RU2758257C1 - Method for the synthesis of metal phosphates in oxidation state iii - Google Patents

Abstract

FIELD: ceramic insulators production.

SUBSTANCE: invention can be used in the manufacture of ceramic insulators and ferromagnets, matrices for the immobilization of toxic industrial waste. The method for the synthesis of metal phosphates in the oxidation state III includes the preparation of solutions of metal salts from the initial reagents, which are used as nitrates, metal chlorides, chromium acetate. Solutions of metal salts are mixed with a solution of H₃PO₄ and stage-by-stage drying of the reaction mixture is carried out at 90°С and 200°С for complete dehydration of the product. Then stage-by-stage heat treatment of the reaction mixture is carried out at 600-1000°С for 12-48 hours, dispersion and control of the chemical and phase composition by methods for electron probe microanalysis and X-ray diffraction after each heat treatment. Combinations of the following metal cations are used in the chemical composition of polycrystalline RR'(PO₄)₃ phosphates: R - Bi, Sb_xBi_1-x (0 ≤ x ≤ 1), R' - Fe₂, Cr₂, Al_xCr_2-x (0 ≤ x ≤ 0,5) or R - Sb, La, Ce, Pr, Nd, R' - Cr₂.

EFFECT: invention improves the reactivity of the charge, to reduce the duration of the process and the temperature of obtaining the product.

4 cl, 2 dwg, 5 tbl, 4 ex

Description

The present invention relates to the field of chemistry, concerns a method for the synthesis of metal phosphates in the oxidation state III of the type RR '(PO ₄ ) ₃ and R _1-x R' _x (PO ₄ ) ₃ , where R, R 'is Al, Cr, Fe, Sb, La, Ce, Pr, Nd, Bi, which have chemical resistance and thermal stability. High stability allows them to be used in special electrical engineering as ceramic insulators and ferromagnets, in chemical technology as matrices for the immobilization of toxic industrial waste.

Bordeaux 1, 2013. https://tel.archives-ouvertes.fr/tel-00914178/document) в виде поликристаллического порошка методом совместного осаждения солей из водного раствора и монокристаллов, выращенных расплавным методом. Для получения поликристаллического фосфата стехиометрические соотношения Bi₂O₃, Fe₂O₃ и (NH₄)₂HPO₄ тщательно перемешивали в агатовой ступке. К исходной смеси добавили 2%-ный избыток (NH₄)₂HPO₄ из-за неизбежного присутствия воды в нем. Затем смесь помещали в 3D планетарную мельницу вместе с этанолом для образования однородной суспензии. Полученную суспензию помещали в печь и выдерживали при температуре 110°С в течение 12 часов для удаления этанола и воды. Затем полученную смесь постепенно нагревали на воздухе до 400, 600 и 975°С в течение 15 часов с промежуточным измельчением. Для всех температурных ступеней скорость нагрева составляла 2°C⋅мин^-1. После окончательной термообработки образец закаливали на воздухе до комнатной температуры. Недостатком метода является трудоемкость, значительные затраты времени, в том числе связанные с использованием трехмерной планетарной мельницы, использование исходных реагентов в синтезе не обосновано.Known synthesis of BiFe ₂ (PO ₄ ) ₃ (Anand Theerthan. Effect of electric and magnetic stresses on ferroelectric single crystals and ceramics: Docteur.

Bordeaux 1, 2013. https://tel.archives-ouvertes.fr/tel-00914178/document) in the form of a polycrystalline powder by co-precipitation of salts from an aqueous solution and single crystals grown by the melt method. To obtain polycrystalline phosphate, the stoichiometric ratios of Bi ₂ O ₃ , Fe ₂ O _3, and (NH ₄ ) ₂ HPO _{4 were} thoroughly mixed in an agate mortar. A 2% excess of (NH ₄ ) ₂ HPO _{4 was} added to the initial mixture due to the inevitable presence of water in it. The mixture was then placed in a 3D planetary mill along with ethanol to form a homogeneous slurry. The resulting suspension was placed in an oven and kept at 110 ° C for 12 hours to remove ethanol and water. Then the resulting mixture was gradually heated in air to 400, 600 and 975 ° C for 15 hours with intermediate grinding. For all temperature steps, the heating rate was 2 ° C⋅min ^-1 . After the final heat treatment, the sample was quenched in air to room temperature. The disadvantage of this method is laboriousness, significant time consumption, including those associated with the use of a three-dimensional planetary mill, the use of starting reagents in the synthesis is not justified.

