  Muons implanted into ionic fluorides often lead to a so-called F-mu-F state,
in which the time evolution of the muon spin contains information about the
geometry and nature of the muon site. Nuclei more distant from the muon than
the two nearest-neighbor fluorine ions result in decoherence of the F-mu-F
system and this can yield additional quantitative information about the state
of the muon. We demonstrate how this can be applied to the determination of
muon sites within the ionic fluorides alpha-PbF2 and YF3 which contain fluoride
ions in different crystallographic environments. Our results can be used to
distinguish between different crystal phases and reveal the presence of anion
Frenkel defects in alpha-PbF2.
