Features of the Phase Formation of Cr/Mn/Fe/Co/Ni/Cu Codoped Bismuth Niobate Pyrochlore

Abstract

The phase formation process of Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ containing 3d-ions of transition elements in equimolar quantities was studied in a wide temperature range (400–1050 °C). The complex oxide crystallizes in the structural type of pyrochlore (sp. gr. Fd-3m:2, a = 10.4937(2) Å). The investigation of the multi-element pyrochlore phase formation process showed that the synthesis goes through a series of successive stages, during which the transition from Bi-rich to Bi-depleted compounds takes place. The predecessor of the pyrochlore phase is bismuth orthorhombic modification orthoniobate (α-BiNbO₄) with an equimolar ratio of Bi(III)/Nb(V) ions. The pyrochlore phase is formed as a result of bismuth orthoniobate doping with transition element ions. The complex oxides Bi₁₄CrO₂₄, Bi₂₅FeO₄₀, BiNbO₄, and Bi₅Nb₃O₁₅ appeared as intermediate phases during the synthesis. The interaction between the initial oxide precursors is fixed at temperatures above 500 °C. The phase transition of α-Bi₂O₃ into β-Bi₂O₃ near 500 °C is observed. Varying the heat treatment duration at each synthesis step did not qualitatively change the phase composition of the sample but had an effect on the quantitative phase ratio. Phase-pure pyrochlore of the given composition by solid-phase synthesis method can be obtained at a temperature no lower than 1050 °C. Ceramics are characterized by low-porous dense microstructure with blurred outlines of grain boundaries.

1. Introduction

Sustained interest in synthetic pyrochlores is due to the wide range of their practically useful properties. The most promising are dielectric properties (low dielectric losses and high dielectric permittivity, tunable temperature coefficient of capacitance) and photocatalytic properties [1,2,3]. A useful addition is the relatively low synthesis temperature and thermal stability of pyrochlores [4]. The crystal structure of pyrochlores is traditionally described by the chemical formula A₂B₂O₆O’, in which two interpenetrating cationic sublattices A₂O’ and B₂O₆ are distinguished [5,6]. The cationic sublattice A₂O’ is similar to the structure of antichristobalite; the B₂O₆ sublattice forms a three-dimensional framework of octahedrons connected by their vertices. The B octahedral positions are occupied by relatively small cations (Ta⁵⁺, Ru⁴⁺, Zr⁴⁺, Nb⁵⁺), and the larger ions (Bi³⁺, Ln³⁺, Ca²⁺) are distributed in the eight coordinated positions A. In bismuth pyrochlores, the Bi³⁺ cations shift from ideal 16d positions, which are typical for A cations in ideal pyrochlore structure, to 96h or 96g positions due to stereoactive 6s² lone pair of electrons. This leads to the formation of pyrochlores with an unfilled bismuth sublattice. Meanwhile, the distortion of the octahedral sublattice arising during doping by the 3d-element atoms leads to the partial distribution of the transition element ions in the bismuth(III) position [5,6,7,8,9,10]. The flexibility of pyrochlore crystal structure to substitutions of cations in bismuth/niobium sublattices and to oxygen vacancies in A₂O’ sublattice allows their functional properties to be adjusted.

