Dependence of the CaOx and MgOx growth rate on solution stoichiometry. Non-Kossel crystal growth

Rates of elementary growth step propagation on the CaOx and MgOx crystal faces were measured by AFM in flowing Solution maintaining kinetic growth mode. Dependence of the step rate on the stoichiometry ratio r = [cation]/[anion] in solution ill the range 5 x 10(-2)< r < 20 for CaOx and 0.2 < r < 5 for MgOx was found. The dependence is nearly symmetrical with respect to the replacement r by l/r or vice versa, i.e. v(r) congruent to v(l/r), with a clear maximum for stoichiometric solution, r = 1. It turns out that v = v(r(1/2) + r(-1/2)), in approximate agreement with the kink growth rate theory for binary and other non-Kossel crystals [J. Zhang, G.H. Nancollas, J. Colloid Interface Sci. 200 (1998) 131; A.A. Chernov, J. Mater. Sci. Mater. Electron. 12 (2001) 437], under the assumption that the cation and anion attachment frequencies are proportional to concentrations of these species only and that the proportionality coefficients, the frequency factor, is independent of the ion type. The fitting frequency factor is similar to 10(5) l/mol s, leading to a slightly exaggerated vibration frequency of species in activated state, 4 x 10(12) l/s, at activation energy 8 kcal/mol. Possible reasons for the observed deviations from the theoretical predictions are discussed. Kink rate calculation for a ternary system is presented in Appendix A. (c) 2005 Published by Elsevier B.V.