What Simulated NMR Chemical Shifts can teach us about Hydrogen Species in Perovskite BaTiO₃

Numerous studies investigating hydrogen incorporation in titanate ATiO₃ perovskites find species including protons as interstitials and on A-site positions, hydride ions on oxygen sites, surface OH groups, and trapped H₂ molecules on Ti positions depending on the chemical potential of oxygen. Which species dominate depends on synthesis and processing conditions; yet, fingerprinting the density and distribution of species experimentally remains challenging. Nuclear Magnetic Resonance (NMR) spectroscopy is sensitive to the local chemical environment and chemical shifts arise from the electron density surrounding the hydrogen nuclei. This makes NMR a well-suited probe for determining the hydrogen species present; however, it does not directly provide the charge state of the nuclei, but a relative shift which must be interpreted. In recent literature on BaTiO₃-derived oxyhydrides, the hydride peak is assigned to chemical shifts ranging from 4.4 to -60 ppm, while the additional peaks between 6.5 and 1 ppm are attributed to surface OH or residual Ca(OH)₂ in the samples from fabrication; protons have not been identified. To that end, we present results from density functional theory simulations of NMR chemical shifts for the BaTiO₃-derived oxyhydrides, with a focus on testing each possible hydrogen species to unambiguously identify each peak. Our work shows how solid-state NMR can be used to identify the nature of the hydrogen present in titanate perovskites and determine their relative concentrations.