Crystal structure and proton conductivity of BaSn0.6Sc0.4O3−δ: insights from neutron powder diffraction and solid-state NMR spectroscopy

Abstract

The solid-state synthesis and structural characterisation of perovskite BaSn_1−xSc_xO_3−δ (x = 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 Pmm) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. ¹¹⁹Sn and ⁴⁵Sc solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D₂O. It also indicates that the Sn⁴⁺ and Sc³⁺ 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 (x ≤ 0.2) and in the tin coordination shell at high concentrations (x ≥ 0.3). ¹⁷O NMR spectra on ¹⁷O enriched BaSn_1−xSc_xO_3−δ 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_0.6Sc_0.4O_3−δ refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 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 × 10⁻⁴ to 1.1 × 10⁻³ S cm⁻¹ 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.