Fe,Mg-Codoped Bismuth Tantalate Pyrochlores: Crystal Structure, Thermal Stability, Optical and Electrical Properties, XPS, NEXAFS, ESR, and ⁵⁷Fe Mössbauer Spectroscopy Study

Abstract

The effect of Fe and Mg-codoping on the crystal structure, optical and dielectric properties of bismuth tantalate-based pyrochlores has been studied. Samples of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x ≤ 0.7) are characterized by a porous dendrite-like microstructure. Fe,Mg-codoped bismuth tantalate pyrochlores are thermally stable up to a temperature of 1140 °C (x = 1). The Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ thermal expansion coefficient increases uniformly and weakly from 3.6 to 9.3 × 10⁻⁶ °C⁻¹ (30–1050 °C). The unit cell parameter of solid solutions increases uniformly from 10.5009(1) Å (x = 0.3) up to 10.5225(7) Å (x = 0.7). The structural parameters of disordered pyrochlore are determined by the Rietveld method (sp. gr. Fd$\overline{3}$m:2 (227), Z = 8). According to near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy data, ions in solid solutions are in the charge states Bi (+3), Mg (+2), Fe (+3), Ta (+5-δ). The Mössbauer spectrum is represented by a symmetric doublet with parameters IS = 0.365 ± 0.0020 mm/s, QS = 0.604 ± 0.034 mm/s, related to Fe³⁺ ions in regular axial octahedral positions. The samples exhibit the properties of dielectrics. The permittivity and the tangent of dielectric losses at 20 °C increases with the growth of iron content in the samples in the range of 28.5–30.5 and 0.001 (1 MHz). The width of the band gap of the obtained materials for direct allowed electronic transitions is in the range of 2.16(5)–2.41(5) eV. The studied samples satisfy the condition of efficient conversion of solar energy into an electrical one and are promising as catalysts and light-absorbing elements for solar panels.

1. Introduction

Compounds with a pyrochlore-type structure have been attracting the close attention of scientists for many years. These compounds are of great interest due to a wide range of practically useful properties. They are known as photocatalysts, dielectrics, ionic and metallic conductors and exhibit ferro- and ferrimagnetism, giant magnetoresistance, superconductivity and spin glass state [1,2,3,4,5]. Oxide pyrochlores are described by the general formula A₂B₂O₇ with a combination of tri-and quadri- (A³⁺₂B⁺⁴₂O₇) or di- and pentavalent elements (A²⁺₂B⁺⁵₂O₇) in cationic sublattices A and B [6,7]. Such a diverse composition of compounds of this structural type is due to the crystal lattice tolerance to substitutions of cations of both sublattices and to defects in the anionic sublattice. The face-centered cubic structure of pyrochlores consists of two independent and interpenetrating B₂O₆ and A₂O’ sublattices. The cationic sublattice B₂O₆ is formed by octahedra connected at the apex of the angle [BO₆]. The A₂O’ sublattice has an anticrystobalite structure formed by tetrahedra [O’A₄]. Relatively small cations (Ti⁴⁺, Hf⁴⁺, Ta⁵⁺, Sb⁵⁺, Nb⁵⁺) occupy cationic sites B, and large A ions (Ca²⁺, Bi³⁺, Pb²⁺) are located in the octahedron formed by oxygen atoms of the A₂O’ and B₂O₆ sublattices [8,9,10,11]. The flexibility of the pyrochlore crystal structure to substitutions of cations of both sublattices and to vacancies in the anionic sublattice makes it possible to significantly vary the composition of compounds and obtain hundreds of compounds of this structural type with various functional properties [12,13,14,15,16].

To describe the stability of the resulting compositions, the concept of “pyrochlore stability field” [6] is used, based on the ratio of cationic radii. The range of values rA/rB = 1.46–1.80 for A³⁺₂B⁴⁺₂O₇ and 1.40–2.20 for A²⁺₂B⁵⁺₂O₇ limits the stability of pyrochlores. For pyrochlore containing bismuth (III) ions in A-sites and tantalum(V) in B-ones, the ionic radii ratio rA/rB gives a large value equal to 1.83, where r(Bi³⁺) = 1.17 Ǻ (c.n. = 8); r(Ta⁵⁺) = 0.64 Ǻ (c.n. = 6). For this reason, bismuth tantalate with an equimolar amount of bismuth (III) and tantalum(V) ions does not form a pyrochlore structure and crystallizes in the BiTaO₄ structural type [17]. The structure of pyrochlore can be stabilized with an equal ratio of Bi (III) and Ta(V) atoms by doping with ions which ionic radius is smaller than for Bi³⁺ (atoms of 3d elements), as it is shown in studies on triple systems Bi₂O₃–Ta (Nb,Sb)₂O₅–MO(M₂O₃) [18,19,20,21,22,23]. In this case, as a rule, the largest part of the dopant atoms is placed in the octahedral position B, making the bismuth sublattice A₂O’ unfilled, which increases the pyrochlore stability.

