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.
The Nature of One-sided Intergrowths on (001) of Triclinic Centrosymmetric K2Cr2O7 (II)†
SHUBNIKOV first observed in 1912 that centrosymmetric K2Cr2O7 crystals (P1) exhibit a growth anomaly in highly supersaturated solutions. The (001) faces of these crystals are always smooth, the parallel opposite faces are always rough. Electron micrographs show these rough surfaces to be crystallites that have grown in stacks. In Part I of this publication, the crystallite orientation was determined using electron-induced diffraction methods. As a rule, these crystallites are parallel intergrowths. Only a small number of crystals near the surface of the host crystal are twinned according to [010]. Part II sought to ascertain the cause of these one-sided intergrowths. To determine the most favourable energetic arrangement of two intergrown individuals, interactions across the intergrowth face as a function of shifts parallel to (001) and to (001) were calculated, 64 theoretically possible cases were considered. Only in the [010]-twin and the seldom-occurring [110]-twin is an approximation of the 2/m symmetry of the macroscopic twin arrangements also found in the intergrowth structure, and this is the case only when the (001) faces of the host and guest crystal are intergrown. 
 
 
 
Bereits 1912 beobachtete SCHUBNIKOW, das zentrosymmetrische K2Cr2O7-Kristalle (P1) in stark ubersattigten Losungen eine Wachstumsanomalie zeigen. Die (001)-Flachen dieser Kristalle sind stets glatt, die parallelen Gegenflachen immer aufgerauht. Diese Aufrauhungen erweisen sich im Elektronenmikroskop als aufgewachsene Kristallstapel. Im Teil I dieser Veroffentlichung wurde die Orientierung der Kristalle mit Hilfe von elektronenstrahlangeregten Beugungsmethoden bestimmt. In der Regel handelt es sich um parallelverwachsene Kristallite. Nur wenige in der Nahe der Oberflache des Wirtkristalls befindliche Kristalle sind nach [010] verzwillingt. Die Aufklarung der Ursachen fur die nur einseitigen Aufwachsungen war Aufgabe des Teils II. Zur Ermittlung der energetisch gunstigsten Anordnung von zwei Verwachsungsindividuen wurde in Abhangigkeit von einer Verschiebung parallel (001) und (001) die Wechselwirkung uber die Verwachsungsflache berechnet. Es wurden 64 theoretisch mogliche Falle betrachtet. Nur fur den [010]- und den selten auftretenden [110]-Zwilling liegt die 2/m Symmetrie der makroskopischen Zwillinge-naherungsweise auch in der Verwachsungsstruktur vor. Dies ist aber nur dann der Fall, wenn die (001)-Flachen von Wirt- und Gastkristall miteinander verwachsen.
Effects of impurities on crystal growth processes
The mechanisms of adsorption of impurities during the growth of bulk crystals are surveyed. The impurities considered are foreign substances deliberately added to or inherently present in a growth medium. After a brief introduction to different growth models, the nature of impurities and types of impurity effects on growth kinetics, adsorption of impurities on F faces at relatively low and high impurity concentrations is first discussed. Here mechanisms of adsorption at kinks in steps and surface terrace in the presence of low concentrations of one impurity and two competitive impurities are presented. Then the concepts of adsorption of impurities involving the formation of two-dimensional impurity layer and three-dimensional impurity clusters as growth promotors on a growing face are developed. Thereafter the mechanism of adsorption of impurities on S and K faces and the determination of the adsorption mechanism on a growing face from the kinetic data are discussed. Finally, impurity effects on surface morphology of bulk crystals, and growth morphodroms of alkali halides are briefly reviewed.