Lightly stuffed pyrochlore structure of single-crystalline Yb2Ti2O7 grown by the optical floating zone technique

Recent neutron scattering and specific heat studies on the pyrochlore Yb2Ti2O7 have revealed variations in its magnetic behavior below 265 mK. In the best samples, a sharp anomaly in the specific heat is observed at T = 265 mK. Other samples, especially single crystals, have broad features in the specific heat which vary in sharpness and temperature depending on the sample, indicating that the magnetic ground statemay be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverized single crystal of Yb2Ti2O7, grown by the floating zone technique, to a sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a stuffed pyrochlore, Yb-2(Ti2-xYbx)O7-x/2 with x = 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb3+ on the nonmagnetic Ti4+ sublattice would introduce random exchange bonds and local lattice deformations. These are expected to be the mechanism leading to the variation of the delicate magnetic ground state of Yb2Ti2O7. Determination of the cubic cell length a could be useful as a method for characterizing the stoichiometry of nonpulverized single crystals at room temperature.