COMPOSITIONAL INHOMOGENEITY AND IMPERFECTION OF CRYSTALS OF K₂SO₄- (NH₄)₂SO₄ SOLID SOLUTIONS

Yu. Punin, O. Smetannikova, A. Zhogina, G. Demidova.

Dept. of Crystallography, Saint Petersburg State University, Universitetskaya nab. 7/9 199034 Saint Petersburg, Russia E-mail olga@cryst.geol.pu.ru

Keywords: crystals, crystal growth, solid solution, crystal imperfections, isomorphism, optical properties, crystal habitus, twinning.

Complete crystallographic study of (K,NH₄)₂SO₄ - mixed crystals was performed. Ternary phase diagram K₂SO₄-(NH₄)₂SO₄-H₂O reveals two anomalous regions at 8-12 and 50-60 mol% (NH₄)₂SO₄ in solid, where solidus and solutus lines respectively have singularities. At these compositions the solid solution crystals change radically their habitus and optical properties, and display abnormal behaviour of unit cell dimensions in relation to crystal composition.

Computer simulation of diffraction patterns was carried out for various distributions of K⁺ and NH₄⁺ ions over two nonequivalent lattice positions. Comparision of calculated and experimental data allows to propose the cation ordering, when (NH₄)₂SO₄-content in crystal exceeds 60 mol%. The origin of the phase transition at 10 mol% (NH₄)₂SO₄ is not yet understood.

The mixed crystals of (K,NH₄)₂SO₄ display sectorial inhomogeneity of birefringence which was proved to be induced by constitutional sector zoning. This inhomogeneity occurs only in 10- 60 mol% (NH₄)₂SO₄ compositional range - the range with disordered state of solid solution. The compositional difference in adjacent growth sectors leads to lattice mismatch and in consequence to high inner stress exceeding the breaking strength of material. It results in intensive fracture of crystals, the arrangement and orientation of cracks being in agreement with estimates of tensile stress distribution over the crystal.

The consequence of growth strains is autodeformational twinning of mixed crystals. There was found gradual transition from deformation twin lamellae through autonomous twin subunits to complex penetration twins. The probability of twinning as well as the complexity of twin appregates are maximal in the middle of isomorphous seria, in the region of maximal inner stress in crystals. This, together with morphological data, confirms the autodeformational origin of growth twins of (K,NH₄)₂SO₄ - mixed crystals.

This work was supported by RFBR, project Ľ 98-05-64151.