Synthesis and NEXAFS and XPS Characterization of Pyrochlore-Type Bi_1.865Co_1/2Fe_1/2Ta₂O_9+Δ

Abstract

A cubic pyrochlore with the composition Bi_1.865Co_1/2Fe_1/2Ta₂O_9+Δ (space group Fd-3m, a = 10.5013(8) Å) was synthesized from oxide precursors using solid-phase reactions. These ceramics are characterized by a porous microstructure formed by randomly oriented grains of an elongated shape with a longitudinal size of 0.5–1 µm. The electronic state of cobalt and iron ions in oxide ceramics was studied by NEXAFS and XPS spectroscopy. The parameters of the XPS spectra of Bi4f, Bi5d, Ta4f, Co2p, and Fe2p ionization thresholds for a complex pyrochlore were compared with the parameters of the corresponding oxides of the transition elements. The energy position of the XPS-Ta4f and -Ta5p spectra is shifted towards lower energies compared to the binding energy in tantalum(V) oxide by 0.75 eV. According to XPS spectroscopy, bismuth and tantalum cations have the corresponding effective charge of +3 and +(5-δ). The NEXAFS-Fe2p spectrum of ceramics coincides with the spectrum of Fe₂O₃ in its main spectrum characteristics and indicates the content of iron ions in the oxide ceramics in the form of octahedral Fe(III) ions, and according to the character of the Co2p spectrum, cobalt ions are predominantly in the Co(II) state.

1. Introduction

Synthetic pyrochlores based on bismuth tantalate attract close attention from scientists due to their promising dielectric properties. Showing low values of dielectric losses and high permittivity, an adjustable temperature of the coefficient of capacitance, and chemical compatibility with low-melting metal conductors, materials based on bismuth-containing pyrochlores are promising as multilayer ceramic capacitors and tunable microwave dielectric components [1,2]. The cubic crystal structure of the pyrochlores A₂B₂O₆O’ are formed by the weakly interacting cationic sublattices A₂O’ and B₂O₆ (Figure 1). The three-dimensional framework of the octahedral sublattice B₂O₆ ensures the stability of the pyrochlore structure with respect to iso- and hetero-valent substitutions of ions in the cation sublattices and to oxygen vacancies in A₂O’ [3], which makes it possible to significantly vary the chemical composition and study the effect of chemical modification of the composition of pyrochlores on the physicochemical properties of pyrochlores. As a rule, the octahedral positions B of the B₂O₆ sublattice are occupied by relatively small and electronegative cations (Ti⁴⁺, Nb⁵⁺, Ta⁵⁺); larger ions (Bi³⁺, Ca²⁺) are distributed in the eight-coordinated A positions. As numerous studies have shown, doping of pyrochlores with 3d-element ions often leads to the formation of so-called mixed pyrochlores [4,5,6,7,8]. A feature of bismuth-containing pyrochlores doped with 3d-element ions is the partial vacancy of the bismuth sublattice and the distribution of dopants in the cation sublattices of both bismuth and tantalum [4,5,6,7,9].

