Neutron-diffraction study of Zr(Ca,Y)O/sub 2-x/: Evidence of differing mechanisms for internal and external distortions

We have deduced from measurements of the neutron elastic cross section that below the order-disorder transition in Zr(Ca)O/sub 2-x/ and Zr(Y)O/sub 2-x/ the oxygen atoms are shifted from their fluorite positions. We find quantitative agreement with experiment for a model containing a rearrangement (internal shear deformation with amplitude 0.23 A) of the oxygen sublattice. The (110) internal shear deformation is the same (in symmetry and magnitude) for Zr(Ca)O/sub 2-x/ and Zr(Y)O/sub 2-x/, however, only Zr(Y)0/sub 2-x/ exhibits an external distortion. These observations are consistent with the fact that the internal rearrangement is an inhomogeneous deformation and does not require the presence of an external strain. A successful test of Cooper's search criterion leads us to an internal rearrangement mode corresponding to a q = (0,0,2π/a) longitudinal phonon, and points to the dominance of cation-anion interactions in determining the nature of the lattice internal rearrangement