Protonic Conduction in the BaNdInO₄ Structure Achieved by Acceptor Doping

Abstract

The potential of calcium-doped layered perovskite compounds, BaNd_1-<i>x</i> Ca _<i>x</i> InO_4-<i>x</i>/2 (where <i>x</i> is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1-2 orders of magnitude higher than those of the pristine material, BaNdInO₄. The highest total conductivity of 2.6 × 10^-3 S cm^-1 was obtained in the BaNd_0.8Ca_0.2InO_3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the <i>x</i> = 0.1 and <i>x</i> = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250-750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H₂O) was 1.3 × 10^-4 S cm^-1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd_0.8Ca_0.2InO_3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd_0.8Ca_0.2InO_3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the <i>x</i> = 0.2 sample, we conclude that experimental evidence indicates that BaNd_1-<i>x</i> Ca _<i>x</i> InO_4-<i>x</i>/2 is a fast proton conductor.