Etude vibrationnelle des oxydes PbO α et PbO β: Interpretation en relation avec la dilatation thermique
A normal coordinate analysis of the Raman and ir vibrational spectra of PbO α and PbO β allowed us to calculate the force field for both phases. The cohesion between the structural layers is realized by means of attractive interactions involving the lone pair of the Pb II atoms. The analysis of the potential energy distribution shows that these interactions are much weaker than the PbO bonds in the layers. The calculated force constants for these bonds vary inversely as a function of their distance and allow one to explain the value of the thermal expansion coefficients in the layers. On the other hand, the abnormally small value of the thermal expansion along the normal direction to the layers, particularly for PbO β, may be explained only in assuming the existence of a negative contribution due to a modification of the relative orientation of the dipoles (Pb II-lone pair) situated on two adjacent layers.