Mixed pyrochlores based on bismuth antimonate, tantalate, and niobate [7,8,9,10,11,12,13,14,15] are actively studied due to their promising electrical and catalytic properties. A feature of the pyrochlores under consideration is a vacant bismuth sublattice and the placement of heterovalent ions of transition elements (Cu, Co, Mn, Zn), which are disproportionate to the B sublattice ions into the bismuth sublattice. This causes relaxation processes in ceramics [15,16,17,18,19]. Studies of bismuth tantalate-based pyrochlores containing transition 3d ions (Cu, Ni, Fe, Cr, Co, Zn) [10,11,12,14,17,20,21,22,23] have shown the formation of porous ceramics with low dielectric losses and moderate dielectric permittivity values. Copper-containing bismuth tantalates exhibiting multiple dielectric relaxation at temperatures above room temperature are characterized by a specific behavior [20,21]. Bi_2.48+yCu_1.92−xTa_3.6+x−yO_{14.6+3x/2−y;} solid solutions [12] exhibit moderate dielectric constants of 60–80 and dielectric losses (tan δ = 0.01–0.20, RT, 1 MHz). As shown in [24], iron-containing pyrochlores in the Bi₂O₃-Fe₂O₃-Nb₂O₅ system exhibit high values of dielectric constant 141–151 and dielectric loss 0.2 at 30 °C and 1 MHz [24]. Iron-containing pyrochlores Bi_3.36Fe_2.08+xTa_2.56−xO_14.56−x (−0.32 ≤ x ≤ 0.48) based on bismuth tantalate show lower values of dielectric constant ∼78–92 and dielectric loss tangent ∼10⁻¹ (MHz, ∼30 °C) [14], which is due to the high porosity of ceramics. Magnesium-containing pyrochlores Bi_3+5/2xMg_2−xTa_3−3/2xO_14−x (0.12 ≤ x ≤ 0.22) are characterized by close values of ε ~70–85 and a low dielectric loss tangent δ ~10⁻³ at 1 MHz and 30 °C [25]. For pyrochlore Bi_1.5ZnTa_1.5O₇, the dielectric permittivity is 58, dielectric loss is 0.0023 at 30 °C and 1 MHz; temperature coefficient of capacitance (TCC) 156 ppm/°C in the range of 30–300 °C at a frequency of 1 MHz [26,27]. The electrical characteristics of cobalt pyrochlores are comparable to those of nickel- and magnesium-containing preparations [10]. For magnesium pyrochlore Bi₂MgTa₂O₉, the permittivity and dielectric loss tangent are 20 and 2·10⁻³ (RT, 1 MHz) [4], and for nickel pyrochlore permittivity and dielectric loss, tangents are 32 and 6·10⁻² at room temperature and frequency 1 MHz, respectively [11]. As shown in [28], Bi_1.4(Mg_1−xNi_x)_0.7Ta_1.4O_6.3 (x = 0.3, 0.5, 0.7) samples are insulators and have high activation energies of ~2.0 eV; moderately high dielectric constants ∼24–28 and dielectric losses ∼0.002 at 1 MHz and 21 °C.

The study of the phase formation process of multielement bismuth tantalate with pyrochlore structure revealed a number of peculiarities [29]. The most significant is the recognition of α-BiTaO₄ as an important intermediate in the synthesis of the pyrochlore phase [29,30,31]. An uneven change in the parameter of the pyrochlore cell constant associated with the formation of nonstoichiometric pyrochlore was noted [31]. The formation of intermediate synthesis products containing Cr(VI) ions was established [30]. The influence of zinc and magnesium ions on the phase relations during the synthesis of pyrochlore was shown [30,31].

In the present work, we aimed to identify similarities and differences in the phase formation stages of multielement tantalates and niobates with pyrochlore structure. This allowed us to note the specific features of the phase formation associated with the nature of niobium(V) and tantalum(V) oxides to determine the optimal conditions for the ceramic synthesis. In this regard, the presented work determines the synthesis conditions and the phase formation stages of multielement pyrochlore based on bismuth niobate and analyzes the phase composition of the samples at each synthesis stage.

2. Experimental Part

Multielement pyrochlore was synthesized by standard ceramic technology from the corresponding oxides of Bi(III), Nb(V), Ni(II), Co(II,III), Cu(II), Mn(III), Cr(III), Fe(III) of analytical grade. The stoichiometric mixture of precursors was finely ground and homogenized in a jasper mortar for one hour. The resulting homogeneous charge was compacted in the form of disks using a plexiglas mold. Air calcination of samples was carried out step by step in the temperature range of 400–1050 °C (step 50 °C) for 10 and 15 h at each stage of thermal treatment. After each stage of calcination, the samples were reground again and pressed in the form of disks. X-ray powder diffraction phase analysis was performed using a Shimadzu 6000 X-ray diffractometer (CuKα radiation; 2θ = 10–60°; scanning speed 2.0°/min). The unit cell parameters have been refined by the Topas 5.0 program employing the Pawley approach and using CSD software [32]. The background was described by 12 Chebyshev polynomials, and the peak shape was modeled by the Thompson–Cox–Hastings pseudo-Voigt function. Surface morphology studies and local quantitative elemental analysis of the samples were performed using a scanning electron microscope (Tescan VEGA 3LMN) and an energy-dispersive X-ray spectrometer (INCA Energy 450).

