DISORIENTATION EFFECTS AT EPITAXY IN AQUEOUS MEDIA

A.Glikin, J.Plotkina

Universitetskaya Nab., 7/9, St.Petersburg University, 199034, St.Petersburg, Russia.
E-mail: glikin@crystgen.geol.pu.ru

Keywords: epitaxy, disorientation factors, kinetics, adhesion

A perfection of epitaxy was estimated for a series of substances pairs in aqueous solutions at the original experimental conditions: «simultaneous growth of a seed and excrescences» and «simultaneous dissolution of a seed and growth of excrescences» (both for the pairs KCl-NaCl and K₂Cr₂O₇-KBr) as well as «growth of a seed and deposition of inactive cleavage grains» (for the pair NaNO₃-CaCO₃ presenting the opposite case to the classical display of epitaxy). The two texture types of deposits were observed: «epitaxial» and «vertical». The first type observed at the crystal intergrowth by the structure-similar surfaces is formed by an arbitrary disorientation from the epitaxial position. The second type is formed by the crystal intergrowth with a sufficient (up to 90⁰) angle between the structure-similar surfaces. Disorientation effects depend on the nucleation type and growth-dissolution rates.

For the pairs KCl-NaCl and K₂Cr₂O₇-KBr, the epitaxy perfections were estimated as 0.8-1.0 and 0-0.6 respectively. These characteristics display the action of the crystallochemical factor well-known as the basis of epitaxy, determined through a structure parameter correspondence which is closer in the first case. On the other hand, the disorientation degree of crystals composing epitaxial textures is increased with a seed growth-dissolution rate rise but it is decreased with a deposit growth rate rise; this is interpreted as a consequence of a disorientation of heterogeneously nucleated crystals by moving growth layers. The vertical texture was revealed in the both of these pairs; for the second one it was obtained when a seed growth rate was sufficiently high and one of the way of its formation appeared to be the consequence of than overturn of initially epitaxial excrescences by an action of moving growth layers. Both of the cases display the kinetic factor of epitaxy.

For the epitaxial texture of the pair NaNO₃-CaCO₃ (14 seeds; the range of deposited crystal sizes is 20-400 microns), there are the three areas of the excrescence distribution in the angle disorientation. The first one located at a=0±1⁰ (the epitaxial position) consists of 40 % excrescences; the second one located in the range 0<a<10⁰ is characterised by the absence of excrescences; the third one located in the range 10<a<50⁰ consists of 60 % excrescences characterised by a weak expressed normal distribution. The vertical texture was also obtained in this case (its ratio with the epitaxial texture was 14:86). It contained 12 % and 2 % CaCO₃ crystals fastened by vertexes and edges respectively; the first ones were isometric, but the second ones were substantially anisometric ('prismatic»). The disorientation inhomogeneity and the shape anisometry influence on the orientation features display the adhesion mechanism acting with a long-range forces containing balanced attraction and repulsion components.

Thus, the analysis of these patterns leads to the definition of the three factors determining mutual orientation-disorientation of epitaxial crystals: crystallochemical, kinetic and adhesive. Two the last of them were revealed by the authors for the first time.

This research was supported by INTAS (grant 93-2498Ext) and RFFI (grant 96-05-150).