalyat 86 K is typical for a second-order magnetic transition.Magnetic field has no effect on Cp(T ) at T > 90 K, but thepeak profile representing the entropy change associated withthe magnetic ordering becomes broad with the peak positionmoving to lower temperatures under magnetic field, which iscommonly seen for an AF transition.

The positive thermoelectric power S of CaCoO3 shown inFig. 6(a) appears not to be consistent with a less than 1/4-filledσ ∗ band in the HS (t5−xex) state of CaCoO3. The hybridizationwith a partially filled π∗ band may be a factor for a positivethermoelectric power. The S(T ) of SrCoO3 also given inFig. 6(a) is positive at room temperature. In comparison withSrCoO3, a much lower thermoelectric power value of CaCoO3

at room temperature would indicate a broader bandwidth in

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HAILIANG XIA et al. PHYSICAL REVIEW MATERIALS 1, 024406 (2017)

FIG. 2. Magnetic and transport properties of CaCoO3. Temperature dependence of (a) magnetization in both zero-field cooling (ZFC) andfield cooling (FC), and (b) specific heat. (c), (d) Field dependence of magnetization at different temperatures. Temperature dependence of(e) resistivity measured at zero field and 5 T, and (f) the magnetoresistance MR(%) = 100%[ρ(5 T) − ρ(0)]/ρ(0). The inset in (a) shows anenlarged view for the AFM transition. The red line in the inverse susceptibility in (a) shows the fit of χ−1(T ) to a Curie-Weiss law between 200and 300 K. The derivative of magnetization in (b) is used to determine the magnetic phase transition temperatures. The red curve in the inset(b) shows the fit of specific heat to the formula Cp = γ T + βT 3/2 + αT 3, giving γ = 66.3(2) mJ mol−1 K−2, β = 4.8(6) mJ mol−1 K−5/2, andα = 0.083(3) mJ mol−1 K−4.

CaCoO3 than that in SrCoO3, which is also consistent withthe comparison of resistivity in these two cobalt oxides. WhileS increases monotonically with increasing temperature, S(T )does not fit the Mott diffusive model of the thermoelectricpower for a metal. The thermal conductivity κ(T ) of CaCoO3

in Fig. 6(b) is low and glassy, which is contrary to that ofmetallic perovskites, such as SrRuO3 [45] and LaNiO3 [46].

30

20

10

0

M (

emu/

g)

806040200

H (103Oe)

84K 74K 54K

96 K46 K

64 K

86 K

FIG. 3. The field dependence of the magnetization in a tem-perature range from 46 K to 64 K with �T = 2 K. The M(H ) atTN ∼ 86 K and TC ∼ 54 K are highlighted by the dash-dotted lines.

As mentioned in the introduction, the spin-state transitionfrom LS to HS in RCoO3 is associated with an anomalouslyhigh thermal expansion. This observation motivated us toperform the structural study at different temperatures. Thecubic phase remains stable to 20 K, the lowest temperature inthe structural study. The temperature dependence of the Co-Obond length from the refinement of the XRD pattern is shownin Fig. 7 together with those of other perovskite oxides forcomparison.

In the structural study under high pressure, we found thatCaCoO3 becomes unstable under high pressure; diffractionpeaks become broad and new peaks not from the cubic phase

600

580

560

R (μ

Ω)

806040200H (10

3 Oe)

30

20

10

0

M (e

mu/

g)

(a)

(b)

FIG. 4. The field dependence of (a) the magnetization and (b) theresistance for CaCoO3 at 40 K.

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40

38

36

34

Cp(

J/m

ol K

)

1009080T (K)

0T 5T 7T

FIG. 5. The temperature dependence of specific heat for CaCoO3

at different magnetic fields

20

15

10

5

0

S (μ

V /K

)

CaCoO3 SrCoO3

60

40

20

0

κ (m

Wcm

-1K

-1)

4003002001000T (K)

(a)

(b)

FIG. 6. Temperature dependence of (a) the thermoelectric powerand (b) the thermal conductivity for CaCoO3. The data of SrCoO3

are after Ref. [30].

1.008

1.006

1.004

1.002

1.000

Reno

rmal

ized

M-O

bon

d le

ngth

4002000T (K)

LaCoO3CaCoO3 PrCoO3 PrCrO3 PrGaO3

FIG. 7. Temperature dependence of renormalized M-O bondlength of CaCoO3 together with those of other perovskite oxidesCaMO3 for comparison.

