The highest proton-conductivities in polycrystalline samples of perovskite-struc — tured oxides are generally found in barium cerates (BaCeO3-based compounds), however, these materials react with CO2 and/or H2O at intermediate temperatures to form BaCO3 (or Ba(OH)2) and CeO2 and therefore degrade with time. Hence, they are poorly suited for use in fuel cells [26]. Strategies to increase the chemical stability of such materials include substitution (doping) of different types of atoms, but improvements are generally small. In comparison, barium zirconates (BaZrO3- based compounds) show excellent chemical stability in CO2 and H2O-containing atmospheres and are in this respect more suitable for use in fuel cells from an application point of view. The conductivity of one specific barium zirconate, namely 10 % Y-doped BaZrO3 (included in Fig. 9.2), indeed exhibits the highest bulk proton-conductivity reported for any oxide material [26]. However, barium zirconates are difficult to sinter, which implies that barium zirconate samples contain a relatively-large volume of grain boundaries, which decrease the total conductivity [26, 35-38]. The sinterability of barium zirconates may be enhanced with the use of sintering aids [36, 39] or by the introduction of a second dopant at the B site of the perovskite [40, 41], however, the bulk conductivity is then typically lowered.

9.1 Neutron Scattering of Proton-Conducting Perovskites

The development of new perovskite structures with improved conductivity, thermodynamical stability, and sinterability, depends on the exploration of new classes of compounds as well as an increased understanding of the basic science of those materials already known. Such investigations should elucidate key material detail such as crystal structure, proton sites, proton concentrations, hydrogen­bonding interactions, and the mechanics of proton dynamics on different time — and length-scales, as well as to clarify how these details correlate with each other. For this purpose, neutron scattering offers the unique potential to access simultaneously information in both space and time, through the momentum (hQ) and energy (hE) transferred in the scattering event, respectively. This combination makes neutron scattering a powerful tool for investigating structures (using neutron diffraction (ND)), vibrational dynamics and hydrogen-bonding interactions (using inelastic neutron scattering (INS)), and diffusional dynamics (using quasielastic neutron scattering (QENS)). Neutrons can also be used to obtain details concerning the proton concentration in the sample through prompt-gamma activation analysis (PGAA). Therefore, neutrons offer good opportunities to advance the understanding of state-of-the-art proton-conducting perovskites. In this context, the remainder of this chapter aims to give a flavour of the important role that neutron methods play in providing a deep insight into the functionality of these materials. Emphasis is put on barium zirconates, due to their great promise for fuel-cell applications.

Examples of recent neutron work on proton-conducting perovskites, which are here briefly reviewed, include studies of proton sites and local structures using ND (Sect. 9.3.1), studies of vibrational proton dynamics and hydrogen-bonding inter­actions using INS (Sect. 9.3.2), studies of proton diffusion using QENS (Sect. 9.3.3), and studies of proton concentrations using PGAA (Sect. 9.3.4). No attempt has been made to be complete in this work, with the aim rather to highlight the different types of information that can be obtained using neutrons.