To obtain single crystals, the powder obtained in the previous method was placed in a platinum crucible and heated to complete melting at 1125 ° C in air for 6 h at a rate of 2 ° C / min. Then the melt was slowly cooled to 1000 ° C at a rate of 1.8 ° C / h. Then the sample was cooled to room temperature for 24 hours. The crystals grown were generally less than a millimeter and were in the shape of needles.

The disadvantages of this method are the high temperature and time consuming, which make it difficult to control the state of the synthesis product.

The task of the invention is to create a new method for the synthesis of metal phosphates in oxidation state III of the composition RR '(PO ₄ ) ₃ and R _1-x R' _x (PO ₄ ) ₃ , where R, R '- Al, Cr, Fe, Sb, La, Ce, Pr, Nd, Bi.

The technical result from the use of the invention is to increase the reactivity of the charge, reduce the duration of the process and reduce the temperature of obtaining the product.

The task is achieved by the fact that the method for the synthesis of metal phosphates in the oxidation state III includes the preparation of solutions of metal salts from the initial reagents, which are used as nitrates, metal chlorides, chromium acetate, mixing metal salt solutions with an acid solution H ₃ PO ₄ , stage-by-stage drying of the reaction mixture at temperatures of 90 ° C and 200 ° C for complete dehydration of the product, further step-by-step heat treatment of the reaction mixture at a temperature of 600-1000 ° C for 12-48 hours, dispersion and control of the chemical and phase composition by methods of electron probe microanalysis and X-ray diffraction after each heat treatment, while using combinations of the following metal cations in the chemical composition of polycrystalline RR '(PO ₄ ) ₃ phosphates R - Bi, Sb _x Bi _1-x (0 ≤ x ≤ 1), R' - Fe ₂ , Cr ₂ , Al _x Cr _2-x (0 ≤ x ≤ 0.5) or R - Sb, La, Ce, Pr, Nd, R '- Cr ₂ ; dispersion is carried out after each heat treatment for 30 minutes; additional heat treatment of the sample is carried out at the temperature of the last heating if an amorphous halo appears on the X-ray diffraction pattern, the presence of fuzzy diffraction maxima and asymmetry of the diffraction line profiles on the X-ray diffraction pattern, indicating the presence of an amorphous phase and imperfections of the crystal structure in the sample; to obtain BiFe ₂ (PO ₄ ) ₃ single crystals, polycrystalline phosphate is used as a charge, previously synthesized by the method according to claim 1, which is heated to the melt temperature, single crystals are obtained by lowering the temperature of the BiFe ₂ (PO ₄ ) ₃ melt from 1080 ° C to 950 ° С at a rate of 2.7 deg / h and subsequent cooling of the sample to room temperature for 20 h.

FIG. 1 shows the X-ray diffraction patterns of phosphates, where: 1 - BiFe ₂ (PO ₄ ) ₃ , 2 - BiCr ₂ (PO ₄ ) ₃ , 3 - Sb _0.25 Bi _0.75 Cr ₂ (PO ₄ ) ₃ , 4 - Sb _0.5 Bi _0.5 Cr ₂ ( PO ₄ ) ₃ , 5 - Sb _0.75 Bi _0.25 Cr ₂ (PO ₄ ) ₃ , 6 - SbCr ₂ (PO ₄ ) ₃ .

FIG. 2 shows the structure of phosphate BiFe₂(PO₄)₃, analogue (α-CaMg₂(SO₄)₃).

The proposed method for the synthesis of metal phosphates in oxidation state III is as follows.

Preliminary structural and chemical design of possible new phosphates is carried out on the basis of crystal-chemical principles. Taking into account the ionic radii of metals in oxidation state III, the probability of finding atoms in similar positions of the crystal structure is predicted (see the book by VS Urusov. Theoretical crystal chemistry, Moscow: Moscow State University Publishing House, 1987. - 275 p.). The proximity of the ionic radii of metals and their identical oxidation state III allows us to count on their isomorphism in phosphates, that is, mixing in the composition of a single-phase product - phosphate in equivalent positions of the crystal structure of metals of close ionic radii that differ by 10-15%.

