The fluorite–pyrochlore transformation of Ho2−yNdyZr2O7

Twelve members of the Ho2−yNdyZr2O7 series, prepared using conventional solid state methods, have been characterised by neutron powder diffraction. Ho2Zr2O7 has a defect fluorite structure whereas Nd2Zr2O7 is found to adopt the ordered pyrochlore structure with the composition induced fluorite–pyrochlore transformation occurring near y=1. Rietveld analysis on the neutron data for all the compositions reveals an increase in lattice parameter as a function of y across the entire series, with a small discontinuity associated with the transformation. The neutron profile results suggest that domains of pyrochlore-type initially begin to form before crystallising into a separate phase, and therefore that anion and cation ordering processes are distinct. There is a strong correlation between the extent of disorder in the anion sublattice and the x-parameter of 48f oxygen. These results point the way to a better understanding of the stability observed in pyrochlore structures.

Graphical abstractNeutron diffraction profiles for Nd2−yHoyZr2O7 type oxides reveal details of the transformation from the ordered pyrochlore structure (y=0) to the disordered fluorite structure (y=2).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Structures of twelve members of the Ho2−yNdyZr2O7 series studied using neutron powder diffraction. ► Domains of pyrochlore-type materials form at low doping levels. ► Higher doping stabilises the pyrochlore. ► Anion and cation ordering processes are distinct.