Comparison of the force field in various pyrochlore families. II. Phases presenting structural defects

A previous study of various A₂B₂O ₆O' pyrochlore families by vibrational spectroscopy allowed the analysis of the ir and Raman spectra and the force fields of pyrochlore phases presenting structural defects. The spectra of the following compounds were compared: (i) ideal pyrochlore Cd ₂Ta ₂O ₇, (ii) lacunary Tl ₂Ta ₂O ₆, (iii) nonstoichiometric Pb _1.5Ta ₂O _6.5, and (iv) Pb _2.3Ta ₂O _7.3 exhibiting regular shear planes. If the tridimensional network of TaO ₆ octahedra is not modified and the perturbations concern only the A₄O' tetrahedra network, a slight modification of the vibrational spectra and a weak decrease in the TaO stretching and OTaO bending force constants are observed. But, if the octahedra network is perturbed, large modifications appear on the spectra, revealing unambiguously the existence of new types of bonds created by the defects in the structure. The Raman intense lines observed in the low frequency range in lead compounds is also discussed.