Effect of Compressive Strain on the Raman Modes of the Dry and Hydrated BaCe0.8Y0.2O3 Proton Conductor

Abstract

The BaCe0.8Y0.2O3−δ proton conductor under hydration and under compressive strain has been analyzed with high-pressure Raman spectroscopy and high-pressure X-ray diffraction. The pressure-dependent variation of the Ag and B2g bending modes from the O–Ce–O unit is suppressed when the proton conductor is hydrated, affecting directly the proton transfer by locally changing the electron density of the oxygen ions. Compressive strain causes a hardening of the Ce–O stretching bond, with the pressure coefficient Δν/Δp = 4.32 ± 0.05 cm–1/GPa being the same for the dry and hydrated sample. As a result of this hardening of the lattice vibrations, the activation barrier for proton conductivity is raised, in line with recent findings using high-pressure and high-temperature impedance spectroscopy. Hydration also offsets slightly the Ce–O B1g and B3g stretching modes by ∼2 cm–1 toward higher wave numbers, revealing an increase in the bond strength of Ce–O. The (20–2) Bragg reflections do not change during pressurizing and thus reveal that the oxygen occupying the O2 site displaces only along the b axis. The increasing Raman frequency of the B1g and B3g modes thus implies that the phonons become hardened and increase the vibration energy in the a–c crystal plane upon compressive strain, whereas phonons are relaxed in the b axis and thus reveal softening of the Ag and B2g modes. Lattice toughening in the a–c crystal plane raises therefore a higher activation barrier for proton transfer and thus anisotropic conductivity. Particularly for the development of epitaxial strained proto-conducting thin film devices with lower activation energy, such anisotropy has to be taken quantitatively into account. The experimental findings of the interaction of protons with the ceramic host lattice under external strain may provide a general guideline for yet to develop epitaxial strained proton-conducting thin film systems with high proton mobility and low activation energy.