The “extra” scattering between Bragg peaks adds information about the structure on a local scale and is therefore of high importance for structural studies of proton­conducting perovskites when the material is not fully periodic, such as when dis­order is present. In particular, the pair-correlation function G(r) is sensitive to key structural details, such as bond distances and angles, the symmetry of structural distortions, and oxygen and/or cation ordering, for example. The initial neutron total-scattering experiments coupled to PDF analysis of proton-conducting per — ovskites were done by Malavasi et al. [50, 51] on undoped and Y-doped BaCeO3. For the undoped material, the neutron total-scattering data suggests no difference between the long-range orthorhombic Pnma structure as determined by Rietveld refinement and the short-range structure as determined from PDF analysis, regardless if the sample is hydrated or not [51].

The good agreement between the long-range and short-range structures may be appreciated from Fig. 9.6, which, for the dry undoped material, shows the good fit of the Pnma model obtained from Rietveld refinement to G(r). For the dry Y-doped material, however, the fit of the Pnma model is less satisfactory, c. f. Fig. 9.7a. For this material, the authors instead found an excellent agreement between the G(r) and a structural model based on the lower-symmetry space group P21 (see Fig. 9.7b), indicating local regions around the Y dopant of such symmetry [51]. Moreover, it was found that the Y-doped material returns to the orthorhombic Pnma structure upon hydration, suggesting that the source of local structural distortion is mainly due to Y-induced oxygen vacancies and not linked directly to the substi­tution of cations [51]. Since the local structure around the proton can be expected to correlate strongly with the mechanistic detail of proton dynamics, such local structural information is of high interest and indeed crucial for the tailoring of new materials with higher proton-conductivities. The importance of understanding the details of local structure may be exemplified by the 10 % Y — and Sc-doped BaZrO3 materials, which both exhibit an average cubic structure, but for which the proton conductivity differs by several orders of magnitude [52].