ROTATIONAL DEGREES OF FREEDOM OF MOLECULES IN SOLIDS .3. THERMAL PROPERTIES OF RBCL - CN

The thermal conductivity and the specific heat of RbCl containing RbCN in concentrations between 1017 and 1019 cm-3 in solid solution has been measured above 0.05°K. Resonance scattering of phonons is found in the thermal conductivity around 0.2 and 10°K. The resonance frequencies, ω1=1.28×1011 rad sec-1 (0.68 cm-1) and ω25.6×1012 rad sec-1 (26 cm-1), agree with the frequencies derived by Seward and Narayanamurti through infrared spectroscopic studies: ω1 is equal to the tunneling frequency of the CN- molecule; ω2 agrees with the frequencies of the states of almost free rotation near the top of the potential barrier, to which the molecule is subjected. This agreement is taken as further evidence that the Devonshire model of molecular rotation in solids, on which Seward's and Narayanamurti's calculations were based, is basically correct for RbCl: CN. The additional specific heat caused by the tunneling states shows two maxima at about 0.4 and 0.08°K. The one at 0.4°K can be explained with the Devonshire model, whereas the one at 0.08°K cannot. A comparison of the phonon scattering caused by the rotational states of the CN- impurity with the scattering caused by impurity modes associated with certain monatomic impurities impurities shows striking similarities. At present one cannot give more than a qualitative model to explain this observation. © 1968 The American Physical Society.