Growth Mechanism and Crystal Form of Potassium Dichromate

The structure of potassium dichromate (P , 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 (00 ). The layers are also nearly coincident with each other through pseudo-twofold axes parallel to [110] and [ 10]. These axes do not act in the (100) or (010) projections. In the projections along the [110] and [ 10] directions, the lattice directions are then extended by a factor of approximately , 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.