As studies show, the reason for the dual distribution of transition element ions in the bismuth sublattice is the strain of the octahedral framework caused by the placement of heterovalent and incommensurate 3d-element ions in the tantalum(V)/niobium(V) sublattice. This conclusion is confirmed by the percentage ratio of cations occupying positions in the bismuth/tantalum sublattices. According to numerous experiments [4,5,6,7], this ratio does not exceed 1/3. The paper [10] shows the possibility of the formation of iron-containing pyrochlores of the general composition Bi_3.36Fe_2.08+xTa_2.56−xO_14.56−x (−0.32 ≤ x ≤ 0.48). Using the data of magnetic susceptibility, and Mössbauer and electron energy loss spectroscopy (EELS) analysis of iron-containing pyrochlores, it was found that iron ions are in the high spin state Fe(III), occupying mainly octahedral positions Nb/Ta/Sb and Nb/Ta/Sb [10,11,12,13,14,15]. It was also found that some of the Fe(III) ions can be placed in the bismuth positions. In particular, depending on the composition of the ceramics, only 4–15% of the A-sites are occupied by Fe³⁺ ions in the pyrochlores of the Bi₂O₃-Fe₂O₃-Nb₂O₅ system [11]. For the Bi₂O₃–Fe₂O₃–Sb₂O_x and Bi_1.8Fe_0.2(FeSb)O₇ systems the occupation value equals to 7–25% [12] and 10% [15], respectively. At the same time, it was indicated in [15] that all compositions of Bi_2−xFe_x(FeSb)O₇ (x = 0.1, 0.2, 0.3) demonstrate the state of spin glass due to the presence of some Fe(III) ions in crystallographic positions A. In the work [16], a sample of Bi_1.6Co_0.8Ta_1.6O_7.2 (sp. gr. Fd-3m:2, 10.5526(2) Å, Z = 8 (No. 227)) containing an admixture of BiTaO₄ was obtained and characterized for the first time by the solid-phase synthesis method. The sample is characterized by a dense microstructure with a small number of pores formed by partially intergrown grains. The electronic state of the atoms was studied by the XPS and NEXAFS methods. According to spectroscopy data, bismuth atoms have an effective charge of +3, cobalt atoms +2, and tantalum ions +(5-δ). Co-doped bismuth magnesium tantalate with a pyrochlore structure (space group Fd-3m) was synthesized in the work [17]. Using X-ray phase analysis, it was established that single-phase Bi₂Mg_1−xCo_xTa₂O₉ samples are formed at x < 0.7. At a higher cobalt content in the samples, the impurity phase β-BiTaO₄ is detected, the amount of which is proportional to the degree of cobalt doping. The unit cell parameter of the Co,Mg co-doped bismuth tantalate phase increases with increasing content of cobalt ions in the samples from 10.5412(8) (x = 0.3) to 10.5499(8) Å (x = 0.7). The electronic state of ions in Bi₂Mg_1−xCo_xTa₂O₉ was studied using X-ray spectroscopy. According to NEXAFS and XPS data, it was established that doping with cobalt and magnesium does not change the oxidation state of bismuth and tantalum in the pyrochlore. The ions have the oxidation states Bi(+3), Mg(+2), and Ta(+5). Meanwhile, in the Ta4f-, Ta5p-, and Ta4d-spectra of the Bi₂Mg_1−xCo_xTa₂O₉ samples, an energy shift of the absorption bands towards lower energies is observed, which is characteristic of tantalum ions with an effective charge (+5-δ). In the XPS-Bi4f and -Bi5d spectra, a shift of bands to lower energies is detected, due to the distribution of some low-charge Co(II) ions in the bismuth position. According to NEXAFS spectroscopy data, the oxidation state of cobalt ions is predominantly 2+ and 3+. Similar cobalt pyrochlores based on bismuth niobate have been studied in detail in a wide concentration range for the Bi₂O₃–CoO–Nb₂O₅ system [7,18,19]. The single-phase pyrochlores were found to have Bi-deficient stoichiometry, with 8 to 25% of the A positions occupied by Co ions. In addition, the Bi and O atoms are shifted from ideal positions to the 96g and 32e positions by 0.34 Å and 0.45 Å, respectively. It was noted that a similar disordering of the structure is observed for pyrochlores in the Bi–M–Nb–O system (M = Zn, Fe, or Mn). It was also shown that a weak antiferromagnetic exchange is realized between paramagnetic atoms, and cobalt atoms have an effective charge of +(2.2).