FIG. 8. Spin-polarized partial density of states of Co 3d for thecubic perovskite CaCoO3 using GGA + U (U = 4 eV) with (a) spin-up, and (b) spin-down components. (c) Co magnetic moment as afunction of isotropic lattice compression in CaCoO3 in which ε =(a/a0 − 1) × 100%.

appear. However, fitting the curve of V versus pressure atlow pressures (to 2 GPa) in which the cubic phase componentcan still be identified, to the Birch-Murnaghan equation givesa bulk modulus Bm = 165 ± 8 GPa. The Bm value is muchsmaller than that found in most perovskite oxides, but identicalto that of PrCoO3 where the unusually small Bm has beeninterpreted as a consequence of the pressure-induced spin-statetransition from the HS to LS at room temperature [16].

Results of our GGA + U calculation of Figs. 8(a) and 8(b)and a previous DFT calculation [47] show that the Fermienergy cuts into both π∗ and σ ∗ electron bands, which isconsistent with the higher spin state. However, the calculatedoverlap between π∗ and σ ∗ bands appears too large, whichleads to a high saturation moment 2.4−3.0 μB close to that for

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TABLE II. Structure and physical properties of CaCoO3 andSrCoO3. The theoretical calculation results of SrCoO3 are afterRef. [47], whereas the results of CaCoO3 are from this work.

SrCoO3 CaCoO3

Space group Theory Pm-3m Pm-3m

Experiment Pm-3m Pm-3m

Lattice Theory 3.78/1.89 3.765/1.8825parameters/Co-O Experiment 3.83/1.915a 3.73431(6)/1.867bond length (A) 3.83/1.915b

Saturation Theory 2.448 2.396 (Ueff = 0 eV)moment (μB) (U = 0) 2.791 (Ueff = 3 eV)

3.050 (Ueff = 4 eV)Experiment 2.50 0.75c

μeff (μB) TheoryExperiment 3.58

γ (mJ K−2 mol−1) Theory 7.3 8.1 (Ueff = 0 eV)6.4 (Ueff = 3 eV)5.7 (Ueff = 4 eV)

Experiment 45.7 53–71d

A (10−5 m� cm K−2) TheoryExperiment 2–9 2.7e

aData from Ref. [9].bData from Ref. [30].cA continuous increase of the magnetization is due to the field-induced spin state transition; this is a cutoff value at H = 4 T.dThis value is sample dependent.eFitting to the Fermi-liquid behavior cannot be performed at verylow temperature since there is an obvious resistivity downturn atT <∼ 10 K; this value was obtained from a fitting at T > 10 K.

the IS state. The lattice parameter is also slightly higher thanthe experimental value at 20 K. By reducing the cell parameter,our calculation successfully predicts a transition from the ISspin state to LS spin state as shown in Fig. 8(c).

IV. DISCUSSION

Physical properties of CaCoO3 and SrCoO3 are summarizedin Table II together with the results from the electronicstructure calculations for comparison. Co(IV) in the cubicSrCoO3 is in the IS state; the paramagnetic susceptibility ofCaCoO3 suggests that CaCoO3 is also in the IS state. Theevolution of electrons in the π∗ and σ ∗ band from SrCoO3

to CaCoO3 would be a key to understanding the peculiartransport and magnetic properties found in CaCoO3. Thechange of electronic behavior is normally reflected in thecrystal structure.

There is no doubt that electrons in the σ ∗ band of SrCoO3

are itinerant. Ferromagnetism in the perovskite is mediatedby the direct exchange interaction in a system with localizedelectrons on t2 orbitals and itinerant electrons in the σ ∗band. In contrast to all AMO3 perovskites (A = Ca, Sr, Ba,M = transition metals, Ge, Sn, Zr) where the replacementof Sr by Ca always ends up with a cubic to orthorhombictransition and/or an enlarged octahedral-site rotation and M-Obond length in the Pbnm structure [39], the same chemicalsubstitution in ACoO3 does not lead to a change in the crystal

2.00

1.95

M-O

bon

dlen

gth

(Å)

1.61.41.2IR (Å)

1.92

1.90

1.88

1.86Ca Sr Ba

V

Co

Ir

RuOs

Ti

FIG. 9. The M-O bond length as a function of ionic radius (IR)of alkaline-earth element in perovskite oxides AMO3.

symmetry, but to a collapse of the Co-O bond length withinthe cubic space group. The Co-O bond length in ACoO3

versus the ionic radius (IR) of the A cation is plotted inFig. 9 together with other transition-metal perovskites AMO3

