XPS and NEXAFS Characterization of Mg/Zn and Mn Codoped Bismuth Tantalate Pyrochlores

Abstract

Two series of the bismuth tantalate pyrochlore samples, codoped with Mg,Mn and Zn,Mn, were synthesized via solid-phase reaction. It was established that the Bi₂Mg(Zn)_xMn_1−xTa₂O_9.5−Δ (x = 0.3; 0.5; 0.7) samples contain the main phase of cubic pyrochlore (sp. gr. Fd-3m) and an admixture of triclinic BiTaO₄ (sp. gr. P-1). In both sets, the amount of BiTaO₄ is proportional to the amount of manganese doping, however, zinc-containing samples have a higher level of impurities than magnesium-containing ones. The unit cell parameter of the Zn,Mn codoped bismuth tantalate phase increases with an increasing content of zinc ions in the samples from 10.4895(5) (x = 0.3) to 10.5325(5) Å (x = 0.7). The unit cell parameter of Mg,Mn codoped bismuth tantalate pyrochlores increases uniformly with an increasing index x(Mg) from 10.4970(8) at x = 0.3 to 10.5248(8) Å at x = 0.7, according to the Vegard rule. The NEXAFS and XPS data showed that the ions were found to have oxidation states of Bi(+3), Ta(+5), Zn(+2) and Mg(+2). In the Ta 4f XPS spectrum of both series of samples, a low energy shift of the absorption band characteristic of tantalum ions with an effective charge of (+5-δ) was observed. The XPS spectra of Bi4f7/2 and Bi4f5/2 also show a shift of bands towards lower energies which is attributed to the presence of some low-charge ions of transition elements in the bismuth position. The NEXAFS spectroscopy data showed that manganese ions in both series of samples have predominantly 2+ and 3+ oxidation states. XPS data indicate that in zinc-containing preparations the proportion of oxidized manganese ions is higher than in magnesium-containing ones.

1. Introduction

In 2026, it will be 200 years since the first information was obtained about natural minerals of the niobium group (Na,Ca)₂Nb₂O₆(OH,F), which were called pyrochlore from the Greek words “fire” and “green” due to their green color when heated [1]. Pyrochlores are an important class of materials in various technological applications due to a wide range of practically useful properties, including metallic and ionic conductivity, superconducting and ferroelectric properties [2,3]. They exhibit ferro- and antiferromagnetism, magnetoresistance and a spin glass state [4,5,6]. Materials based on pyrochlores are used in solid-state devices as thin-film resistors, thermistors, communication elements and photocatalysts, and are used as components of ceramic molds for radioactive waste. Due to their excellent dielectric properties, their ability to regulate dielectric properties using an electric field, their low sintering temperature and chemical compatibility with low-melting Ag and Cu conductors, oxide pyrochlores are promising as dielectrics for multilayer ceramic capacitors, tunable microwave dielectric components, resonators, and devices for microwave applications [7,8,9].