After the theoretical stage, an empirical stage is carried out, which includes carrying out various analyzes that make it possible to establish the optimal intermediate heat treatments, their duration, and the temperature boundary of synthesis. For this, a synchronous differential thermal and thermogravimetric analysis (DTA-TG) is used, which makes it possible to estimate in dynamics the temperatures of exo- and endothermic effects associated with phase transformations in the sample. In addition to the dynamic determination of the temperatures of intermediate isothermal treatments and the possible synthesis temperature, the static heating method is used to determine the minimum (optimal) time of heat treatment at each isothermal stage. The essence of the method lies in the step-by-step isothermal heating of the sample, after which X-ray phase and electron-microprobe analyzes are carried out to determine the change in the phase and chemical composition of the sample.

At the empirical stage, reagents are also determined, the use of which will allow the subsequent solid-phase reaction to be carried out. Advantageously, nitrates, carbonates, metal chlorides, salts of organic acids, for example chromium acetate, are used as such substances. Their decomposition upon heating leads to a rearrangement of the crystal structure, causing a large number of crystal lattice defects, which favorably affect the rate of the subsequent solid-phase reaction. The use of thermally stable substances (oxides) requires higher temperatures for their activation and the start of the reaction.

Subsequently, solutions of metal salts are prepared from the starting reagents, solutions of metal salts in oxidation state III are mixed with solutions of acid H ₃ PO ₄ or its salt NH ₄ H ₂ PO ₄ , stage-by-stage drying of the reaction mixture until the product is completely dehydrated, first at a temperature of 90 ° C , then at a temperature of 200 ° C, further heat treatment of the reaction mixture with dispersion and control of the chemical and phase composition by the methods of electron probe microanalysis and X-ray diffraction after each heating.

Heat treatment of the reaction mixture is carried out for 6-48 hours. Heating the phosphate for less than 6 hours results in incomplete crystallization, and therefore the sample may be non-single-phase. Heat treatment for more than 48 hours does not cause noticeable changes in the formed single-phase crystalline sample, and is an unjustified waste of resources.

Dispersion of the mixture is carried out after each heat treatment, for example, for 30 minutes.

If an amorphous halo appears on the X-ray diffraction pattern, the presence of fuzzy diffraction maxima and asymmetry of the diffraction line profiles on the X-ray diffraction pattern, indicating the presence of an amorphous phase and imperfections in the crystal structure in the sample, additional heat treatment of the sample is carried out at the temperature of the last heating.

Below are examples of specific implementation of the invention.

Example 1.

и P-O 1.508-1.559

(рис. 2). Тетраэдры PO₄ двумя вершинами скрепляют вершины двух сдвоенных гранями октаэдров FeO₆, образуя колонку, и двумя другими вершинами присоединяются к соседним колонкам, образуя смешанный каркас. В пустотах каркаса располагаются атомы Bi, которые окружены шестью атомами кислорода (координационные полиэдры - тригональные призма Bi1 и антипризма Bi2). Расстояния Bi-О находятся в пределах 2.155-3.192

.Phosphate BiFe ₂ (PO ₄ ) ₃ with the structure α-CaMg ₂ (SO ₄ ) ₃ was predicted on the basis of crystal chemical data. The structural basis of phosphate I is the {[Fe ₂ (PO ₄ ) ₃ ] ^3- } _{3∞ framework,} in which Fe atoms are coordinated by six oxygen atoms from six PO ₄ tetrahedra with a Fe-O distance of 1.897-2.140

and PO 1.508-1.559

(fig. 2). _{PO 4} tetrahedra with two vertices fasten the vertices of two double-faceted FeO ₆ octahedra, forming a column, and two other vertices join adjacent columns, forming a mixed framework. In the voids of the framework, Bi atoms are located, which are surrounded by six oxygen atoms (coordination polyhedra - trigonal prism Bi1 and antiprism Bi2). The Bi-O distances are in the range 2.155-3.192

...

For the synthesis of phosphate, Fe ₂ O ₃ , Bi ₂ O ₃ , H ₃ PO ₄ were chosen as starting reagents. Their use made it possible to carry out the process at the lowest temperature.

To obtain 2 g of phosphate BiFe ₂ (PO ₄ ) _3, at the first stage, a _{solution of hydrochloric acid was added to the stoichiometric sample of Fe 2} O ₃ until the sample was completely dissolved. A stoichiometric sample of Bi ₂ O _{3 was} added to the resulting solution. Then, with constant stirring, a solution of phosphoric acid was added dropwise, taken also in accordance with the stoichiometry of the sample (see table 1).