(M = Ti, V, Ru, Os, Ir) [40,48–56] for comparison. Themetallic AVO3 is the only other family to show a bondlength contraction from SrVO3 to CaVO3. Electrons in theπ∗ band of these vanadate oxides are itinerant. However, thebond length contraction from SrMO3 to CaMO3 is much moredramatic in ACoO3 than that in AVO3, as is displayed. Thischange places CaCoO3 to be the perovskite oxide havingthe smallest unit cell and M-O bond length in the literatureso far. The most conducting perovskite ReO3 has all the A

sites vacant, but the M-O bond length (1.873 A) in ReO3

[57] is slightly larger than that in CaCoO3. This structuralanalysis signals that electrons in both t2 and eg orbitals may bedelocalized in CaCoO3. If so, the low-temperature magneticphases in CaCoO3 should be attributed to itinerant electronmagnetism.

Linear behavior in the Arrott plot of magnetization is adirect consequence of itinerant electron ferromagnetism [58].For this test, we have converted the isothermal M(H ) of Fig. 3into the Arrott plot in Fig. 10. The obvious anomaly at highfields is due to a field-induced transition; more discussionabout the field-induced transition will be given in a followingparagraph. The majority of the isothermal near Tc can be fittedby straight lines; the extension of the line corresponding toTc passes through the origin. All Heisenberg magnets withlocalized moments would have all isotherms with concavedcurvature in the Arrott plot. SrRuO3 is the only perovskiteoxide showing a straight line in the Arrott plot so far [59].However, interpreting the observation as evidence of itinerantelectron ferromagnetism in SrRuO3 is not straightforward.Interested readers are referred to a previous study on this[60]. In another test for itinerant electron ferromagnetism,we have placed the magnetic properties of CaCoO3 in aRhodes-Wohlfarth (RW) plot of μeff/Ms versus Tc in Fig. 11.Existing ferromagnets are located on two wings in the RW plot;

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800

600

400

200

0

M2 (e

mu/

g)2

3000200010000H/M (Oe g/emu)

46K

64K

Tc = 54K

FIG. 10. The Arrott plot of isothermals for CaCoO3 in thetemperature range 46–64 K with �T = 2 K.

those with itinerant electron magnetism are on the wing with alarge μeff/Ms ratio and lower Tc, whereas those with localizedmoments are close to the line with a small μeff/Ms ratio andhigher Tc [61]. SrCoO3 becomes unstable at T > Tc, so thatthe μeff cannot be determined. Since the saturation momentof SrCoO3 is consistent with that of the IS state of Co4+, itwould be a good assumption to take a μeff = 3.87 μB of the ISstate for SrCoO3. Therefore, a much higher Tc and a higher Ms

make SrCoO3 in line with localized moment ferromagnetism.In contrast, CaCoO3 is close to the line of itinerant electronferromagnetism. As a matter of fact, the spot of CaCoO3 onthe plot is near that for ZrZn2, a prototype material for itinerantelectron ferromagnetism.

While fitting the paramagnetic susceptibility of CaCoO3 toa Curie-Weiss law gives a μeff that is close to the value for the ISstate, the saturation moment at 2 K is even smaller than that forthe LS state. In order to solve this puzzle, we have monitoredthe crystal structure of CaCoO3 at low temperatures. Thetemperature dependence of the Co-O bond length (normalizedto that at lowest temperature) is plotted in Fig. 7 together withother perovskite oxides for comparison [14]. We start fromPrGaO3 and PrCrO3 in this analysis. PrGaO3 serves as thebase for the thermal expansion of the M-O bond length in a

14

12

10

8

6

4

2

0

μ eff

/Ms

120010008006004002000T (K)

(FeCo)Si

Pd-Co

ZrZn2

CoB

NiFe

SrCoO3

CaCoO3

FIG. 11. The Rhodes-Wohlfarth plot of typical magnets; the dataof magnets other than SrCoO3 and CaCoO3 are after Ref. [61].