The general formula of oxide pyrochlores A₂B₂O₇ describes a large family of compounds isostructural with the mineral pyrochlore. In the crystal structure of pyrochlore, two cationic sublattices with the anticristobalite structure A₂O’ and the octahedral B₂O₆ are distinguished. The positions of cations A with octaoxygen coordination are occupied by large ions (Ca²⁺, Bi³⁺). The three-dimensional framework of B₂O₆ is formed by [BO₆] octahedra connected at the vertices, in the positions of which cations with a smaller ionic radius (Ti⁴⁺, Ta⁵⁺) are located [10,11]. There are known cases of mixed pyrochlores with three or more types of cations located at two nonequivalent cation positions A and B. These include pyrochlores based on bismuth tantalate, doped with ions of 3 elements [12,13,14]. Taking into account the polarization properties of these ions and the proximity of their radii to the ionic radius of Ta(V), it is logical to assume their distribution in the octahedral positions of tantalum ions. However, as studies by many scientists have shown [15,16], depending on the chemical nature, a small fraction of transition element ions replaces the Bi(III) positions. Apparently, the reason for this is the tension of the octahedral framework due to the introduction of incommensurate and heterovalent dopant ions (Zn²⁺, Mn²⁺, Co²⁺, Cu⁺) in relation to Ta(V) ions, as well as the preference of some of them for tetrahedral coordination. Such doping options lead to the formation of a pyrochlore structure deficient in A cations, as is observed for bismuth-containing pyrochlores and is the reason for the relaxation properties of oxide ceramics [12,17]. Currently, almost all pyrochlores based on bismuth tantalate containing 3 ions (Cr,Fe,Co,Ni,Cu,Zn) are known and studied [18,19,20,21,22,23,24,25,26,27]. Due to their excellent dielectric properties, such pyrochlores are promising as multilayer ceramic capacitors, resistors, resonators, sensors and microwave filters. It has been established that moderate values of dielectric constant ∼78–92 and dielectric loss ~10⁻¹ at 30 °C and 1 MHz are exhibited by iron-containing pyrochlores Bi_3.36Fe_2.08+xTa_2.56−xO_14.56−x (−0.32 ≤ x ≤ 0.48) [27]. Magnesium-containing pyrochlores Bi_3+5/2xMg_2−xTa_3−3/2xO_14−x (0.12 ≤ x ≤ 0.22) are characterized by comparable values of ε ~70–85 and low dielectric loss tangent δ ~10^–3 at 1 MHz and 30 °C [28,29]. For zinc-containing pyrochlores Bi_1.5ZnTa_1.5O₇, the dielectric constant under standard conditions does not exceed 60 [25,26]. We have not found any information on manganese-containing pyrochlores based on bismuth tantalate. There is one known work [15] devoted to the study of analog pyrochlores in the Bi₂O₃-Mn₂O₃-Nb₂O₅ system. The authors of the article established that a significant concentration region of bismuth-deficient manganese-containing pyrochlores is formed in the system, in which 14–30% of the A-positions are occupied by Mn²⁺ ions. As the authors of [15] showed, X-ray powder diffraction data confirmed that all Bi–Mn–Nb–O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. According to [15], the displacive disorder is crystallographically analogous to that reported for Bi_1.5Zn_0.92Nb_1.5O_6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra of manganese-containing pyrochlores showed the presence of Mn²⁺ and Mn³⁺ ions. As is known, manganese in high-temperature ceramics can have a complex ionic composition (the oxidation state of manganese varies from 2+ to 4+), which affects the physicochemical properties of the ceramics. The ionic state of manganese is influenced by many variable factors, among which is the symmetry and strength of the crystal field, and the nature of the ligands; distortions and the size of the coordination polyhedron are particularly prominent. Studies have shown that the combined use of X-ray spectroscopy methods (XPS, NEXAFS) make it possible to most accurately determine the ionic composition of complex oxides [30]. As part of our work, we studied the electronic state of manganese ions in Mg(Zn), Mn codoped bismuth tantalate pyrochlores using NEXAFS and XPS spectroscopy. The influence of zinc and magnesium and the degree of substitution of Ta(V) ions on the proportion of oxidized manganese ions in pyrochlores has been established.

2. Experimental Part

Bi₂Zn(Mg)_xMn_1−xTa₂O_9.5−Δ (x = 0.3, 0.5, 0.7) samples were synthesized using the solid-phase reaction method from the oxides ZnO, MgO, Bi₂O₃, Mn₂O₃, Ta₂O₅. A finely ground and homogeneous mixture of oxides in stoichiometric quantities was pressed into disk-shaped compacts (diameter 10 mm, thickness 3–4 m) using a hand press. High-temperature treatment of the samples was carried out in stages, at temperatures of 650, 850, 950, 1050 °C for 15 h at each calcination stage. The phase composition was determined by X-ray phase analysis using a Shimadzu 6000 X-ray diffractometer (CuKα radiation). The microstructure and local elemental composition of the samples were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy (electron scanning microscope Tescan VEGA 3LMN, Japan, energy dispersion spectrometer INCA Energy 450, Japan). XPS studies were carried out using the equipment of the resource center of the Science Park of St. Petersburg State University “Physical methods of surface research.” XPS analysis was performed on a Thermo Scientific ESCALAB 250Xi X-ray spectrometer, Great Britain. An X-ray tube with AlKα radiation (1486.6 eV) was used as a source of ionizing radiation. To neutralize the sample charge in the experiments, an ion-electronic charge compensation system was used. All peaks were calibrated relative to the C1s peak at 284.6 eV. The processing of experimental data was carried out using the software of the ESCALAB 250Xi spectrometer (Avantage v5.9925, Great Britain). The samples were studied using NEXAFS spectroscopy at the NanoFES station of the KISS synchrotron source at the Kurchatov Institute (Moscow, Russia) [30]. NEXAFS spectra were obtained by recording the total electron yield (TEY) with an energy resolution of 0.5 eV.