New studies of pyrochlores based on bismuth tantalate/niobate have shown that pyrochlores are successfully synthesized in which the octahedral sublattice can simultaneously accommodate ions of seven different transition elements. The possibility of synthesizing phase-pure pyrochlores doped with Cr, Mn, Fe, Co, Ni, Cu, and Zn ions was described for the first time in the literature [20,21,22]. Several studies of complex oxide tantalates have shown [20,22] that they are low-porosity ceramics with moderate values (compared to complex niobates) of relative permittivity and low dielectric losses. As studies have shown [22], the permittivity of the complex oxide Bi_2-1/3Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Zn_1/6Ta₂O_9+Δ is 49, and the dielectric losses are 0.004, showing average values of dielectric parameters compared to monodoped pyrochlores. The authors of the work suggested that the individual effect of transition element ions in multi-element pyrochlores is leveled by the combined effect of all dopants. Despite the growing interest of researchers in multi-element oxide systems, the degree and nature of the interaction of dissimilar ions with each other and their effect on the properties of oxide ceramics as a whole have not been finally clarified. It should be noted that the properties of two- and three-component (by the number of transition elements as dopants) oxide systems are currently being actively studied as promising catalysts and anodes for lithium batteries [23,24,25,26]. In [27], it was shown that the two-component pyrochlore Fe,Co-doped bismuth tantalate exhibits tunable dielectric relaxation due to variable oxidation states of the Fe³⁺/Co²⁺ ions. In addition, the samples exhibit photocatalytic activity with respect to organic dyes under the influence of visible radiation. It was found that a pyrochlore with a low cobalt content showed better photocatalytic activity, and a pyrochlore with a high cobalt content demonstrated a higher photoinduced current density. In our work, the possibility of solid-phase synthesis of the phase-pure pyrochlore Bi_2−xFe_1/2Co_1/2Ta₂O_9+Δ with an equivalent content of cobalt and iron was demonstrated; oxidation states of transition element ions in the ceramics were analyzed based on X-ray spectroscopy data (Near Edge X-ray Absorption Fine Structure (NEXAFS) and X-ray Photoelectron Spectroscopy (XPS)).

2. Experimental Part

For solid-phase synthesis of Bi_2−xFe_1/2Co_1/2Ta₂O_9+Δ (x = 0; 0.135) samples, stoichiometric amounts of bismuth (III), tantalum (V), iron (III), and cobalt (II, III) oxides of analytical grade were used (Acros Organics BVBA, Geel, Belgium). The stoichiometric mixture of oxides was thoroughly homogenized in an agate mortar for one hour, then pressed into disks using a manual plexiglass press (pressure 5 MPa). The samples were successively calcined in 4 stages at temperatures of 650, 850, 950, and 1050 °C for 15 h at each stage of heat treatment. The temperature regime was selected based on the results of a study of the phase formation of pyrochlores based on bismuth tantalate [9,20]. After each calcination stage, the sample was thoroughly homogenized and pressed again in the form of disks to ensure tight contact of the ceramic grains. X-ray data were obtained using a Shimadzu 6000 X-ray diffractometer (Kyoto, Japan) (Cu_Kα radiation; 2θ = 10–80°; Δ2θ = 0.05°; scanning rate 2.0°/min). Analysis of the experimental the X-ray diffraction pattern was performed using the Powder Cell program V. 2.3, Berlin, Germany [28]. To calculate the unit cell parameter, the WinCSD software package, V. 4.0, Russia, was used [29]. The analysis of the impurity content in the samples was carried out by the Rietveld method using the Topas V. 5.0 software package [30].

The microstructure, local chemical analysis, and elemental mapping of the sample surface were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy (Tescan VEGA 3LMN scanning electron microscope, INCA Energy 450 energy-dispersive spectrometer, Tescan, Czech Republic). NEXAFS spectroscopy studies were carried out at the NanoPES beamline of the KISI synchrotron radiation source at the Kurchatov Institute (Moscow, Russia) [31]. NEXAFS spectra were obtained by total electron yield (TEY) recording with an energy resolution of 0.5 eV and 0.7 eV in the regions of the Fe2p and Co2p absorption edges, respectively. To construct NEXAFS spectra we used the built-in software (Atom V. 1.0, Russia) on the NanoPES experimental station. The X-ray Photoelectron Spectroscopy (XPS) study was performed on a Thermo Scientific ESCALAB 250Xi X-ray spectrometer (Thermo Fisher Scientific, Great Britain, UK). An Al Kα X-ray tube (1486.6 eV) was used as a source of ionizing radiation. In the experiments, an ion-electron charge compensation system was used to neutralize the charge of the sample. All peaks were calibrated with respect to the C1s peak at 284.6 eV. The experimental data were processed using the ESCALAB 250 Xi software (Thermo Fisher Scientific, Great Britain, UK). Standards for XPS are precursors for solid-phase synthesis; purchased oxides were α-Bi₂O₃ (monoclinic, sp. gr. P21/c), β-Ta₂O₅ (orthorhombic, sp. gr. Pna2), and Co₃O₄ (sp. gr. Fd-3m), α-Fe₂O₃ (sp. gr. R-3c), grade pure for analysis, from Acros Organics BVBA, Belgium.