Chromium-, zinc-, iron- and copper-containing compounds were among the first pyrochlores obtained on the basis of bismuth tantalum [24,25,26,27,28]. A distinctive feature of such pyrochlores is the mixed placement of transition metal cations in two nonequivalent cation sublattices A and, to a greater extent, B. This leads to the formation of a bismuth-defective pyrochlore structure. Moreover, in [29,30] it was shown that the deficiency of bismuth atoms in the A₂O’ sublattice cannot be more than 1/3 mole percent and closer to 1/4 due to the stereoactive 6s² electron pair of bismuth atoms. The formation of iron-containing pyrochlores does not satisfy the stability parameter and, for example, for the compositions Bi₂FeTaO₇ and Bi₂FeTa₂O_9.5 it is equal to 1.82, provided that Fe (III) ions are placed in octahedral positions. In the case when some of the iron (III) ions are placed in the bismuth position, the value of the stability parameter will be within the required interval. The paper [25] 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 Nb/Ta/Sb [22,23,25,29,31,32,33]. 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 ceramics, only 4–15% of A-sites are occupied by Fe³⁺ ions in the pyrochlores of the Bi₂O₃-Fe₂O₃-Nb₂O₅ system [22]. For the Bi₂O₃–Fe₂O₃–Sb₂O_x and Bi_1.8Fe_0.2 (FeSb)O₇ systems the occupation value is equal to 7–25% [23] and 10% [34], respectively. At the same time, it was indicated in [34] 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. It was found that for pyrochlores in the Bi₂O₃-Fe₂O₃-Nb₂O₅ system, the value of the dielectric constant ε lies in the range of 141–151 and dielectric losses are close to 0.2 at a temperature of 30 °C and frequency of 1 MHz [35]. For pyrochlore Bi_1.657Fe_1.092Nb_1.150O₇ the dielectric constant remains high ~120 at 300 K and 1 MHz [22]. Iron-containing pyrochlores Bi_3.36Fe_2.08+xTa_2.56−xO_14.56−x (−0.32 ≤ × ≤ 0.48) are characterized by lower values of the dielectric constant ~78–92 and the dielectric loss tangent ∼10⁻¹ (MHz, ~30 °C) [25]. Meanwhile, solid solutions Bi_3.36Fe_2.08+xSb_2.56−xO_14.56−x (0.00 ≤ × ≤ 0.64) have lower permittivity values in the range of 24–35 and dielectric losses of the order of 10⁻¹ at room temperature and frequency of 1 MHz [36].

Within the work the possibility of the formation of Mg,Fe codoped bismuth tantalate pyrochlores and the influence of magnesium ions on the structure, thermal behavior, symmetry of the local coordination environment of iron (III) ions and optical properties of compounds were demonstrated.

2. Experimental Section

Solid solutions of Bi₂Mg_xFe_1−xTa₂O_9,5±Δ (x = 0, 0.3, 0.5, 0.7) were synthesized by the solid-phase method according to the procedure described in detail in [29]. It should be noted that the synthesis was carried out in stages, at temperatures of 650, 850, 950, 1050 °C for 10 h at each calcination stage. The precursors for the solid-phase reaction were the oxides MgO, Bi₂O₃, Fe₂O₃, Ta₂O₅. The microstructure and local elemental composition of the samples were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) using electron scanning microscope Tescan VEGA 3LMN, (Tescan, Czech), combined with the energy-dispersive spectrometer INCA Energy 450, (Tescan, Czech). X-ray photoelectron spectroscopy (XPS) analysis was performed by the Thermo Scientific ESCALAB 250Xi X-ray spectrometer, (Thermo Fisher Scientific, USA). An X-ray tube with Al K_α radiation (1486.6 eV) was used as a source of ionizing radiation. To neutralize the charge of the sample, an ion-electronic charge compensation system was used. All peaks are calibrated relative to the intensity of the C1s peak at 284.6 eV. The experimental data were processed using the ESCALAB 250 Xi software. The near edge X-ray absorption fine structure (NEXAFS) spectra were measured with a resolution of about 0.45 eV at the NanoPES beamline [37] of the Kurchatov Synchrotron Radiation Source (NRC Kurchatov Institute) using the total electron yield (TEY) mode.

