Growth Mechanism and Crystal Form of Potassium Dichromate
The structure of potassium dichromate (P1, Z = 4) consists of two layers parallel to (001), between which no crystallographic symmetry relation exists. These layers are nearly coincident with each other when rotated through 90° about c * . One question remained unanswered: do the crystals grow parallel to {001} with d(002) or d(001) layers? The pseudo-fourfold axis does not run parallel to a crystallographic direction and acts only between two layers. This means that crystal growth parallel to {001} occurs in d(001) layers, which corresponds to the occurrence of one-sided intergrowths on (001). The layers are also nearly coincident with each other through pseudo-twofold axes parallel to [110] and [110], These axes do not act in the (100) or (010) projections. In the projections along the [110] and [110] directions, the lattice directions are then extended by a factor of approximately √2, and the formula units in the cell of the d(002) layer are doubled. This indicates a further possible growth mechanism. The theoretical growth forms of the crystals were calculated with the Fourier transform method of morphology.
Crystal morphology of sucrose influenced by rotation axes parallel to growth planes
Three different types of growth forms of sucrose (P21) were found by calculating with the Fourier transform method of crystal morphology. The observed central distances of the (100) and (001) faces are smaller than those calculated. It will be shown that the two-fold screw axis, which runs parallel to these faces, influences the rate of growth. The effectiveness of these symmetry elements is relative to the rotation angle around the face normal.
Calculated bulk and surface properties of sulfates
Atomistic simulation techniques are used to model a range of sulfates. Two widely different sets of potentials have been developed. The first is based on shell model, electron-gas potentials; the second is a rigid ion model which treats inter- and intra-molecular forces differently. The success of the potential models has been demonstrated by comparing calculated and experimental lattice parameters and elastic constants. The structures and energetics of surfaces of barite (BaSO4) are examined in detail, allowing for the effects of surface relaxation. The two lowest-energy surfaces are {001} and {210} which dominate the calculated equilibrium morphology.