3. Results and Discussion

3.1. Phase Composition of the Bi₂Zn(Mg)_xMn_1−xTa₂O_9.5−Δ

The choice of the stoichiometry of ceramic is based on our previous research on the doping of magnesium-containing pyrochlores based on bismuth tantalate with transition 3d elements [21,22]. X-ray phase analysis revealed that samples with the composition Bi₂Zn_xMn_1−xTa₂O_9.5−Δ (x = 0.3–0.7) are two-phase. In addition to the main cubic phase, they contain triclinic bismuth orthotantalate BiTaO₄ as an impurity (Figure 1) [31]. The analysis of the reflection extinctions of the cubic phase confirmed that the symmetry of the crystal structure is cubic with the space group Fd-3m [10,11]. The unit cell parameter of cubic pyrochlore in Bi₂Zn_xMn_1−xTa₂O_9.5−Δ increases uniformly with an increase in the zinc ion content (and a decrease in the manganese ion content) in the samples from 10.4895(5) (x = 0.3) to 10.5325(5) Å (x = 0.7). In addition, X-ray spectroscopy indicates that the ionic radius of zinc(II) (R(Zn(II))c.n-6 = 0.74 Å) is smaller than that of Mn(II) ions (R (Mn(II))c.n-6 = 0.83 Å) present in the samples (Figure 1) [32]. The observed unusual behavior can be explained by several factors. Firstly, the amount of impurity decreases with increasing zinc content, resulting in a higher pyrochlore cell parameter. Secondly, Mn(II) ions occupy bismuth(III) positions, which have a similar ionic radius to bismuth(III) (R(Bi(III))c.n-8 = 1.17 Å, R(Mn(II))c.n-8 = 0.90 Å), and do not significantly contribute to the cell parameter. The increase in the cell parameter occurs due to the distribution of large zinc(II) ions into the octahedral positions of tantalum(V) (R(Zn(II))c.n-6 = 0.74 Å, R(Ta(V))c.n-6 = 0.64 Å). Quantitative assessment of the bismuth orthotantalate content showed that the impurity content varies from 11.9 (x(Zn) = 0.7) to 20.0 wt.% (x = 0.3) and increases with increasing manganese content in the samples. As previously demonstrated [20], the presence of impurities in the samples can be associated with the placement of Mn(II) ions both in the octahedral sublattice of tantalum(V) and the bismuth(III) sublattice, due to the larger size of Mn(II) ions compared to Zn(II). It is probable that most of the relatively large manganese ions are distributed within the octahedral sublattice of tantalum(V) leading to oxygen vacancies and stress in the octahedral framework as a whole. In order to relieve stress in the crystal structure, some manganese(II) ions are placed in the bismuth(III) position. This causes the system to create vacancies in the bismuth sublattice, resulting in the release of the bismuth orthotantalate phase as an impurity [20]. In most cases, the amount of bismuth orthotantalate impurity is typically proportional to the number of 3 ions located in the bismuth position [33]. These ions may include manganese(II) and zinc(II) ions, since they have similar ionic radii (R(Zn(II))c.n-8 = 0.90 Å, R(Mn(II))c.n-8 = 0.96 Å) [32]. Additionally, zinc(II) ions are typically characterized by tetrahedral coordination rather than octahedral. On the other hand, manganese(II) ions have a larger ionic radius than Zn(II). Therefore, considering the change in the cell parameter, it can be assumed that all manganese ions occupy the bismuth position, especially at high values of x(Mn). The inset in Figure 1 shows the dependence of the change in the unit cell parameter on the manganese content and the amount of BiTaO₄ impurity in Bi₂Zn_xMn_1−xTa₂O_9.5−Δ.