The crystal structure of Fe,Mg-codoped bismuth tantalate was investigated by the powder high-temperature X-ray diffraction (HTXRD) using an X-ray diffractometer Rigaku Ultima IV (Co K_α radiation, air atmosphere, 40 kV/30 mA, Bragg-Brentano geometry) equipped with plastic scintillator strips detector D/teX Ultra. The correctness of 2θ at room temperature (RT) was checked before every measurement using silicon as external standard; the change in zero shift never exceeded ±0.02° 2θ. The unit-cell parameters were calculated at every temperature step by the Pawley approach and the crystal structure of Fe,Mg-doped bismuth tantalate was refined at 25 °C by the Rietveld method using the Topas 5.0 software package [38]. According to HTXRD data, the crystal structure for the Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ composition was refined. The Thompson–Cox–Hastings pseudo-Voigt function was used to describe the reflex profile. Scattering factors of neutral atoms were applied for all atoms. The ideal structure of pyrochlore (space group Fd$\overline{3}$m) was used as the initial structure model. The filling of the positions was determined in accordance with the composition stoichiometry.

For the electron spin resonance (ESR) spectroscopy of the Fe,Mg-codoped bismuth tantalate polycrystalline samples an X-band spectrometer RadioPAN SE/X 2547 was used. The spectra were recorded using a rectangular resonator (RX102, TE 102 mode) at RT as the first derivative, at the high frequency modulation of 100 MHz with the amplitude of 0.25 mT and the microwave power of 35 mW. The pyrochlore sample (45–70 mg) was put into a thin-walled quartz test tube (internal diameter of 2.5 mm) together with the reference sample (anthracite, singlet line g₀ = 2.003, peak to peak distance ∆B_PP = 0.5 mT) in an ampoule. For each sample, the spectrum in the magnetic field range of 0–700 mT and the reference line g₀ = 2.003 in the scan range of 5 mT were separately recorded. The intensity of the reference line served as a measure of the gain of the instrument and, when processing spectra, was used to accurately remove background signals from the test tube and ampoule. The spectra were normalized to the reference line intensity and then to 60 mg of the sample. The ESR spectra were recorded with an X-band radiospectrometer SE/X-2547 (RadioPAN) in the Shared Services Center “Geonauka” at the Institute of Geology FRC Komi SC UB RAS.

In order to study electrical properties, metallic Ag electrodes were applied onto both sides of the ceramic discs and sintered at 650 °C for 1 h. The measurements were carried out with an E7-28 impedancemeter (frequency range of 25–10⁷ Hz) at temperatures from 25 up to 450 °C. The thickness and diameter of the studied disk-shaped sample were 1.7–2.3 mm and 14.2–14.3 mm, respectively.

The ⁵⁷Fe Mössbauer spectra were obtained using an Mössbauer spectrometer Wissel (Wissel, Germany, Starnberg) at the rates of −11–+11 mm/s at RT. The 6 × 10⁸ Bq ⁵⁷Co in chrome matrix (Ritverc GmbH, St. Petersburg, Russia) at RT was used. To eliminate the texturing effects in the spectra, the samples were prepared in the form of finely ground powder. The duration of spectrum accumulation was about 270 h. The isomeric shift was determined relative to α-Fe.

3. Results and Discussion

3.1. Thermal Behavior, Morphology and Crystal Structure

According to the X-ray phase analysis data, it was found that samples of the composition Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.3, 0.5, 0.7, 1.0) crystallize in cubic syngony. The analysis of reflection loss confirmed that the symmetry of the crystal structure is cubic with the space group Fd$\overline{3}$m. It should be noted that the samples of this series were synthesized twice. On the X-ray diffraction patterns of the first series of samples (Figure S1) an admixture of the bismuth orthotantalate was detected, the amount of which decreased with increasing the iron content. The content of bismuth orthotantalate was assessed from X-ray diffraction patterns. For the Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.7) sample with the highest content of bismuth orthotantalate, the amount of impurity is no more than 4.86 mass. percent (Figure S2).

The appearance of an impurity and its dependence on the concentration of iron in the samples may be associated with the tendency of Mg (II) ions to be located in two cationic positions of bismuth (III) and tantalum (V), which weakens with an increase in iron and a decrease in magnesium content. Apparently, magnesium ions, being distributed in the Ta(V) positions and having a smaller charge and a larger ionic radius, are capable of causing distortions in the pyrochlore crystal structure due to oxygen vacancies. In order to reduce the degree of stress of the crystal structure, some of the magnesium ions are placed in positions Bi (III) and to create vacancies in bismuth positions, bismuth orthotantalate is released as an impurity. A similar reaction was observed in the case of cobalt-containing pyrochlore [30]. With a decrease in the content of Mg (II) ions in the samples, the stresses of the octahedral frame become weaker and the need for such a process disappears, which can be seen in the experiment. The repeated synthesis of a series of preparations is performed more efficiently. An intermediate and longer homogenization was performed. As a result, pure preparations of solid solutions that do not contain bismuth orthotantalate were synthesized (Figure 1). Based on this, one can conclude that the absence of impurities is due to careful and repeated homogenization of the preparations. Apparently, homogenization contributed to the reduction in local stresses of the structure caused by heterovalent substitution and uniform distribution of ions in the structure, including oxygen vacancies.

