  Pyrochlore systems are ideally suited to the exploration of geometrical
frustration in three dimensions, and their rich phenomenology encompasses
topological order and fractional excitations. Classical spin ices provide the
first context in which it is possible to control emergent magnetic monopoles,
and anisotropic exchange leads to even richer behaviour associated with large
quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a
quantum spin liquid or a ferromagnetic phase induced by a Higgs transition
appears to be sample dependent. Here we have determined the role of structural
defects on the magnetic ground state via the diffuse scattering of neutrons. We
find that oxygen vacancies stabilise the spin liquid phase and the stuffing of
Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been
eliminated by annealing in oxygen exhibit a transition to a ferromagnetic
phase, and this is the true magnetic ground state.
