SU953018A1 - Method of growing colorless lead molybdate single crystals - Google Patents

Description

(54) METHOD OF CULTIVATION OF COLORLESS MONO-CRYSTAL CHOV

LEAD MOLIBDAT

one

This invention relates to a process for making colorless single crystals of lead molybdate of chemical composition PbMoO.

Along with using as an optical medium, PbMoO single crystals are used because of their very good 5 effects of interaction between acoustic waves and electromagnetic radiation in the visible and infrared spectral ranges from 400 to 4000 in various ways in optical and acoustic-optical, Q-cic filter devices. , reflective and accumulating systems of high power and resolution.

It is known to create single crystals of lead molybdate by the Czochralski method by growing from a melt onto a rotating crystallographically oriented seed from a PbMoO melt. For these applications it is necessary to grow large single crystals with a diameter of about 30 mm and a length of 50 mm, with impeccable optical homogeneity and without internal stresses. These requirements are realized, according to the prior art, while observing the optimum ratios of the components of the mixture of starting materials PbO

and MoOS for the composition of a high-purity melt, as well as certain preferred orientations of the seed crystal under growing conditions, as they are known for the Czochralski method and followed by heat treatment at 800-900 ° C to eliminate stresses.

However, the disadvantage of this method is that the crystals obtained are more or less colored in color from yellow to orange. This color change is a consequence of broadband absorption with a maximum of 400-430 nm. It is attributed to the formation of Pb ions, which is enhanced in many cases by the accumulation of traces of impurities, such as Fe compounds (formation of Fe).

Until now, known methods for eliminating this disadvantage are limited to using PbO and MoOj as starting materials of very high purity (at least 99.99 / 0th) and in exactly the equimolar ratio of the components of the mixture in the melt composition.

However, until now it was impossible to completely prevent or eliminate these discolorations, since the present

reasons for the emergence of broadband absorption between L 400 and 500 nm by more high-quality metal ions in the crystal lattice (Pb Fe, etc.) did not eliminate this. As soon as an excess of PbO in the initial mixture attained color clarification, in any case, other defects, such as the formation of trash, opacities, and the formation of bubbles, simultaneously appeared.

Studies are known in which an excess of MoOj or PbO does not affect the color of PbMoO4 crystals.

The disadvantage of all these methods is unreliability.

Starting from observations of Pb ions causing color changes and other multi-charge ions as hole centers that arise depending on the nature of the heat treatment atmosphere, it was proposed to remove PbMoO single crystals by thermal treatment at low partial oxygen pressure to eliminate the interfering absorption region. or in an inert gas atmosphere.

Since the proposed method encompasses the elimination of color change only during the heat treatment, its scope is limited.

The purpose of the invention is not only to eliminate the occurrence of a color change in single crystals of PbMoO4, but already first to prevent it from changing the growing atmosphere.

In this connection, the aim is to prevent the broadband absorption region in PbMoO single crystals, the maximum of which lies within L 400-430 nm, which especially at large thicknesses of the layer reduces the permeability well below the theoretical value given by the refractive index. This improves the performance of the corresponding optical or acoustic-optical structural groups, especially if they are loaded with light, the wavelength of which lies between 400 and 500 nm, as is the case, for example, for an argon ion laser (L 488 nm).

The application of the invention excludes subsequent heat treatment.

In the proposed method, the initial components of PbO and MoO3 can deviate by ± IVo from an equimolar quantitative ratio for the composition of the melt and reduce the requirements for purity to at least 99.9% without detecting deficiencies in the final product.

The task of the invention is to create colorless PbMo04 single crystals without interfering absorbing layers in the region of 400-4000 nm. without lowering other valuable properties such as optical uniformity and lack of voltage. It is necessary to change the Czochralski method so as to prevent the possibility of oxygen exposure to the growing crystal. Considering the high vapor pressure at temperatures higher than 900 ° C and the evaporation losses in vacuum associated with this, the problem is solved by a method for creating colorless molybdate single crystals (PbMoO4) for optical and acoustic-optical purposes according to the Czochralski method, which is different in that Growing lead molybdate single crystals to prevent color changes and interfering absorption takes place in an atmosphere of a chemically inactive gas, such as nitrogen, inert gases, under a pressure of 500-760 torr.

In the drawing, the curves denote: curve 1 — the dependence of the absorption coefficient KI on the wavelength of an L PbMoO crystal created at the level of the technique; curve 2 is the dependence of the absorption coefficient K2 on the wavelength A of a BMo04 crystal produced according to the invention; curve 3 is the dependence of the decrease in the absorption coefficient of AK on the wavelength L when applying the proposed method in comparison with the level of the technique.

In the framework of the first stage of the process, the composition of the PbMoO4 melt is made of 99.9% purity in PBO and MoOj in a platinum crucible of 35 mm diaivfeTpoM and a length of 70 mm, equipped with a subsequent and lower preheater. In the next phase of growing from this PbMoO melt (melting point 1060-1065 ° C) using a crystallographically oriented PbMoO4 seed crystal rotating at 20 rpm, at a lifting speed of 4 mm / h in a nitrogen or inert gas atmosphere at 760 Torr PbMoO monocrystal is grown. After cooling at a rate of 8 ° C per hour between 1060 ° C and 900 ° C, and 20 ° C per hour between 900 ° C and 760 ° C, an optically homogeneous, stress-free, colorless PbMoO4 single crystal with a diameter of 25 mm and a length is obtained 60 mm.

If you choose a large platinum crucible (diameter 40-50 mm, length 70 mm), you can grow crystals of the same quality with a diameter of 30-35 mm and a length of 60-70 mm.

The efficiency of the proposed method can be seen from the absorption coefficient curves shown in the drawing, depending on the wavelength in the range from D 400 to 600 nm.

Claims (1)

The invention The method of growing colorless monocrystals of lead molybdate (PbMoO) according to the Czochralski method, characterized in that the cultivation is carried out in an inert gas at a pressure of 500-760 Torr. It is recognized as an invention according to the results of the examination carried out by the Office for the Invention of the German Democratic Republic. K (cM-V about -O.Z5-0 .5 М (cm-9 Q.15