Colossal enhancement in ionic conductivity of La2Mo2O9 thin films: Role of lattice strain and oxygen vacancy

Abstract

Search for high oxygen-ion conducting electrolyte forms a crucial step towards lowering the operating temperature of conventional solid oxide fuel cell. Here, we report an investigation of conductivity in polycrystalline films of Er and W codoped La_1.8Er_0.2W_0.3Mo_1.7O₉ (WEr0.2), deposited on four different substrates Si(111), SrTiO₃(100), LaAlO₃(100) and MgO(100) with different growth orientation and lattice parameters. While all the films show an increase of conductivity by several orders compared to its bulk counterpart, a magnificent increase of about six orders (13 S/cm @ 375 °C) has been observed in films on Si substrate. Films with SrTiO₃ and LaAlO₃ substrate further show steep rise in conductivity around 450 °C, mimicking the undoped bulk La₂Mo₂O₉ which is triggered by structural phase transition. This large modification of conductivity is believed to be induced by interfacial strain and consequent oxygen vacancy that increases from MgO to Si substrate due to their difference in lattice parameters and growth orientation. The oxygen vacancy concentration calculated using Nernst-Einstein analysis of frequency dependent impedance data further supports an increasing oxygen vacancies in film on Si substrate. The present results show an effective way of tuning conductivity through substrate strain which would hopefully have considerable effect on much sought low temperature operation of fuel cell.