Lanthanum barium gallates, i. e. LaBaGaO4-based compounds, represent a novel class of proton-conducting oxides [89]. The structure of these materials consists of discrete GaO4 tetrahedra which are charge balanced by Ba/La ions, as shown in Fig. 9.18a. Increasing the Ba:La ratio results in the formation of oxygen vacancies, and similarly to perovskite-type oxides, such vacancies can be filled with — OH groups in a humid atmosphere [89]. In this regard, Kendrick et al. [89] raised the question of how the oxygen vacancies are accommodated in the structure, with the knowledge that the inclusion of oxygen vacancies may result in energetically unfavourable three-coordinated Ga atoms, which would limit the oxygen vacancy concentration and hence the degree of hydration. Using computer-modelling methods, the authors found that the oxygen vacancies are accommodated via con­siderable relaxation of neighbouring GaO4 tetrahedra, resulting in the formation of

Ga2O7 groups so that the Ga retains tetrahedral coordination (see purple unit in Fig. 9.18a) [89]. These computational results were supported by ND results which showed a splitting of the oxygen site, consistent with the presence of both GaO4 and Ga2O7 [89]. Hydration of the material can lead to the breaking of such units [89].

On the basis of these results, later investigations focussed on proton positions within the hydrated form of the lanthanum barium gallate structure, using ND coupled with difference Fourier-density analysis of protonated and deuterated samples. Notably, three stable proton positions, one located adjacent to the O3 site and two located adjacent to the O4 site, were found (Fig. 9.18a). The presence of several proton sites is in agreement with modelling work suggesting little difference in energy between proton sites for different oxygens [89]. Further, determination of Ga-O-H bond angles indicated that protons point almost perpendicularly to the Ga-O bond direction and that the protons experience a mixture of intra — and inter­tetrahedral hydrogen-bonding, as shown in Fig. 9.18b [91]. The highest degree of hydrogen bonding is found for O3-H — O3 linkages, in agreement with previous modelling work [91]. Modelling also suggests facile inter-tetrahedron proton-con­ducting pathways due to hydrogen bonding, whereas intra-tetrahedron pathways are found to be less favourable and hence rate-limiting for proton conduction [89].