With increasing magnesium concentration, the unit cell parameter increases almost uniformly from 10.5009 ± 0.0001 Å (x = 0.3) up to 10.5225 ± 0.0007 (x = 0.7). The concentration dependence is almost linear and obeys Vegard’s law quite well (Figure 1) [39]. This fact indicates the formation of a continuous series of solid solutions and the distribution of iron and magnesium ions in the same system of crystallographic positions.

It should be noted that the unit cell parameters previously determined for the extreme terms of the series of the studied solid solution Bi_1.5Mg_0.75−xFe_xTa_1.5O_7±Δ are a = 10.4871(2) Å (x = 0.75) [29] and a = 10.54607 Å (x = 0) [40], which is in good agreement with the parameters of iron- and magnesium-containing solid solutions. The calculated parameters of the solid solution cell are in satisfactory agreement with the cell constant for the border compositions of solid solutions Bi₂FeTa₂O_9.5 (a = 10.4871 Å) and for Bi₂MgTa₂O₉ (a = 10.5461 Å), and are also close to the values given in [25] for iron-containing pyrochlores based on bismuth tantalate Bi_3.36Fe_2.08+xTa_2.56−xO_14.56−x (−0.32 ≤ × ≤ 0.48) (10.4979–10.5033 Å) and bismuth niobate (Bi_1.721Fe_0.190(Fe_0.866Nb_1.134)O₇) a = 10.508 Å and Bi_3.36Fe_2.08+xNb_2.56−xO_14.56−x (−0.24 ≤ × ≤ 0.48), for which the parameter varies from 10.5071(4) to 10.5107(7) Å [22,35]. The proximity of the polarization properties and ion radii of Fe (III) and Ta(V) explains the tendency of Fe (III) ions to occupy octahedral positions, repeatedly proven by physicochemical analysis methods [41,42,43].

Based on powder data, using the software package Topas 5.0, the crystal structure was refined for the compositions Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5, 0.3). The ideal structure of pyrochlore (sp. gr. Fd$\overline{3}$m) served as the initial structure model. The best agreement between the experimental and calculated data was obtained for the disordered structure model, in which the Bi atoms are displaced from the highly symmetrical (16d) positions to the (96g) positions. Tantalum(V) and iron ions are located in the same system of crystallographic positions (16b). The oxygen atoms are disordered and are located in two crystallographic positions, one of which (48f) is completely occupied, the other (8a) is in short supply and filled by 56 (66)%, respectively. For example, the stoichiometric formula of the nominal composition Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ (or the normalized formula to 7 oxygen atoms Bi_1.4Mg_0.35Fe_0.35Ta_1.4O_6.65), determined as a result of structure refinement, corresponds to the composition with a deficient sublattice of bismuth cations—Bi_1.38Fe_0.34Mg_0.32Ta_1.34O_6.69.

Table 1. Parameters of atoms in the Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5; 0.3).

			x = 0.5
Atom	Wyckoff site	x	y	z	SOF	Biso, Å2
Bi	96g	0	−0.02489(10)	0.02489(10)	0.1146(7)	1.26(6)
Ta	16b	0.5000	0.5000	0.5000	0.67(6)	0.60(3)
Fe	16b	0.5000	0.5000	0.5000	0.17(6)	0.60(3)
Mg	16b	0.5000	0.5000	0.5000	0.16(6)	0.60(3)
O1	48f	0.1250	0.1250	0.4302(4)	1	1.78(15)
O2	8a	0.1250	0.1250	0.1250	0.56(3)	1.78(15)
			x = 0.3
Atom	Wyckoff site	x	y	z	SOF	Biso, Å2
Bi	96g	0	−0.02516(8)	0.02516(80)	0.1125(4)	0.94(5)
Ta	16b	0.5000	0.5000	0.5000	0.67(6)	0.54(2)
Fe	16b	0.5000	0.5000	0.5000	0.22(6)	0.54(2)
Mg	16b	0.5000	0.5000	0.5000	0.11(6)	0.54(2)
O1	48f	0.1250	0.1250	0.4317(3)	1	1.72(12)
O2	8a	0.1250	0.1250	0.1250	0.66(2)	1.72(12)

shows the results of the refinement of the pyrochlore structure for the compositions Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5, 0.3) by the Rietveld method in the space group Fd$\overline{3}$m:2 (227).

The experimental, calculated, and difference diffraction patterns of Bi₂Mg_0.3Fe_0.7Ta₂O_9.5−Δ are shown in Figure 2. Atomic and geometric parameters of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5, 0.3) are presented in Table 1 and

Table 2. Crystallographic data of the Bi₂Mg_xFe_1−xTa₂O_9.5−Δ.

