Transition-metal oxides with unusual valence states show fascinating physical properties. For example, an exotic charge-ordering transition that involves competing dimer and trimer orderings is observed in the recently discovered compound Fe₄O₅ obtained by high-pressure synthesis methods.^[1,2] Both temperature and pressure induced intersite charge transfers are found to occur in the A-site ordered quadruple perovskite LaCu₃Fe₄O₁₂ with the presence of unusually high Cu³⁺ and Fe^3.75+ valence states, accompanying with a series of sharp variations in unit cell, magnetism, and electrical transport properties as well as negative thermal expansion.^[3–5] Charge disproportionation of Fe⁴⁺ takes place in the distorted perovskite CaFeO₃ , leading to metal-to-insulator and orthorhombic-to-monoclinic structural phase transitions.^[6,7] On the other hand, in the cubic perovskite SrFeO₃, the high Fe⁴⁺ state is stable and there is not any charge variation at different temperatures. However, the antiferromagnetic (AFM) ordering of SrFeO₃ is very sensitive to chemical doping. A small amount of Fe⁴⁺ substituted by another high Co⁴⁺ or Ni⁴⁺ can lead to strong ferromagnetic (FM) interactions with high Curie temperature above room temperature.^[8–10]

As for the element of chromium, the valence states such as Cr³⁺ with half-filled orbitals and Cr⁶⁺ with empty 3d orbitals exist widely. Although the Cr⁴⁺ state ( can occur in the simple binary oxide CrO₂ under ambient pressure, high pressure is necessary to prepare ternary ACr⁴⁺O₃ perovskites such as CaCrO₃, SrCrO₃, and PbCrO₃, where interesting physical properties are observed. The crossover from local to itinerant electron features gives rise to anomalous electronic behaviors for the orthorhombic CaCrO₃ and cubic SrCrO₃ perovskites.^[11–15] Large volume collapse and Pb–Cr intermetallic charge transfer emerge in PbCrO₃.^[16–18] By comparison, the chromium oxide with Cr⁵⁺ is rare to know. Though the zircon-type RCr⁵⁺O₄ ( earth and Y) compounds with tetrahedral coordinated CrO₄ units can form by adopting low-temperature wet chemical methods, the oxygen is readily to release on heating.^[19–23] At higher temperatures, these compounds always decompose to Cr³⁺-based RCrO₃ perovskites. At present, there is no report for the discovery of Cr⁵⁺-involved perovskite oxide with octahedral coordinated Cr⁵⁺O₆ unit ( .

In this paper, we report a new perovskite CaCr_0.5Fe_0.5O₃ (CCFO) synthesized at 8 GPa and 1473 K. Since the well-studied CaCrO₃ and CaFeO₃ respectively possess quadrivalent Cr⁴⁺ and Fe⁴⁺ at room temperature,^{[6,7,11,12,24,25]} one may think, at first glance, that the Cr and Fe ions in CCFO should also possess similar charge states (i.e., ). In sharp contrast, however, our experimental results indicate the presence of Cr⁵⁺ and Fe³⁺ states. CaCr_0.5Fe_0.5O₃ thus provides the first example with unusually high Cr⁵⁺ state in octahedral coordinated environment of perovskite.

The solid-state reaction synthesis of polycrystalline CCFO was carried out using a cubic-anvil-type high pressure apparatus. Highly pure ( ) CaO, Fe₂O₃, CrO₂, and CrO₃ powders were used as the starting materials. These reactants with a stoichiometric mole ratio of 4:1:1:1 were mixed thoroughly in an agate mortar within an argon-filled glovebox. The mixed powders were then sealed into a platinum capsule with 2.8 mm diameter and 4.0 mm length. The capsule was treated at 8 GPa and 1473 K for 30 min in the high-pressure apparatus. When the heating process was finished, the sample was quenched to room temperature and then the pressure was slowly released to ambient pressure. The crystal quality and structure of CCFO were examined by powder x-ray diffraction (XRD) using a Huber diffractometer with Cu radiation at 40 kV and 30 mA. The XRD data were collected in the angle (2θ) range from 10° to 100° with steps of 0.005° and analyzed by the Rietveld refinement using the GSAS program.^[26] Thermogravimetric (TG) measurement was performed using a Setaram TG-DTA system in mixed H₂ (4%) and N₂ (96%) gas flow on heating from 300 K to 1473 K with a speed of 5 K/min. The selected area electron diffraction (SAED) was carried out using a Philips-CM200 field emission transmission electron microscopy operated at 200 kV. The Fe and Cr K- and L-edge x-ray absorption spectroscopy (XAS) data were collected at beamline 14W of the Shanghai Synchrotron Radiation Facility and beamline 16A1 of the “National” Synchrotron Radiation Research Center of Taiwan. The dc magnetic susceptibility (χ_dc) and magnetization were measured using a Quantum Design superconducting quantum interference device magnetometer. The ac magnetization ( ) and resistivity were measured on a physical property measurement system (Quantum Design, PPMS-9T).

