  Rb2Ti2O5 (RTO) has recently been demonstrated to be a solid electrolyte,
producing colossal capacitance when interfaced with metals. In order to
understand the mechanisms leading to such colossal equivalent permittivity (up
to four orders of magnitude above state-of-the-art values), the charge
distribution in RTO is a key feature to be investigated. In the present
article, this charge distribution is probed using the pressure-wave-propagation
method, in devices made of RTO single crystals or polycrystals sandwiched
between two metallic electrodes. Remarkably enough, in both types of samples,
negative charges are found to accumulate inside RTO, near the anode, while the
electric field near the cathode remains zero. This proves that the ionic
carriers are majoritarily negatively charged and provides an explanation for
the colossal capacitance. The latter takes place only at the anode while the
cathode is virtually shifted into the solid electrolyte.
