Method of growing lithium-magnesium molybdate monocrystals

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of growing lithium-magnesium molybdate Li2Mg2(MoO4)3 crystals. The method involves melting lithium-magnesium molybdate in a molten solvent, crystallising while cooling the melt and cooling the grown crystals, wherein the solvent used is lithium molybdate Li2MoO4 with molar ratio of lithium-magnesium molybdate to lithium molybdate Li2MoO4 of 2:3, respectively; crystallisation is carried out on an inoculating crystal revolving at a rate of 35 rpm, oriented on the [010] direction, rate of drawing rate of the inoculating crystal of 1-3 mm/day while simultaneously cooling the melt at a rate of 0.2-5 degrees/day and then separating the grown crystals from the melt and cooling at a rate of 30 degrees/hour.

EFFECT: method enables to obtain optically homogeneous lithium-magnesium molybdate crystals which do not contain inclusions, blocks and cracks.

 

The invention relates to the field of chemical technology, namely the growing of single crystals, and the receipt of large optically homogeneous crystals of lithium-magnesium molybdate Li2Mg2(MoO4)3.

A method of obtaining fine particles of a lithium-magnesium molybdate Li2Mg2(MoO4)3by spontaneous crystallization (Sebastian L., Y. Piffard, A.K. Shukla, F. Taulelle, Gopalakrishnan, J. Synthesis, structure and lithium-ion conductivity of Li2-2xMg2+x(MoO4)3and Li3M(MoO4)3(MIII=Cr, Fe), Journal of Materials Chemistry (2003), 13, 1797-1802). The crystals obtained from the melt of a mixture of Li2CO3, MgO Moo3and NH4F, with Li2CO3and NH4F take a 10% excess (by weight). The reaction is carried out at 750 for 18 hours and then gradually cooled down to room temperature (cooling rate is not specified). Get small crystals (0.170.0450.045 mm

Lithium-magnesium molybdate, Li2Mg2(MoO4)3that melts with decomposition at 1060C and therefore can not be obtained in the form of bulk homogeneous crystals conventional Czochralski method. A method of obtaining crystals of lithium-magnesium molybdate Li2Mg2(MoO4)3by spontaneous crystallization from the melt of a mixture of Li2Mo2O7and Li2Mg2(MoO4)3taken in the rate of 2:1 (Vgentoo, Pavlecov, "Synthesis of crystals of double molybdates lithium with divalent metals of Mg, Ni, Co and Zn", J. of neorganic. chemistry. 22 (1977) 1713-1715), selected as a prototype. Spontaneous crystallization of Li2Mg2(MoO4)3conducted by lowering the temperature of the solution-melt with the speed of 3-5 deg/HR in the temperature range from 1000 to 930 degrees. The size of the crystals in a length of 5-7 mm

These methods get small crystals unsuitable for practical application.

The objective of the invention is to increase the size of the crystals of lithium-magnesium molybdate, preserving their optical quality.

This object is achieved in that in the method of growing crystals of lithium-magnesium molybdate from the solution in the melt comprising a molten lithium-magnesium molybdate in the melt solvent, crystallization during cooling of the melt and cooling the grown crystals in the solvent used molybdate lithium Li2MoO4when the molar ratio of lithium and magnesium molybdate and lithium molybdate Li2Moo4equal to 2:3, respectively, crystallization lead to rotating a seed crystal at a speed of 35 rpm, oriented in the direction [010], the rate of withdrawal of the seed from 1 to 3 mm/day while cooling the melt at a speed ranging from 0.2 to 5 deg/day, and will follow them Department of the grown crystals from the melt and cooled at a speed of 30 deg/hour thus, pulling of the seed are with automatic weight control.

Distinctive features of the proposed method are:

as the solvent used molybdate lithium Li2MoO4,

- the ratio of the lithium-magnesium molybdate and lithium molybdate Li2MoO4,

rotation of the seed and its orientation in the direction [010],

- speed drawing a dose from 1 to 3 mm/day,

the cooling rate of the melt from 0.2 to 5 deg/day,

- rotation starters with a speed of 35 rpm,

- cooling crystals with a speed of 30 deg/h

the process is carried out by automatic weight control.

The use of such parameters cultivation allows you to get large optically homogeneous single crystals of lithium-magnesium molybdate Li2Mg2(MoO4)3.

