Homogeneous nucleation and the phenomenon of crystalloluminescence

The validity of the Thomson and Volmer relations when applied to the nucleation of highly soluble salts from aqueous solution is examined and it is shown that Volmer's theory of the rate of three-dimensional nucleation applies if ‘local’ supersaturations are considered. Of the alkali chlorides only the sodium salt shows true crystalloluminescence, e.g. with lead impurities, whereas the other salts require supersaturations so high as to be experimentally unattainable. With silver impurities however all alkali chlorides exhibit marked crystalloluminescence due to the formation of giant clusters of sub-nuclei which require a much lower supersaturation for three-dimensional nucleation to occur. Surface active anions displace lead impurities, confirming the model suggested earlier for the structure of the crystal nucleus. Using apparatus of greater resolution than previously available, the time of click-over for the nucleus has been shown to be less than 5 nanosec. The mechanism of light emission appears to be akin to piezoluminescence, and to be caused by phonons produced at the time of click-over.