The magnesium-containing samples of Bi₂Mg_xMn_1−xTa₂O_9.5−Δ (x = 0.3–0.7) are also two-phase and contain β-BiTaO₄ as an impurity (Figure 2). The amount of bismuth orthotantalate is proportional to the content of manganese ions and varies from 7.9 (x(Mn) = 0.3) to 23.8 wt.% (x(Mn) = 0.7). The proportionality of the impurity amount to the manganese content in the samples may indicate the distribution of some manganese ions in the bismuth position and magnesium ions in the octahedral positions of tantalum(V). This was previously demonstrated in the example of solid solutions Bi₂Mg_xM_1−xTa₂O_9.5−Δ (M-Ni, Cr, Fe) [21]. The analysis of the reflection extinctions in the cubic phase confirmed that the symmetry of the crystal structure is cubic and is described by the space group Fd-3m, which is characteristic of cubic pyrochlore [10]. The unit cell parameter of cubic pyrochlore in Bi₂Mg_xMn_1−xTa₂O_9.5−Δ decreases uniformly with an increase in the content of manganese ions (and a decrease in magnesium ions) in the samples from 10.5248(8) (x(Mn) = 0.3) to 10.4970(8) Å (x(Mn) = 0.7), despite the fact that the ionic radius of magnesium(II) is smaller than that of the Mn(II) ions (Figure 2 insert) (R(Mg(II))c.n-6 = 0.72 Å, R(Mn(II))c.n-6 = 0.83 Å) [32]. This increase is due to the difference in ionic radii between magnesium and tantalum(V) (R(Mg(II))c.n-6 = 0.72 Å, R(Ta(V))c.n-6 = 0.64 Å). When analyzing the phase composition of Bi₂Mg(Zn)_xMn_1−xTa₂O_9.5−Δ (x = 0.3; 0.5; 0.7) samples, it is noteworthy that the only impurity present in both series with zinc and magnesium is BiTaO₄. The amount of impurity increases with increasing manganese content in x(Mn) samples, which may be due to an increase in the content of large Mn(II) ions and their distribution in the Bi(III) position. Based on Figure 3a, zinc preparations have a higher impurity level at low concentrations of x(Mn) < 0.5, while magnesium pyrochlores have a higher impurity level at higher concentrations of x(Mn) > 0.5. There are several reasons for this phenomenon. In accordance with earlier X-ray diffraction studies of magnesium-containing preparations Bi₂Mg_xM_1−xTa₂O_9.5−Δ (M-Ni, Cr, Cu, Fe) showed the distribution of magnesium ions in the octahedral sublattice, which means that with a combination of Mn/Mg in bismuth positions there will be manganese ions in the entire concentration interval x(Mn). In Mn/Zn samples, it was previously noted that both manganese and zinc ions have the potential to occupy bismuth positions. In this regard, the higher amount of impurities in zinc preparations at x(Mn) < 0.5 can be explained by the fact that with a high content of zinc ions, some of them move to the bismuth position, and with a decrease in zinc ions content, manganese ions are distributed in the bismuth(III) position (II). This statement is supported by the fact that the composition of Bi₂MgTa₂O₉ is a single-phase pyrochlore, while the Bi₂ZnTa₂O₉ sample is a non- single phase with an admixture of BiTaO₄ in addition to the main pyrochlore phase. It is interesting to note that, despite having the same charge and comparable ionic radii of magnesium and zinc in six and eight coordinate positions (R(Bi(III))c.n-8 = 1.17 Å, R(Mg(II))c.n-8 = 0.89 Å, R(Zn(II))c.n-8 = 0.90 Å), their distribution in the bismuth position is different and zinc ions have a greater tendency to occupy it. This phenomenon is mainly influenced by the polarization properties of the ions. In particular, the electronegativity of bismuth and zinc atoms according to Allred–Rochow coincides and is equal to 1.67, and the electronegativity of magnesium is significantly less at 1.23 and is closer to the electronegativity of tantalum [34].

