CRYSTAL STRUCTURE AND DEFECTS IN CZOCHRALSKI- GROWN LiCaAlF₆. D. Klimm, K. Seiranian, P. Reiche, Institut für Kristallzüchtung, Rudower Chaussee 6, Geb. 18.46, D-12489 Berlin, Germany

Two colquiriite type fluorides, Cr³⁺:LiCaAlF₆ and Cr ³⁺:LiSrAlF₆, were reported as new tunable laser hosts (Lin + Keszler, Chem. Mater. 4 (1992), 645). The Cr³⁺ substitutes Al ³⁺ ions isomorphic, and thus very high doping levels can be obtained. Moreover, the segregation coefficient k=c_sol/c_liqu for the lasing ion Cr³⁺ is close to 1, resulting in a homogeneous Cr distribution along the length of the growing crystal. Accordingly, both substances are suspected to have considerable advantages over conventional high power dye lasers.

Many authors reported on the occurrence of scattering centers in the Czoch ralski (CZ) grown crystals, that reduce the lasing efficiency. Usually these particles are attributed to CrO_x resulting from the moisture contained in the ambient atmosphere reacting with the CrF₃ dopant during crystal growth.

This work reports on the automatizised CZ growth of Cr³⁺:LiCaAlF₆ single crystals. Crystal growth was performed with RF heating using Pt crucibles under flowing nitrogen atmosphere (<10 ppm H₂O). The quality of the boules was characterized by light microscopy and by means of EDX and X-ray topography. As usual, the near surface region of the crystals turns out to be free from scattering centers. Scattering centers in the interior of the crystals are investigated by transmission electron microscopy and by positron annihilation, respectively, and are discussed in terms of extrinsic (CrO_x ) as well as intrinsic defects. The relation between the ideal structure and the defect structure of the colquiriite type crystals is discussed.

This work was sponsored by BMBF (contract 624-4013-13N6750/2).