Beyond average crystal structures: understanding extended and local environments in proton-conducting Sc-substituted BaTiO3 perovskites
Doctoral thesis, 2019
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
Chalmers, Chemistry and Chemical Engineering, Energy and Material, Environmental Inorganic Chemistry 2
Insight into the dehydration behaviour of scandium-substituted barium titanate perovskites via simultaneous in situ neutron powder thermodiffractometry and thermogravimetric analysis
Solid State Ionics,; Vol. 324(2018)p. 233-240
The influence of cation ordering, oxygen vacancy distribution and proton siting on observed properties in ceramic electrolytes: the case of scandium substituted barium titanate
Dalton Transactions,; Vol. 46(2017)p. 8387-8398
Torino N., Sławiński W. A., Knee C. S.,Henry P. F., Eriksson S. G. - Reverse Monte Carlo modelling reveals the local ordering in hexagonal and cubic scandium-substituted BaTiO3 protonconducting perovskites
Perrichon A., Torino N., Jedvik Granhed E., Lin, Y.-C., Parker S. F, Jiménez-Ruiz M., Karlsson M., Henry P. F. - Proton sites in hexagonal and cubic Sc-doped BaTiO3 proton-conducting oxides
In order to incorporate protons, these materials interact with water vapour absorbing water like a sponge would: they expand when hydrated and contract when dehydrated. The system object of this thesis, Sc-substituted BaTiO3, is alternative to the most known proton conducting materials and was investigated because it behaves differently. The expansion caused by water absorption is remarkably small in one member of the series of compounds, a characteristic that can be crucial for building durable fuel cells. For these studies, I mainly used a combination of light (X-rays) and particles (neutrons) in order to know where the atoms are (the structure) and what they do (their dynamics). Particularly interesting was the combination of neutrons experiments with a scale, so that atoms positions could be followed closely during dehydration.
It was found that local structural deviations are present and that the interactions between species at the local level deeply influence the global properties of these materials. Local structures became even more relevant and were studied in conjunction with the global structure. In doing so, this research contributed further understanding of the factors that influence proton transport, from the chemical composition to the immediate vicinities of an atom.
Condensed Matter Physics
Areas of Advance
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4648
Chalmers University of Technology
Lecture hall KB
Opponent: Dr. Kirsten Marie Ørnsbjerg Jensen, Department of Chemistry, University of Copenhagen