Features of phase formation of pyrochlore-type Bi 2 Cr 1 /

The study of the process of phase formation of multicomponent pyrochlore (Bi 2−


Introduction
Bismuth-containing pyrochlores exhibit a wide range of practically useful properties, among which are dielectric properties (low dielectric losses and high dielectric constant, adjustable temperature coefficient of capacitance), catalytic properties in the UV and visible range [1][2][3].A useful addition is the relatively low synthesis temperature and thermal stability of bismuth pyrochlores [4].In the crystal structure of pyrochlore A 2 B 2 O 6 О', two interpenetrating and weakly interacting sublattices are distinguished [5,6].The cationic sublattice А 2 O' is similar to the structure of anticristobalite, the sublattice B 2 O 6 forms a threedimensional framework of vertex-linked octahedrons.Octahedral positions B are occupied by relatively small cations (Ta +5 , Ru +4 , Zr +4 , Sb +5 ), larger ions (Bi +3 , Sm +3 , Pb +2 ) are distributed in eight-coordinated positions A. The flexibility of the pyrochlore crystal structure to cation substitutions in the bismuth / tantalum sublattices and to oxygen vacancies in the А 2 O' sublattice makes it possible to control their functional properties [5 -10].Currently, pyrochlores based on bismuth tantalate and niobate are being actively studied due to their promising dielectric and catalytic properties [7 -15].A feature of the pyrochlores under consideration is the unfilled bismuth sublattice due to the 6s 2 electron pair and the ability of transition element ions to simultaneously reside in the cationic sublattices of bismuth (А 2 O') and tantalum (niobium) (B 2 O 6 ), causing relaxation processes in ceramics [15 -19].Studies of pyrochlores based on bismuth tantalate containing 3d transition ions (Cu, Ni, Fe, Cr, Co, Zn) [10 -12,14,17, 21-23] have shown that porous ceramics are formed with low dielectric losses and moderate values of dielectric permittivity.Copper-containing bismuth tantalates are characterized by a special behavior, which exhibit multiple dielectric relaxation at temperatures above room temperature [20,21].
As established earlier, monocomponent pyrochlores containing ions of one 3d-element (Cu, Ni, Fe, Cr, Co, Mn) [4,10,11, 20 -23] exhibit unique properties depending on the nature of the transition element.It is of interest to study the mutual influence of a combination of atoms of transition elements on the physicochemical properties, on the microstructure of multicomponent pyrochlore of the same stoichiometry.In this regard, in the presented work, an attempt was made to synthesize a multicomponent pyrochlore.In this work, the process of phase formation was studied and the optimal conditions for synthesis by the solid-phase method were established.The features of phase formation are revealed, the density and microstructure of ceramics are studied.

Experimental
High-entropy pyrochlore was synthesized by the standard ceramic technology from the corresponding Bi (III) (99.5 %), Ta (V) (99.99 %), Ni (II) (≥99 %), Co (II, III) (99.99),Cu (II) (≥99 %), Mn (III) (≥99 %), Cr (III) (99.00 %), Fe (III) (98.70 %) oxides.The purity of the reagent is shown in parentheses.The stoichiometric mixture of precursors was finely ground and homogenized in a jasper mortar for one hour.The resulting homogeneous mixture was compacted in the form of disks using a Plexiglas mold.A manual press was used to press the sample.The pressing pressure was 20 Atm.The samples were calcined in air in stages at a temperature of 650, 850, 950, 1050°C for 60 hours.At each stage, the samples were again ground and pressed into tablets.X-ray phase analysis was performed using a Shimadzu 6000 X-ray diffractometer (CuK α radiation; 2θ =10 -60°; scanning speed 2.0° / min).The unit cell parameters of pyrochlores were calculated using the CSD software package [24].The phase formation process was studied using X-ray phase analysis of samples sequentially calcined in the temperature range from 650 to 1050°С (step 50°С) for 15 h at each stage of heat treatment.After each calcination, the sample was carefully homogenized and pressed again in the form of disks to ensure tight contact between the ceramic grains.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).

