PhD Pavlova A. P.¹,

Doctor of Chemical Sciences Mittova I. Ya.¹,

PhD Mittova V. O.²,

PhD student Dinh Van Tac¹.

¹ Voronezh State University
² Voronezh State Medical Academy named after N.N. Burdenko

The properties of nanocrystals Y₃Al₅O₁₂, Y₃Fe₅O_12, Y_3-xLa_xFe₅O₁₂ garnet structure, synthesized by chemical precipitation method.

Nanocrystalline garnets Y₃Al₅O₁₂ (YAG), Y₃Fe₅O_12, (YIG) and solid solutions based on them attract great interest due to their optical properties. Iron garnets are under scrutiny because of their magneto-optical properties and prospects for practical use as functional materials in microwave engineering, recording and storing information, nanocomposites, and various magneto-optical devices [1, 2]. The crystals of yttrium aluminum garnet due to their relatively stable lattice structure and high thermal conductivity are used as laser materials and coatings for electronic devices.

Out of the possible chemical methods of obtaining these nanomaterials , the sol-gel method is very promising since it allows to reduce the duration of sintering, decrease the temperature of synthesis and to provide better control of the size of the particles obtained.

Developed processes for the synthesis of nanopowders Y₃Al₅O₁₂, Y₃Fe₅O₁₂, Y_3-xLa_xFe₅O₁₂ include the following steps: deposition of the metal hydroxides, Fe (III), Y, Al, La in water by the method of [1, 3, 4], filtration, drying, their subsequent firing in air to complete the dehydration, and the formation of the final product.

The method consists of obtaining Y₃Fe₅O₁₂ by preparing an aqueous solution containing 0.008 M Fe(NO₃)₃, 0.008× 3/5 M YCl₃, boil it for 5 minutes and then cool it to room temperature. Then into the resulting solution was poured drop by drop with constant stirring with a mechanical stirrer (speed of 3000 rev/min) 0.3 M aqueous solution of ammonia in an amount necessary for complete precipitation of the cations Y³⁺ and Fe³⁺. After the introduction of ammonia, stirring was continued for another 15 min. Then the precipitation was filtered, washed and dried at room temperature until its weight becomes constant. Nanocrystals are Y₃Fe₅O₁₂ calcination of the precipitate in a muffle furnace at 1000 °C for 4 h. The process of doping Y_3-xLa_xFe₅O₁₂ performed on the same technology, taking into account the values of x = 0.2, 0.4, 0.6 [1, 3].

For the synthesis of liquid-phase samples Y₃Al₅O₁₂ the method of using aqueous solutions of nitrates Al, Y (0.15 M and 0.15 M, respectively) and 1.5 M solution of NH₄HCO₃ as precipitator. The speed of adding of salt solution to the precipitant in the magnetic stirrer 3 ml/min at room temperature, processing time - 21 minutes. After aging the suspension (1 h) and filtration, the precipitate is dried in the air 24 hours, the regime of calcination - 955 °C for 1 hour.

Phase composition of samples determined by means of X-ray diffraction (XRD, DRON-4 diffractometer, СоК_а - radiation, λ = 0,17902 nm) with the calculation of the size of coherent scattering regions (CSR) by Scherrer for the determination of particle size. Alternative methods for the determination of particle size - dynamic and static light scattering (spectrometer Photocor Complex) [1, 3] and high-voltage transmission electron microscopy (TEM, PC-100 BR, fig.). The results of the calculations the average particle size obtained from the three methods used are summarized in the table.

The magnetic properties of nanocrystals Y_3-xLa_xFe₅O₁₂ tested for vibration magnetometer at room temperature [1, 3]. It is shown that with increasing lanthanum content of the saturation magnetization first increases and then decreases, the maximum corresponds to x = 0.2 (Y_2.8La_0.2Fe₅O₁₂) and amounts to 28,123 Am²/kg. For samples Y_2.6La_0.4Fe₅O₁₂ (x = 0.4) and Y_2.4La_0.6Fe₅O₁₂ (x = 0.6) of the saturation magnetization are respectively 25,747 and 24,758 Am²/kg, which is lower than that of the sample in the absence of dopant Y₃Fe₅O₁₂ (26.141 Am²/kg).

Fig. TEM images of nanopowders Y_3-xLa_xFe₅O₁₂ (a), Y₃Fe₅O₁₂ (b), Y₃Al₅O₁₂ (c)

Table. The average diameter of the obtained nanocrystals.

	XRD	dynamic and static light scattering	TEM
Y3Fe5O12	54 nm	57 nm	68 nm
Y3Al5O12	26 nm	40 nm	35 nm
Y3-xLaxFe5O12 х = 0 – 0.6	54 nm – 44 nm	57 nm – 49 nm	for x= 0.6 65 nm

From the data of XRD and LRSMA (local X-ray microanalysis in the electron microscope JEOL-JSM-6380LV), it follows that the substitution of yttrium,

lanthanum, limiting the formation of solid solutions of Y_3-xLa_xFe₅O₁₂ up to 0.8. For x = 0.8 phase appears LaFeO₃, consequently, for x ≥ 0.8 the region of homogeneity breaks down and released the second phase LaFeO₃. With increasing content of lanthanumin the solid solutions Y_3-xLa_xFe₅O₁₂ decreases the size of the particles and the cell parameters increase. When x varies from about 0.6 to the cell parameter (a) increased from 12,328 to 12.421 Å, which is associated with an increase in the ionic radius of La³⁺ compared to Y³⁺.

References:

1. Dinh Van Tac, Mittova V. O., Almyasheva O. V., Mittova I. Ya. Synthesis, structure and magnetic properties of nanocrystalline Y_3-xLa_xFe₅O₁₂ (0 ≤ x ≤ 0.6) // Inorg. materials. 2012. V. 48. № 1. Р. 81–86.

2. Eliseev А. А, Lukashin A. V. Functional Nanomaterials. M.: FIZMATLIT, 2010. P. 456.

3. Dinh Van Tac, Mittova V. O., Mittova I. Ya. Effect of lanthanum content and annealing temperature on the size and magnetic properties Y_1-xLa_xFeO₃, obtained by the sol-gel method // Inorg. materials. 2011. V. 47. № 5. P. 590-595.

4. Ji-Guang Li et al. Co-precipitation synthesis and sintering of yttrium aluminum garnet (YAG) powders: the effect of precipitant // Journal of the European Ceramic Society. 2000. № 20. P. 2395-2405.