RU2681641C1 - Method of growing lithium tribolate crystal (options) - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention relates to the field of obtaining a crystal of lithium triborate LiBO(LBO), that is a highly efficient non-linear optical material used for passive frequency conversion of laser radiation. Method of growing a lithium triborate crystal involves loading the initial charge into a growth crucible, melting it and homogenizing it, bringing the point with the lowest temperature on the solution-melt surface to the center of the crucible, introduction of an oriented seed crystal in contact with the surface of the solution-melt in the center of the crucible, the search for an equilibrium temperature and the subsequent crystal growth with a decrease in temperature in the three-zone growth furnace, its middle and lower zones consist of eight heating elements, switching over time of which provides control of convective flows in the melt. Crystal is grown by lowering the temperature in the lower zone relative to the middle and upper zones, thus, so that at the end of the growth cycle, the temperature interval of lowering the lower zone increased by 5–10 %. At that in the first embodiment, the expansion is carried out with an increase in the process of increasing the shutdown time of 2, 3, 6 and 7 heating elements of the middle and lower zones, located along the crystallographic direction "b", from 5 to 7 seconds and from 20 to 23 seconds, respectively, while simultaneously decreasing the turn-off time of 1, 4, 5 and 8 heating elements of the middle and lower zones, located along the crystallographic direction "a", with 5 to 3 seconds and from 20 to 17 seconds, respectively. According to the second variant, the growth of the crystal is carried out when the shutdown time 5 and 8 of the heating elements of the lower zone changes from 30 to 35 seconds during the growth process, 6 and 7 from 30 to 37 seconds, 1 and 4 from 30 to 27 seconds and 2 and 3 from 30 to 23 seconds. According to the third variant, the growth of the crystal is carried out with an increase in the growth process of the turn-off time of 1, 8, 4 and 5 heating elements of the middle and lower zones, located along the crystallographic direction "a", from 6 to 11 seconds and from 30 to 37 seconds, respectively, and at the same time reducing the turn-off time 2, 3, 6 and 7 of the heating elements of the middle and lower zones, located along the crystallographic direction "b", from 6 to 1 second and from 30 to 23 seconds, respectively.EFFECT: achieved technical result: the creation in the process of crystal growth dynamically changing, asymmetric thermal fields by controlling the heat flow in the melt from the heating elements of the growth furnace and obtaining LBO crystals of the required size in the specified crystallographic directions, providing the possibility of manufacturing nonlinear optical elements of increased size for various types of conversion of laser radiation.3 cl, 11 dwg, 5 ex, 2 tbl

Description

; b=7.3798

; с=5.1408

. Наиболее эффективным применением данного кристалла является преобразование частоты лазеров на неодиме с длиной волны 1064 нм во вторую (532 нм) и третью (533 нм) гармоники. Высокий порог разрушения и эксплуатационные характеристики данного кристалла делают его весьма перспективным материалом для применения в широкоапертурных лазерных системах высокой мощности. В настоящее время в данных системах преимущественно применяются н/о элементы из кристаллов калия дигидрофосфата (KDP), размер которых в апертуре достигает 40×40 см². Кристаллы LBO имеют неоспоримое преимущество по всем параметрам по сравнению с кристаллами KDP за исключением размеров. Поэтому выращивание кристаллов LBO больших размеров с целью изготовления нелинейно-оптических элементов увеличенных размеров представляет собой важную и актуальную научно-техническую задачу.The invention relates to the field of obtaining optical materials for nonlinear optics. The lithium triborate crystal LiB ₃ O ₅ (LBO) is a highly efficient nonlinear optical material used for passive frequency conversion of laser radiation. LBO crystallizes in rhombic syngony with unit cell parameters: а = 8.447

; b = 7.3798

; c = 5.1408

. The most effective application of this crystal is to convert the frequency of neodymium lasers with a wavelength of 1064 nm into the second (532 nm) and third (533 nm) harmonics. The high fracture threshold and operational characteristics of this crystal make it a very promising material for use in high-power wide-aperture laser systems. Currently, these systems mainly use n / a elements from potassium crystals of dihydrogen phosphate (KDP), the size of which in the aperture reaches 40 × 40 cm ² . LBO crystals have an undeniable advantage in all respects compared to KDP crystals with the exception of size. Therefore, the growth of large LBO crystals with the aim of manufacturing non-linear optical elements of increased dimensions is an important and relevant scientific and technical task.

