  Charged domain walls form spontaneously in Cu-Cl boracite on cooling through
the phase transition. These walls exhibit changed conductivity compared to the
bulk and motion consistent with the existence of negative capacitance. Here, we
present the dielectric permittivity and DC resistivity of bulk Cu-Cl boracite
as a function of temperature (-140 {\deg}C to 150 {\deg}C) and frequency (1 mHz
to 10 MHz). The thermal behaviour of the two observed dielectric relaxations
and the DC resistivity is discussed. We propose that the relaxations can be
explained by the existence of point defects, most likely local complexes
created by a change of valence of Cu and accompanying oxygen vacancies. In
addition, the sudden change in resistivity seen at the phase transition
suggests that conductive domain walls contribute significantly to the
conductivity in the ferroelectric phase.
