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 TaO stretching and OTaO 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.