where r a b (0) is the unpaired electron density in the s-orbital and c is the susceptibility per paramagnetic centre. We notice from Equation 2 that the shift in the case of the polaron depends on the transferred spin density to the hydrogen s-orbital r a b (0), therefore this contribution is additive meaning that a hydride adjacent to two electron polarons would give double the shift compared to a hydride in the proximity of a single polaron. Therefore, we expect with increasing hydride content (potentially increasing electron polaron concentration) for a sec- For the bandstate the FC part of shift is proportional to the density of states at the Fermi level as shown in Equation ), and so depends on the number of electrons in the conduction band (3dband of Ti) and the number of charge carriers. In a bandstate configuration substitution of an oxide ion with a hydride introduces an additional electron in the Ti 3d-band, therefore an increase in the number of charge carriers is expected. A prior study has confirmed that higher hydride concentrations in the structure result in a larger number of charge carriers for BaTiO 3 x H y with y = 0.14 0.35. Thus, we attribute the isotropic shift becoming more negative with the increasing hydride content (see Table ) to the metallic character of the material. Furthermore, the negative shift is consistent with the polarization mechanism. More specifically, according to the Goodenough-Kanamori rules the conduction electrons in the t 2g -band polarize the e g -band, resulting in a negative spin density in the hydride s-band and consequently a negative contact shift. We speculate that this mechanism may not occur for the double occupied bandstate due to the different bonding geometry of the two hydrides, which allows direct overlap between the hydride s-orbitals and the t 2g -orbital giving rise to the delocalisation mechanism and a positive shift. Until now we only considered the FC term of the Knight shift, however, it also possesses orbital and spin-dipolar parts, which are orientation dependent and contribute to the shift anisotropy. The spindipolar term depends on the density of states at the Fermi level and so increases with higher hydride concentrations. This explains the directly proportional relation between shift anisotropy (see Table ) and the hydride concentration.