3. Results and Discussion

3.1. Synthesis and Microstructure

As shown by X-ray phase analysis, as a result of solid-phase synthesis of a sample of the Bi₂Fe_1/2Co_1/2Ta₂O_9+Δ composition, a two-phase preparation was reproducibly formed, containing, in addition to the pyrochlore, an impurity phase of bismuth orthotantalate of the triclinic modification β-BiTaO₄ (sp. gr. P-1) in an amount of 6.56 mol.% (Figure 2). As shown by early studies [22], the formation of bismuth orthotantalate is associated with the placement of a part of the dopants—ions of transition elements in the cationic sublattice of bismuth(III). Low-charge cations, the ionic radius of which is significantly larger than the ionic radius of tantalum(V), preferring tetrahedral coordination (Zn²⁺, Co²⁺, Cu⁺, and Mn²⁺), are distributed into the crystallographic positions of bismuth. In works [9,20] it was demonstrated that the synthesis of a single-phase sample is possible if additional vacancies are created in the bismuth sublattice by an amount proportional to the amount of the β-BiTaO₄ impurity. For our case, based on the exact value of the mole fraction of the impurity, 6.56 mol.% β-BiTaO₄, the defective bismuth composition of the single-phase ceramics Bi_2−xFe_1/2Co_1/2Ta₂O_9+Δ were calculated. Solid-phase synthesis of the Bi_2−xFe_1/2Co_1/2Ta₂O_9+Δ sample (x = 0.135) was carried out under the same conditions as Bi₂Fe_1/2Co_1/2Ta₂O_9+Δ, described in the experimental part. The X-ray diffraction pattern of the sample is shown in Figure 1. As shown by the X-ray phase analysis, the synthesized sample Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ does not contain impurity phases. It is interesting to note that the synthesis of the pyrochlore of the Bi₂CoTa₂O₉ composition also led to the formation of a two-phase preparation. Taking into account the amount of the β-BiTaO₄ impurity, it was possible to synthesize a single-phase pyrochlore Bi_1.862CoTa₂O₉ in [9]. Comparing these two compositions, we see that the coefficients for the bismuth atom absolutely coincide within the error limits of calculating the impurity content. In this case, several conclusions can be made. Obviously, Co(II) ions are distributed in the bismuth position in the Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ and Bi_1.862CoTa₂O₉ samples; these ions have a large ionic radius compared to the ionic radius of tantalum (V) (R(Co(II))_c.n-6 = 0.745 Å and R(Ta(V))_c.n-6 = 0.64 Å)) [29]. Cobalt(II) ions are distributed in the bismuth position in equal amounts for the two compositions Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ and Bi_1.862CoTa₂O₉. This indicates a non-random (non-statistical) distribution of ions over the cation sublattices of bismuth/tantalum and the presence of a pattern in the distribution of cations. It should be noted that the data are highly reproducible, since the samples were not synthesized simultaneously, but with a difference of several years. Similar behavior of the cobalt(II) ions in the two compositions can be explained by the proximity of the ionic radii of Ta(V) and Fe(III) (R(Fe(III))_c.n-6 = 0.645 Å and R(Ta(V))_c.n-6 = 0.64 Å)) in octahedral coordination. Moreover, it can be assumed that the ionic and spin composition (Co(II),Co(III)) of cobalt is almost the same in both samples.

Calculation of the unit cell parameter of the pyrochlore Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ showed a value of 10.5013(8) Å, which is significantly less than the unit cell parameter of 10.54051(3) Å for the cobalt-containing pyrochlore based on bismuth tantalate Bi_1.49Co_0.8Ta_1.6O_7.0 (Bi_1.862CoTa₂O₉) [9], which, with the same bismuth content, is associated with a significant difference in the radii of iron (III) and cobalt (II) ions (R(Fe(III))_c.n.-6 = 0.645 Å and R(Co(II))_c.n.-6 = 0.645 Å) [32]. The unit cell parameter of 10.4969 Å for the iron-containing pyrochlore Bi₂FeTa₂O_9+Δ is comparable with that calculated for Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ [33]. This is due to the fact that the lack of bismuth(III) ions in the composition of the studied sample is compensated by the difference in the radii of the iron(III) and cobalt(II) ions. The unit cell parameter of the pyrochlore Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ calculated by us corresponds to the values given in the article [10] for iron-containing pyrochlores based on bismuth tantalate (10.4979–10.5033 Å). Elemental mapping of the Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ sample showed a uniform distribution of element atoms over the sample surface (Figure 3), and EDS analysis confirmed that the experimental sample composition corresponded to the specified one. According to the EDS spectrum, the chemical composition of the sample corresponds to the formula Bi_1.74Fe_0.47Co_0.48Ta₂O_9+Δ and corresponds to the nominal composition.