The molar ratio of the components of Li2Mg2(MoO4)3and Li2MoO4equal to 2:3. The choice of this molar ratio of the components of the system caused by the optimal modes of operation of the equipment and energy consumption.

The choice of lithium molybdate due to the fact that when using this solvent are more homogeneous, with no inclusions crystals (best quality).

Optimal conditions for crystal growth: the rate of extraction of seed from 1 to 3 mm/day obul who go to those the pulling of the seed crystal when the crystal is grown at a faster speed than 3 mm/day, does not match the speed stable homogeneous crystal growth under these conditions. The decrease in the rate of withdrawal is less than 1 mm/day is impractical because it leads to an increase in process time. The rotation of the seed crystal at a given speed (35 rpm) promotes uniform growth, thus avoiding the occurrence of defects in the crystal that affect its optical properties.

Cooling the melt at a speed ranging from 0.2 to 5 deg/day due to the fact that the decrease of the cooling rate is less than 0.2 deg/day at the beginning of the process leads to the decrease of mass crystallization rate, reduction of the sizes of the grown crystal and increase the process time. Increasing the cooling rate is above 5 deg/day in the end leads to the formation of concentration overcooling and, as a consequence, the capture of the solvent, the formation of blocks and other defects.

The orientation of the seed crystal in the direction [010] provides under these conditions the formation of the most homogeneous crystals compared to other crystallographic orientations.

Automatic weight control allows you to monitor the growth process all over crystal growth. The grown crystals is then gradually cooled, th is would not happen shrinkage, and the rate of cooling depends on their size.

Example.

In a platinum crucible with a diameter of 70 mm and a height of 120 mm was placed a mixture of compounds of Li2Mg2(MoO4)3and Li2MoO4synthesized in a known manner by means (solid phase synthesis) of Li2CO3, MgO Moo3while Li2Mg2(MoO4)3- 130,15 g and Li2MoO4- 62,57 g, or in the ratio 2:3, which corresponds to the concentration of the solution-melt 40 mol.%. The mixture is melted at 1000C in air in a resistive furnace installation for growing crystals. For homogenizing the solution-melt is stirred platinum stirrer, the temperature was then reduced until the point of equilibrium crystal with a solution-melt for a given concentration of Li2Mg2(MoO4)3(994C) and to the surface of the melt down of a rotating (35 rpm) of the seed crystal which is oriented in the direction [010].

After establishing the temperature at which there is visible growth of the seed, carry out the pulling of the seed at a rate of 1-3 mm/day, at the same time lower the temperature of the solution-melt with an initial velocity of 0.2 to 5 deg/day.

In the growth process by increasing the mass of the crystal speed pulling smoothly reduced to 1 mm/day, and a cooling rate of 5 deg/day in accordance with the schedule solubility crystal is s Li 2Mg2(MoO4)3in the melt Li2MoO4.

30 days grow the single crystal of lithium-magnesium molybdate weighing 80 g dimensions: length (cone + cylinder) to 45 mm and diameter up to 25 mm optical quality.

At the end of the process of growing the single crystal is separated from the solution-melt and cooled to room temperature at a speed of 30 deg/hour.

The optical quality of the grown crystals was determined visually under a microscope. In a crystal with inclusions of other phases not identified blocks and other defects.

Thus, the proposed method allows to obtain optically homogeneous crystals of lithium-magnesium molybdate, Li2Mg2(MoO4)3not containing inclusions, blocks and cracks with dimensions 2545 mm, sufficient to study the physical properties and practical use.

1. The method of growing crystals of lithium-magnesium molybdate from the solution in the melt comprising a molten lithium-magnesium molybdate in the melt solvent, crystallization during cooling of the melt and cooling the grown crystals, characterized in that the solvent used molybdate lithium Li2MoO4when the molar ratio of lithium and magnesium molybdate and lithium molybdate Li2MoO4equal to 2:3, respectively, lead to crystallization of rotations is muusa the seed to a speed of 35 rpm, oriented in the direction [010], the rate of withdrawal of the seed from 1 to 3 mm/day while cooling the melt at a speed ranging from 0.2 to 5 deg/day and the subsequent separation of the grown crystals from the melt and cooled at a speed of 30 deg/h

2. The method according to claim 1, characterized in that the pulling of the seed are with automatic weight control.



 

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