High precision lattice parameter determination of KDP with different crystal perfection

Abstract

Potassium dihydrogen phosphate (KDP) single crystals are important because of their laser applications (LYUTSKANOV et al.), optical activities (GILVARG, KOLESHOV), electrooptical effect (BELOUET 1976) and pyroelectric properties (HADNI et al.). The application in optics and quantum electronics requires optically homogeneous crystals. Several authors (LUTSAU et al.; MELLE, MEYER; BEL~UET 1981) have reported their studies of perfection and other aspects of KDP crystals. The purpose of this paper is to investigate the real structure of KDP with different crystal perfection and to oppose the induced birefringence in the crystal.The investigations were carried out on a Z-cut 2 mm thick crystal plate. The surface of the cjstal plate was polished and cleaned. The applied experimental methods are the precision lattice parameter determination, X-ray topography, X-ray microanalysis and the measurement of the birefringence.The precision measurement of the lattice parameter was carried out according to the "Bond-method" (BOND) with the measuring device which has been established at the Sektion Physik of the Friedrich-Schiller-Universitat Jena represented by GroDwig (GROSSWIG et al.). The symmetrical (008) reflection was used to determine the c-lattice parameter. Optimized conditions of measuring (GROSSWIG et al.) guarantee a relative error less than 1.5 x The main part of this error is due to the great influence of the temperature variations because of the large linear thermal expansion coefficient of KDP. A temperature variation of T = 0.02 K causes a relative lattice parameter change of about 1 -X-ray topography studies were carried out by the "Lang-method" (LANG) in transmission case with MoK,-radiation using the (200)-and the (020) reflection. The topographs were taken by an X-ray generator RU 200 Z from Rigaku Denki and on Ilford L4 plates with emulsion thickness 50 microns. The Burgers vector direction of the dislocations and the dislocations density were determined from the topographs (MELLE, MEYER).X-ray microanalysis (MOENKE) was carried out by the electron beam microprobe unit JEOL JXA-5OA which has been established at the Institute of Physics of the Polish Academy of Sciences (IFPAN) Warsaw to determine the impurity content of the most important impurities in KDP crystals: Ca, Cr, Fe and A1 (MULLIN et al. ; BELOUET et al.). This device allows to determine the impurity content with a detection sensitivity of lo-'* g/cm3. The investigated volume of the crystal specimen comes out to about lo-' cm3. Therefore a local investigation of the impurity content is possible. The measuring result is the ratio of the magnitude of the analysed line to that of the noise. The amount of the signal-to-noise ratio of the analysed line is proportional to the impurity content. A signal-to-noise ratio equal or less than one means that an indication within the detectable limit is not possible.Perfect KDP crystals show no birefringence in [001]direction, but in practice the crystal imperfection causes a birefringence also in [Ool] direc...