Observation of Singular Points in Defect-Induced Far-Infrared Spectra of KBr

Far-infrared absorption spectra of KBr crystals containing ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, ${\mathrm{Sm}}^{++}$, ${\mathrm{OH}}^{-}$, and ${\mathrm{F}}^{-}$ are shown all to have sharp discontinuities at 74.8, 75.2, 85.5, and 89.5 ${\mathrm{cm}}^{-1\mathrm{}}$. It is shown by comparison with dynamical models for the lattice and the defect that these frequencies correspond closely to expected van Home singularities in the host lattice. These experiments, when extrapolated to zero defect concentration, provide accurate measurements of certain phonon frequencies in the pure lattice.