Due to incongruent melting, LBO crystals are grown by solution-melt crystallization according to the Stepanov method [A.E. Koch, V.A. Vlezko, K.A. Koch. Control of the heat field symmetry in an installation for growing LBO crystals by the Kyropoulos method - Instruments and experimental equipment, 2009, No. 5, p. 145-149; A. Kokh, N. Kononova, G. Mennerat, Ph. Villeval, S. Durst, D. Lupinski, V. Vlezko, K. Kokh. Growth of high quality large size LBO crystals for high energy second harmonic generation - J. Crystal Growth, vol. 312, No. 10, 2010, pp. 1774-1778; I. Nikolov, D. Perlov, S. Livneh, E. Sanchez, P. Czechowicz, V. Kondilenko, D. Loiacono, Growth and morphology of large LiB3O5 single crystals, Journal of Crystal Growth, 331 (2011) 1-3] . The solvent used is molybdenum oxide. Basically, LBO crystals are grown along the crystallographic direction “c” or [001], such crystals are characterized by high optical quality.

In [Zh. Hu, Y. Zhao, Y. Yue, X. Yu. Large LBO crystal growth at 2 kg-level // J Crystal Growth, 335 (2011) 133-137] a method for growing LBO crystals along the crystallographic direction [011] is proposed, however, the results of this work show that the grown crystals contain visible inclusions that their use does not allow for the manufacture of large-sized nonlinear optical elements.

In [A. Kokh, V. Vlezko, K. Kokh, N. Kononova, Ph. Villeval, D. Lupinski. Dynamic control over the heat field during LBO crystal growth by high temperature solution method - J. Crystal Growth, vol. 360, 2012, pp. 158-161] describes the design of the installation and method of growing crystals of LBO. The heating furnace consists of three vertical heating zones, this makes it possible to control the axial temperature gradient. The middle and lower heating zones, in turn, each consist of eight heating elements. In the temperature control system there are load switches of the middle and lower zones, with the help of which the effect on the heat flux from each of the heating elements is provided by changing the connection / disconnection time of the heating elements individually or in groups. This design of the heating furnace and the corresponding control system of the growth unit provide the ability to control convective flows in the melt, as well as to bring the so-called cold point to the center of the surface of the solution-melt for contact of the seed crystal with the solution-melt at the point with the lowest temperature.

The volume and weight of the grown crystal depend on the size of the growth crucible used in this installation, on the initial charge of the charge, on the concentration of the useful component, and on the value of the total temperature decrease in the heating furnace during the crystal growth. Obviously, with the above parameters fixed, the weight (volume) of the obtained crystals will also be the same. The LBO crystal according to the known method is grown in a symmetric thermal field, which provides a crystal whose size ratio in the directions "a" and "b" is close to the ratio of the unit cell parameters in these directions ("a" / "b" = 1.14). In this case, the crystal size along the c direction depends on the axial gradient, which does not affect the crystal size in the a and b directions, keeping their ratio close to 1.14. It follows from this that with fixed technological parameters, under conditions of a symmetric thermal field, it is not possible to change the dimensions of the crystals in the directions "a" and "b" in order to achieve an increase in the size of nonlinear optical elements for different types of synchronism.

The objective of the invention is to create conditions for growing LBO crystals of the required size in the given crystallographic directions, providing the possibility of manufacturing non-linear optical elements of increased size for various types of laser radiation conversion.

The technical result consists in creating dynamically changing, asymmetric heat fields during crystal growth by controlling heat fluxes in the melt from the heating elements of the growth furnace.

To increase the diameter of the nonlinear optical element for conversion to the second harmonic, it is necessary to ensure the ratio of the crystal sizes in the directions "a" and "b" to close to unity and to increase the relative size of the crystal along the direction "c". For this, in a method for growing a lithium triborate crystal, which includes loading the initial charge into a growth crucible, melting and homogenizing it, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with the surface of the solution-melt in the center of the crucible , the search for equilibrium temperature and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements , the time switching of which provides control of the heat fluxes in the melt from the heating elements of the growth furnace, crystal growth is carried out with a decrease in temperature in the lower zone relative to the middle and upper zones so that at the end of the growth cycle the temperature interval of the lower zone decreases by 5-10% , and when the shutdown time of 2, 3, 6, and 7 heating elements of the middle and lower zones located in the crystallographic direction “b” from 5 to 7 seconds and from 20 to 23 seconds increases with growth Naturally, with a simultaneous additional reduction in the shutdown time of 1, 4. 5 and 8 heating elements of the middle and lower zones located in the crystallographic direction “a” from 5 to 3 seconds and from 20 to 17 seconds, respectively. Thus, the control of flows in the melt by a relative increase in the time of connecting the heating elements of the furnace along the directions "+ a" and "-a" with respect to the directions "+ b" and "-b" provides crystal growth under conditions of an asymmetric thermal field, which allows you to increase the radial temperature gradient along the direction "a" relative to the direction "b" with a total increase relative to the direction "c".

