Crystal structure and proton conductivity of BaSnScO : insights from neutron powder diffraction and solid-state NMR spectroscopy.

The solid-state synthesis and structural characterisation of perovskite BaSn Sc O ( = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group 3) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. Sn and Sc solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with DO. It also indicates that the Sn and Sc local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations ( ≤ 0.2) and in the tin coordination shell at high concentrations ( ≥ 0.3). O NMR spectra on O enriched BaSn Sc O materials show the existence of Sn-O-Sn, Sn-O-Sc and Sc-O-Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSnScO refines the deuteron position at the 24k crystallographic site (, , 0) with = 0.579(3) and = 0.217(3) which leads to an O-D bond distance of 0.96(1) Å and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 °C with total conductivity values in the range 1.8 × 10 to 1.1 × 10 S cm between 300 and 600 °C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 °C temperature region, and a suppression of the conductivity at higher temperature.