Peculiar aspects of crystal growth mechanism in solutions of KDP and ADP tetragonal crystals shall be presented using our published data and some new results. Fractional recrystallization procedure (in two or three steps) was used to improve the quality of the basic substance (H.V.Alexandru 2007). Double walled thermostated systems of 9 and 3 liters have been used and reversible rotation of the crystals tree support. Stirring rates ensured kinetic regime of the grown crystals. Linear (horizontal [001] and vertical [110]) crystals dimensions were registered in-situ versus the supersaturations. This way, pyramidal (011) and prismatic (100) faces, mean growth rate, were registered for the three crystals always grown in the same conditions. Arrhenius corrections of the growth rates, for our data and some literature data, were made for chosen growth temperature, using activation energy of growth.

Growths kinetic of KDP prismatic faces, more sensitive to Me³⁺ impurities, were registered in 9 liters solution volume of the crystallization chamber, up to one month. Growth kinetic in impure solutions was measured and analyzed. At higher supersaturations, (σ > 8%) a dominant heterogeneous 2D-nucleation mechanism of growth was found, where the coverage of ad-molecules q~10^-6 in the surface layer is about of the same order of magnitude as Fe³⁺ impurity concentration in solution. Using 2D-nucleation formalism introduced by Chernov we have found the edge free energy per growth unit γ ≈ 0.4 k_BT, in good agreement with the literature data. The lower limit of the adsorption energy 18 kcal/mol of this impurity was estimated on the prismatic faces of KDP, much higher than ~9.5 kcal/mol the activation energy for growth. The critical coverage q*~10^-3 at the limit of the “dead” growth zone, much higher than previous figure, suggest the segregation coefficient of this impurity increases dramatically towards the lower supersaturations (not published yet).

Macroscopic growth kinetic measurements were performed for pyramidal faces of KDP crystal. The three crystals were simultaneously grown in the same conditions, by temperature decreasing method. Arrhenius corrections were made for 40 ^oC growth temperature. Surface diffusion mechanism of growth appears as essential at small supersaturations, but several particular aspects dominate the growth. Pyramidal faces are less sensitive to impurities and show a large dispersion of the growth kinetic at s = 1÷5 %. There are distinct BCF curves fitting the experimental data on several ranges of supersaturation, corresponding to several efficiencies of growth. Bürgers vector and the linear dimension of the dominant centers of dislocations involved during growth shall be discussed.

Growth kinetic of pyramidal faces of ADP was studied at small supersaturations (s < 1 %.). There is a large dispersion of the kinetic data and apparently there is not a well defined relation between the growth rate and supersaturation, during a long run experiment. A special procedure was used to correlate the kinetic date with the efficiency of growth. The reverse of the estimated efficiencies versus the supersaturation, allowed estimating approximately the Burger’s vector at several stages of growth and the edge free energy ratio γ/k_BT of the steps in the frame of the BCF dislocation mechanism of growth. Unexpected switching effect of efficiencies, found around 0.5 % supersaturations shall be discussed in relation with literature data (Vekilov, Kuznetsov 1992).

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/29816 must be provided.

1. INTERFACE KINETIC IN CRYSTAL GROWTH FROM SOLUTIONS