Another embodiment of the invention is that in order to increase the diameter of the nonlinear optical element for conversion to the third harmonic during the growing process, it is necessary to preferentially grow the crystal in the direction perpendicular to one of the faces (011), to do this, reduce the radial temperature gradient along the + b direction ", relative to the direction" -b ", that is, to provide preferential cooling in the furnace along the direction" + b ", relative to the direction" -b ". This effect is achieved by increasing the connection time of the heating elements of the furnace along the direction "+ b" with respect to the direction "-b". The crystal grown under such conditions will be asymmetric (skewed) with respect to the central axis in the plane perpendicular to the "a" direction. This result is achieved by the fact that in the method of growing a crystal of lithium triborate, including loading the initial charge into a growth crucible, melting and homogenizing it, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with the surface of the solution melt in the center of the crucible, the search for equilibrium temperature and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight by heating elements, the time switching of which provides control of the heat fluxes in the melt, crystal growth is carried out with a decrease in temperature in the lower zone relative to the middle and upper zones so that at the end of the growth cycle the temperature interval for lower zone decrease increases by 5-10%, and when the time of switching off 5 and 8 heating elements of the lower zone changes from 30 to 35 seconds, 6 and 7 from 30 to 37 seconds, 1 and 4 from 30 to 27 seconds and 2 and 3 from 30 to 23 seconds during a change in the growth time.

Along with the cylindrical shape of LBO nonlinear optical elements for converting wide-aperture laser beams when the diameter is much greater than the thickness, nonlinear optical elements from a LBO crystal in the form of a rectangular parallelepiped are often used to convert to the second harmonic under conditions of the so-called non-critical phase matching with increased temperature. The orientation of the element in this case coincides with the crystallographic direction “a” or [100]: (θ = 90 °, ϕ = 0 °). The length of the element, as a rule, significantly exceeds the size of its aperture, i.e. crystals with a large size in the "a" direction are required, up to 100 and more mm. This effect can be achieved by slower cooling of the lower zone relative to the middle and upper, as well as by a relative reduction in the time of connecting the heating elements of the furnace along the directions "+ a" and "-a" with respect to the directions "+ b" and "-b". Thus, another embodiment of the invention is that to increase the length of the nonlinear optical element for conversion to the second harmonic under conditions of the so-called uncritical phase matching at elevated temperature, in a method for growing a lithium triborate crystal, including loading the initial charge into a growth crucible , its melting and homogenization, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with by the temperature of the solution-melt in the center of the crucible, the search for equilibrium temperature, and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements, the time switching of which ensures control of heat fluxes in the melt, crystal growth lead at lower temperatures in the lower zone relative to the middle and upper zones in such a way that at the end of the growth cycle, the temperature interval for lowering the lower zone decreases by 5-10% and and increase in the process of increasing the shutdown time of 1, 8, 4 and 5 heating elements of the middle and lower zones located in the crystallographic direction "a" from 6 to 11 seconds and from 30 to 37 seconds, respectively, and while reducing the shutdown time 2 , 3. 6 and 7 heating elements of the middle and lower zones located in the crystallographic direction "b" from 6 to 1 second and from 30 to 23 seconds, respectively.

In FIG. 1 is a structural diagram of a growth furnace heating furnace described in J. Crystal Growth, vol. 360, 2012, pp. 158-161 and used in the invention.