Density functional theory calculations were performed using the full-potential linearized augmented plane-wave method implemented in WIEN2K code with the generalized-gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE) exchange-correlation energy.^[27] The muffin-tin radii were set to 2.1 a.u. for Ca, 1.70 a.u. for O, and 1.90 a.u. for Cr/Fe. The maximum modulus for the reciprocal vectors was chosen such that and we used 1000 k-point meshes for the Brillouin zone.

Figure 1 shows the XRD pattern of the high-pressure product CCFO. The Rietveld refinement shows that CCFO crystallizes into an ABO₃-type perovskite structure with orthorhombic space group Pbnm, where the B-site Cr and Fe are randomly distributed at the special atomic site 4b(0, 0.5, 0), forming oxygen six-fold coordinated Cr/FeO₆ octahedra. To further confirm the disordered Cr/Fe distribution, high-resolution SAED was performed along different characteristic axes like [1-10] zone axis (not shown here). We did not find any trace for long-range B-site ordering in CCFO. This observation is essentially different from the theoretical prediction on the analogous oxide Sr₂CrFeO₆, which is claimed to be a B-site ordered double perovskite composed of high-spin Cr⁴⁺ and low-spin Fe⁴⁺ charge states.^[28] Table 1 lists the refined structural parameters as well as the satisfied goodness factors for the Rietveld analysis of CCFO.

Fig. 1.

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Fig. 1. (color online) X-ray diffraction pattern and structure refinement results of CaCr0.5Fe0.5O3. The observed (circles), calculated (line), and difference (bottom line) are shown. The top ticks indicate the allowed Bragg reflections of CCFO with space group Pbnm. The bottom ticks indicate the impurity phase of CaFe2O4. The two-phase refinement shows that the amount of impurity phase is 4.1 wt.%.

Table 1.

Table 1.

Table 1. Refined structure parameters of CaCr0.5Fe0.5O3 at room temperature. Space group: Pbnm; atomic positions: Ca (x, y, 0.25), Cr/Fe (0, 0.5, 0), O1 (x, y, , O2 (x, y, 0.25). Occup.: occupancy factor. . Parameters CaCr0.5Fe0.5O3 Parameters CaCr0.5Fe0.5O3 a/Å 5.3993(2) Biso(Ca)/Å2 5.33(1) b/Å 5.4230(1) Biso(Cr/Fe)/Å2 3.37(4) c/Å 7.6637(1) Biso(O1)/Å2 7.6(2) Ca (x) 0.990(1) Biso(O2)/Å2 9.1(6) Ca (y) 0.0202(8) Cr/Fe-O1/Å2 1.98(1) (×2) O1 (x) 0.724(2) Cr/Fe-O1/Å2 1.89(2) (×2) O1 (y) 0.288(2) Cr/Fe-O2/Å2 1.968(3) (×2) O1 ( 0.015(2) Cr/Fe-O1-Cr/Fe/(°) 163.9(4) (×4) O2 (x) 0.082(2) Cr/Fe-O2-Cr/Fe/(°) 153.5(7) (×2) O2 (y) 0.484(3) Occup.(Ca) 0.95(1) Rwp/% 1.85 Occup.(Cr/Fe) 0.90(2) Rp/% 1.40 Occup.(O1) 1.0 Occup.(O2) 1.0

Table 1. Refined structure parameters of CaCr0.5Fe0.5O3 at room temperature. Space group: Pbnm; atomic positions: Ca (x, y, 0.25), Cr/Fe (0, 0.5, 0), O1 (x, y, , O2 (x, y, 0.25). Occup.: occupancy factor. .

