Perovskite‐type metal oxides such as Y‐doped BaMO3 (M = Zr/Ce) have drawn considerable attention as proton‐conducting electrolytes for intermediate temperature ceramic electrochemical cells. Improving the proton conductivity at lower temperatures requires a comprehensive understanding of the proton conduction mechanism. By applying high pressure or varying the Ce content of Y‐doped BaMO3, it is demonstrated that the proton conductivity follows the Meyer–Neldel rule (MNR) well. In the Arrhenius plot, the conductivities intersect at an isokinetic temperature, where the proton conductivity is independent of activation energy. Considering the relationship between isokinetic temperature and lattice vibration frequency, a high isokinetic temperature is observed in materials with stiff lattices, consisting of light atoms and small MO bond length. Based on consideration of the MNR, it is suggested that the enhancement of proton conductivity at low temperature can be well achieved by tuning lattice vibration frequency toward a desired isokinetic temperature.

Proton conduction is an important property for fuel cell electrolytes. The search for molecular details on proton transport is an ongoing quest. Here, we show that in hydrated yttrium doped barium zirconate using X-ray and neutron diffraction that protons tend to localize near the dopant yttrium as a conjugated superstructure. The proton jump time measured using quasi-elastic neutron scattering follows the Holstein-Samgin polaron model, revealing that proton hopping is weakly coupled to the high-frequency O-H stretching motion, but strongly coupled to low-frequency lattice phonons. The ratio of the proton polaron effective mass, m*, and the proton mass is m*/m = 2, when coupled to the Zr-O stretching mode, giving experimental evidence of proton pairing in perovskites, as a result of proton-phonon coupling. Possible pathways of a proton pair are provided through Nudge Elastic Band calculations. The pairing of protons, when jumping, is discussed in context of a cooperative protonic charge transport process.

Proton-conducting metal oxides such as Y-doped BaZrO3 have a wide application prospect as electrochemical energy converters at intermediate temperature. Understanding their proton transport mechanisms is important for improving the proton conductivity. Here, we investigate the influence of the crystal structure and the lattice dynamics of the BaZr0.9Y0.1O3‑δ proton conductor using high-pressure Raman spectroscopy and high-pressure X-ray diffraction. A linear increase in the Raman shifts with pressure between 2.7 and 7.5 cm–1GPa–1 is observed for the vibrational modes higher than 450 cm–1 but almost invariant for vibrational modes less than 450 cm–1, highlighting the more significant phonon hardening effect of Zr–O stretching motions at high pressure than other motions. When the pressure increases from ambient pressure to 2 GPa, despite the 4 orders of magnitude enhancement in the Arrhenius prefactor and attempt frequency, the 2-fold increase in the activation energy results in a smaller proton conductivity at low temperature upon phonon hardening. A scenario of proton transfer combined with the Zr–O stretching motion to overcome the energy barrier is illustrated. According to the proton polaron model, the calculated polaron radius lies in the range of lattice parameters, suggesting the formation of lattice polarons in BaZr0.9Y0.1O3‑δ. Our findings suggest that a softer lattice can enhance the low-temperature proton conductivity with a reduced energy barrier.

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