In FIG. Figure 2 shows the projection of an LBO crystal grown under a symmetric thermal field (according to the conditions of the well-known solution: J. Crystal Growth, vol. 360, 2012, pp. 158-161) in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements (1-8) in a platinum crucible (9) from a solution-melt (10). The ratio of crystal sizes in the directions "a" and "b" is close to 1.14, which corresponds to the ratio of the unit cell parameters in these directions. The size of the crystal along the c direction depends on the axial gradient. The orientations of nonlinear optical elements for converting a wavelength of 1064 nm into the second (532 nm: type I phase matching type; orientation: θ = 90 °, ϕ = 11.4 °) (11), and the third (533 nm: type II type synchronization are also presented; orientation: θ = 42.6 °, ϕ = 90 °) (12) harmonics, to the second harmonic of non-critical phase matching at elevated temperature (orientation: θ = 90 °, ϕ = 0 °) (13) and the location (cutting) of the corresponding nonlinear optical elements in the form of a cylinder and a rectangular parallelepiped in the volume of the grown crystal.

In FIG. 3 shows projections of a crystal of the same volume as the crystal in FIG. 2 with an increased size of the nonlinear optical element to the 2nd harmonic. Projections of an LBO crystal grown in an asymmetric thermal field under the cutting of a nonlinear optical element under the 2nd harmonic. The ratio of the dimensions in "a" and "b" is close to 1. The inscribed nonlinear optical element under 2ω (Type I, θ = 90 °, ϕ = 11.4 °). The designations are the same as in FIG. 2.

In FIG. 4 shows projections of a crystal of the same volume as the crystal in FIG. 2 with an increased size of the nonlinear optical element under the 3rd harmonic. The projections of an LBO crystal grown in an asymmetric thermal field under the cutting of a nonlinear optical element to the 3rd harmonic are the predominant crystal growth in the [001] crystallographic direction. An inscribed nonlinear optical element under 3ω (Toure II, θ = 42.6 °, ϕ = 90 °). The designations are the same as in FIG. 2.

In FIG. 5 shows a projection of a crystal of the same volume as the crystal in FIG. 2 with an increased length of the nonlinear optical element to the second harmonic of non-critical phase matching. Projections of an LBO crystal grown in an asymmetric thermal field under the cutting of a nonlinear optical element at 2 ° C under conditions of the so-called non-critical phase matching at elevated temperature. The ratio of "a" to "b" is close to 1.33. The orientation of the inscribed nonlinear optical element coincides with the crystallographic direction “a” or [100]: (θ = 90 °, ϕ = 0 °). The designations are the same as in FIG. 2.

In FIG. Figure 6 shows graphs of temperature decrease (a, b) and change in the switching time of heating elements (s, d)

In FIG. Figures 7a and b show a photograph of an LBO crystal grown in a symmetric thermal field (according to the conditions of the known solution) with a ratio of crystal sizes in directions "a" and "b" close to the ratio of unit cell parameters 1.14 in Example 1, table. 1, No. 1.

In FIG. 8 a, b are photographs of an LBO crystal grown in an asymmetric thermal field (according to the present method) with a ratio of crystal sizes in directions "a" and "b" close to unity, as in example 2, table. 1, No. 2.

In FIG. 9 a, b show photographs of a LBO crystal grown in a symmetric thermal field according to the conditions of a known solution (J. Crystal Growth, vol. 360, 2012, pp. 158-161), according to Example 3, table. 2, No. 1.

In FIG. 10 a, b, c presents photographs of a crystal grown in an asymmetric thermal field (according to the present method) with predominant growth of the crystal in the crystallographic direction [001], as in example 4, table. 2, No. 2.

In FIG. 11a, b are photographs of a crystal grown in an asymmetric thermal field (according to the present method) with predominant growth of the crystal in the direction "a", as in example 5, table. 2, No. 3.

The growth of lithium triborate crystals (LiB ₃ O ₅ ) under conditions of symmetric thermal fields (according to the method of the known solution) is demonstrated in Examples 1 and 3, and in conditions of asymmetric thermal fields (according to the three claimed methods), examples 2,4,5.

