Charge Transport in High-Entropy Oxides

This work presents the results of research on the transport properties of the high-entropy BaZr1/8Hf1/8Sn1/8Ti1/8Y1/8In1/8Sm1/8Yb1/8O3–x perovskite oxide with special focus on proton transport. The presented study is part of broader work in which we focus on multiple different chemical compositions with the cation number varying from 5 up to 12 (in B-sublattice). The presence of proton defects is analyzed with thermogravimetry, whereas the results of electrochemical impedance spectroscopy in dry, H2O-, and D2O-containing synthetic air in the 300–800 °C temperature range enable the evaluation of the proton and deuterium conductivities. The isotope effect is observed and discussed. The obtained data allow us to establish the contribution of proton conductivity to the total one and the transport numbers for proton/deuterium conductivity. Based on the bulk and grain boundary conductivities, the potential at a grain boundary φ0, Debye length LD, and space-charge layer (SCL) thickness λ for proton defects is calculated. The potential barrier heights were found to be significantly lower than those observed for typical polycrystalline-doped barium zirconates. For the first time in the case of high-entropy oxides, the electrical conductivity relaxation (ECR) studies are performed, allowing the calculation of water kinetic coefficients. The ECR in the 300–600 °C temperature range revealed a single-fold nature, which indicates a negligible component of the electronic hole conductivity in the hydrated material. The chemical diffusion coefficient of water DOH• and the chemical surface exchange coefficient of water kOH• along with their activation energies are determined. The chemical diffusion coefficient DOH• is in a range of 10(1)−8–10(1)−6 cm2 s–1, and the chemical surface exchange coefficient kOH• is in a range of 10(1)−6–10(1)−4 cm s–1.