Knowledge about proton sites is essential to understand the properties of proton­conducting oxides, whether it be the local proton-dynamics, hydrogen-bonding interactions, or macroscopic proton-conductivity. Some early examples of ND measurements on proton-conducting perovskites in this area include by Knight [42] who reported on BaCe0.9Y01O2.95, Sata et al. [43] who reported on Sc-doped SrTiO3, Sosnowska et al. [44] who reported on Ba3Ca1.18Nb182O9_,5, and Kendrick et al. [45] who reported on La06Ba04ScO2 8. More recently, Ahmed et al. [46] reported on BaZr050In050O3_d, whilst Azad et al. [47] reported on BaCe0.4Zr0.4Sc0.2O2.9. One may note that most of these studies were performed on samples which were deuterated rather than hydrated, in order to reduce the inco­herent scattering and transform the scattering into a useful signal, which increases the chance of locating the positions of protons even in systems with low proton- concentration.

To give a representative example of the previous work we turn to the neutron powder diffraction measurements by Ahmed et al. [46] on BaZr0 50In0 50O3_d. As pointed out by the authors, the Rietveld analysis of the ND patterns (Fig. 9.4a) indicated no departure from cubic Pm3m symmetry. However, further analysis showed that the material is phase-separated into a deuterium-rich and non-deuter — ated phase with phase fractions of 85 and 15 %, respectively.

To determine the deuterium positions in the deuterium-rich phase, the authors calculated the Fourier-difference maps by taking the difference between simulated (with no deuterium in the structural model) and experimental data in reciprocal space; the Fourier-difference map taken at z = 0 is shown in Fig. 9.4b. Here, the positive peak at approximately (0.5, 0.2, 0) in Fig. 9.4b is consistent with the positive scattering-length of deuterium, suggesting that this is the deuterium site. However, from a closer analysis of the neutron data, it was found that the missing scattering density is distributed anisotropically within the ab plane, which suggests instead delocalization of the deuterium atom at the 24k site. It follows that there are eight equivalent deuterium-sites around each oxygen, which are tilted towards a neighbouring oxygen [46]. Such tilting increases the tendency for the formation of a strong hydrogen-bond between the deuterium and the oxygen towards which it is

tilted (c. f. Fig. 9.5a) and is consistent with results obtained both from first-principles calculations and infrared spectroscopy [48].

Further structural refinements based on the diffraction patterns revealed highly- anisotropic atomic displacement parameters (ADPs) of the oxygen atoms, as illustrated in Fig. 9.5b, as well as the deuterium site occupancy. The large ADPs

reflect static, localized, displacements of the oxygen ions around their ideal site in a cubic structure and most likely result from the structural disorder as induced by the difference in size between Zr4+ and In3+ [46]. Similarly, large ADPs have been obtained elsewhere for the composition BaZr0 33In0 67O3_a and those results were also attributed to static-disorder effects [49]. To conclude, the authors not only succeeded in determining the location and concentration of protons (deuterons) in the perovskite structure, but also revealed the presence of pronounced short-range structural distortions of the average cubic perovskite-structure, which are likely to affect considerably the proton transport in the material. More detailed information about the local structure, however, needs the use of more local probes, such as PDF analysis of neutron total-scattering data, which is highlighted below.