3. Results and Discussion

According to the X-ray powder diffraction phase analysis data, the sample with the Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ composition is single-phase and crystallized in the structural type of pyrochlore (sp. gr. Fd-3m, R_br = 0.234) (Figure 1) [5,6]. Its unit cell parameter a = 10.4937(2) Å is comparable with the bismuth-deficient tantalum pyrochlore analogue Bi_2-1/3Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Ta₂O_9+Δ (a = 10.48106 Å) [33]. The images of the sample surface obtained by means of scanning electron microscopy in the mode of elastically reflected electrons (BSE mode) do not contain areas of contrast in color associated with the presence of impurity phases. Energy dispersive spectroscopy (EDS) analysis showed (Figure S1) (see Supplementary Materials) that the chemical composition of the sample is close to the specified one.

Since the tantalum analog of the Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ composition is multiphase and contains an admixture of the bismuth orthotantalate BiTaO₄, one can conclude that the tantalum and niobium pyrochlore have a different set of heterovalent transition element ions. This refers to the ions of each transition element in the pyrochlore composition. For example, for manganese, there may be a set of Mn(II), Mn(III), and Mn(IV) ions. The difference can appear in the quantitative ratios of one element’s ions. Since the BiTaO₄ impurity accompanies the bismuth tantalate-based composition, this may mean that there are more transition element ions disproportionate to Ta(V) ions in the tantalum pyrochlore than in the niobium one, while the ionic radii of niobium and tantalum are equal (R(Ta⁵⁺)_cn-6 = R(Nb⁵⁺)_cn-6 = 0.064 nm) [34].

The process of Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ phase formation was studied by comparing the phase composition of samples calcined every 50 °C in the temperature range of 400–1050 °C. Analysis of phase composition and microstructure and elemental mapping of samples were performed at each stage of heat treatment. X-ray powder diffraction patterns of calcined samples in the temperature intervals of 400–650 °C and 650–1050 °C are shown in Figure 2 and Figure 3.

The phase composition of the sample significantly depends on the calcination temperature.

Table 1. Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ phase composition depending on the synthesis temperature.

Synthesis Temperature, °C	Phase Composition
400	η-Nb2O5, α-Bi2O3
450	η-Nb2O5, α-Bi2O3
500	η-Nb2O5, α-Bi2O3, β-Bi2O3
550	η-Nb2O5, β-Bi2O3, Bi5Nb3O15, Bi14CrO24
600	η-Nb2O5, β-Bi2O3,Bi5Nb3O15, Bi14CrO24, Bi25FeO40
650	Bi25FeO40,β-Nb2O5, Bi5Nb3O15, Bi14CrO24, BiNbO4, pyrochlore
700	Bi25FeO40(traces),β-Nb2O5, Bi5Nb3O15, Bi14CrO24, BiNbO4, pyrochlore
750	β-Nb2O5(traces), Bi14CrO24(traces), Bi5Nb3O15, BiNbO4, pyrochlore
800	Bi5Nb3O15(traces), BiNbO4, pyrochlore
850	pyrochlore (68), BiNbO4 (32)
900	pyrochlore (70), BiNbO4 (30)
950	pyrochlore (76), BiNbO4 (24)
1000	pyrochlore (89), BiNbO4 (11)
1050	Bi2Cr1/6Mn1/6Fe1/6Co1/6Ni1/6Cu1/6Nb2O9+Δ (100)

Note. The numbers in parentheses indicate the mass fraction (wt.%) of the phases in the sample.

contains the results of studies of the phase composition in the sample by X-ray powder diffraction patterns. The results of elemental mapping (for all elements—Bi, Nb, O, atoms of 3d elements) of samples calcined in the temperature interval of 650–1050 °C (in 50 °C increments) are shown in Figure 4 and Figure S2.