To confirm the oxygen content in CCFO, the temperature dependence of TG measurement was carried out. As shown in Fig. 2, the compound is very sensitive to heating and starts to lose some weight above 350 K, which may reflect the metastable state of the Cr⁵⁺/Fe³⁺ charge combination with the presence of unusually high Cr⁵⁺ valene state as described later. According to the TG loss between 350 K and 1473 K as well as the residual products (CaO, Fe, and CaCr₂O₄), the oxygen content is determined to be 2.97 ± 0.02. This value is very close to the stoichiometric oxygen composition for CCFO.

Fig. 2.

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	Fig. 2. Temperature dependence of thermogravimetric loss for CaCr0.5Fe0.5O3.

We now turn to the determination of the charge states of Fe and Cr in CCFO by hard XAS K-edge measurements. Figure 3(a) shows the Fe K-edges of CCFO together with those of Fe₂O₃ and SrFeO₃, which are used as standard Fe³⁺ and Fe⁴⁺ references with oxygen octahedral coordination, respectively. The chemical shift of the Fe K-edge defined at –0.8 of the normalized intensity can be used to identify the Fe valence state.^[29,30] We find that the absorption edge of CCFO shifts to lower energy by 1.5 eV relative to that of SrFeO₃, but at the same energy as that of Fe₂O₃, revealing the Fe³⁺ valence state of CCFO. The Cr K-edge of CCFO is shown in Fig. 3(b), where Cr₂O₃ and SrCrO₃ are used as Cr³⁺ and Cr⁴⁺ references with octahedral coordination, respectively. Unfortunately, at this stage there is no octahedral coordinated Cr⁵⁺ reference that can be applied for comparison. Anyway, in Fig. 3(b) one can see that the Cr K-edge shifts to higher energy by 1.3 eV from Cr³⁺ oxide Cr₂O₃ to Cr⁴⁺ oxide SrCrO₃ and by another 1.0 eV from SrCrO₃ to CCFO. Taking into account the confirmed Fe³⁺ state as well as the nearly stoichiometric chemical composition of CCFO, the systematical energy shift observed in the Cr K-edge strongly suggests the formation of an unusual Cr⁵⁺ state fulfilling the charge balance requirement for the charge combination.

Fig. 3.

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	Fig. 3. (color online) The hard XAS of CaCr0.5Fe0.5O3 measured at room temperature for (a) the Fe-K edge, and (b) the Cr-K edge. The spectra of some related references are also shown for comparison. The rectangles show the normalized intensity –0.8, which can be used to judge the valence state.

The unusual Cr⁵⁺/Fe³⁺ charge combination in CCFO can be further confirmed by soft XAS at the Fe- and Cr-L_2,3 edges. As shown in Fig. 44(a), the energy position of the Fe-L_2,3 edges in CCFO is remarkably lower than that of SrFe⁴⁺O₃ by about 0.8 eV, ruling out the formation of Fe⁴⁺. By comparison, the Fe-L_2,3 XAS spectrum of CCFO situates at similar energy and has comparable spectral feature with that of LaFeO_3, confirming the presence of high-spin Fe³⁺ valence state in CCFO. Since there is no Cr⁵⁺ reference with an octahedral coordination, the Cr-L_2,3 XAS of Cr₂O₃ and SrCrO₃ were measured as Cr³⁺ and Cr⁴⁺ references with octahedral coordination, respectively. As shown in Fig. 4(b), with increasing valence state from Cr³⁺ in Cr₂O₃ to Cr⁴⁺ in SrCrO₃, the main peak positions of both L₃ and L₂ edges shift toward higher energy by ∼1.0 eV. As the absorption spectrum of CCFO is concerned, it also experiences a similarly systematic energy shift, but displays substantially different spectral features from that of the Cr⁴⁺ reference SrCrO₃ probably due to the enhanced p–d hybridization between Cr and the coordinating oxygen atoms, suggesting that the unusually high Cr⁵⁺ state instead of Cr⁴⁺ occurs in CCFO, in agreement with the K-edge measurements mentioned above. Both the hard and soft XAS results thus demonstrate the charge combination. The Cr⁵⁺/Fe³⁺ charge states presented in the current CCFO greatly differ with the Cr⁴⁺ state observed in CaCrO₃ and Fe⁴⁺ in CaFeO₃. The newly synthesized oxide CCFO provides a rare example possessing an unusual Cr⁵⁺ state in a perovskite structure with Cr⁵⁺O₆ octahedral coordination.

