Ab initio study of La10-xSrx(Si,Ge)(6)O27-0.5x apatite for SOFC electrolyte

Apatite structure materials have outstanding performances as electrolytes for SOFC. Among them, La10-xSrx(Si,Ge)(6)O27-0.5x (LSO) series have attracted more attention for a higher ionic conductivity. Two doping positions have been investigated, Sr dopant at La position and Ge dopant at Si position. Doped LSO conductivity was studied using first principle density functional theory (DFT) calculations. With the increase of Sr-doped ratio, the DOS (density of states) peaks above Fermi level shift to a higher energy level. Furthermore, the electron energy gap broadening reduces the electron migration, while ion migration was limited. However, few changes of DOS with Ge substitution can be observed. The activation energies of these two dopants are simulated by the energy barriers of the migration channel. We have observed that the two dopant positions affect the apatite ionic conductivities with different mechanisms. Indeed, La doped position change the electron energy level and activation energy. However, the Si position can affect the density of electrons since it is surrounded by the oxygen and away from the migration channel. The calculation results of ionic conductivity are also consistent with those obtained by experiments. (C) 2015 Elsevier B.V. All rights reserved.