Site Selectivity of Hydride in Early-Transition-Metal Ruddlesden-Popper Oxyhydrides.

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH reduction from Ruddlesden-Popper Sr Ti O phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered SrTiOD material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system SrTiOH is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered SrTiOH material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the SrTiOH model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V electronic polaron is suggested to play a role in stabilizing H on the equatorial site in SrVOH for x = 0.125.