Rh4Si5formation occurs by two different mechanisms, depending on the film geometry. Thermal annealing of a Rh film on a silicon substrate results in the formation of Rh4Si5at about 850 °C by a process described as ‘‘nucleation controlled.’’ Alternatively, when the film consists of thin Rh/Si multilayers, Rh4Si5can form at room temperature in a very rapid, self‐sustaining fashion that is the chemical‐reaction analog of the explosive crystallization process. This mechanism of explosive silicide formation, reported recently for the first time [J. A. Floro, J. Vac. Sci. Technol. A4, 631 (1986)], apparently occurs in the multilayer geometry due to the large driving forces and reduced diffusion lengths associated with this configuration.