Results and discussion
According to X-ray-phase analysis, a sample of the complex composition of Bi 2 Cr 1 / 6 Mn 1 / 6 Fe 1 / 6 Co 1 / 6 Ni 1 / 6 Cu 1 / 6 Ta 2 O 9+Δ is crystallized in the structural type of pyrochlore (sp.gr.Fd-3m) and, regardless of the conditions of the synthesis, contains an admixture (7.9 mol.%) orthotantalate of bismuth of the triclinic modification (β-BiTaO 4 ) [25].In order to obtain phase-pure preparations, samples with a deficiency of atoms in both cationic sublattices of bismuth and tantalum, and separately were synthesized.Meanwhile, absolutely single-phase highly entropic chemicals are reproducibly synthesized in the case of compositions with a deficiency of bismuth atoms in the cationic sublattice, e.g., Bi 2−x Cr 1 / 6 Cu 1 / 6 Ni 1 / 6 Co 1 / 6 Fe 1 / 6 Mn 1 / 6 Ta 2 O 9±Δ , x =1 / 3. The calculation of the unit cell parameter of the Bi 2-1 / 3 Cr 1 / 6 Cu 1 / 6 Ni 1 / 6 Co 1 / 6 Fe 1 / 6 Mn 1 / 6 Ta 2 O 9+Δ pyrochlore showed the value a =10.48106Å, which is close to the unit cell parameter of iron-containing pyrochlores based on bismuth tantalate [14,22].Figure S1 (supplementary material) shows, that the ceramic microstructure is porous, reticulate, and consists of slightly melted randomly oriented elongated grains of small size 0.5 -1 µm.In places, crystallites in the form of large agglomerates are observed on the ceramic surface.As microphotographs show, a noticeable fusion of grains is characteristic of ceramics calcined at a temperature of 1050°С.
In order to establish the possibility of influencing the microstructure of ceramics by choosing the optimal conditions for sintering of samples, the stages of phase formation of multicomponent pyrochlore were studied.The process of phase formation was studied by comparing the phase composition of the samples calcined every 50 degrees in the temperature range of 650 -1050°С.At each stage of calcination, an analysis of the phase composition and microstructure was performed, as well as elemental mapping of the samples.The X-ray diffractions patterns of the calcined preparations are shown in Fig. 1.
It can be seen from the presented X-ray diffraction patterns that the phase composition of the sample significantly depends on the heat treatment temperature.The results of studies of the qualitative composition of the phases in the sample X-ray phase analysis are presented in Table 1.
Determination of traces of impurities in the samples was carried out using electron scanning microscopy.
The results of elemental mapping of samples calcined in the range of 650 -1050°С are shown in Fig. 2 and Fig. S2 (supplementary material).According to the analysis data, the samples calcined at 650°C have the most complex phase composition.Phases of the initial precursors Ta 2 O 5 , Co 3 O 4 , NiO and oxide interaction products are identified on the X-ray diffraction pattern -a compound with the sillenite structure Bi 25 FeO 40 (sp.gr.I23), Bi 16 CrO 27 (sp.gr I / 4m), α-BiTaO 4 (sp.gr.Pnna), Bi 3 TaO 7 (sp.gr.Fm-3m) and the pyrochlore phase [26 -29].Elemental mapping of a sample calcined at a temperature of 650°С showed (Fig. 2) that cobalt and nickel are unevenly distributed on its surface, which does not contradict the XRD data (Fig. 3).No other oxides of 3d-element precursors were found as impurity phases, which indicates their entry into chemical interaction.Meanwhile, broadened reflections of the pyrochlore phase are fixed on the X-ray diffraction pattern.Broad reflections indicate the imperfection of the crystal structure, probably due to the nonstoichiometric composition.Apparently, the formation of pyrochlore became possible due to the phase transformation of the α-Bi 2 O 3 phase into the more active δ-Bi 2 O 3 form [30].In general, bismuth oxide, as an independent phase, did not appear on X-ray diffraction patterns of samples calcined at 650°С, which indicates its high reactivity and is confirmed by the results of [14].Part of the tantalum (V) oxide did not enter into the reaction, probably because part of the bismuth (III) oxide was spent on the reaction with iron (III) and chromium (III) oxides to form intermediate products -complex oxides Bi 16 CrO 27 , Bi 25 FeO 40 , respectively.Above 700°С, Bi 16 CrO 27 is not fixed and, in trace amounts, Bi 25 FeO 40 is present, the Bi 3 TaO 7 phase disappears.Cobalt and nickel oxide phases are still fixed, as shown by elemental mapping (Fig. S2, supplementary material).As shown by the results of elemental mapping, impurities of nickel and cobalt oxides are not fixed in samples calcined at temperatures above 900°С.With an increase in the calcination temperature, the phase composition of the samples becomes simpler due to the formation of the pyrochlore phase with the participation of intermediate compounds, for example, BiTaO 4 .A special feature in the process of evolution of the high-entropy pyrochlore phase is the occurrence of an intermediate reaction of the formation of cobalt (nickel) tantalate, which is fixed in samples up to a temperature of 1000°С.We believe that cobalt tantalate was formed, since the position of the reflections on the X-ray diffraction pattern most of all corresponds to this compound.In addition, cobalt oxide is fixed in samples as an impurity up to 900°С.Meanwhile, when studying the phase formation of nickel pyrochlore based on bismuth tantalate, the nickel tantalate phase was not detected [31].By the way, in [12], when studying the phase formation of copper pyrochlore, an intermediate phase of copper (II) tantalate was recorded, which does not contradict the results of our study.Hightemperature treatment of the sample (at temperatures above 800°С) leads to the gradual interaction of BiTaO 4 with oxides of nickel, cobalt, cobalt tantalate and the formation of a pyrochlore phase of a given stoichiometry.X-ray diffraction patterns of samples calcined in this temperature range  show a decrease in the intensity of reflections of bismuth orthotantalate.Phase-clean pyrochlore, without admixture of bismuth orthotantalate, is formed at a temperature of 1000 -1050°С.Scheme S1 (supplementary material) shows an exemplary route for the synthesis of pyrochlore from precursors.
Figure 3 shows the change in the elementary cell parameter of pyrochlore with an increase in the calcination temperature of the sample.As can be seen from the Fig. 3, the elementary cell parameter changes nonmonotonically during synthesis.It is characteristic that during the synthesis of bismuth orthotantalate (650 -750°С), the cell constant of pyrochlore decreases quite sharply with a subsequent increase to a practically constant value a =10.4811Å at 1050°С.A sharp decrease in the unit cell parameter in the range from 650 to 750°С can be associated with the removal of structural deformation due to the nonstoichiometric composition of low-temperature pyrochlore.The strongly