Index x	x = 0.5	x = 0.3
a (Å)	10.51036(3)	10.49929(4)
α, β, γ (°)	90, 90, 90
V (Å 3)	1161.053(11)	1157.389(12)
Dcalc (g/cm3)	7.57(2)	7.58(1)
RB	0.63	0.67
Rwp Rp Rexp GOF	3.16 2.26 2.16 1.46	3.68 2.71 2.14 1.72

, respectively.

According to the simulation results, the tantalum/iron atoms form a regular TaO₆ octahedron with a Ta–O bond length of ~1.9869 Å. Individual interatomic distances in the weakly ordered BiO₈ polyhedron vary from 2.30 to 2.99 Å (

Table 3. Selected bond lengths in the structure of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ.

Index x	x = 0.5	x = 0.3
Bond	Length (Å)	Length (Å)
Bi1–O1 × 2	2.306(2)	2.304(2)
–O1 × 2	2.344(4)	2.350(3)
–O1 × 2	2.683(3)	2.688(2)
–O2 × 2	2.983(4)	2.987(3)
<Bi1VIII–O>	2.58	2.58
Ta1–O1 × 6	1.9959(16)	1.9898(12)
<Ta1VI–O>	2.00	1.99

), with 4 out of 8 bonds noticeably shorter than the others. The asymmetry of the polyhedron of bismuth atoms is due to the distribution of the stereoactive 6 s² pair of bismuth ions.

As noted earlier, in the case of pyrochlores based on bismuth tantalate containing atoms of 3d elements [29,30], the formation of a symmetric tantalum–oxygen polyhedron is typical, and the length of the Ta–O bond in the octahedron changes depending on the nature of the 3d atom. In the case of iron ions, the average Ta–O bond length is shorter than, for example, for nickel compositions and the degree of distortion of the BiO₈ polyhedron is lower. Apparently, this is due to the close radii of the Fe (III) and Ta(V) ions [29], distributed in the same system of octahedral sites, and the degree of covalence of the M–O bond (M–Fe (III), Ta(V)).

The samples have a yellow color characteristic of iron (III) compounds, which becomes more intense with increasing iron content (Figure 3).

The microstructure of the samples is porous, formed by weakly aggregated elongated particles (Figure 4). No significant dependence of the crystallite size on the magnesium/iron ratio has been established. The average size of crystallites determined by the Scherrer method for solid solutions is ∼59.6 nm, meanwhile, larger grains with a longitudinal size of 1–2 μm were recorded by scanning electron microscopy (SEM). In some places, local coalescence of grains with the formation of larger aggregates is observed. It can be noted that an increase in the magnesium content in solid solutions contributes to the appearance of a larger number of grain aggregates. The porosity of the samples according to SEM data varies from 22 (x(Mg) = 0.7) to 28 (x(Mg) = 0.3) percent. Local quantitative analysis by the EDS method showed that the experimental composition of the samples corresponded to the specified one (Figure S3). Elemental mapping of the samples showed a uniform distribution of atoms in the composition of the sample (Figure S4).

The study of thermal behavior was carried out by the HTXRD method in the range of 30–1200 °C (Tables S1 and S2). Figure 5a shows the temperature dependence of the cubic unit cell parameter Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5) for the range 30–1200 °C. The unit cell parameter a increases uniformly from 10.50183 Å (30 °C) to 10.57607 Å (1110 °C). A uniform change in the cell constant indicates the absence of phase transformations and the thermal stability of pyrochlore in the considered temperature range, as was previously revealed for pyrochlores based on bismuth tantalate containing magnesium or 3d ions [29,40,43]. Above 1100 °C, the thermal dissociation of the solid solution probably occurs, as shown for Bi₂FeTa₂O_9.5 [43]. It is interesting to note that the limiting temperature of the phase stability of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.5) decreases with an increase in the magnesium content from 1140 °C (x = 0) to 1050 °C (x = 1), which may be due to a change the nature of the MO (M-Fe(III), Mg(II) bond in the octahedron from covalent to ionic and the number of oxygen vacancies destabilizing the structure.

The Figure 5b shows the temperature dependences of the thermal expansion coefficient (TEC) for Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ calculated as a result of approximation of the temperature dependences of the unit cell parameters.

As can be seen from the Figure 5b, the TEC values for Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ increases uniformly and weakly from 3.6 to 9.3 × 10⁻⁶ °C⁻¹ in the temperature range 30–1050 °C. In this regard, iron–magnesium pyrochlores can be attributed to weakly expanding compounds with isotropic thermal expansion. The average value of TEC (6.4 × 10⁻⁶ °C⁻¹) for Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ in the studied temperature range is comparable to the thermal expansion coefficient of compounds with a framework structure like pyrochlore [44,45,46], including pyrochlores based on tantalate bismuth containing 3d ions [43]. Taking into account that transition ions are mainly located in octahedral positions, one can speak of a weak effect of the nature of dopants distributed in the three-dimensional cationic sublattice B on the thermal expansion of pyrochlores.

