  The charge ordered structure of ions and vacancies characterizing rare-earth
pyrochlore oxides serves as a model for the study of geometrically frustrated
magnetism. The organization of magnetic ions into networks of corner-sharing
tetrahedra gives rise to highly correlated magnetic phases with strong
fluctuations, including spin liquids and spin ices. It is an open question how
these ground states governed by local rules are affected by disorder. In the
pyrochlore Tb$_2$Hf$_2$O$_7$, we demonstrate that the vicinity of the
disordering transition towards a defective fluorite structure translates into a
tunable density of anion Frenkel disorder while cations remain ordered.
Quenched random crystal fields and disordered exchange interactions can
therefore be introduced into otherwise perfect pyrochlore lattices of magnetic
ions. We show that disorder can play a crucial role in preventing long-range
magnetic order at low temperatures, and instead induces a strongly-fluctuating
Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.