Example 1. In a platinum crucible with a diameter of 140 mm load the mixture to obtain 3 kg of the finished melt. The ratio of the components of the flux 2Li ₂ O: 3B ₂ O ₃ : 3MoO ₃ and the amount of the melt solution allow to grow crystals depending on the temperature range weighing from 600 to 700 g. After homogenization of the melt solution for 5-7 days, the temperature in the furnace is reduced to values are 10-15 degrees higher than the expected temperature of the equilibrium point and establish the necessary ratio between the temperatures of the upper, middle and lower zones. The switching times of the heating elements of the middle and lower zones, selected experimentally, are 5 and 20 seconds, respectively. By adjusting the switching time of the heating elements, the point with the lowest temperature, “cold point”, is moved to the center of the crucible. Then, the seed fixed in the crystal holder and oriented relative to the heaters of the growth furnace is lowered into the furnace, as shown in FIG. 2. The seed is brought closer to the surface of the solution-melt and the equilibrium temperature of the onset of growth is determined by melting the seed after several touches of the surface of the melt and lowering the temperature, depending on the amount of fusion of the seed crystal. After finding the equilibrium temperature, the crystal begins to grow. The graph of the temperature reduction rate is determined experimentally and is presented in FIG. 6a. The switching times of the heating elements, taking into account the corrective additives, remain unchanged throughout the growing process. At the end of the growth process, the crystal is raised above the melt and cooled at a rate of 15 deg / h to room temperature.

An LBO crystal grown in a symmetric thermal field according to the known method (Table 1, No. 1) with a ratio of crystal sizes in directions "a" and "b" close to the ratio of unit cell parameters in these directions ("a" / "b" = 1.14), shown in FIG. 7a, b. From such a crystal it is possible to make (cut out) a nonlinear optical element for the 2nd harmonic with a diameter of 30 mm and a thickness of 12 mm (Fig. 2).

Example 2. LBO crystals were grown from a melt solution weighing 3 kg at the ratios of the flux components specified in Example 1. The growth procedure to find the equilibrium temperature corresponds to that described in Example 1. Crystal growth was carried out at lower temperatures in the lower zone according to the program (1) and in the upper and middle zones according to the program (2) (Fig. 6b), so that at the end of the growth cycle, the temperature interval of the lower zone decrease increases by 5-10%. In the process of lowering the temperature, an additional adjustment of the switching time of the heating elements (Fig. 6c, dashed lines) is also performed by turning off 2, 3, 6 and 7 heating elements of the middle and lower zones located in the crystallographic direction "b" from 5 to 7 seconds and s 20 to 23 seconds, respectively, while simultaneously reducing the shutdown time of 1, 4, 5, and 8 heating elements of the middle and lower zones located in the crystallographic direction a from 5 to 3 seconds and from 20 to 17 seconds, respectively of course.

An LBO crystal grown in an asymmetric thermal field according to the first declared embodiment (Table 1, No. 2) is shown in FIG. 8a, b. It is possible to make a nonlinear optical element from such a crystal under the 2nd harmonic with a diameter of 40 mm and a thickness of 12 mm (Fig. 3).

Example 3. In a platinum crucible with a diameter of 180 mm load the mixture to obtain 7.5 kg of the finished melt. The ratio of the components of the flux 2Li ₂ O: 3B ₂ O ₃ : 3MoO ₃ and the amount of the melt solution allow to grow crystals weighing from 1350 to 1450 g. The procedures for preparing the melt, seeding and growing are shown in example 1. Temperatures in three zones are reduced in accordance with the program shown in FIG. 6a (2), and the switching time of the heating elements of the middle and lower zones, selected experimentally, is 6 and 30 seconds, respectively.

An LBO crystal grown by a known method (Table 2, No. 1) in a symmetric thermal field with a ratio of crystal sizes in the directions "a" and "b" close to the ratio of unit cell parameters ("a" / "b" = 1.14) shown in FIG. 9 a, b. It is possible to make a nonlinear optical element from a crystal under the 3rd harmonic with a diameter of 55 mm and a thickness of 12 mm (Fig. 2).

Example 4. LBO crystals were grown from a melt solution weighing 7.5 kg at the ratios of the flux components specified in Example 3. The growth procedure until the search for equilibrium temperature corresponds to that described in Example 1. Crystal growth was carried out at lower temperatures in the lower zone according to the program ( 3) and with a decrease in temperature in the upper and middle zones according to the program (4) (Fig. 6b) so that at the end of the growth cycle, the temperature interval for lowering the lower zone increases by 5-10%. In the process of lowering the temperature, an additional adjustment of the switching time of the heating elements (Fig. 6a) is also carried out by changing, in the process of increasing the time, the shutdown of the corresponding heating elements of the lower zone, namely, 5 and 8 from 30 to 35 seconds, 6 and 7 from 30 to 37 seconds , 1 and 4 from 30 to 27 seconds, 2 and 3 from 30 to 23 seconds.

