One of the fruits of the recent developments in far-infrared techniques has been the knowledge of the whole vibrational spectrum of a solid and its evolution with temperature, We consider here the particular case of potassium bromide, which has been studied elsewhere by neutron diffraction, and we give all the information obtained in the laboratory by the diferent methods of far-infrared spectrometry now available:(1) the reflectivity of potassium bromide at liquid-helium, liquid-nitrogen, and room temperature, including the effect of polarization, grazing incidence, and the quality of the surface, and interpreted according to a Kramers-Kronig analysis and a Lorentz analysis with two linear damped oscillators; (2) the transmission of thin layers at, normal and oblique incidence, and a determination of the complex index of refraction, the compressibility coefficient, and the effective charge of the ions; (3) the very-far-infrared transmission of thick samples and its temperature dependence explained by phonon-diGerence processes; and (4} the 6rst spectroscopic observation of the phonon spectrum of an ionic crystal, provided by sodium-induced absorption in potassium bromide, with peaks at 101, 83.5, and 71.5 cm, having the same frequencies as the three maxima in the lattice-mode density, which correspond approximately to characteristic TQ, LA, and TA phonons, The effect of other impurities and of neutron irradiation is also considered.

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The existence of solid solutions of composition CaFZ-,H, (0 < x 2 1.24) is recently established, crystallizing in CaF2-type cubic symmetry. There is a statistical substitution of fluoride ions b y hydride ions in the CaF2 unit cell. The infrared spectroscopy gives informations on these compounds b y the behaviour of the Hions localized vibration and that of the fundamental TO mode when x varies.Une etude structurale par diffraction des rayons X e t par diffraction des neutrons a recemment mis en evidence l'existence de solutions solides CaF2-,H, (0 < z 5 1,34). Elles cristallisent dans la symBtrie cubique type fluorine. La spectroscopie infrarouge apporte des informations sin ces composes par le comportement de la vibration 1ocalisBe des ions He t par celui du mode fondamental de CaF, lorsque x varie.