The pyrochlore to 'defect fluorite' transition in the Y ₂(Zr _yTi _1-y) ₂O ₇ system and its underlying crystal chemistry

A careful investigation of the previously reported single phase, pyrochlore structure type Y ₂(Zr _yTi _1-y) ₂O ₇, 0≤y≤0.9, (YZT) solid solution has been carried out. Given the known slow rate of diffusion of cations in fluorite-related stabilized zirconia systems and the consequent difficulty in achieving equilibrium, careful attention was paid to synthesis procedures while Guinier XRD and electron diffraction were used to investigate the synthesized materials. As a consequence, a subtle but nonetheless clear two-phase region separating a pyrochlore-type solid solution field (from 0≤y≤∼0.54) from a "defect fluorite" type solid solution field (from ∼0.68≤y≤1) has been found. The underlying crystal chemistry of the system has been investigated using the bond valence sum approach. The dielectric properties have also been measured as a function of composition. The dielectric constant and dielectric loss of the lowest y compounds are potentially quite useful but deteriorate rapidly with increasing Zr content. A plot of twice the refined cubic fluorite, parent unit cell dimension (corresponding to the pyrochlore unit cell dimension) of the MOD synthesized samples as a function of Zr content y in the Y ₂(Zr _yTi _1-y) ₂O ₇ system. The open triangles and filled squares correspond to 5 and 20 h annealed specimens, respectively.