$\mathrm{O}\text{−}\mathrm{H}$ wag vibrations in hydrated ${\mathrm{BaIn}}_{\mathit{x}}{\mathrm{Zr}}_{\mathbf{1}-\mathit{x}}{\mathrm{O}}_{\mathbf{3}-\mathit{x}∕\mathbf{2}}$ investigated with inelastic neutron scattering

The nature of $\mathrm{O}\text{−}\mathrm{H}$ wag vibrations in the cubic proton conducting perovskite-type oxides, ${\mathrm{BaIn}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_{3-x∕2}$ $(x=0.20–0.75)$ have been investigated by means of inelastic neutron scattering. Our spectroscopic results show that the $\mathrm{O}\text{−}\mathrm{H}$ wag vibrations are manifested as a broad band between 600 and $1300{\mathrm{cm}}^{-1}$. With increasing In concentration, the $\mathrm{O}\text{−}\mathrm{H}$ wag band increases in intensity and broadens with an increased weight of the high frequency part. The intensity increase of the $\mathrm{O}\text{−}\mathrm{H}$ wag band is a result of the increasing concentration of protons in the perovskite structure while the increased weight of the high frequency part results from an increased fraction of protons in strongly hydrogen bonding configurations. Such strongly hydrogen bonding configurations of the proton is primarily a result of the presence of oxygen vacancies and dopant atoms, which act as charged defects, but also of local structural distortions caused by the introduction of dopant atoms. From the temperature dependence of the $\mathrm{O}\text{−}\mathrm{H}$ wag band we find that the mean square displacement of the protons increases only slightly as the temperature is raised from 30 to $300\mathrm{K}$, which implies that it is unlikely that the protons undergo long-range diffusion for temperatures below $300\mathrm{K}$. Comparing the In concentration dependence of different components of the $\mathrm{O}\text{−}\mathrm{H}$ wag band with the proton conductivity of the reported compositions indicates that protons in weak or intermediate hydrogen bonding configurations are likely the main charge carriers.

Figure 1

INS spectra of hydrated ${\mathrm{BaIn}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_{3-x∕2}$ ($x=0.20$, 0.50, and 0.75) and dry ${\mathrm{BaIn}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_{3-x∕2}$ $(x=0.50)$ at $30\mathrm{K}$. The spectra have been vertically offset for clarity.

Figure 2

Gaussian fit of the baseline corrected $\mathrm{O}\text{−}\mathrm{H}$ wag band for hydrated ${\mathrm{BaIn}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_{3-x∕2}$ $(x=0.75)$.

Figure 3

Fit parameters: (a) and (b) show the peak positions and relative integrated intensities of the three Gaussian bands. (c) shows the sum of the integrated intensities of the three Gaussians, i.e., the total intensity of the $\mathrm{O}\text{−}\mathrm{H}$ wag band. Linear fits serve as guides for the eye. In (c), the fit has been extrapolated to $x=0$.

Figure 4

Temperature dependence of the INS spectra of hydrated ${\mathrm{BaIn}}_x{\mathrm{Zr}}_{1-x}{\mathrm{O}}_{3-x∕2}$ $(x=0.20)$. The spectra have been vertically offset for clarity.