Infrared (IR) absorption spectra for O-H (vOH) and O-D (vOD) stretching vibration regions of Ca2+-, Sr2+-, and Ba2+-doped La2Zr2O7 proton conductors were recorded. The spectra consisted of three to six vOH bands at 3200-3600 cm-1. The vOH bands were positioned at higher wavenumbers and were sharper than the case for the well-known high-temperature proton conductors with perovskite and related structures. The three to six vOH bands observed were attributable to different sites of protons dissolved in the crystal. The dissolving sites of protons were discussed in terms of the dopant alkaline-earth species. The observed vOH bands were reasonably interpreted assuming that six sites for protons exist in La2Zr2O7-based proton conductors. The dissolving site of protons itself did not depend on the dopant species, but the population of protons in each site did depend on the dopant species. The protons preferred the site corresponding to the vOH at the lower wavenumber, i.e., the site with strong hydrogen bonding strength. This tendency was discussed from the viewpoint of the basicity of the oxygen near the dopant and structural distortion and relaxation introduced by doping. Because the population of protons in each site strongly depended on the dopant species despite the very small concentration of ∼1 atom %, it was shown that the protons dissolved at the nearest neighbor of the alkaline-earth dopant, at least near room temperature at which the IR spectra were recorded.
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