Beyond average crystal structures: understanding extended and local environments in proton-conducting Sc-substituted BaTiO3 perovskites
Doctoral thesis, 2019
Proton conducting ceramics are very promising for applications concerned with energy sourcing with cleaner, safer, more abundant and cheaper alternatives to fossil fuels. These materials are still in development and advances in the field depend on a better understanding of the role of defects, their identification and location in the host framework, and the assessment of their short- and long-range dynamics and kinetics. With that aim, the work included in this thesis focussed on investigations of the effect of Sc substitution on the long- and short-range structure, oxygen vacancies and protons distribution, and their link to proton conductivity, in BaTiO3 materials. The system BaTi1–xScxO3–x/2 with x = ⅙, 20, 50 and 70 was studied with a combination of thermogravimetric, scattering, spectroscopic and computational methods.
Neutron powder diffraction (NPD) provided the first representations of hexagonal and cubic members of the solid solution BaTiO3-Sc2O3. They revealed the different ordering of oxygen vacancies, protons and transition metal ions in the two structural types as a function of the Sc concentration and justified the large improvement in proton conductivity from hexagonal to cubic structures, due to the localised nature of protonic defects in the former. The combination of thermogravimetric and NPD methods applied simultaneously to study the dehydration of cubic members of the series suggested that vacancy-vacancy interactions are attenuated by higher Sc levels where the size difference between oxygen vacancy and protonic defect is larger. The Reverse Monte Carlo method revealed the local ordering of Ti in cubic types, a local symmetry-breaking effect that has repercussions on the physical properties of these materials, causing anomalously small volume changes upon hydration in low-Sc phases. Computer simulations, and spectroscopic methods employing radiation (IR, Raman) and neutrons (Inelastic Neutron Scattering) provided further insight into the structural features and offered a detailed characterisation of the proton sites and their dynamics, suggesting that higher Sc levels are associated to weaker hydrogen bonding and to configurations more favourable for proton transport.
The present work contributed further understanding of the factors influencing proton transport in highly defective perovskite-structured materials. It was found that high Sc concentrations in the cubic host lattice of BaTiO3 yield highly stable phases where transport of protonic defects is favoured by a crystal site of high symmetry and multiplicity. Alongside the study of the peculiarities of the BTS system, recommendations for candidate systems identification and doping strategy were provided.
proton conducting oxide
Reverse Monte Carlo