Abstract
Hydration of oxygen-deficient metal oxides causes filling of oxygen vacancies and formation of hydroxyl groups with interstitial structural protons, rotating around the oxygen in localized motion. Thermal activation from 500 to 800 K triggers delocalization of the protons by jumping to adjacent oxygen ions, constituting proton conductivity. We report quantitative analyses of proton and lattice dynamics by neutron-scattering data, which reveal the interaction of protons with the crystal lattice and proton–phonon coupling. The motion for the proton trapped in the elastic crystal field yields Eigen frequencies and coupling constants, which satisfy Holstein’s polaron model for electrons and thus constitutes first experimental evidence for a proton polaron at high temperature. Proton jump rates follow a polaron model for cerium-oxygen and hydroxyl stretching modes, which are thus vehicles for proton conductivity. This confirms that the polaron mechanism is not restricted to electrons, but a universal charge carrier transport process.
Highlights
Hydration of oxygen-deficient metal oxides causes filling of oxygen vacancies and formation of hydroxyl groups with interstitial structural protons, rotating around the oxygen in localized motion
Observations on the elastic properties made by experimenters Slodczyk et al.[8] with vibration spectroscopy made them speculate over the potential polaronic nature of the proton conductivity: ‘The question of the vibrational signature of isolated proton and its dynamic nature is open’[8]
This indicates that the quasi-elastic neutron scattering (QENS) contribution from the non-pressurized hydrated BCY20 due to proton diffusivity disappears upon pressurizing the sample
Summary
Hydration of oxygen-deficient metal oxides causes filling of oxygen vacancies and formation of hydroxyl groups with interstitial structural protons, rotating around the oxygen in localized motion. Proton jump rates follow a polaron model for cerium-oxygen and hydroxyl stretching modes, which are vehicles for proton conductivity This confirms that the polaron mechanism is not restricted to electrons, but a universal charge carrier transport process. Observations on the elastic properties made by experimenters Slodczyk et al.[8] with vibration spectroscopy made them speculate over the potential polaronic nature of the proton conductivity: ‘The question of the vibrational signature of isolated proton (for example, the ionic proton, a proton sharing its interaction with more than two acceptors) and its dynamic nature (proton gas, polaron and so on) is open’[8] Such a proton polaron was not identified so far in hard inorganic matter such as metal oxides. Comparison with our vibration spectroscopy data suggest that it is the cerium-oxygen and hydroxyl stretching modes that propel the protons as charge carriers
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