  Magnetostriction drives a rhombohedral distortion in the cubic rock salt
antiferromagnet MnO at the N\'eel temperature $T_{N}=118$ K. As an unexpected
consequence we show that this distortion acts to localize the site of an
implanted muon due to the accompanying redistribution of electron density. This
lifts the degeneracy between equivalent sites, resulting in a single observed
muon precession frequency. Above $T_{N}$, the muon instead becomes delocalized
around a network of equivalent sites. Our first-principles simulations based on
Hubbard-corrected density-functional theory and molecular dynamics are
consistent with our experimental data and help to resolve a long-standing
puzzle regarding muon data on MnO, as well as having wider applicability to
other magnetic oxides.
