We report 63Cu and 65Cu NMR on Cu2Se, a superionic conductor of interest for
thermoelectric and other applications. Results demonstrate an initial
appearance of ionic hopping in a narrow temperature range above 100 K,
coinciding with the recently observed low-temperature phase transition.
At room temperature and above, this goes over to rapid Cu-ion hopping and
a single motionally narrowed line both above and below the alpha-beta structural
transition. The low-temperature alpha-prime phase exhibits a sizable metallic shift,
in contrast to the high-temperature structures. The large variation in
native carrier density, significantly enhanced at positions associated
with Cu vacancies, indicates the strong influence of an impurity band
on low-temperature electronic transport.