The lower amount of impurities in zinc preparations at x(Mn) > 0.5 is explained by the lower content of Mn(II) ions compared to magnesium preparations (Figure 3). X-ray spectroscopy data shows that the amount of oxidized manganese(III) ions throughout the entire concentration range is greater in zinc preparations than in magnesium preparations. Earlier, we observed this interesting experimental fact while studying the magnetic behavior of solid solutions of heterovalent substitution Bi₅Nb_3−3xMn_3xO_15−δ and Bi₂BaNb_2−2xMn_2xO_9−δ [35,36]. It has been shown that with an increase in manganese dopant ions, the proportion of oxidized manganese ions increases in order to neutralize the effect of heterovalent substitution and reduce the number of oxygen vacancies that destabilize the crystal structure. In this case, the difference between magnesium and zinc preparations is based on the degree of distortion of the octahedral framework when replacing Ta(V) positions. Apparently, zinc ions, unlike magnesium ions, cause significant geometric distortions of the polyhedral environment due to a tendency towards tetrahedral coordination. These distortions affect the oxygen environment of manganese ions, resulting in an asymmetrical environment. In order to stabilize the oxygen framework, manganese ions pass into an oxidized state. It is interesting to note that in the competition to occupy bismuth positions, zinc ions are less effective than manganese(II) ions. This can be attributed to the difference in ionic radii between zinc and manganese(II) (R(Zn(II))c.n-6 = 0.74 Å, R(Mn(II))c.n-6 = 0.83 Å).

The unit cell parameter of pyrochlore in magnesium compositions Bi₂Mg_xMn_1−xTa₂O_9.5−Δ is greater than in similar zinc compositions (Figure 3). This can occur due to two reasons. Firstly, BiTaO₄ impurities in zinc compositions are significantly higher than in magnesium compositions, but only for compositions with x(Mn) < 0.5. Secondly, in zinc compositions, the proportion of oxidized manganese (+3) ions is higher than in magnesium compositions. The unit cell parameter decreases when octahedrally coordinated and oxidized manganese ions are present, due to their radius being small compared to magnesium Mg(II) and Ta(V) (R(Mn(IV))c.n-6 = 0.53 Å, R(Mn(III))c.n-6 = 0.645 Å, R(Mg(II))c.n-6 = 0.72 Å, R(Ta(V))c.n-6 = 0.64 Å). NEXAFS studies show that zinc compositions have a higher proportion of oxidized manganese ions than magnesium compositions, with comparable amounts of impurities.

3.2. NEXAFS and XPS of the Bi₂Mg(Zn)_xMn_1−xTa₂O_9.5−Δ

The electronic state of ions in Zn(Mg),Mn-codoped bismuth tantalate pyrochlores was studied using X-ray spectroscopy methods (NEXAFS and XPS). XPS spectra of Bi₂MgTa₂O₉ and Bi₂Mg_xMn_1−xTa₂O_9.5−Δ (x = 0.5, 0.7) precursor oxides are presented in Figure 4a–f.

Figure 4a shows XPS spectra in a wide binding energies range (20–1400 eV) and in the region of the Bi4f-, Bi5d-, Ta4f-, Ta4d- and Mn2p-ionization thresholds of Bi₂Mg_0.5Mn_0.5Ta₂O_9.5−Δ and Bi₂MgTa₂O₉ (Figure 4a–f). The spectral dependences were decomposed into individual peaks by Gaussian –Lorentzian curves and the background lines were modeled by Shirley or smart approximation. The XPS survey spectrum exhibits a C1s peak indicating the presence of organic contaminants on the sample surface. Since the signal from surface contaminants contributes to the intensity of the O1s peak, the composition of the sample surface was characterized using metal ion spectra. The energy positions of the components in the measured XPS spectra of the samples are presented in

Table 1. Energy positions of the components of XPS spectra depending on the chemical composition of the compounds.

	Chemical Composition of Compounds
Peak	Bi2Mg0.5Mn0.5Ta2O9.5−Δ	Bi2MgTa2O9	Bi2Zn0.3Mn0.7Ta2O9.5−Δ	Bi2Zn0.7Mn0.3Ta2O9.5−Δ
	Energy (eV)
Bi4f7/2	158.80	159.03	158.81	158.79
Bi4f5/2	164.12	164.35	164.13	164.10
Bi5d5/2	25.82	26.11	25.81	25.87
Bi5d3/2	28.72	29.08	28.71	28.78
Ta4f7/2	25.31	25.66	25.35	25.41
Ta4f5/2	27.21	27.56	27.25	27.31
Mg1s	1302.49	1303.19
Mn2p3/2	640.83		640.98	640.71
Mn2p1/2	652.39		652.75	652.51
Zn2p3/2			1021.07	1020.98
Zn2p1/2			1044.10	1043.99