The data of X-ray powder diffraction phase analysis show that in the temperature range of 400–450 °C the phases of the initial precursors coexist, in particular, the Bragg reflections of monoclinic η-Nb₂O₅ (sp. gr P2/m, PDF Card-01-071-0005) [35] and α-Bi₂O₃ [36,37] phases are clearly observed. The phase transition from monoclinic α-Bi₂O₃ (sp. gr. P21/c, PDF Card-01-070-8243) to tetragonal β-Bi₂O₃ phase (sp. gr. P-421c, PDF Card-01-077-5341) occurs near 500 °C.

At 550 °C, in addition to the Bragg reflections of β-Bi₂O₃ and η-Nb₂O₅ oxides, the Bragg reflections of the interaction products of the initial precursors—Bi₅Nb₃O₁₅ (sp. gr. P4/mmm, PDF Card-00-055-0528) [38] and chromium(VI) compound Bi₁₄CrO₂₄ (sp. gr. I4/m, PDF Card-01-070-4457) [39] are detected. The formation of intermediate chromium(VI) compounds was observed when pyrochlores of Bi₂Mg(Zn)_1−xCr_xTa₂O_9.5−Δ up to 700 °C were obtained [30].

X-ray powder diffraction patterns of samples calcined at 600 °C contain sillenite reflexes Bi₂₅FeO₄₀ (sp. gr. I23, PDF Card-00-046-0416) [40]. At 650 °C the bismuth oxide phase does not manifest itself, and the niobium oxide content decreases; at the same time, the Bi₅Nb₃O₁₅, Bi₁₄CrO₂₄, and Bi₂₅FeO₄₀ content increases, as well as the BiNbO₄ and pyrochlore phases first appear. An increase or decrease in the phase content in the sample was judged from the change in the relative intensities of the Bragg reflections. In Table 1, in the temperature range from 650 to 1000 °C, the presence of the pyrochlore phase is noted. The composition of the phase is unknown because the synthesis is incomplete. We believe that nonstoichiometric pyrochlore is formed at this stage of the synthesis. At 700 °C, the intensity decreases, and at 800 °C and 750 °C, the reflexes of Bi₁₄CrO₂₄ and Bi₂₅FeO₄₀ phases disappear, which may be related to the intensive synthesis of pyrochlore. No niobium oxide reflexes are detected in X-ray powder diffraction patterns of samples synthesized at 800 °C, and Bi₅Nb₃O₁₅ is not present at 850 °C. On this basis, one can note the chemical inertness of niobium oxide compared to bismuth one, which does not form an independent phase already at 650 °C. During high-temperature treatment of the sample, the transition from Bi-rich to Bi-depleted compounds is observed. The intermediate phase in the pyrochlore synthesis is Bi₅Nb₃O₁₅, whereas the Bi-rich analog in the bismuth tantalate-based pyrochlore synthesis is Bi₃TaO₇ (sp. gr Fm-3m, PDF Card-01-076-8514) [41], while its isostructural analogue is Bi₃NbO₇ [42,43]. This is partly due to the fact that the Bi₅Ta₃O₁₅ compound is not stable. Moreover, one can assume that the reactivity of oxides of transition elements, such as chromium(III) or iron(III), is higher than that of niobium(V), so bismuth oxide is consumed primarily and in significant amounts by interacting with them. From 850 °C to 1050 °C the samples are biphasic and consist of pyrochlore and bismuth orthoniobate of α-BiNbO₄ (Pnna, PDF Card-01-072-7513) modification [44,45]. It should be noted that the BiNbO₄ fraction begins to decrease in the samples calcined at 950 °C and higher, which is associated with the activation of the reactivity of transition element oxides and pyrochlore synthesis. The quantitative ratios of pyrochlore and bismuth orthoniobate phases in the range of 850–1000 °C are shown in Table 1. The precursors—oxides of transition elements—are not detected on the samples’ X-ray powder diffraction patterns due to their low content in the initial charge and high reactivity. In order to investigate the distribution of transition element ions, the elemental mapping of samples synthesized at 650–1050 °C was carried out (Figure 4 and Figure S2). According to the elemental mapping data, transition element ions are unevenly distributed in the sample synthesized at 650 °C due to its multiphase. A more even distribution of elements is fixed in the samples calcined above 700–750 °C due to the active formation of the pyrochlore phase as a result of the interaction of bismuth niobate and oxides of transition elements. At 1050 °C, ions are uniformly distributed over the sample surface. Meanwhile, the distribution of cobalt and chromium ions should be noted. When at 650 °C, their distribution over the surface is independent, then, starting from the calcination at 750 °C, they form single centers of concentration. This may indicate the formation of a complex chromium-cobalt-bearing oxide (Figure 5).

