Polytypism and the vibrational properties of PbI2

The vibrational properties of six different polytypes of Pb${\mathrm{I}}_2$ crystals have been studied experimentally by means of Raman spectroscopy and x-ray diffraction. The crystals were prepared either by gel growth or by thermal annealing. The Raman spectra are used to determine features of the phonon dispersion curves, in the crystallographic $c$ direction, which are common to all of the polytypes. The results are correlated with existing information obtained from measurements of Brillouin spectra and inelastic scattering of neutrons and then analyzed within the framework of a six-parameter force-constant model. Thermodynamic data, including new measurements of lattice parameters in the range $100<T<\mathrm{}295\mathrm{}$ K, are used to discuss anharmonic and two-dimensional contributions to the vibrational properties.