The pyrochlore magnet ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ has been proposed as a quantum spin ice candidate, a spin liquid state expected to display emergent quantum electrodynamics with gauge photons among its elementary excitations. However, ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$'s ground state is known to be very sensitive to its precise stoichiometry. Powder samples, produced by solid-state synthesis at relatively low temperatures, tend to be stoichiometric, while single crystals grown from the melt tend to display weak “stuffing” wherein $\sim 2\%$ of the ${\mathrm{Yb}}^{3+}$, normally at the $A$ site of the $A_2{B}_2{\text{O}}_7$ pyrochlore structure, reside as well at the $B$ site. In such samples ${\mathrm{Yb}}^{3+}$ ions should exist in defective environments at low levels and be subjected to crystalline electric fields very different from those at the stoichiometric $A$ sites. Neutron scattering measurements of ${\mathrm{Yb}}^{3+}$ in four compositions of ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7-y}$ show the spectroscopic signatures for these defective ${\mathrm{Yb}}^{3+}$ ions and explicitly demonstrate that the spin anisotropy of the ${\mathrm{Yb}}^{3+}$ moment changes from $XY$-like for stoichiometric ${\mathrm{Yb}}^{3+}$ to Ising-like for “stuffed” $B$ site ${\mathrm{Yb}}^{3+}$ or for $A$ site ${\mathrm{Yb}}^{3+}$ in the presence of oxygen vacancies.

• Received 18 October 2017
• Revised 23 February 2018

DOI:https://doi.org/10.1103/PhysRevB.97.224409