Feynman path-integral Monte Carlo simulations have been performed to s
tudy finite-temperature properties of isolated hydrogenlike impurities
in crystalline silicon. The Si-H interaction was modeled following th
e results of earlier pseudopotential-density-functional calculations f
or this system. Hydrogen is found to be stable at the bond-center (BC)
site. However, the same potential model predicts that muonium is high
ly delocalized in the interstitial region. This mass-induced effect is
shown to be a consequence of the uncertainty principle, which prevent
s localization of muonium at the BC site due to an increase of its kin
etic energy.