Structure and dehydration mechanism of the proton conducting oxide Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$

The structure and dehydration mechanism of the proton conducting oxide Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ are investigated by means of variable temperature Raman spectroscopy together with inelastic neutron scattering. At room temperature, Ba$_{2}$In$_{2}$O$_{5}$(H$_{2}$O)$_{x}$ is found to be fully hydrated ($x=1$) and to have a perovskite-like structure, which dehydrates gradually with increasing temperature and at around 600 $^{circ}$C the material is essentially completely dehydrated ($x=0$). The dehydrated material exhibits a brownmillerite structure, which is featured by alternating layers of InO$_{6}$ octahedra and InO$_{4}$ tetrahedra. The transition from a perovskite-like to a brownmillerite-like structure is featured by a hydrated-to-intermediate phase transition at $ca.$ 370 {deg}C. The structure of the intermediate phase is similar to the structure of the fully dehydrated material, but with the difference that it exhibits a non-centrosymmetric distortion of the InO$_{6}$ octahedra not present in the latter. For temperatures below the hydrated-to-intermediate phase transition, dehydration is featured by the release of protons confined to the layers of InO$_{4}$ tetrahedra, whereas above the transition also protons bound to oxygens of the layers of InO$_{6}$ are released. Finally, we found that the O-H stretch region of the vibrational spectra is not consistent with a single-phase spectrum, but is in agreement with the superposition of spectra associated with two different proton configurations. The relative contributions of the two proton configurations depend on how the sample is hydrated.

Short-Range Structure and Phonon Assignment of the Brownmillerite-Type Oxide Ba$_{2}$In$_{2}$O$_{5}$ and its Hydrated Proton-Conducting Form BaInO$_{3}$H

The vibrational spectra and short-range structure of the brownmillerite-type oxide Ba$_{2}$In$_{2}$O$_{5}$ and its hydrated form BaInO$_{3}$H, are investigated by means of Raman, infrared, and inelastic neutron scattering spectroscopies together with density functional theory calculations. For Ba$_{2}$In$_{2}$O$_{5}$, which may be described as an oxygen deficient perovskite structure with alternating layers of InO$_{6}$ octahedra and InO$_{4}$ tetrahedra, the results affirm a short-range structure of $Icmm$ symmetry, which is characterized by random orientation of successive layers of InO$_{4}$ tetrahedra. For the hydrated, proton conducting, form, BaInO$_{3}$H, the results suggest that the short-range structure is more complicated than the $P4/mbm$ symmetry that has been proposed previously on the basis of neutron diffraction, but rather suggest a proton configuration close to the lowest energy structure predicted by Martinez et al. [J.-R. Martinez, C. E. Moen, S. Stoelen, N. L. Allan, J. of Solid State Chem. 180, 3388, (2007)]. An intense Raman active vibration at 150 cm$^{-1}$ is identified as a unique fingerprint of this proton configuration.

Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins

We report on a polarized-neutron Laue diffraction experiment on a single crystal of neodynium doped lanthanum magnesium nitrate hydrate containing polarized proton spins. By using dynamic nuclear polarization to polarize the proton spins, we demonstrate that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.

Using neutron spin-echo to investigate proton dynamics in proton-conducting perovskites

We demonstrate the applicability of studying the proton dynamics in proton-conducting perovskites using neutron spin-echo spectroscopy, a powerful method hitherto neglected for studies of the proton dynamics in ceramic proton conductors. By combining our neutron spin-echo results of hydrated BaZr0.90Y0.10O2.95 with results obtained from kinetic modeling based on first-principles calculations we show that already over a length-scale as short as 2 nm the long-range proton self-diffusion is observed, and the likely effect of dopant atoms acting as trapping centers has averaged out.

Quasielastic neutron scattering of hydrated BaZr_{0.90}A_{0.10}O_{2.95} (A = Y and Sc)

Proton motions in hydrated proton conducting perovskites BaZr_{0.90}A_{0.10}O_{2.95} (A = Y and Sc) have been investigated using quasielastic neutron scattering. The results reveal a localized motion on the ps time scale and with an activation energy of ~10-30 meV, in both materials. The temperature dependence of the total mean square displacement of the protons suggests an onset of this motion at a temperature of about 300 K. Comparison of the QENS results to density functional theory calculations suggests that for both materials this motion can be ascribed to intra-octahedral proton transfers occurring close to a dopant atom. The low activation energy, more than ten times lower than the activation energy for the macroscopic proton conductivity, suggests that this motion is not the rate-limiting process for the long-range proton diffusion, i.e. it is not linked to the two materials significantly different proton conductivities.

Short-range structure of proton conducting perovskite BaIn_{x}Zr_{1-x}O_{3-x/2} (x = 0-0.75)

In a systematic study we investigate the effect of dopant level and hydration on the short-range structure of the proton conducting perovskite-type oxide BaIn_{x}Zr_{1-x}O_{3-x/2} (x = 0-0.75), using infrared and Raman spectroscopy. The results show that doping leads to significant local distortions of the average cubic structure of these materials. By increasing the In concentration from x = 0 to x = 0.75 new bands appear and grow in intensity in both the IR and Raman spectra, showing that the local distortions become successively more and more pronounced. The structural distortions are largely uncorrelated to the presence of oxygen vacancies, but instead are mainly driven by the size difference between the In^{3+} and Zr^{4+} ions, which leads to displacements of the cations and to tilting of the (In/Zr)O_{6} octahedra. Based on our results, we suggest that there is a threshold between x = 0.10 and x = 0.25 where the local structural distortions propagate throughout the whole perovskite structure. Comparison of our spectroscopic data with the proton conductivity reported for the same materials indicates that the presence of extended structural distortions are favorable for fast proton transport.