Behaviour of steps on the (001) face of K2Cr2O7 crystals

The behaviour of (100) steps, originating from spiral centres, on the (001) face of potassium bichromate (K2Cr2O7) crystals growing from aqueous solutions has been studied by in situ and ex situ optical microscopy as well as by ex situ atomic force microscopy. After considerations concerning the centrosymmetry of K2Cr2O7, the (001) and (001) faces are discriminated and a general view of the (001) surface morphology is given. The bunching, blocking and velocity of the (100) steps as a function of supersaturation have been measured and are discussed in terms of models for step propagation influenced by time-dependent impurity adsorption. Blocking is observed for both growth and dissolution at super (under) saturations below a critical value. By atomic force microscopy steps of half, one and several unit cells height are measured. For the supersaturation region where blocking occurs cusps are found in the steps, which correspond to the sites of the impurities that act as pinning points hindering step propagation. Bunching of lower steps to macrosteps and a proportional dependence of the mutual separation of these steps on their distance from the growth centre is found for supersaturations above the critical value for blocking.