THE NATURE OF ONE-SIDED INTERGROWTHS ON (00(1)OVER-BAR) OF TRICLINIC CENTROSYMMETRIC K(2)CR(2)O7(II)

SHUBNIKOV first observed in 1912 that centrosymmetric K2Cr2O7 crystals (<P(1)over bar>) exhibit a growth anomaly in highly supersaturated so lutions. The (001) faces of these crystals are always smooth, the para llel opposite faces are always rough. Electron micrographs show these rough surfaces to be crystallites that have grown in stacks. In Part I of this publication, the crystallite orientation was determined using electron-induced diffraction methods. As a rule, these crystallites a re parallel intergrowths. Only a small number of crystals near the sur face of the host crystal are twinned according to [010]. Part II sough t to ascertain the cause of these one-sided intergrowths. To determine the most favourable energetic arrangement of two intergrown individua ls, interactions across the intergrowth face as a function of shifts p arallel to (001) and to (00(1) over bar) were calculated. 64 theoretic ally possible cases were considered. Only in the [010]-twin and the se ldom-occurring [1(1) over bar0$]-twin is an approximation of the 2/m s ymmetry of the macroscopic twin arrangements also found in the intergr owth structure, and this is the case only when the (00(1) over bar) fa ces of the host and guest crystal are intergrown.