Topotactic reaction of thin silver films with selenium

It is shown that the selenidation of thin silver films at elevated temperature is a topotactic reaction, caused by epitactic nucleation of high temperature silver selenide on the silver surface, followed by phase transformation to the low temperature phase. The occurrence of two or even three different orientations of the selenide, the preference of one of them in extended single crystalline areas, and the presence of prominent planar defects in a specific orientation, are qualitatively explained by geometrical arguments.