The precursor of Sr₂CoFeO₆ was prepared by the sol– gel method with citric acid as a complexant. Stoichiometric Sr(NO3)₂, Co(NO₃)₂· 6H₂O, and Fe(NO₃)₃· 9H₂O of analytical grade were dissolved in water, and then citric acid and ethylene glycol were added. The mixed solution was slowly evaporated at 80– 90 ° C until a gel was formed. Subsequently, the gel was dried at 130 ° C and then heated at 850 ° C in air for 10 h. After being uniformly ground in an agate mortar, the mixture was pressed into a disk with a diameter of 8 mm and a thickness of 2 mm. The disk-shaped samples were assembled for the high-pressure and high-temperature (HPHT) synthesis in a cubic anvil high-pressure apparatus at 1473 K and 5 GPa for 1 h.

The crystal structure and the phase purity of Sr₂CoFeO₆ were examined using conventional X-ray powder diffraction at room temperature. X-ray diffraction (XRD) measurement was performed using a Rigaku Rotaflex X-ray diffractometer with Cu K_α radiation in the range of 20° – 80° . High-pressure angle dispersive XRD measurement was performed at beamline 3W1A of the Institute of High Energy Physics, CAS.

X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250 spectrometer using an Al K_α X-ray source. The obtained binding energies were standardized and analyzed using C_1s as the reference at 284.9 eV.

Measurements of the electric resistivity were performed with the four-point probe method from 10 K to 300 K under an applied magnetic field of 8 T in a commercial Quantum Design physical property measurement system (PPMS).

At ambient pressure, Sr₂CoFeO₆ is found to have a cubic crystal structure (space group Fm-3m). As shown in Fig. 1, it can be viewed as a regular arrangement of the corner-sharing CoO₆ and FeO₆ octahedra, alternating along the crystal axes, with the large Sr cations occupying the voids between CoO₆ and FeO₆ octahedra. The X-ray diffraction pattern at the ambient condition is displayed in Fig. 2, and the calculated lattice parameter a = 7.7323(5) Å is in good agreement with the previous reports.^{[12, 25]}

Fig. 1.

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	Fig. 1. Crystal structure of Sr2CoFeO6.

Fig. 2.

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	Fig. 2. X-ray diffraction pattern of Sr2CoFeO6 at the ambient condition.

Figure 3 shows the results of XPS measurements on Fe 2p and Co 2p in Sr₂CoFeO₆. Lorenz– Gaussian fitting of the peaks to the components of Fe³⁺ , Fe⁴⁺ , Co³⁺ , and Co⁴⁺ is also given in the figure. The relative peak intensities of the fitting curves are summarized in Table 1. The peaks at 712.51 eV and 725.63 eV identify the existence of Fe⁴⁺ ; those at 710.56 eV and 723.64 eV identify the existence of Fe³. The binding energies of 780.59 eV and 795.77 eV correspond to Co³⁺ ; while those of 781.66 eV and 797.55eV correspond to Co⁴⁺ . These results are in good agreement with the previous reported values.^{[26, 27]}

Fig. 3.

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	Fig. 3. XPS spectra of (a) Fe 2p and (b) Co 2p in Sr2CoFeO6.

Table 1.

Table 1.

Table 1. Relative XPS peak intensities of Fe3+ , Fe4+ , Co3+ , and Co4+ components. Component Fe3+ Fe4+ Co3+ Co4+ Intensity/% 57.73 42.27 46.52 53.48

Table 1. Relative XPS peak intensities of Fe3+ , Fe4+ , Co3+ , and Co4+ components.