perovskite oxide without any transitions associated with spinand charge [62]. PrCrO3 behaves like PrGaO3 except for anarrow temperature range near room temperature where themagnetorestriction associated with an antiferromagnetic spinordering appears to alter the thermal expansion. In LaCoO3

where the thermally driven spin-state transition from the LS toHS state starts at 25 K, a thermal expansion of the Co-O bondlength that is much higher than that of either PrGaO3 or PrCrO3

has been reported [7]. In PrCoO3, the onset temperature of theLS to HS transition moves to 200 K where an extra gain of thethermal expansion can be clearly seen [14]. The unusuallyhigh thermal expansion, especially at T > 130 K found inCaCoO3, suggests continuous spin-state transition from theLS to the IS state, i.e., continuously pumping electrons fromthe π∗ band to the σ ∗ band, as temperature increases. Thescenario of a temperature-driven spin-state transition is alsowell supported by a low and glassy thermal conductivity foundin CaCoO3 displayed in Fig. 6(b) since the thermally drivenspin-state change is associated with bond-length fluctuations.A similar thermal conductivity has been reported in LaCoO3

and PrCoO3 [10]. The LS state is stabilized below roomtemperature in NdCoO3 where the phonon thermal conduc-tivity is restored [10]. The observation of a thermally drivenspin-state transition also implies that the electronic states inthe π∗ band and σ ∗ band are nearly degenerate in CaCoO3,which opens the possibility of a field-induced spin-statetransition.

Figure 12 illustrates a schematic diagram of the spin stateand the band structure of CaCoO3. The IS state can be definedfrom the free-ion model with a cubic field on the left. The Co-Obonding in the perovskite structure transforms the localizedorbital energies into bonding and antibonding bands; theFermi energy is located inside and antibonding π∗ and σ ∗bands are shown in the middle. The exchange splitting in theferromagnetic phase shifts the band of up-spin relative to thatof down-spin. The net moment is M = μB(nπ↑ + nσ↑ − nπ↓)that is parallel to the external field for a H > Hc (the coercivefield of the magnet). Therefore, the effect of an external field isto further stabilize the up-spin bands relative to the down-spinbands. This field-induced band shift will lead to transferringelectrons from the π∗(↓) band to σ ∗(↑). The field-inducedmetamagnetic transition to the state with higher magnetizationshown in Fig. 2(d) and Fig. 3 supports the argument very well.Since the σ ∗ bandwidth is broader than that of the π∗ band,the magnetization increase should be coupled with a gain ofthe conductivity, which is consistent with the observation inFig. 4. The field-induced electron transfer from the π∗ to σ ∗band is also the origin of a giant negative magnetoresistanceshown in Fig. 2(f).

A clear deviation from a Curie-Weiss law fitting in theinverse magnetic susceptibility in Fig. 2(a) can be found near150 K. Moreover, a nonlinear magnetization curve M(H )indicates that magnetic fluctuations take place below 150 K. Aremarkable change of M(H ) on cooling through TN is that themagnetic field induces an obvious transition at HT ∼ 5 T inthe M(H ) isothermals below TN , as illustrated in Fig. 3. Thisfield-induced transition moves to a higher field as temperaturefurther decreases from TN . There is no noticeable change ofthe M(H ) curve on cooling through Tc. We may look for aclue to explain these anomalous magnetic properties from the

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FIG. 12. The schematic energy diagram of the IS state for CaCoO3.

structural data. Although there is no structural transition oncooling through these magnetic transitions, the temperaturedependence of Co-O bond length undergoes a major change atT ∼ 100 K, as shown in Fig. 7. A negligible thermal expansionof Co-O bond length below 100 K suggests that CaCoO3 haspredominately the LS state at T < TN , whereas an unusuallyhigh thermal expansion coefficient of the Co-O bond lengthat T > TN means a continuous increase of the IS state astemperature increases. The LS state at low temperatures isconsistent with the saturation moment Ms < 1 μB observedand the HS state at high temperatures is close to the μeff

extracted from the paramagnetic susceptibility. Therefore,TN actually marks an onset temperature for the spin-statetransition in addition to ordering spins into an AF structure.The AF spin structure evolves into a ferromagnetic phase(perhaps still with some spiral-spin component) on coolingthrough Tc. In the magnetically ordered phases either below TN

or Tc, the peculiar band structure in CaCoO3 where the Fermienergy cuts into both degenerate π∗(↓) and σ ∗(↑) makes itpossible that a magnetic field induces a transition from the LSstate to the higher spin state.

One may be curious about (1) whether the Curie-Weiss lawremains valid for CaCoO3 which is now identified as exhibitingitinerant electron magnetism and (2) μeff and Ms in the RW plothave been measured at temperatures where different spin stateshave been identified. The temperature dependence of magneticsusceptibility for ZrZn2 can be fitted by the Curie-Weiss lawalmost perfectly at T > 100 K [63]. The Curie constant fromthe fitting can be generally expressed as C = gq. Although theq value is not far from [S(S + 1)]1/2 for localized spins, it isactually related to the electronic density of state (EOS) at theFermi energy as derived from the unified picture of magnetismby Moriya [64]. Since the Curie constant C is related to theEOS of itinerant electrons in CaCoO3, the spin-state changein the band picture may not induce a change in the C valueas much as that for a localized spin system. Moreover, thespin-state transition from the LS to the IS state at T > TN is

progressive. The dramatic difference of the RW ratio C/Ms

between CaCoO3 and SrCoO3 is actually dominated by thechange of the saturation moment Ms .

