Melting and crystal structure: Effects of thermal transformations of ionic crystals on their ultra-violet absorption

Abstract

Using a new design of absorption cell, spectroscopic studies in the range 4000 to 2000 Å have been made on films of salts about 100 μ thick, in order to investigate changes of ionic environment that accompany phase transformations in the crystals, and melting. Behaviour which is theoretically simplest to interpret is shown by lithium iodide. This salt shows changes similar to those previously reported for other alkali halides, though the techniques now developed give much more detailed information. In brief, the wavelength of the absorption maximum shifts to longer wavelengths as the positional disorder increases with rise in temperature of the solid and on melting. Accompanying this shift, the intensity of the absorption maximum decreases and band edges (defined in various ways) shift to longer wavelengths. A group of ‘low melting’ salts previously investigated in other ways shows behaviour in marked contrast with that of the alkali halides. For example, on melting, the absorption maximum may shift either to shorter or longer wavelengths, but in any case the shift is much smaller. Maxima of absorption intensity actually increase, or show only slight decreases on melting. These and other differences between the alkali halides and low melting salts such as KCNS, AgNO 3 , LiNO 3 , NaNO 3 , KNO 3 , are attributed to the formation of ionic complexes (including ion pairs) when these salts are melted. This can lead to closer approach between anion and cation, than in the crystal at low temperatures, and thus accounts for the contrast in optical behaviour. Whereas the halides undergo mainly positional disorder on melting, for salts such as the nitrates complex formation on fusion is an important additional mechanism of melting. For KCNS the exceptionally large premelting previously reported for the crystals on the basis of volume and conductance measurements is also observed in ultra-violet absorption measurements.