During recent years the authors have investigated the kinetics of displacement rates of individual faces of ammonium oxalate monohydrate [(NH₄)₂C₂O_4·H₂O; AO] single crystals from aqueous solutions containing bi- and trivalent cationic impurities. However, displacement rates of different faces are a resultant of layer displacements on them, it is of interest to study the displacement rates of layers as a function of solution supersaturation and concentration of different impurities. The present work is addressed to this subject. Growth kinetics of growth layers was studied on the {110} faces of specially prepared seeds of ammonium oxalate monohydrate (abbreviated as AO) crystals in a specially designed growth cell in which growth temperature was controlled to ±0.05^oC. Employing appropriate programmes and series of four photographs recorded by a digital camera in different growth experiments in a solution containing a known concentration c_i of a selected impurity at a constant growth temperature, the displacement rate in the [00-1] direction of growth layers oriented along the [-110] and [-111] directions were determined. The experimental set up used earlier for growth kinetics was similar to that used for the study of the motion of steps on the {010} faces of potassium acid phthalate single crystals. Using the values of the average displacement rate v of steps in the investigated supersaturation s interval, the kinetic coefficient β (= dv/dσ) for step motion at different concentrations c_i of four impurities (i.e. Cu(II), Mn(II), Fe(III) and Cr(III)) was determined. The experimental data were analysed according to the theoretical dependence: β/β₀ = 1 - αKc_i/(1+Kc_i), where β₀ is the kinetic coefficient for impurity-free system (β₀ = 2.55 × 10^-5 m/s), K is the Langmuir constant and α is the effectiveness parameter for an impurity. The values of constants α, K and differential heat of adsorption for different impurities are given in Table 1. As found from the data of impurity concentration dependence of face growth rates, Cu(II) and Fe(III) are practically ineffective in growth but Mn(II) and Cr(III) are effective growth inhibitors.

Table 1. Constants β₀, α, K and Q_diff

Acknowledgements. This work was financially supported by Ministry of Science and Higher Education under research project PB 1041/P03/2004/26. The authors also thank Dr E. Mielniczek-Brzóska for growing crystal seeds for this study.

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