The LBO crystal grown in an asymmetric thermal field according to the second claimed embodiment (Table 2, No. 2) is shown in FIG. 10a, b, p. It is possible to make a nonlinear optical element from a crystal under the 3rd harmonic with a diameter of 65 mm and a thickness of 12 mm (Fig. 4).

Example 5. The growth of LBO crystals was carried out from a melt solution weighing 7.5 kg at the ratios of the components of the flux specified in example 1. The growing procedure to find the equilibrium temperature corresponds to that described in example 1. Crystal growth is carried out at lower temperatures in the upper and middle zones program (4) and lowering the temperature in the lower zone according to program (5) (Fig. 6b) so that at the end of the growth cycle, the temperature interval for lowering the lower zone is reduced by 5-10%. In the process of lowering the temperature, an additional adjustment of the switching time of the heating elements (Fig. 6c, solid lines) is also made by increasing the shutdown time of 1, 8, 4, and 5 heating elements of the middle and lower zones located in the crystallographic direction “a”, c 6 to 11 seconds and from 30 to 37 seconds, respectively, and while reducing the shutdown time of 2, 3, 6 and 7 heating elements of the middle and lower zones located in the crystallographic direction "b" from 6 to 1 second and from 30 to 23 seconds, respectively.

An LBO crystal grown in an asymmetric thermal field with predominantly crystal growth in the direction “a” according to the third claimed embodiment (Table 2, No. 3) is shown in FIG. 11 a, b. It is possible to make a nonlinear optical element in the form of a rectangular parallelepiped for the 2nd harmonic for noncritical phase matching with a length of 100 ÷ 120 mm and an aperture size of 15 × 15 ÷ 12 × 12 mm from this crystal (Fig. 5).

Claims (3)

1. A method of growing a crystal of lithium triborate, including loading the initial charge into a growth crucible, melting and homogenizing it, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with the surface of the solution-melt in the center of the crucible, search for equilibrium temperature and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements, switching in time which provides control of convective flows in the melt, characterized in that the crystal growth is carried out at lower temperatures in the lower zone relative to the middle and upper zones so that at the end of the growth cycle the temperature interval for lower zone decrease increases by 5-10% with an increase in the process of increasing the shutdown time of 2, 3, 6 and 7 heating elements of the middle and lower zones located in the crystallographic direction "b", from 5 to 7 seconds and from 20 to 23 seconds, respectively, while additional reduction of the shutdown time of 1, 4, 5 and 8 heating elements of the middle and lower zones located in the crystallographic direction "a" from 5 to 3 seconds and from 20 to 17 seconds, respectively. 2. A method of growing a crystal of lithium triborate, including loading the initial charge into a growth crucible, melting and homogenizing it, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with the surface of the solution-melt in the center of the crucible, search for equilibrium temperature and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements, switching in time which provides control of convective flows in the melt, characterized in that the crystal growth is carried out at lower temperatures in the lower zone relative to the middle and upper zones so that at the end of the growth cycle, the temperature interval for lowering the zone increases by 5-10% with a change in the process of increasing the shutdown time of the heating elements of the lower zone 5 and 8 from 30 to 35 seconds, 6 and 7 from 30 to 37 seconds, 1 and 4 from 30 to 27 seconds, 2 and 3 from 30 to 23 seconds. 3. A method of growing a lithium triborate crystal, including loading the initial charge into a growth crucible, melting and homogenizing it, bringing the point with the lowest temperature on the surface of the solution-melt to the center of the crucible, introducing an oriented seed crystal into contact with the surface of the solution-melt in the center of the crucible, search for equilibrium temperature and subsequent crystal growth with a decrease in temperature in a three-zone growth furnace, the middle and lower zones of which consist of eight heating elements, switching in time which provides control of convective flows in the melt, characterized in that the crystal growth is carried out at lower temperatures in the lower zone relative to the middle and upper zones so that at the end of the growth cycle the temperature interval for lower zone reduction decreases by 5-10% with an increase in the process of increasing the shutdown time for 1, 8, 4 and 5 heating elements of the middle and lower zones located in the crystallographic direction “a” from 6 to 11 seconds and from 30 to 37 seconds, respectively, and at the same time m reducing the shutdown time of 2, 3, 6 and 7 heating elements of the middle and lower zones located in the crystallographic direction "b", from 6 to 1 second and from 30 to 23 seconds, respectively.

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