Scattering of Phonons by Monatomic Impurities in Potassium Halides

Abstract

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.

Excess thermal resistivity in N2–CO solid solution at low carbon monoxide concentration

“…This also explains earlier success of the approach, in which foreign impurities in dielectric crystals were regarded as pure isotopic admixtures, e.g., Ref. 4. Concluding, the thermal conductivity of pure and carbon-monoxide-doped nitrogen crystals has been measured for the samples containing 0.2, 0.25, 0.3, 0.5 and 0.7% of CO molecules, in the temperature range 1.2-26 K. The simple analysis has shown that the scattering of phonons on admixture molecules possessing the same mass as the host molecule is relatively weak when compared with the scattering on a molecule featuring different mass.…”

Analysis of the Effect of Resonant Phonon Scattering on the Thermal Conductivity of CaF2

“…A concentration of 1·0 % Y in CaF 2 was necessary to produce an anomalous thermal conductivity dip, while Baumann and Pohl (1967) found dips for ,....,0·02% TI in KCI. The purpose of the present paper is to analyse the thermal conductivity of the doped CaF 2 in order to suggest a reason for such differences.…”

“…Only the latter is discussed here in detail, and for details of the others the reader is referred to Klemens (1955), Berman and Brock (1965) and Klein (1966). Baumann and Pohl (1967) have discussed the relative difficulty found in fitting experimental data by resonant scatter relaxation rates. Many forms have been used which are theoretically justifiable (Schwartz and Walker 1967).…”

“…Schwartz and Walker (1966) observed resonant behaviour due to lattice vacancies in KCI. Baumann and Pohl (1967) and Schwartz and Walker (1967) observed resonant behaviour due to monatomic impurities in alkali halides. True localized vibrations have also been observed: Elliott et al (1965) discussed the absorption spectra ofH-and D-ions in CaF 2 , SrF2 and BaF 2 .…”

“…A great deal of work has been carried out with alkali halides as the host material, and the effect on the thermal conductivity has been found to be quite dramatic (Baumann and Pohl 1967). Harrington and Walker (1970) chose to investigate doped XF 2 (where X = Ca, Sr, and Ba) for resonant phonon scattering, as Moore and Klein (1969) had been unsuccessful with doped CdS using impurity concentrations of up to 500 p.p.m.…”