. It should be noted that the doping with manganese and magnesium atoms has almost no effect on the spectral characteristics of bismuth and tantalum atoms (Figure 4b,d,e). The XPS spectra of the oxides Bi₂O₃ and Ta₂O₅ (Figure 4 and Figure 5) were obtained by our research group. The comparative analysis of the XPS Bi4f spectra of the investigated sample and Bi₂O₃ oxide (Figure 4b,d) highlights that the energy position and width of the peaks in the sample spectrum correlate with the corresponding spectra of Bi₂O₃ oxide. It is notable that the Bi5d3/2, Bi5d5/2, Bi4f7/2, and Bi4f5/2 absorption bands of the Bi2Mg_0.5Mn_0.5Ta₂O_9.5−Δ sample are slightly shifted to a lower energy region compared to the Bi₂MgTa₂O₉ sample, from 164.12 eV to 164.35 eV (for Bi4f5/2 band), respectively. This fact indicates a decrease in the effective charge of bismuth ions in the samples. Apparently, the shift of the peaks is associated with the placement of a certain fraction of divalent manganese(II) ions in the bismuth position demonstrated by X-ray phase analysis. The energy positions of the peaks in the XPS Mg1s spectra shown in Figure 4c are typical for the divalent magnesium atom [37]. The analysis of the peak shape in the spectrum of tantalum atoms (Figure 4d,e) indicates that all tantalum atoms are in the same charge state. However, the energy position of the peaks shows a characteristic shift towards lower energies compared to the binding energy in pentavalent tantalum oxide Ta₂O₅. This shift towards lower energies indicates a decrease in the effective positive charge. Specifically, Ta4f and Ta5p spectra exhibit an energy shift of ΔE = 0.7 eV, while the Ta4d spectra exhibit an energy shift of 1 eV. This suggests that tantalum atoms have the same effective charge +(5-δ) as observed in similar spectra of tantalum in bismuth tantalates doped with Cr, Fe, Co, Ni, Cu atoms [21,22,24]. The shift of the Ta4f and Ta5p spectra for Bi₂Mg_0.5Mn_0.5Ta₂O_9.5−Δ and Bi₂MgTa₂O₉ may also be associated with the presence of a certain fraction of heterovalent manganese and magnesium ions in the tantalum(V) position.

