Vibration Spectra of the Alkali Halides

Abstract

As is well known, ammonium chloride crystallizes in the cubic system, its structure being of the rock-salt type above 450° T. and of the cæsium chloride type below that temperature. Menzies and Mills¹ found that at liquid air temperatures it shows in its Raman spectrum a strong line with a frequency shift of 183 cm.⁻¹. This observation is significant, since the frequency shift corresponds to the Reststrahlen frequency of the crystal² and hence indicates that the oscillation of the NH₄ groups against the Cl ions is Raman-active. The crystal powder technique employed by Menzies and Mills is, however, not well suited for the study of Raman spectra in the region of such small frequency shifts. Accordingly, a fresh investigation has been made with a well-grown single crystal and the λ2536·5 excitation and the same technique as in my recent investigations with various other crystals. To enable the entire sequence of changes to be observed, spectra were recorded with the crystal held at a series of seven temperatures down to that of liquid air.