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

A previous study of various A₂B₂O₆O′ 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 Cd₂Ta₂O₇, (ii) lacunary Tl₂Ta₂O₆, (iii) nonstoichiometric Pb_1.5Ta₂O_6.5, and (iv) Pb_2.3Ta₂O_7.3 exhibiting regular shear planes. If the tridimensional network of TaO₆ 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.