  The crystal structures of the A2B2O7-x Niobium-based pyrochlores
Y2(Nb0.86Y0.14)2O6.91, CaYNb2O7, and Y2NbTiO7 are reported, determined by
powder neutron diffraction. These compounds represent the first observation of
B-site displacements in the pyrochlore structure: the B-site ions are found to
be displaced from the ideal pyrochlore positions, creating electric dipoles.
The orientations of these dipoles are fully analogous to orientations of the
magnetic moments in Ising spin based magnetically frustrated pyrochlores.
Diffuse scattering in electron diffraction patterns shows that the
displacements are only short range ordered, indicative of geometric frustration
of the collective dielectric state of the materials. Comparison to the crystal
structure of the Nb5+ (d0) pyrochlore La2ScNbO7 supports the prediction that
charge singlets, driven by the tendency of Nb to form metal-metal bonds, are
present in these pyrochlores. The observed lack of long-range order to these
singlets suggests that Nb4+-based pyrochlores represent the dielectric analogy
to the geometric frustration of magnetic moments observed in rare earth
pyrochlores.