3.2. XPS, ESR, NEXAFS and Mössbauer Spectroscopy

The studies of the electronic state of atoms in Mg,Fe-codoped bismuth tantalate pyrochlore were carried out by NEXAFS, XPS, ESR and Mössbauer spectroscopy methods. The chemical state of the surface of the studied samples was investigated by XPS. The obtained XPS spectra of bismuth–magnesium tantalate, Mg,Fe-codoped bismuth tantalate (on the example of a composition with x = 0.5) and corresponding oxides are shown in Figure 6a–d: XPS spectra in a wide energy range of binding energies (20–1400 eV) and spectral dependences in the region of Bi4f-, Bi5d-, Ta4f-, Ta4d-, Mg1s and Fe2p ionization thresholds. The graphs show the results of the decomposition of spectral dependencies into individual peaks, which were modeled by Gauss–Lorentz curves, and the background by Shirley approximation. To study the chemical composition of the samples, only the spectra of metals were analyzed. This is explained by the fact that in the Survey XPS spectrum there is a C1s peak caused by surface contamination of the sample, which can give an indefinite contribution to the intensity of the O1s peak.

It should be noted that the analyzed spectra in the main details and their energy positions coincide with the previously obtained spectra for the Bi₂MTa₂O₉ (M-Co,Ni,Fe)_, Bi₂MgTa₂O₉ pyrochlores [29,30,40,43]. Therefore, it is sufficient to present an analysis of the spectra for one composition at x = 0.5. The energy positions of the Bi₂MgTa₂O₉ (1) and Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ (2) XPS spectra features are given in

Table 4. Energy positions of the components of the XPS spectra of Bi₂MgTa₂O₉ (1) and Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ (2).

Peak	Energy (eV)
	1	2
Bi4f7/2	158.99	159.03
Bi4f5/2	164.31	164.35
Bi5d5/2	25.83	26.11
Bi5d3/2	28.85	29.08
Ta4f7/2	25.39	25.66
Ta4f5/2	27.29	27.56
Ta4d5/2	229.57	229.78
Ta4d3/2	241.32	241.44
Mg1s	1302.99	1303.19
Fe2p3/2	710.47
Fe2p1/2	724.06
Fe2p sat	718.92
Fe2p sat	733.10

.

Comparison of the observed features with chemical states was carried out on the basis of [47,48]. We note only some features of the newly obtained spectra. First of all, doping with iron ions does not significantly change the spectral characteristics of bismuth, tantalum and magnesium ions (Figure 6a–d). Consequently, the electronic state of these ions remains unchanged and corresponds to the ions Mg (II), Bi (III). In the Ta4f and Ta5p spectra of tantalum atoms, the energy position of the peaks has a characteristic shift towards lower energies compared to the binding energy in pentavalent tantalum oxide Ta₂O₅, which is characteristic in the case of a decrease in the effective positive charge. The energy shift ΔE in the Ta4f and Ta5p spectra is equal to 0.5 eV, and near the Ta5d edge—to ~1 eV. This suggests that tantalum atoms can have the identical effective charge smaller than five +(5-δ). Apparently, the observed shift is due to the substitution of tantalum positions with magnesium (II) and iron (III) ions with a lower effective charge. Since any change in the chemical environment of an element affects the spatial redistribution of the charge of its valence electrons, the binding energy of the electrons is consequently changed. The XPS Fe2p spectra of Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ (Figure 6d) demonstrate two wide bands Fe2p_1/2 and Fe2p_3/2 and their satellites with a characteristic binding energy of 710.47 eV (Fe2p_3/2), 724.06 eV (Fe2p_1/2) (Table 4). The coincidence of the Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ and Fe₂O₃ oxide spectra by the number and energy positions of the main peaks suggests that iron atoms have an effective charge of +3.

A similar conclusion about the charge state of Fe³⁺ iron atoms in Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ follows from the analysis of NEXAFS Fe2p_3/2 spectra of ceramics and iron oxides shown in Figure 7. In the spectrum there are two broad lines at 707.5 eV and 709 eV, which correlate well with the spectrum of iron in iron (III) oxide in terms of the energy position and shape of the lines. Iron-containing pyrochlore Bi₂FeTa₂O_9.5 has a similar spectrum shape [29,43].

The state of iron (III) ions is confirmed by the data of the ESR spectrum of Bi₂Fe_0.5Mg_0.5Ta₂O_9.5−Δ, in which there is an intense dipole-broadened band from Fe³⁺ ions, split into two components with g ~2.1 and 2.01 with widths of about 80 and 20 mT, respectively. Figure 8 shows for comparison two spectra normalized to one microwave and one gain. They are completely aligned. The spectrum of Bi₂FeTa₂O_9.5 is naturally wider than the spectrum of iron in Bi₂Fe_0.5Mg_0.5Ta₂O_9.5−Δ due to the iron content. The vertical line in the figure shows the position of the reference signal with g = 2.003.