Fig. 4.

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	Fig. 4. (color online) The soft XAS of CaCr0.5Fe0.5O3 measured at room temperature for (a) the Fe-L2,3 edges, and (b) the Cr-L2,3 edges. The spectra of some related references are also shown for comparison.

The unexpected Cr⁵⁺/Fe³⁺ charge states of CCFO are also consistent with the magnetism as characterized by both dc and ac magnetization measurements. Figure 5(a) shows the temperature dependence of χ_dc measured at 0.1 T. With decreasing temperature to , the χ_dc sharply increases, whereas there is no visible anomaly in ac magnetization (Fig. 5(b)). Moreover, the zero-field-cooling (ZFC) and field-cooling (FC) χ_dc curves start to separate from each other below T_SR. These behaviors may imply the presence of some short-range FM interactions below T_SR due to the random distribution of Fe³⁺ and Cr⁵⁺ spins. On further cooling to , both the ZFC and FC χ_dc curves experience a remarkable kink, suggesting an AFM phase transition. When the ac magnetization is measured at different frequencies (see Fig. 5(b)), we find that the AFM cusp occurring around 50 K is not dependent on the measurement frequency at all, confirming that the AFM ordering should be moderately long-range in nature. Above 200 K, the inverse χ_dc can be well fitted by the Curie–Weiss law (Fig. 5(a)), producing the Curie constant and the Weiss temperature . The negative Weiss temperature is consistent with the low-temperature long-range AFM ordering, and the considerably large value of θ compared with T_N may imply some spin frustrations caused by the competing FM and AFM interactions mentioned above. Accordeing to the Curie constant, the effective magnetic moment of CCFO is calculated to be /f.u., which is roughly comparable to the spin-only theoretical value for charge conbination (4.36 /f.u).

Fig. 5.

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	Fig. 5. (color online) Temperature dependence of (a) dc magnetic susceptibility and its inverse measured at 0.1 T, and (b) ac magnetization ( ) measured at different frequencies for CaCr0.5Fe0.5O3. (c) Field dependence of magnetization measured at different temperatures. The purple line in (a) shows the Curie–Weiss fitting.

Figure 5(c) shows the dc magnetization measured at different temperatures. Above T_SR, the linear magnetization behavior is coherent with the paramagnetism. Below T_SR, however, one can find a small amount of magnetic hysteresis due to the short-range FM coupling as well as the possible canted antiferromagnetism (below T_N). In CCFO, there exist two distinct magnetic ions (i.e., Cr⁵⁺ and Fe³⁺). According to the Goodenough-Kanamori rules,^[31,32] the Cr⁵⁺–O–Fe³⁺ and Fe³⁺–O–Fe³⁺ interaction pathways with the average band angle ∼159° (see Table 1) will generate FM and AFM superexchange interactions, respectively. Our first-principles calculations also show the FM interaction for the 3d¹ electrons of Cr⁵⁺ via the Cr⁵⁺–O–Cr⁵⁺ superexchange pathways (not shown here). Consequently, both Cr⁵⁺–O–Fe³⁺ and Cr⁵⁺–O–Cr⁵⁺ interactions are responsible for the short-range FM correlation of CCFO, whereas the AFM ordering should arise from the Fe³⁺–O–Fe³⁺ superexchange interaction.

Because of the strong electronic correlated effects, the 3d⁵-electron perovskite systems of Fe³⁺ such as RFeO₃ usually exhibit Mott insulating behaviors.^[33] For the 3d¹-electron systems like LaTiO₃ and YTiO₃ with distorted perovskite structure, the electrical insulating behavior is also observed,^[34–36] whereas the undistorted cubic 3d¹ perovskite SrVO₃ displays metallic conductivity.^[37] In the current CCFO perovskite oxide, there exists considerable structural distortion. The compound also shows the insulating behavior, as featured by the increasing resistivity on cooling as well as the well fitted 3D Mott variable-range-hopping model (see Fig. 6 and the inset), although the B-site Fe³⁺ (3d⁵) and Cr⁵⁺ (3d¹) are distributed disorderly. Moreover, as seen from Table 1, the Cr-O distances in a single Cr/FeO₆ octahedron in CCFO can be differed by about 4.5%. This significant difference may suggest possible orbital ordering for the 3d¹ electrons of Cr⁵⁺ ions, as observed in another 3d¹ system Sr₂VO₄ with tetragonal distorted perovskite structure.^[38]

Fig. 6.

