We present ab initio phonon dispersion relations for the three lead chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good agreement with inelastic neutron-scattering data and calculations of the specific heat give good agreement with experimental data. The pronounced minimum of the transverse-optical branch at ⌫ due to the near ferroelectricity of the lead chalcogenides is qualitatively reproduced. In addition, we find a pronounced dip in the longitudinal-optical branch at ⌫. This dip was previously explained as the effect of "free carriers" ͑due to the presence of impurities͒. The calculations demonstrate that it persists also in the case of pure lead chalcogenides. We explain the dip as a "near Kohn anomaly" which is associated with the small electronic band gap at the high-symmetry point L.

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm −1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement.