Symmetry and light stuffing of Ho sub(2) Ti sub(2) O sub(7), Er sub(2) Ti sub(2) O sub(7), and Yb sub(2) Ti sub(2) O sub(7) characterized by synchrotron x-ray diffraction

The Ho sub(2) Ti sub(2) O sub(7), Er sub(2) Ti sub(2) O sub(7), and Yb sub(2) Ti sub(2) O sub(7) pyrochlores were studied by synchrotron x-ray diffraction to determine whether the (002) peak, forbidden in the pyrochlore space group Fd - 3m but observed in single crystal neutron scattering measurements, is present due to a deviation of their pyrochlore structure from Fd - 3m symmetry. Synchrotron diffraction measurements on precisely synthesized stoichiometric and nonstoichiometric powders and a crushed floating zone crystal of Ho sub(2) Ti sub(2) O sub(7) revealed that the (002) reflection is absent in all cases to a sensitivity of approximately one part in 30 000 of the strongest x-ray diffraction peak. This indicates to high sensitivity that the space group of the crystal structure of these rare earth titanate pyrochlores is Fd - 3m, and that, thus, the (002) peak observed in the neutron scattering experiments has a nonstructural origin. The cell parameters and internal strain for lightly stuffed Ho sub(2+x) Ti sub(2-x) O sub(7) are also presented.