Adsorption of an Al (III) Impurity onto the (100) Face of a Growing KDP Crystal in Supersaturated Solution

The growth-starting supercooling Δ T G, defined as a supercooling at which a crystal starts to grow on cooling, was measured for the (100) face of a potassium dihydrogen phosphate (KDP) crystal on which an Al (III) impurity had been adsorbed beforehand. When adsorption was made onto the nongrowing (100) face at a low supercooling value of Δ T A = 0.6 °C, the measured values of Δ T G were successfully explained with a mathematical model considering the Langmuir equilibrium (or instantaneous) adsorption. On the contrary, smaller values of Δ T G were obtained when adsorption was made on the growing (100) face at a high supercooling value of Δ T A = 6.0 °C. These smaller values obtained for the adsorption at Δ T A = 6.0 °C are concluded to be due to an insufficient amount of the adsorbed Al (III) impurity, and the adsorption is shown to proceed more slowly onto the growing (100) face compared to the nongrowing face.