The reaction mixture was initially dried at a temperature of 90 ° C to remove water and then at 200 ° C for complete dehydration of the product. Subsequently, the reaction mixture was heat treated at 600 ° C for 24 hours, after which the sample was dispersed in an agate mortar for 30 minutes using isopropyl or ethyl alcohol to ensure homogenization of the mixture. Then the sample was again fired in an oven at a temperature of 800 ° C. Each heating step was alternated with dispersion.

, V=1318.12(8)

Данные электронно-зондового микроанализа показали однородность состава зерен, их состав отвечал формуле Bi_1.01(2)Fe_2.00(1)P_3.00(2)O₁₂ (таблица 5).BiFe ₂ (PO ₄ ) ₃ , according to X-ray phase analysis (XPA) (Fig. 1), crystallizes in the structure of α-CaMg ₂ (SO ₄ ) ₃ (space group P 6 ₃ / m ), the parameters of its unit cell: a = 14.3115 (4), c = 7.4311 (2)

, V = 1318.12 (8)

The data of electron probe microanalysis showed the homogeneity of the grain composition, their composition corresponded to the formula Bi _{1.01 (2)} Fe _{2.00 (1)} P _{3.00 (2)} O ₁₂ (Table 5).

Bordeaux 1, 2013) тем, что он включает выбор исходных реагентов, благоприятно влияющих на скорость протекания последующей твердофазной реакции, приготовление растворов солей металлов из исходных реагентов, смешение растворов солей металлов с раствором кислоты H₃PO₄, поэтапную сушку реакционной смеси при температурах 90°С и 200°С для полного обезвоживания продукта, дальнейшую термообработку реакционной смеси в течение 12-48 часов и диспергирование в агатовой ступке (отличается от мельницы простотой очищения) на каждой стадии, меньшее число стадий синтеза, образование целевого продукта при более низкой температуре.This synthesis method is superior to that previously described (Anand Theerthan. Effect of electric and magnetic stresses on ferroelectric single crystals and ceramics: Docteur.

Bordeaux 1, 2013) by the fact that it includes the choice of starting reagents that favorably affect the rate of the subsequent solid-phase reaction, preparation of metal salt solutions from starting reagents, mixing metal salt solutions with an H ₃ PO ₄ acid solution, step-by-step drying of the reaction mixture at temperatures of 90 ° C and 200 ° C for complete dehydration of the product, further heat treatment of the reaction mixture for 12-48 hours and dispersion in an agate mortar (differs from the mill in ease of purification) at each stage, fewer synthesis stages, formation of the target product at a lower temperature.

To obtain BiFe single crystals₂(PO₄)₃ used as a charge pre-synthesized polycrystalline phosphate. Single crystals were obtained by lowering the temperature of the BiFe melt₂(PO₄)₃ from 1080 to 950 ° C at a rate of 2.7 deg / h. Then the sample was cooled to room temperature for 20 h.

Bordeaux 1, 2013) тем, что нами используются более низкая температура расплава (1080°С вместо 1125°С), более высокая скорость его охлаждения от 1080 до 950°С (2.7 град/ч вместо 1.8 град/ч), меньшее последующее время охлаждения до комнатной температуры.This synthesis method is superior to that previously described (Anand Theerthan. Effect of electric and magnetic stresses on ferroelectric single crystals and ceramics: Docteur.

Bordeaux 1, 2013) by the fact that we use a lower melt temperature (1080 ° C instead of 1125 ° C), a higher cooling rate from 1080 to 950 ° C (2.7 deg / h instead of 1.8 deg / h), and a shorter subsequent time cooling to room temperature.

Example 2.

Phosphates Sb _x Bi _1-x Cr ₂ (PO ₄ ) ₃ , (0 ≤ x ≤ 1) with the expected structure α-CaMg ₂ (SO ₄ ) ₃ were predicted based on crystallochemical data. In this case, the framework of the structure is formed by CrO ₆ octahedra and PO ₄ tetrahedra. The extraframework positions are occupied by cations of a larger radius - these are Sb ³⁺ and Bi ³⁺ (the same oxidation states, the ionic radii are close).

To obtain this series of phosphates, the following reagents were used: Sb ₂ O ₃ , Bi ₂ O ₃ , Cr (CH ₃ COO) _3, and H ₃ PO ₄ .

