  Recent neutron scattering and specific heat studies on the pyrochlore
Yb2Ti2O7 have revealed variations in its magnetic behavior below 265mK. In the
best samples, a sharp anomaly in the specific heat is observed at T=265mK.
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 state may be qualitatively different in
such samples. We performed detailed comparisons of the chemical structure of a
pulverised 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, Yb2(Ti(2-x)Yb(x))O7-(x/2) with x =
0.046(4), despite perfectly stoichiometric starting material. Substituting
magnetic Yb3+ on the non-magnetic Ti4+ sublattice would introduce random
exchange bonds and local lattice deformations. These are expected to be the
mechanism leading to of 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 non- pulverised single crystals
at room temperature.
