  The structure and dehydration mechanism of the proton conducting oxide
Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ are investigated by means of variable
temperature Raman spectroscopy together with inelastic neutron scattering. At
room temperature, Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ is found to be fully
hydrated ($x=1$) and to have a perovskite-like structure, which dehydrates
gradually with increasing temperature and at around 600 $^{\circ}$C the
material is essentially completely dehydrated ($x=0$). The dehydrated material
exhibits a brownmillerite structure, which is featured by alternating layers of
InO$_{6}$ octahedra and InO$_{4}$ tetrahedra. The transition from a
perovskite-like to a brownmillerite-like structure is featured by a
hydrated-to-intermediate phase transition at $ca.$ 370 {\deg}C. The structure
of the intermediate phase is similar to the structure of the fully dehydrated
material, but with the difference that it exhibits a non-centrosymmetric
distortion of the InO$_{6}$ octahedra not present in the latter. For
temperatures below the hydrated-to-intermediate phase transition, dehydration
is featured by the release of protons confined to the layers of InO$_{4}$
tetrahedra, whereas above the transition also protons bound to oxygens of the
layers of InO$_{6}$ are released. Finally, we found that the O-H stretch region
of the vibrational spectra is not consistent with a single-phase spectrum, but
is in agreement with the superposition of spectra associated with two different
proton configurations. The relative contributions of the two proton
configurations depend on how the sample is hydrated.
