  A series of Yb2Ti2O7 doped samples demonstrates the effects of
off-stoichiometry on Yb2Ti2O7's structure, properties, and magnetic ground
state via x-ray diffraction, specific heat, and magnetization measurements. A
stoichiometric single crystal of Yb2Ti2O7 grown by the traveling solvent
floating zone technique (solvent = 30 wt% rutile TiO2 and 70 wt% Yb2Ti2O7) is
characterized and evaluated in light of this series. Our data shows that upon
positive x doping, the cubic lattice parameter a increases and the Curie-Weiss
temperature decreases. Heat capacity measurements of stoichiometric Yb2Ti2O7
samples exhibit a sharp, first-order peak at T = 268(4) mK that is suppressed
in magnitude and temperature in samples doped off ideal stoichiometry. The full
entropy recovered per Yb ion is 5.7 J/K ~ Rln2. Our work establishes the
effects of doping on Yb2Ti2O7's physical properties, which provides further
evidence indicating that previous crystals grown by the traditional floating
zone method are doped off ideal stoichiometry. Additionally, we present how to
grow high-quality colorless single crystals of Yb2Ti2O7 by the traveling
solvent floating zone growth method.