To obtain 2 g of the product at the first stage, a _{solution of hydrochloric acid was added to a stoichiometric sample of Bi 2} O ₃ until the oxide was completely dissolved. A stoichiometric portion of Sb ₂ O _{3 was} added to the resulting solution, and after its dissolution, a stoichiometric portion of Cr (CH ₃ COO) _{3 was} added. Then, with constant stirring, a solution of phosphoric acid was added dropwise, taken also in accordance with the stoichiometry of the sample (see table 2).

The reaction mixture was initially dried at a temperature of 90 ° C to remove water and then at 200 ° C for complete dehydration of the product. Subsequently, the reaction mixture was heat treated at 600 ° C for 24 hours, after which the sample was dispersed in an agate mortar for 30 minutes using isopropyl or ethyl alcohol to ensure homogenization of the mixture. Then the sample was again fired in an oven at temperatures of 800, 900, and 1000 ° C. Each heating step was alternated with dispersion. The final temperature of obtaining the target product depended on the composition of the solid solution.

According to X-ray diffraction data, all samples of the composition Sb _x Bi _1-x Cr ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 1) (Fig. 1) crystallize in the structure of α-CaMg ₂ (SO ₄ ) ₃ and form an unlimited solid solution. The data of the electron probe microanalysis showed the homogeneity of the composition of their grains and the correspondence of the expected (predicted) and actual compositions, taking into account the error of the method (Table 5).

Example 3.

Phosphates BiAl _x Cr _2-x (PO ₄ ) ₃ , 0 ≤ x ≤ 0.5 with the expected structure of α-CaMg ₂ (SO ₄ ) ₃ were predicted on the basis of crystal chemical data. In this case, the framework of the structure is formed by CrO ₆ and AlO ₆ octahedra (the same oxidation states, ionic radii are close), PO ₄ tetrahedra. Extraframework positions are occupied by cations of a larger radius - Bi ³⁺ .

To obtain this series of phosphates, the following reagents were used: Bi ₂ O ₃ , AlCl ₃ , Cr (CH ₃ COO) _3, and H ₃ PO ₄ .

To obtain 2 g of the product at the first stage, a _{solution of hydrochloric acid was added to a stoichiometric sample of Bi 2} O ₃ until the oxide was completely dissolved. A stoichiometric weighed portion of AlCl _{3 was} added to the resulting solution, and after its dissolution, a stoichiometric weighed portion of Cr (CH ₃ COO) _{3 was} added. Then, with constant stirring, a solution of phosphoric acid was added dropwise, taken also in accordance with the stoichiometry of the sample (see table 3).

The reaction mixture was initially dried at a temperature of 90 ° C to remove water and then at 200 ° C for complete dehydration of the product. Subsequently, the reaction mixture was heat treated at 600 ° C for 24 hours, after which the sample was dispersed in an agate mortar for 30 minutes using isopropyl or ethyl alcohol to ensure homogenization of the mixture. Then the sample was again fired in an oven at temperatures of 800 and 1000 ° C. Each heating step was alternated with dispersion.

According to X-ray diffraction data, all samples of the composition BiAl _x Cr _2-x (PO ₄ ) ₃ 0 ≤ x ≤ 0.5 crystallize in the structure of α-CaMg ₂ (SO ₄ ) ₃ and form a limited solid solution. The data of the electron probe microanalysis showed the homogeneity of the composition of their grains and the correspondence of the expected (predicted) and actual compositions, taking into account the error of the method (Table 5).

Example 4.

Phosphates RCr ₂ (PO ₄ ) ₃ , where R = La, Ce, Pr, Nd, with the expected structure of α-CaMg ₂ (SO ₄ ) ₃ were predicted based on crystal chemical data. In this case, the framework of the structure is formed by CrO ₆ octahedra and PO ₄ tetrahedra. Extraframework positions are occupied by cations with a larger radius - R ³⁺ .

To obtain this series of phosphates, the following reagents were used: LaF ₃ ⋅0.5H ₂ O, Ce (NO ₃ ) ₃ ⋅6H ₂ O, Pr (NO ₃ ) ₃ ∙ 6H ₂ O, Nd ₂ O ₃ , Cr (CH ₃ COO) ₃ and H ₃ PO ₄ .

