  Using extensive first-principles molecular dynamics calculations, we
characterize the superionic phase transition and the lattice and electronic
structures of the archetypal Type-I superionic conductor alpha-AgI. We find
that superionicity is signalled by a phase transition of the silver ions alone.
In the superionic phase, the first silver shell surrounding an iodine displays
a distinct dynamical structure that would escape a time-averaged
characterization, and we capture this structure in a set of ordering rules. The
electronic structure of the system demonstrates a unique chemical signature of
the weakest-bound silver in the first shell, which in turn is most likely to
diffuse. Upon melting, the silver diffusion decreases, pointing to an unusual
entropic contribution to the stability of the superionic phase.
