Structured diffuse scattering as an indicator of inherent cristobalite-like displacive flexibility in the rare earth zirconate pyrochlore La delta Zr1-delta O2-delta/2, 0.49 <delta < 0.51

A detailed electron and X-ray powder diffraction study has been made of the compositional and displacive flexibility of the rare earth zirconate pyroc hlore (1 - delta)ZrO2 . delta LaO1.5 = (La3+)(delta)(Zr4+)(1-delta)O2-delta /2, delta similar to 0.5. The extent of the pyrochlore "solid solution" in specimens rapidly quenched from 1500 degrees C is shown to be considerably narrower than previously reported and certainly less than 2 mol.% in width. A weak but highly structured and quite reproducible diffuse intensity dist ribution in the form of {110}* polarized sheets of diffuse intensity perpen dicular to each of the sis [110] real space directions is observed in elect ron diffraction patterns and interpreted in terms of beta-cristobalite-like [110] tetrahedral edge rotation of essentially rigid O(2)(La)(4) tetrahedr a. (C) 1999 Academic Press.