Let us move on to consider the Mn2p spectra presented in Figure 4f. The comparative analysis of the composite spectra with the spectra of the oxides MnO [38], Mn₂O₃ [39] and MnO₂ [40] from the literature data shows that the peak energy positions of the spectrum of Bi₂Mg_0.5Mn_0.5Ta₂O_9.5−Δ correlates well with the spectrum of MnO, while the spectra of Mn₂O₃ and MnO₂ are shifted towards higher energies. This suggests that manganese ions predominantly have an oxidation state of (2+). Figure 4f shows the incomplete energy coincidence of the components of the spectrum of manganese in MnO and Bi₂Mg_0.5Mn_0.5Ta₂O_9.5−Δ, indicating the presence of oxidized manganese ions (3+, 4+). The degree of oxidation of manganese in solid solutions was clarified by NEXAFS studies. Figure 4h shows the NEXAFS Mn2p spectra of Bi₂Mg_0.5Mn_0.5Ta₂O_9.5−Δ and Bi₂Mg_0.3Mn_0.7Ta₂O_9.5−Δ and manganese oxides. The correlation between the spectra shape and energy position of the main peaks (a-c) in the NEXAFS Mn2p spectra of all samples and in oxides indicates that the manganese atoms primarily have a charge state of +2. It should be noted that band c, which can be identified as a separate peak in the spectrum of MnO, is visible in the spectra of the samples as a shoulder. In addition, the relative intensity of the b bands also increases compared to the MnO spectrum. Shoulder c coincides in energy position with a broad band in the spectrum of MnO₂, and bands a and b correlate well with the corresponding features in the spectra of Mn₂O₃, but with different intensities. This suggests that the manganese atoms in the composites are both in the +2-charge state and partially in the +3/+4 state. To clarify this conclusion, we decomposed the XPS Mn2p spectra of the Bi₂Mg_0,5Mn_0,5Ta₂O_9.5−Δ sample into individual components (Figure 4g) using the spectra of the oxides presented in Figure 4f. The decomposition occurred in multiple stages: (1) subtraction of the background lines obtained using the Shirley approximation, (2) the “background-free” XPS spectra were normalized by equal areas (in this case, one was taken), (3) a model spectrum was constructed as the sum of the XPS spectra of MnO, Mn₂O₃ and MnO₂ with the corresponding coefficients α, β and γ (α + β + γ = 1). The value of coefficients was selected in order to achieve the best possible agreement with the composite spectrum. The Fisher F-criterion was used to determine optimal agreement between intensities of the XPS spectra of the composite and the model spectrum. The results of optimal modeling are presented in Figure 4h, with α = 0.77, β = 0.23 and γ = 0 (Fisher’s F test is 0.9998). The obtained data suggest that the Bi₂Mg_0,5Mn_0,5Ta₂O_9.5−Δ composite has two nonequivalent states of manganese atoms: about 77% are in the +2-charge state, and the remaining 23% are in +3 state. This fact allows us to conclude that the intense b band presented in the NEXAFS spectra (Figure 4h) indicates the presence of Mn³⁺ ions in the composite. Moreover, with increasing manganese content in Bi₂Mg_xMn_1−xTa₂O_9.5−Δ, the intensity of this peak also increases, indicating an increase in Mn³⁺ ion content. Figure 5a–f shows the XPS spectra of Bi₂Zn_xMn_1−xTa₂O_9.5−Δ (x = 0.3 and 0.7) and the initial metal (Bi and Mn) oxides used in the sample synthesis. Table 1 summarizes the energy positions of the separated peaks in the measured spectra. As in the case of composition with Mn,Mg, doping with manganese atoms does not change the spectral characteristics of bismuth, tantalum and zinc atoms (Figure 5). Comparing the XPS Bi4f spectra of the sample under study and Bi₂O₃ oxide (Figure 5b) highlights the energy position and width of the peaks in the spectra of the samples almost completely coincides and correlates well with the corresponding spectra of the oxide. In the case of Bi₂Zn_xMn_1−xTa₂O_9.5−Δ samples, the position of the Bi4f7/2 and Bi4f5/2 bands correlates with the magnesium compositions. The sample spectrum shows a slight shift of the Bi4f7/2 and Bi4f5/2 absorption bands relative to the Bi₂O₃ spectrum, both relative to each other and to the lower energy region in general. For example, the Bi4f5/2 band shifts from 164.10 eV (x = 0.7) to 164.13 eV (x = 0.3), indicating a decrease in the effective charge of bismuth ions with increasing x(Zn). This shift is associated with the placement of a certain fraction of divalent ions in the bismuth position. The observed concentration dependence also suggests the possible distribution of zinc ions in the bismuth position compared to manganese ions at large values of x(Zn). The energy positions of the peaks in the XPS Zn2p spectra shown in Figure 5c are characteristic of the divalent zinc ion [41]. When analyzing the spectra of tantalum ions (Figure 5d), it is important to note that the peaks are not split or distorted. This clearly indicates that all tantalum atoms are in the same charge state. However, the energy position of the peaks has a characteristic shift towards lower energies compared to the binding energy in pentavalent tantalum oxide Ta₂O₅ [42,43]. The shift towards lower energies is characteristic of a decrease in the effective positive charge; in particular, for the presented Ta4f and Ta5p spectra, the energy shift is ΔE = 0.65 eV, which suggests that tantalum ions have the same effective charge +(5-δ). Figure 5e shows the XPS Mn2p spectra of Bi₂Zn_xMn_1−xTa₂O_9.5−Δ (x = 0.3 and 0.7) samples and manganese oxides presented in a comparison of the composite spectra, with different manganese oxides spectra showing a strong correlation in the energy position of the peaks in both composite and MnO spectra. This led to the conclusion that manganese ions predominantly have an oxidation state of (2+). Figure 5e shows that there is no complete energy coincidence between the components of the spectrum of manganese in the composites and MnO, indicating the presence of oxidized manganese ions (3+, 4+). To clarify this conclusion, we attempted to decompose the XPS Mn2p spectra of the Bi₂Zn_0.3Mn_0.7Ta₂O_9.5−Δ and Bi₂Zn_0.7Mn_0.3Ta₂O_9.5−Δ samples into individual components according to the method described above, using the spectra of the oxides presented in Figure 5e. The results of optimal modeling for the Bi₂Zn_0.7Mn_0.3Ta₂O_9.5−Δ sample (α = 0.79, β = 0.21 and γ = 0) and the Bi₂Zn_0.3Mn_0.7Ta₂O_9.5−Δ sample (α = 0.48, β = 0.52 and γ = 0) are presented in Figure 5f and in Figure 5e, respectively. The obtained data indicate that the composites contain two nonequivalent states of manganese atoms: +2 and +3. Moreover, it was found that with increasing manganese content, the proportion of states with +3 increases from 0.21 to 0.52 in the total magnesium content. The NEXAFS studies clarified the oxidation degree of manganese in solid solutions. Figure 5h shows the NEXAFS Mn2p spectra of Bi₂Zn_0.3Mn_0.7Ta₂O_9.5−Δ and Bi₂Zn_0.7Mn_0.3Ta₂O_9.5−Δ and manganese oxides. The comparison of the samples and oxides spectra draws our attention tothe shape of the spectra; the energy position of the main peaks of the samples (640.3 and 641.5) correlate with the spectra of MnO. This suggests that the manganese atoms in the studied samples are primarily in +2 charge state. Moreover, some spectra features indicate the presence of oxidized manganese (III, IV) ions in the samples. This is indicated by the absence of clear absorption lines at 644 and 639 eV, which appear in the spectrum of MnO. Additionally, the ratio of the intensities of the lines at 640.3 and 641.5 eV observed in the spectra of the samples is lower than for the lines in the MnO spectrum.