According to the XPS spectral analysis, it can be deduced that in Sr₂CoFeO₆, the B sites are randomly occupied by Fe and Co in the mixed valence states of Fe³⁺ /Fe⁴⁺ or Co³⁺ /Co⁴⁺ . Owing to the coexistence of Fe⁴⁺ , Fe³⁺ , Co⁴⁺ , and Co³⁺ , in order to maintain electric neutrality of Sr₂CoFeO₆, there should be some oxygen vacancies inside Sr₂CoFeO₆. Such oxygen vacancies induced by the random occupancy and the mixed valence may account for the observed spin glass behavior, which is caused by the inhomogeneous magnetic exchange interactions.^[10]

The electric resistance of SFCO in 0 T and 8 T applied magnetic fields is presented in Fig. 4. It exhibits a monotonic increase with decreasing temperature, and the variation of the electric resistance indicates a semiconducting behavior.

Fig. 4.

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	Fig. 4. Temperature dependence of the electric resistance of Sr2CoFeO6 in 0 T and 8 T.

As shown in Fig. 4, unlike Sr₂CoMoO₆, the resistivity curves measured in the fields of 0 T and 8 T are almost identical, indicating that Sr₂CoFeO₆ has a negligible magnetoresistance.

In order to understand the electric transport properties of Sr₂CoFeO₆, the resistivity data are fitted to various models, among which the Mott variable-range hopping (MVRH) model^{[28– 30]} is the best fit to our results, as shown in Fig. 5. The Mott variable-range hopping model gives the temperature-dependent resistivity as follows: ρ = ρ _oexp(T_o/T)^x, with exponent x = 1/4, as expected in three dimensions. The best fitted values are T_o = 3.10 × 10⁸ K, ρ _o = 1.34 × 10^{– 14} Ω · m (at 0 T) and T_o = 3.06 × 10⁸ K, ρ _o = 1.51 × 10^{− 14} Ω · m (at 8 T), where T_o is related to the localization length ξ with the relationship , and N(E_F) denotes the density of states at the Fermi level. It suggests that the resistivity is not only induced by the thermally activated mechanism of carrier hopping, and there is no energy gap opening for the upturn of resistance at low temperature.

Fig. 5.

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	Fig. 5. ln ρ (T) versus (1/T)1/4 normalized to the resistivity at 300 K (ln (ρ (T)/ρ (300 K))) of Sr2CoFeO6 in (a) 0 T and (b) 8 T. The red lines are linear fits to the experimental data, indicating that the 3D variable range hopping mechanism is valid in the measured temperature range.

Various compounds in the double perovskite family are found to have different high pressure behaviors.^{[23, 24]} Further investigations of the structural stability in these compounds would be beneficial for understanding their physical properties at high pressures. This study reports a high pressure X-ray diffraction study of Sr₂CoFeO₆. Figure 6 shows the X-ray diffraction patterns at various pressures up to 31.0 GPa. The lowest diffraction pattern is indexed with respect to the cubic phase and some impurity (unreacted oxides) peaks existing in the pattern are marked by asterisks. All the X-ray diffraction peaks shift toward higher 2θ values, and no new peaks are found with increasing pressure. The variation of the relative intensities and the broadening of the X-ray diffraction peaks upon compression can be attributed to the thinning of the sample and the uniaxial compression in the process. We thus conclude that no structural phase transition has occurred up to the highest pressure.

Fig. 6.

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	Fig. 6. Selected X-ray diffraction patterns of Sr2CoFeO6 at room temperature for various pressures from 2.2 GPa to 31.0 GPa. Some impurity (unreacted oxides) peaks are marked by asterisks. The wavelength λ is 0.6199 Å . In the whole pressure range, the crystal structure of Sr2CoFeO6 remains cubic.

The lattice parameter a and the unit cell volume as a function of pressure are plotted in Fig. 7. The second order Birch– Murnaghan equation of state (BM-EoS) is applied to determine the bulk modulus.^[31] The fitting gives the bulk modulus B_o = 163(2) GPa. The lattice parameter and the unit cell volume of Sr₂CoFeO₆ decrease smoothly with increasing pressure, with no evidence of any phase transition throughout the whole pressure range.

Fig. 7.

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	Fig. 7. (a) Lattice parameters of cubic Sr2CoFeO6 versus pressure up to 31.0 GPa at room temperature. (b) Pressure dependence of unit cell volumes and data fitting using BM-EoS.