The influence of crystal size and crystallographic orientation on decomposition in the solid state: sodium and thallous azides

Abstract
A method of preparing very small crystals of uniform and controlled size has been developed and used to study the decomposition of sodium and of thallous azides. The decomposition is carried out in ultra high vacuum and is coupled with the use of a mass ion spectrometer. The method is a sensitive one and it is possible to follow the kinetics of decomposition of one molecular layer from single crystals which may be only 10$^{-3}$ cm$^2$ in area. The decomposition products are identified by the mass ion spectrometer and the changes in the crystal are observed by scanning electron microscopy. It is shown that the rate of decomposition is proportional to the surface area of the crystals. There is evidence that the decomposition takes place over a surface or interface which moves into the crystal in a preferred crystallographic direction. It is the area of this surface which controls the rate of decomposition.