This indicates that of the oxides of the other six transition elements, only the oxides of cobalt and chromium interact to form a complex oxide. EDS analysis showed that the stoichiometry of such a compound is close to the spinel formula CrCo₂O₄ = Co²⁺Cr³⁺Co³⁺O₄ [45]. The occurrence of the complex cobalt oxide impurity is explained by its small reactivity, which was noted in [29]. A single-phase sample with uniform distribution of all ions over the sample surface was obtained at 1050 °C, which is confirmed by X-ray powder diffraction phase analysis and elemental mapping data. It should be noted that an increase in sample calcination at each synthesis stage from 10 to 15 h did not lead to qualitative differences in the phase composition of the samples; only the quantitative phase ratios changed. This conclusion was made on the basis of the phase analysis of samples calcined for 15 h at each stage of heat treatment in the range of 650–1050 °C (with a step of 50 °C). All X-ray powder diffraction patterns of the samples are shown in Figure S3. The evaluation of the relative intensities of Bragg reflections of pyrochlore and orthoniobate phases showed that the content of impurity-BiNbO₄ was greater in the samples calcined for 15 h (Figure S3).

Apparently, the prolonged calcination of the samples (15 h) contributed to ceramic grain enlargement and hindered the interaction of precursors.

Microphotographs of the synthesized samples’ surfaces in the temperature range of 650–1050 °C are shown in Figure 6. The heterogeneous microstructure with different grains and inclusions of impurity phases is characteristic of samples calcined at 650–1000 °C. The low-porous, dense microstructure is formed in the sample synthesized at a temperature of 1050 °C. Grain intergrowth with the formation of large crystallites and the formation of a monolithic microstructure are observed in ceramics in contrast to pyrochlores based on bismuth tantalate [30,31].

4. Conclusions

The multielement pyrochlore Bi₂Cr_1/6Mn_1/6Fe_1/6Co_1/6Ni_1/6Cu_1/6Nb₂O_9+Δ was synthesized by solid-phase reaction method. The phase-pure pyrochlore was obtained at 1050 °C. The microstructure of the sample is low-porous with implicit grain boundary outlines. The process of pyrochlore phase evolution in the temperature interval of 400–1050 °C is analyzed. A number of peculiarities of the phase formation process are revealed. The phase formation is a multistage process that ends at a temperature of 1050 °C with the synthesis of single-phase pyrochlore. The synthesis reaction of the pyrochlore phase consists of the interaction of orthorhombic bismuth orthoniobate with oxides of transition elements. The interaction between oxides is fixed at temperatures above 500 °C. Near 500 °C bismuth oxide undergoes a phase transition of α-Bi₂O₃ to β-Bi₂O₃. The synthesis is a multistep process, and the transition of Bi-rich to Bi-depleted oxide compounds is observed. During the synthesis, intermediate compounds of Bi₁₄CrO₂₄ and Bi₂₅FeO₄₀ are formed. The duration of calcination (15 and 10 h at each synthesis stage) did not affect the phase composition but had an effect on the quantitative ratios of the phases in the sample.