Annular dynamical disorder of the rare earth ions in a La₂Zr₂O₇ pyrochlore via single crystal synchrotron X-ray diffraction

The reciprocal lattices of a range of rare earth titanate and zirconate pyrochlores have recently been shown to exhibit an extremely characteristic diffuse intensity distribution identical in form to that characteristic of β-cristobalite implying the existence of β-cristobalite-like orientational disorder of essentially rigid, (O2)(R³⁺)₄ tetrahedra within the pyrochlore structure type. A single crystal X-ray diffraction study of La₂Zr₂O₇ has been carried out utilizing synchrotron radiation in order to investigate this proposed orientational disorder and to determine an unknown oxygen atom fractional co-ordinate. An annular distribution of residual electron density is found around the average La ion positions in {111} planes perpendicular to the local O2 - La - O2 directions and interpreted as implying that the La ions never in fact occupy the average 16d positions but rather are dynamically disordered around an annulus perpendicular to that direction in an exactly analogous manner to that which occurs in βcristobalite.