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The polymorphous crystallization kinetics of a melt-quenched amorphous selenium (a-Se) under pressure was investigated by using a piston-cylinder pressure vessel over a pressure range of 0-1.0 GPa. When increasing the applied pressure, the crystallization temperature was found to be enhanced by about 50 K, and the apparent activation energy for the crystallization elevated from 69 kJ/mol to 167 kJ/mol. The pressure-induced enhancement in thermal stability of the a-Se against crystallization was analyzed in terms of the pressure effect on crystal nucleation kinetics, in which formation of the crystalline/amorphous (c/a) interfaces is taken into consideration. Due to the excess volume of the c/a interface, an overall volume expansion is involved in the crystal nucleation process when the nucleus size is very small. This effect leads to a significant increase in the critical work of nucleation when pressure is applied. The model agrees satisfactorily with our experimental observations. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.