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	Fig. 6. (color online) Resistivity as a function of temperature for CaCr0.5Fe0.5O3. The red line in the inset shows the fitting result on the basis of the 3D Mott variable-range-hopping model between 200 K and 300 K.

Since the detailed electronic properties of Cr⁵⁺ in perovskite structure with octahedral coordination are completely absent to date, it is very interesting to further investigate through first-principles calculations based on the density functional theory (DFT). Considering that the DFT is unfavorable to treat a completely disordered system, for simplify, we assume some ordered crystal structures to gain insights into the properties of the experimental CCFO. The electronic structures of CCFO were preliminary calculated for different hypothetical Cr/Fe orderings such as the G-type, A-type, and C-type. By comparison, the G-type (i.e., rocksalt type) Cr/Fe ordered structure is optimized in system energy and we obtain the lattice parameters a = 5.123 Å, b = 5.375 Å, and c = 7.580 Å with space group P2₁/n. In this structure model, we obtain an A-type AFM spin ground state, with a zero total magnetic moment. The band structures and the partial density of states (DOS) of the magnetic ground state were calculated with U_eff = 3 eV for Cr and 4 eV for Fe, following the common choices in the literature,^[39–41] and the results are shown in Fig. 7, indicating an insulating behavior with an energy gap of about 0.3 eV. Both are consistent with the experimental observation in CCFO. Our calculated moment is about 1.0 μ_B/Cr and 4.1 μ_B/Fe, so there is only one d-electron on each Cr ion and a fully polarized and half-filled d-shell on each Fe ion. We therefore conclude that the Cr ions have a valence of +5, while the Fe ions have a valence of +3 in this hypothetical G-type Cr/Fe ordered perovskite structure. In sharp contrast, for other types of Cr/Fe orderings mentioned above, we obtain only Cr⁴⁺ ions. In addition, in the mentioned G-type Cr/Fe ordered structure, if we do not consider the Coulomb interaction for Cr⁵⁺ and Fe³⁺ both, a metallic ground state instead of an insulating one as observed in experiment is obtained. However, one can still obtain an insulating ground state (albeit with a much reduced gap) if U_ef = 0 is set only for Cr⁵⁺, suggesting that the Coulomb interaction on Fe³⁺ ions is crucial for the insulating conductivity of CCFO.

Fig. 7.

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	Fig. 7. (color online) First-principles numerical results for the band structures and the partial density of states of the hypothetical G-type Cr/Fe ordered double perovskite Ca2CrFeO6.

In summary, by using high-pressure and high-temperature synthesis conditions, we obtained a new perovskite oxide CaCr_0.5Fe_0.5O₃ with nearly stoichiometric oxygen content. Although previous theoretical calculation predicted the analogous compound Sr₂CrFeO₆ to be a B-site ordered double perovskite with Cr⁴⁺/Fe⁴⁺ charge combination, our results reveal that CCFO crystallizes in a simple ABO₃ perovskite structure with randomly distributed Cr and Fe ions at the B site. Fe- and Cr-K and L edge XAS measurements suggest that the charge states are Fe³⁺ and Cr⁵⁺ in CCFO, providing the first example that shows an unusual Cr⁵⁺ state in perovskite structure with Cr^{5 +}O₆ octahedral coordination. In accordance with the Cr^{5 +}/Fe³⁺ disordered perovskite structure, the random Cr^{5 +}–O–Fe³⁺ and Cr⁵⁺–O–Cr⁵⁺ interactions cause some short-range FM couplings below ∼120 K. On the other hand, the Fe³⁺–O–Fe³⁺ superexchange interaction generates an AFM ordering at a lower temperature 50 K. The resistivity data of CCFO follows the Mott 3D variable-range-hopping mechanism, indicating the insulating conductivity. Our first-principle theoretical calculations based on the hypothetical G-type Cr/Fe ordered structure further illustrate the formation of Cr⁵⁺/Fe³⁺ charge combination. The electorn correlation effect of Fe³⁺ is closely related to the insulating ground state of CCFO.