Initial electrical conductivity measurements indicated that the bulk phase of BaTiO 3 x H y is semiconducting. However, the experiment was carried out on a powder sample without sintering, which may introduce errors due to grain-boundary effects. Recently, conductivity measurements on epitaxial thin films revealed that BaTiO 3 x H y has metallic conductivity with high hydride concentrations (y > 0.2), while it is semiconducting with lower hydride compositions (y < 0.2). Liu et al. proposed that the semiconductor-like behaviour is a result of electron polaron formation. Polarons are quasiparticles composed of a localized charge carrier within a potential energy well that is self-generated by distorting the local lattice. In BaTiO 3 x H y the substitution of oxygen with hydrogen introduces an electron in the Ti 3dband which can potentially localize and form an electron polaron. Thermally activated polarons can hop to neighboring Ti ions and would display semiconductor-like conductivity in the bulk phase. However, a more recent study using INS and density functional theory (DFT) calculations showed that the extra electron is more likely to delocalize among all Ti 3d-bands generating a bandstate with metallic electrical conductivity. Several investigations on the oxyhydride with solid-state nuclear magnetic resonance (NMR) have also proposed that the electronic structure is more likely a bandstate. In contrast to other reports Misaki and co-workers suggested that the hydride site is occupied by two hydrogens.