The study of the nature of the local environment and the degree of oxidation of iron ions was carried out by the nuclear-gamma-resonance (NGR) method. The Mössbauer spectrum of the Bi₂Mg_0.5Fe_0.5Ta₂O_9+Δ compound is shown in Figure 9, more precisely the paramagnetic part (−4–+4 mm/s) of the full spectrum. About 100% of the area of the spectrum paramagnetic part falls on a symmetrical Fe³⁺ doublet with a chemical shift IS ~0.144 ± 0.009 and a quadrupole splitting QS ~0.600 ± 0.014 mm/s.

According to the data of literature sources and X-ray spectroscopy, the doublet is associated with Fe³⁺ ions in regular axial octahedral positions Ta⁵⁺O₆ + V[O²⁻] → Fe³⁺O₆. This assumption does not contradict the results of studies of the NGR spectra of iron-containing compounds in which Fe (III) ions are in octahedral coordination [29,31,32,33,34,41]. In particular, for pyrochlore Bi₂FeNbO₇, the parameters of the Mössbaur spectrum of Fe (III) ions occupying octahedral B positions were determined: IS = 0.27 mm/s, QS = 0.66 mm/s [49]. For pyrochlores formed in the Bi–Fe–W–O system, the parameters of the Mössbaur spectrum for octahedral positions are IS = 0.38 mm/s, QS = 0.54 mm/s [31]. The Mössbaur spectrum of pyrochlore Bi_1.8Fe_0.2(FeSb)O₇ exhibits two doublets with parameters IS = 0.38 mm/s, QS = 0.54 mm/s and IS = 0.32 mm/s, QS = 1.87 mm/s, assigned to Fe ions (III) in octahedral positions of antimony (90%) and 8-fold positions of bismuth (10%), respectively [32]. Our NGR data do not contradict the results of X-ray diffraction analysis on the presence of iron (III) ions in octahedral positions. The assumption that Fe (III) ions replace Bi (III) positions is not supported by the spectrum parameters. Otherwise, an asymmetric signal should appear in the spectrum, described by a doublet with a large quadrupole splitting [31,32,33], which contradicts our spectrum. It should be noted that the NGR spectrum of iron (III) in Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ is similar to the spectrum of pyrochlore Bi₂FeTa₂O_9.5, which does not contain magnesium ions [40]. For them, a signal is observed with practically the same quadrupole splitting QS = 0.575 ± 0.010 and 0.604 ± 0.034 mm/s and close chemical shift IS ~ 0.378 ± 0.005 and 0.365 ± 0.0020 for Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ and Bi₂FeTa₂O_9.5, respectively. The shift of the signal to strong fields can be associated with the greatest imperfection of the polyhedral environment of the Fe (III) ions in Bi₂Mg_0.5Fe_0.5Ta₂O_9.5−Δ due to the heterogeneous replacement of Ta(V) ions by Mg (II) and Fe (III) ions.

Thus, according to near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) data, Mössbauer spectroscopy study and ESR, iron ions in solid solutions are in the charge state Fe (+3). The Mössbauer spectrum is represented by a symmetric doublet with parameters IS = 0.365(2) mm/s, QS = 0.60(3) mm/s, related to Fe³⁺ ions in regular axial octahedral positions.

3.3. Dielectric and Optical Properties

For samples of solid solutions Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.7, 0.5, 0.3) at room temperature, the electrical characteristics were studied—the permittivity and dielectric loss tangent in the frequency range 25 Hz–10 MHz (Figure 10) depending on the ratio magnesium/iron(III).

