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

The reciprocal lattices of a range of rare earth titanate and zirconate pyr ochlores have recently been shown to exhibit an extremely characteristic di ffuse intensity distribution identical in form to that characteristic of be ta -cristobalite implying the existence of beta -cristobalite-like orientat ional disorder of essentially ri,aid, (O2)(R3+)(4) tetrahedra within the py rochlore structure type. A single crystal X-ray diffraction study of La2Zr2 O7 has been carried out utilizing synchrotron radiation in order to investi gate this proposed orientational disorder and to determine an unknown oxyge n atom fractional co-ordinate. An annular distribution of residual electron density is found around the average La ion positions in (111) planes perpe ndicular 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 d ynamically disordered around an annulus perpendicular to that direction in an exactly analogous manner to that which occurs in beta -cristobalite.