Infrared absorption and thermal conductivity of CaF2containing heavy metal impurities

Abstract

The far-infrared absorption of CaF₂ doped with Co, Y, La, Sm, Gd and Tm has been measured at 1.8 K. A simple lattice-dynamical model incorporating only a mass change at the impurity site and change of nearest-neighbour central force constant has been used to calculate the infrared absorption. A lattice Green function based on a shell model fit to neutron diffraction data was used in the calculation and quite good agreement with experiment was obtained with the rare-earth impurity ions. Ion size was the dominant factor in determining the change of central force constant required to obtain agreement with experiment. Thermal conductivity measurements on CaF₂ containing 1 at% of Y impurity showed a resonance dip at about 30 K and this has been correlated with an infrared resonance at 80 cm^-1.