Measurements of the electrical properties at room temperature (20 °C) showed that the permittivity of the samples is practically independent of frequency and exhibits low values ε ≈ 28.5–30.5 (υ > 10² Hz). In the low-frequency range, the dielectric constant slightly increases by 1–2 units, reaching for x(Mg) = 0.3, values over 32 at 25 Hz. The dielectric loss tangent exhibits a similar course of dependence. Above 10³ Hz, the loss tangent for all samples does not change and has low values of 0.001 (10⁶ Hz). For samples x = 0.5 and 0.7, no noticeable frequency dependence is observed. The exception is the sample with x(Mg) = 0.3, which is characterized by a sharp increase in the dielectric loss tangent at low frequencies. This behavior may be related to the absorption of water by the sample and the tendency of Fe–doped bismuth tantalate pyrochlore [29] to conduct protons. For this reason, the permittivity of a sample with x(Mg) = 0.3 takes on intermediate values of ~29.2 between the values of ε ≈ 28.5 (x(Mg) = 0.5) and 30.5 (x(Mg) = 0.7). It should be noted that the permittivity of Fe,Mg-codoped bismuth tantalates is 1.5 times higher than for magnesium-containing pyrochlores of the same composition [29]. More precisely, for Bi₂MgTa₂O₉ at room temperature and a frequency of 1 MHz, the permittivity and dielectric loss tangent are 20 and 2·10⁻³, respectively. As can be seen from the figure, the dielectric permittivity of the samples depends on the magnesium/iron ratio and takes on the higher values, the more magnesium (II) ions in the samples. This fact conditionally contradicts the concept of the influence of atomic polarizability (α (Mg (II)) = 1.32 ų, α (Fe (III)) = 2.29 ų) [25,29]. We have already stated earlier [29] that for polycrystalline materials the influence atomic polarizability on the permittivity secondarily. The grain boundary area has a significant influence on the permittivity. The larger the area, the higher the permittivity. This is achieved by the smaller ceramic grain size. It should be expected that the effect of atomic polarizability will be of paramount importance in the case of single crystals. Studying the microstructure Fe,Mg–codoped bismuth tantalate pyrochlore, we previously noted that an increase in the magnesium content leads to grain intergrowth and a decrease in porosity, a decrease in the area of grain boundaries. This explains why magnesium pyrochlores have an increased dielectric constant. It can be assumed that the dielectric constant, in addition to microstructure, the adsorbed o sample water, as observed for a sample with x(Mg) = 0.3. Apparently, the permittivity increases with increasing sample moisture. The dielectric loss tangent also depends on the ceramic microstructure. The value of the dielectric loss tangent is the lower, the smaller the grain size [29]. In general, the electrical properties of the sample are typical of dielectrics with medium permittivities and low dielectric losses. Having highly porous and fine-grained Fe,Mg–codoped bismuth tantalate pyrochlore ceramics, we investigated its optical properties. Diffuse reflectance spectra of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ (x = 0.7, 0.5, 0.3) solid solution samples are shown in Figure 11.

The band gap (Eg) of the obtained materials for direct allowed electronic transitions was calculated from the data of the diffuse reflectance spectrum and is in the range of 2.28(5)–2.62(5) eV [49]. All studied samples have a yellow color, the intensity of which increases with an increase in the content of iron (III). This indicates a significant reflection of visible light in the yellow and longer wavelength region of the spectrum. All samples demonstrate a pronounced absorption peak at about 475 nm (~2.63 eV), indicating the presence of iron ions in the trivalent state (Fe³⁺) with distorted octahedral symmetry [36,37]. Calculations showed that the measured energies of direct allowed electronic transitions, which determine the edge of the absorption band of light quanta, are in the range of 2.28(5)–2.62(5) eV. In this case, with an increase in the iron content in the samples, the band gap decreases, which leads to the convergence of the VZ valence band and the conduction band. A similar effect of doping with iron on the band gap of pyrochlores in the systems Bi₂O₃–Fe₂O₃–TeO₃, Bi-Fe-W-O was noted in [19,31]. The underestimated values of the band gap in the studied pyrochlores can be a consequence of the Stark effect, as well as the small grain sizes of ceramics. It is interesting to note that the band gap of the studied ceramics correlates with the energy of solar radiation reaching the Earth’s surface and having a maximum intensity (2.1–2.5 eV). In this regard, the obtained samples satisfy the condition for efficient conversion of solar energy into electrical energy and are promising as light-absorbing elements for solar batteries.

4. Conclusions

In the work, a continuous series of Bi₂Mg_xFe_1−xTa₂O_9.5−Δ solid solutions synthesized by the solid-phase method was characterized. It has been shown that in the case of insufficient homogenization of preparations during the synthesis, an admixture of bismuth orthotantalate appears. If the thermal stability of the samples is higher, the higher the content of iron ions is. Iron–magnesium pyrochlores are characterized by weak and uniform, isotropic expansion. The average value of TEC in the temperature range of 30–1050 °C is 6.4 × 10⁻⁶ °C⁻¹. Solid solutions are characterized by a disordered pyrochlore structure (sp. gr. Fd$\overline{3}$m:2, Z = 8), in which iron (III) and tantalum(V) ions share octahedral positions 16b, bismuth ions are displaced to 96g positions. The microstructure of the samples is porous, dendrite-type. According to X-ray spectroscopy, bismuth, tantalum and iron ions are in the charge state Bi (+3), Ta(+5-δ), Fe (+3). The Mössbauer spectrum is represented by a symmetric doublet with IS = 0.378 ± 0.005, QS = 0.575 ± 0.010 mm/s, associated with Fe³⁺ ions in regular axial octahedral positions of tantalum. The samples exhibit typical properties for dielectrics. The band gap of the studied ceramics (2.28(5)–2.62(5) eV) correlates with the energy of solar radiation reaching the Earth’s surface and having a maximum intensity.