ANOMALOUS BIREFRINGENCE IN ALUMS ISOMORPHIC MIXTURES. A. Shtukenberg, O. Kovalev, Yu. Punin (St.Petersburg Univ., Russia)

Anomalous optical behavior is known for many natural and synthetic crystals. Their optical properties have strong connection with growth kinetics but their nature is vague. The aim of this work is study of optical anomalies in perfect isomorphic mixtures.

The system investigated were three perfect isomorphic rows of alums. The crystals have been grown from aqueous solutions. Unlike the end members of isomorphic rows the mixed crystals display lower symmetry of optical properties then original ones (space group Pa3), i.e. dissymmetryzation take place. In one crystal different growth sectors have different optical indicatrix orientation and birefringence. In <111> sectors optical indicatrix is almost unaxial with main axis nearly parallel to growth direction. Connection between indicatrix orientation and growth hillocks exist. The dependence of birefringence on the composition of crystals is parabolic with maximum (8.10^-5) at the middle of the rows. In <100> and <110> growth sectors indicatrix orientation has complicated character and birefringence attain 1.10^-5. Calculations have shown that lattice misfit strains have no contribution to optical anomalies. These anomalies may be connected with ordering of isomorphic ions, because of nonequivalence of crystallographically equivalent lattice positions on growth surface. We have been investigated the influence of growth conditions on optical anomalies. Increasing of growth temperature leads to birefringence decreasing. High temperature annealing (T=20-82 ^deg.C, T_melt=95 ^deg.C) have shown decreasing of birefringence. The activation energy is about 118+/-7 KJ/Mole. These facts corresponds to cations ordering mechanism. X-ray structure refinement is carried out now. Preliminary data (growth sector <111>) suggest symmetry decreasing to monoclinic (space group Pc) or even lower.