The mechanism of tapering on KDP-type crystals

The anomalous wedge-like shape of KH2PO4-type single crystals, also called tapering, is reconsidered. A reformulation of the problem is given on the basis of a detailed observation of the boundary between prismatic and pyramidal growth sectors by means of transmitted light polarization microscopy. A narrowing of both pyramidal and prismatic sectors can be identified to constitute tapering. The first being due to an overall retardation of prismatic growth layers caused by impurity adsorption, whereas the narrowing of the pyramidal sectors is due to the inability of pyramidal growth layers to reach the prismatic crystal edge. The retardation of pyramidal growth layers can be explained in terms of a change in electrostatic parameters near the crystal edge or by assuming a fundamental roughness of the crystal edge. A connected net analysis shows the presence of two equally stable {022} nets parallel to the pyramidal faces. In a polar environment this equality is broken, favouring a {011} double layer growth mechanism which can also explain in principle the low segregation coefficient for three-valent positive metal ions in the pyramidal growth sector.

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