Microphotographs of the surface of the sample synthesized at 1050 °C are shown in Figure 4. The porous, loose microstructure is formed by partially intergrown grains. Individual small crystallites and large agglomerates up to 4 μm are recorded as a result of grain intergrowth. The average crystallite size determined by X-ray diffraction using the Scherrer formula is ~67 nm, while larger crystallites in the range of 0.5–1 μm were determined using a scanning electron microscope (SEM). Apparently, the crystallites in the micrographs are aggregated ceramic grains of significantly smaller sizes.

It is interesting to note that the microstructure of the cobalt pyrochlore of the same bismuth stoichiometry is less porous, coarse grains are fused with each other, and the grain size is 1–3 μm [9]. Meanwhile, the microstructure of iron-containing preparations is porous [33] and formed by partially fused weakly faceted grains. The sizes of round crystallites, determined by the Scherrer method, are ∼49 nm. Grains with a longitudinal size of 0.2–2 μm were recorded by scanning electron microscopy. Taking into account the above, the microstructure of the ceramics is affected by the relative content of bismuth, iron, and cobalt ions. Apparently, cobalt and bismuth ions contribute to the production of dense, low-porosity ceramics, while iron ions contribute to the production of loose, fine-grained ceramics. This important observation will allow targeted regulation of the microstructure of ceramics by chemical modification of the pyrochlore composition.

3.2. XPS and NEXAFS Spectroscopy

The XPS spectra of bismuth tantalate co-doped with cobalt and iron atoms Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ are shown in Figure 5a–e. The spectra of the initial oxides, the precursors for the synthesis, are also shown for comparison. Figure 5a shows the XPS spectrum in a wide energy range, and Figure 5b–e shows the spectral dependences in the region of the Bi4f, Bi5d, Ta4f, Co2p, and Fe2p ionization thresholds of the studied pyrochlore.

Table 1. Energy positions of the components of the XPS spectra for Bi_1.865Co_1/2Fe_1/2Ta₂O_9+Δ.

Peak	Energy (eV)	Peak	Energy (eV)
Bi4f7/2	158.78	Co2p3/2	780.16
Bi4f5/2	164.10	Co2p1/2	795.93
Bi5d5/2	25.84	Co2p sat	785.27
Bi5d3/2	28.84	Fe2p3/2	710.56
Ta4f7/2	25.41	Fe2p1/2	724.32
Ta4f5/2	27.31	Fe2p sat	718.60
Ta5p3/2	35.78	Fe2p sat	733.22

shows the energy positions of the XPS spectra components. The figures also show the results of decomposition of spectral dependences into individual peaks modeled by Gauss–Lorentz curves, and the background lines were modeled by the Shirley or smart approximation. The Survey XPS spectrum contains a C1s peak, which is associated with the presence of random surface organic contaminants on the surface. The presence of organic impurities contributes to the intensity of the O1s peak and distorts it. In this regard, the analysis of the surface composition of the samples was carried out based on the analysis of the spectra of metal ions.

Comparison of XPS-Bi4f and -Bi5d spectra of the studied sample and Bi₂O₃ oxide (Figure 5b) showed that the energy position and peak width in the spectrum of the sample almost completely coincide with the spectrum of Bi₂O₃ oxide. This suggests that the bismuth cation in the pyrochlore has a charge state of Bi³⁺. The shape of the peaks in the spectrum of tantalum ions (Figure 5c) clearly indicates that all cations are in the same charge state (there is no splitting or distortion of the peaks), while the energy position of the peaks has a characteristic shift toward lower energies compared to the binding energy in pentavalent tantalum oxide Ta₂O₅. The shift toward lower energies is characteristic of a decrease in the effective positive charge; in particular, for the Ta4f and Ta5p spectra presented by us, this energy shift is ΔE = 0.75 eV. This, in turn, allows us to assume that tantalum atoms have the same effective charge +(5-δ).

