Recent neutron scattering and specific heat studies on the pyrochlore Yb${}_2$Ti${}_2$O${}_7$ 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 state may be qualitatively different in such samples. We performed detailed comparisons of the chemical structure of a pulverized single crystal of Yb${}_2$Ti${}_2$O${}_7$, grown by the floating zone technique, to a sintered powder sample of Yb${}_2$Ti${}_2$O${}_7$. Rietveld refinements of neutron powder diffraction data on these samples reveal that the crushed single crystal is best described as a “stuffed” pyrochlore, Yb${}_2$(Ti${}_{2-x}$Yb${}_x$)O${}_{7-x/2}$ with $x$ $=$ 0.046(4), despite perfectly stoichiometric starting material. Substituting magnetic Yb${}^{3+}$ on the nonmagnetic Ti${}^{4+}$ 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 Yb${}_2$Ti${}_2$O${}_7$. Determination of the cubic cell length $a$ could be useful as a method for characterizing the stoichiometry of nonpulverized single crystals at room temperature.

• Received 23 July 2012

DOI:https://doi.org/10.1103/PhysRevB.86.174424