The electronic contribution to specific heat Cp has beenobtained by fitting the Cp(T ) data in Fig. 2(b) to theformula Cp = γ T + αT 3/2 + βT 3 at low temperatures. Aγ = 53 mJ K−2 mol−1 is significantly higher than the γ0 =5.7–8.1 mJ K−2 mol−1 from the band structural calculation inthis work and from the literature [47]; this γ value is alsohigher than those of typical σ∗ band metals like LaNiO3 (γ =18.1 mJ K−2 mol−1) and LaCuO3 (γ = 5.6 mJ K−2 mol−1)[65]. The only possibility for CaCoO3 to have such ahigh γ is that the highly correlated π∗ electrons whichfacilitate ferromagnetism also contribute to the specific-heatand transport properties. A similar γ = 45 mJ K−2 mol−1 hasalso been obtained for the isostructural SrCoO3. The unusuallyhigh γ value of CaCoO3 is rarely observed in 3d oxides.We have performed dynamical mean-field calculations totake into account possible Kondo hybridization, but found noevidence of enhanced renormalization of band structures froma strongly correlation effect. The Van Hove singularity is toofar away from the Fermi energy to yield a large γ . Thus theheavy-fermion behavior in CaCoO3 remains a mystery as inthe case of LiV2O4 [66,67] and demands further investigation.Although CaCoO3 has the highest γ value of the simple metal-lic perovskites, the ratio γ 2/A is not far from the Kadowaki-Woods ratio γ 2/A = 1/a0 ∼ 1011 (mJ2 K−2 mol−2 cm−2 �−2)[68], where A is a coefficient of the formula ρ = ρ0 + AT 2

used to fit the temperature dependence of resistivity at lowtemperatures.

In conclusion, a new perovskite oxide CaCoO3 has beensynthesized by treating the brownmillerite-type precursorCa2Co2O5 under high pressure and high temperature. CaCoO3

is the first CaMO3 (M = transition metal, Ge, Sn, and Zr)perovskite reported which is cubic at room temperature. Likethe perovskites with Co(III) such as LaCoO3 and PrCoO3 atroom temperature, both SrCoO3 and CaCoO3 have the down-

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spin t2 energies close to the up-spin eg energies. The metallicconductivity down to 1.8 K in these perovskites with Co(IV)indicates that the σ ∗ electrons are itinerant. Furthermore,the Co-O bond length in CaCoO3 is the shortest in alltransition-metal perovskites and promises that t2 electrons areitinerant. Itinerant electron ferromagnetism below Tc ≈ 54 Kcan be well supported by (a) nearly linear isothermal near in thevicinity of Tc in an Arrott plot and (2) the location of CaCoO3

which is near that of ZrZn2 in the Rhodes-Wohlfarth plot of themagnetic properties. In contrast, SrCoO3 is located in a regimetypical for localized magnetism in this plot. In CaCoO3, spinsare ordered into an AF structure below TN ≈ 86 K and furtherevolve into a ferromagnetic state below Tc. The magneticordering transition at TN also separates the spin state below TN

that is dominated by the LS state from the high-temperaturephase showing a thermally driven LS-HS transition at T > TN .The peculiar band structure of CaCoO3 makes it possible that

magnetic field induces the LS to HS transition in the phasebelow TN and Tc.

ACKNOWLEDGMENTS

The authors are grateful to Q. F. Zhang, Z. G. Sheng, andJ. Q. Yan for enlightening discussion and to J. B. Goodenoughfor reviewing and commenting on the manuscript. This workwas supported by the 973 Project of the Ministry of Scienceand Technology of China (Grant No. 2014CB921500), theStrategic Priority Research Program of the Chinese Academyof Sciences (Grant No. XDB07030300), and the NationalNatural Science Foundation of China (Grant No. 11574378).J.S.Z. was supported by DOD-ARMY (W911NF-16-1-0559).M.A.M. acknowledges support from the US Department ofEnergy, Office of Science, Basic Energy Sciences, MaterialsSciences and Engineering Division.

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