Dependence of activation energy on temperature and structure in lanthanum gallates

The parent phase of the lanthanum gallate series of electrolytes, LaGaO3, is orthorhombic at room temperature and rhombohedral between 250 and 1000degreesC (1-3). Our high resolution neutron powder studies on the doped phases, La0.9Sr0.1Ga0.8Mg0.2O3 and its Ba-analogue, show significant structural differences from the parent phase that have direct bearing on the ionic conduction process that can easily be understood in terms of the degree of tilting of the GaO6 octahedra. The monoclinic form at room temperature has a higher degree of distortion associated with the octahedra and undergoes phase transitions at elevated temperatures that relate to the change in activation energy for the ionic conduction obtained from AC impedance spectroscopy data. In particular the gradual alignment of the octahedra above 300 (Ba) or 500degreesC (Sr) as the cell heads toward cubic symmetry produces a slow, temperature-dependent decrease in Ea for ionic conduction as the tilting of the GaO6 is reduced, thereby facilitating the migration of the oxide ion. AC impedance studies of the doped lanthanum gallates reveal a similar activation energy dependence for the barium and strontium analogues with an activation energy of I eV below the transition to rhombohedral with a gradual decrease towards 0.6 eV at the highest temperature.