The energy range of the Co2p spectrum (Figure 5d) of the sample includes a peak responsible for the binding energy of the Ta4p_1/2 level, which somewhat complicates the perception of the cobalt spectrum. Meanwhile, when comparing the pyrochlore spectrum with the Co₃O₄ spectrum we obtained and the CoO spectrum known from the literature [34], it can be noted that the energy position of the main peaks in all the spectra presented is almost the same. Bands characteristic of Co(II) are observed at 781 and 797 eV. At the same time, the spectrum of the pyrochlore and cobalt oxides contains satellite peaks, the energy position of which in the oxides is different. In the CoO spectrum, they are shifted toward lower energies and are more intense. Based on the position and intensity of the satellite lines in the pyrochlore spectrum, one can conclude that the spectra of CoO and pyrochlore correspond. This means that the charge state of cobalt ions in the pyrochlore is predominantly +2. In the XPS-Fe2p spectra (Figure 5e) the situation is different from that in the cobalt spectra. Comparison of the Fe2p spectrum in the pyrochlore with the spectra of FeO [35,36] and Fe₂O₃ oxides shows that the binding energy of the Fe2p_3/2 and Fe2p_1/2 levels (711 and 724 eV) and the energy position of the satellite peaks (719 and 733 eV) in the pyrochlore coincide with the spectrum of Fe₂O₃. This allows us to unambiguously determine the charge state of iron atoms in the composites as Fe³⁺. NEXAFS spectra of the Bi_1.865Fe_1/2Co_1/2Ta₂O_9+Δ sample are shown in Figure 6.

When analyzing NEXAFS spectra, it is necessary to take into account that absorption spectra have a richer structure compared to XPS spectra, since they are associated with transitions of internal electrons of atoms (in this case, 2p electrons of iron and cobalt atoms) to free valence states of these atoms [37]. In this case, in accordance with the dipole selection rules, these states should be formed from 3d-orbitals. As is known, it is 3d electrons that participate in the formation of covalent bonds in the compounds under consideration, and accordingly, NEXAFS 2p spectra are most sensitive to changes in the environment of 3d-atoms.

When comparing the NEXAFS Fe 2p_3/2 spectrum of the pyrochlore and the spectra of iron oxides, it is evident (Figure 6a) that the energy position and intensity ratio of the absorption bands at 709 and 708 eV in the pyrochlore coincide with the characteristics of the bands in the Fe 2p_3/2 spectrum of Fe₂O₃ oxide, which also confirms the state of iron oxides as Fe(III). In the Co 2p_3/2 spectrum of the pyrochlore (Figure 6b), a peak at 779 and a shoulder at 781 eV and 778 eV appear, which in terms of energy position and intensity ratio correlate well with the main features of the CoO spectrum. Based on this, it can be concluded that cobalt ions in pyrochlore are predominantly in the Co(II) state.

4. Conclusions

The possibility of forming a Fe/Co co-doped pyrochlore with equivalent iron and cobalt content was investigated. As a result, two samples of Bi_2−xCo_1/2Fe_1/2Ta₂O_9+Δ (x = 0; 0.135) were synthesized by the solid-phase reaction method. According to the XRD data, the Bi₂Co_1/2Fe_1/2Ta₂O_9+Δ sample is not single-phase and contains an impurity of bismuth orthotantalate in the amount of 6.56%. We associate the formation of the impurity with the distribution of a part of the cobalt(II) cations in the bismuth sublattice. In our work, we were able to prevent the formation of the impurity by creating vacancies in the bismuth sublattice proportional to the amount of BiTaO₄ (x = 0.135). In this regard, the Bi_1.865Co_1/2Fe_1/2Ta₂O_9+Δ sample (sp. gr. Fd-3m) turned out to be phase-pure. According to the XPS data, a characteristic shift of the Ta4f and Ta5p spectra to lower energies by 0.75 eV is observed, which may be due to the distribution of low-charge cobalt and iron cations in the octahedral positions of tantalum (V). The effective charge of tantalum cations is +(5-δ) and that of bismuth cations is +3. According to the character of the Co2p XPS spectrum of the complex oxide, cobalt ions are predominantly in the charge state of +2. According to the XPS data, the effective charge of iron cations is +3.