Extending the Momentum Transfer Range in QENS Measurements on Sc-Doped Barium Zirconates

Abstract

In this work, a range of scandium-doped, proton-conducting barium zirconates have been subject to two complementary quasi-elastic neutron scattering (QENS) experiments; one featuring a traditional range of momentum transfer (Q) values (0.5 – 2 Å−1) and one featuring an extended Q-range (1 – 4 Å−1). Recent data suggests that the extended Q-range may allow differentiation between jump and rotational motions of protons within a Grotthuss-type diffusion model. However, so far measurements have been conducted only on the traditional and extended dynamical ranges separately, using different samples. By collecting QENS spectra in both momentum transfer regimes for the same sample, this work aims to achieve a comparison of QENS results over these regimes and their corresponding time-scales. Furthermore, theoretical models for jump and rotational localized motions are compared to experimental data over both Q regimes. It was found that the atomic-scale proton dynamics probed using an extended Q-range are somewhat more rapid (processes with characteristic time-scales of 0.8 – 2.0 picoseconds) than those probed using a traditional Q-range (3.0 – 12.3 picoseconds), as expected from the inherently lower energy-transfer resolution of extended Q-range experiments. QENS measurements further suggested that the more rapid dynamics targeted by the extended Q-range measurement require higher temperatures to initiate. Models for jump-type and rotation-type proton motions both display good fits with the experimental data, preventing discrimination between the models. Nonetheless, it is shown why the extended Q range is useful for ensuring reliability in fitting these models. Activation energies and correlation times are largely consistent with previous measurements on traditional Q-range experiments.