Recent neutron scattering and specific heat studies on the pyrochloreYb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In thebest samples, a sharp anomaly in the specific heat is observed at T=265mK.Other samples, especially single crystals, have broad features in the specificheat which vary in sharpness and temperature depending on the sample,indicating that the magnetic ground state may be qualitatively different insuch samples. We performed detailed comparisons of the chemical structure of apulverised single crystal of Yb2Ti2O7, grown by the floating zone technique, toa sintered powder sample of Yb2Ti2O7. Rietveld refinements of neutron powderdiffraction data on these samples reveal that the crushed single crystal isbest described as a "stuffed" pyrochlore, Yb2(Ti(2-x)Yb(x))O7-(x/2) with x =0.046(4), despite perfectly stoichiometric starting material. Substitutingmagnetic Yb3+ on the non-magnetic Ti4+ sublattice would introduce randomexchange bonds and local lattice deformations. These are expected to be themechanism leading to of the variation of the delicate magnetic ground state ofYb2Ti2O7. Determination of the cubic cell length, a, could be useful as amethod for characterizing the stoichiometry of non- pulverised single crystalsat room temperature.
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