VII. CONCLUSIONSThe optical constants of potassium bromide (VT, 8I, pi, k M , €EO, an d X T) vary linearly with temperature within the experimental accuracy. Lorentz theory permits us to consider two linear oscillators, which represent quite well the behavior of KBr, if they are modified to include a temperature dependence. The damping of the fundamental oscillator decreases when T decreases. On the other hand, its strength does not vary much, while the strength of the second oscillator decreases strongly.
ACKNOWLEDGMENTSWe are grateful to Bernard Jacques and Robert Thomas, who did the laser measurements at 336 /z, and wish also to thank Dr. J. N. Plendl of the Cambridge Research Laboratories for his interest in the investigation. from the defect. Such impurity modes can have frequencies which lie in the frequency spectrum of the unperturbed lattice. If they are strongly coupled to the plane-wave phonons, they will impede the heat flow in the crystal and can thus be detected through measurements of the lattice thermal conductivity. 2-6 By investigating a large number of foreign alkali or halogen ions dissolved in the host lattices KC1, KBr, and KI, and by comparing the results with those obtained by far 2 C.The thermal conductivity of doped KG, KBr, and KI has been measured between 1 and 200°K. Below ^5°K, the phonon scattering by the defects obeys a Rayleigh law in most cases. The magnitude of the scattering is determined by the mass difference alone. In Ag + -doped crystals, however, the Rayleigh scattering is much weaker than predicted by the mass difference. This is explained through the weak coupling between the Ag + and its neighbors. Above 5°K, in the region of the resonance scattering, the conductivity can be qualitatively described with the model of elastic phonon resonant scattering by quasilocalized modes. In KC1:T1,1, Rb, and Br, the mass difference alone suffices to qualitatively describe the data, which means that the resonant mode is odd. The resonant scattering observed in Li + -and Ag + -doped crystals is ascribed to excitations of even resonant modes. It is noted that in many cases thermal-conductivity measurements are complementary to optical-absorption measurements, since the former observe defects strongly coupled to the lattice, whereas the latter observe the ones which are weakly coupled.