To obtain 2 g of the product in the first step to the hitch LaF ₃ ⋅0.5H ₂ O (or Ce (NO _{3) 3} ⋅6H ₂ O, or Pr (NO _{3) 3} ⋅6H ₂ O, or Nd ₂ O ₃₎ was added a solution of hydrochloric acid until the sample is completely dissolved. _{Cr (CH 3} COO) _{3 was} added to the resulting solution. Then, with constant stirring, a solution of phosphoric acid was added dropwise (see table 4).

The reaction mixture was initially dried at a temperature of 90 ° C to remove water and then at 200 ° C for complete dehydration of the product. Subsequently, the reaction mixture was heat treated at 600 ° C for 24 hours, after which the sample was dispersed in an agate mortar for 30 minutes using isopropyl or ethyl alcohol to ensure homogenization of the mixture. Then the sample was again fired in an oven at temperatures of 800, 1000, and 1200 ° C. Each heating step was alternated with dispersion.

According to X-ray diffraction data, all samples of the composition RCr ₂ (PO ₄ ) ₃ crystallize in the α-CaMg ₂ (SO ₄ ) ₃ structure. The data of the electron probe microanalysis showed the homogeneity of the composition of their grains and the correspondence of the expected (predicted) and actual compositions, taking into account the error of the method (Table 5).

Thus, an increase in the reactivity of the charge is achieved due to the use in the synthesis of nitrates, carbonates, metal chlorides, salts of organic acids (for example, chromium acetate), which, undergoing thermal decomposition when exposed to a temperature field, increase the specific surface area and surface activity of the reaction zone due to the rearrangement of the crystalline structure and formation of a large number of crystal lattice defects. As a result, the high activity of such a material leads to an increase in the rate of the subsequent solid-phase reaction and the formation of a product phase at a lower temperature compared to the use of thermally stable reagents (oxides) that initiate the reaction little and require higher temperatures for their activation and the formation of the desired product.

In the presented examples, metal cations were used in the synthesis: Bi³⁺, Fe³⁺, Cr³⁺, Sb³⁺, Al³⁺, La³⁺, Ce³⁺, Pr³⁺, Nd³⁺... However, the synthesis of phosphates can lead to the production of substances with a wider range of metals in the same oxidation states, for example, phosphates that have not yet been studied RR '₂(PO₄)₃ (R, R '- metals in oxidation state III) and solid solutions based on them. Therefore, the presented examples illustrate the invention, but do not limit it.

Claims (4)

1. The method for the synthesis of metal phosphates in the oxidation state III includes the preparation of solutions of metal salts from initial reagents, which are used as nitrates, metal chlorides, chromium acetate, mixing metal salt solutions with an acid solution H ₃ PO ₄ , stage-by-stage drying of the reaction mixture at temperatures of 90 ° C and 200 ° C for complete dehydration of the product, further step-by-step heat treatment of the reaction mixture at a temperature of 600-1000 ° C for 12-48 hours, dispersion and control of the chemical and phase composition by electron probe microanalysis and X-ray diffraction after each heat treatment, while use combinations of the following metal cations in the chemical composition of polycrystalline RR '(PO ₄ ) ₃ phosphates: R - Bi, Sb _x Bi _1-x (0 ≤ x ≤ 1), R' - Fe ₂ , Cr ₂ , Al _x Cr _{2- x} (0 ≤ x ≤ 0.5) or R - Sb, La, Ce, Pr, Nd, R '- Cr ₂ . 2. A method according to claim 1, characterized in that the dispersion is carried out after each heat treatment for 30 minutes. 3. The method according to claim 1, characterized in that additional heat treatment of the sample is carried out at the temperature of the last heating in the case of the appearance of an amorphous halo on the X-ray diffraction pattern, the presence of fuzzy diffraction maxima and asymmetry of the diffraction line profiles on the X-ray diffraction pattern, indicating the presence of an amorphous phase and imperfections of the crystal structure in sample. 4. The method according to claim 1, characterized in that to obtain single crystals of BiFe ₂ (PO ₄ ) ₃ , polycrystalline phosphate is used as a charge, previously synthesized by the method according to claim 1, which is heated to the melt temperature, the single crystals are obtained by lowering the temperature of the BiFe melt ₂ (PO ₄ ) ₃ from 1080 ° C to 950 ° C at a rate of 2.7 deg / h and subsequent cooling of the sample to room temperature for 20 h.

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