The lines in the Mn2p spectra of the samples appear broad and diffuse compared to MnO, which could be due to the superposition of the spectra of manganese ions in different oxidation states. In addition, the ratio of line intensities at 640.3 and 641.5 eV is not constant for the Bi₂Zn_xMn_1−xTa₂O_9.5−Δ samples (x = 0.3 and 0.7). The line intensities ratio decreases with the manganese content x(Mn) increases, which indicates a variable ionic composition of manganese in the samples and a higher concentration of oxidized manganese ions in samples with the highest manganese content, which is consistent with XPS data. Thus, it can be concluded that in the XPS spectra of both series bismuth and tantalum samples there is an energy shift of the absorption bands towards lower energy values, depending on the dopant content. This shift is typical for bismuth and tantalum ions with an effective charge of (+3-δ) and (+5-δ), respectively, due to the distribution of some of the transition elements ions in the position of bismuth and tantalum. According to NEXAFS and XPS spectroscopy data, the manganese ions in the samples have predominantly +2 and +3 oxidation states. According to X-ray spectroscopy data, zinc-containing preparations exhibit a higher proportion of oxidized manganese(III) ions compared to magnesium compounds. In both series of samples, the proportion of oxidized manganese ions increases with increasing manganese content.

4. Conclusions

The Bi₂Mg(Zn)_xMn_1−xTa₂O_9.5−Δ composites were synthesized by the solid-phase method. X-ray diffraction analysis indicates that the composites contain a BiTaO4 impurity which is directly proportional to the amount of manganese content. The unit cell parameter of the pyrochlore phase increases with increasing zinc or magnesium content in the samples. The XPS spectra of both samples’ series show an energy shift towards the lower energy region of the bismuth absorption bands (Bi4f7/2 and Bi4f5/2), which is associated with the presence of some transition element ions in the bismuth position. According to NEXAFS and XPS data, it was established that the charge states of bismuth, zinc, and magnesium ions are Bi(+3-δ), Zn(+2), Mg(+2). The XPS Ta4f spectrum of Bi₂Mg(Zn)_xMn_1−xTa₂O_9.5−Δ demonstrates a lower energy shift of the absorption band by ΔE = 0.7 and 0.65 eV compared to the Ta₂O₅ spectrum. This shift allows us to estimate the oxidation state of tantalum ions as Ta(+5-δ). It has been shown that doping pyrochlores with zinc or magnesium ions leads to the oxidation of some manganese ions to Mn(III). This effect is particularly significant in zinc-containing samples.