Crystal structure and proton conductivity of BaSn 0.6 Sc 0.4 O 3- δ : 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 D O. 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 BaSn Sc O 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.