Growth mechanism and crystal form of potassium dichromate

The structure of potassium dichromate (<P(1)over bar>, Z = 4) consists of t wo layers parallel to (001), between which no crystallographic symmetry rel ation exists. These layers are nearly coincident with each other when rotat ed through 90 degrees about c*. One question remained unanswered: do the cr ystals grow parallel to (001) with d(002) or d(001) layers? The pseudo-four fold axis does not run parallel to a crystallographic direction and acts on ly between two layers. This means that crystal growth parallel to {001} occ urs in d(001) layers, which corresponds to the occurrence of one-sided inte rgrowths on (00(1) over bar). The layers are also nearly coincident with ea ch other through pseudo-twofold axes parallel to [110] and [(1) over bar 10 ]. These axes do not act in the (100) or (010) projections. In the projecti ons along the [110] and [(1) over bar 10] directions, the lattice direction s are then extended by a factor of approximately root 2, and the formula un its in the cell of the d(002) layer are doubled. This indicates a further p ossible growth mechanism. The theoretical growth forms of the crystals were calculated with the Fourier transform method of morphology.