Microstructure and ionic conductivity of LLTO thin films: Influence of different substrates and excess lithium in the target

• Epitaxial growth of Li3xLa2/3 − xTiO3 thin films on oriented SrTiO3, LaAlO3 and MgO substrates.
• Unavoidable growth of La2Ti2O7 secondary phase at the oxygen pressure required to maintain low electronic conductivity.
• Secondary phases act as blocking layer to lithium ion diffusion.
• Up to 10% excess of lithium in the target is necessary to compensate lithium loss.

The deposition of single phase Li3xLa2/3 − xTiO3 (LLTO) thin films remains very challenging. The growth of the perovskite phase is in competition with the insulating La2Ti2O7 phase when prepared at high oxygen pressure by PLD. Nevertheless, we have achieved epitaxial growth of LLTO on different (001) oriented substrates such as LaAlO3, SrTiO3 and MgO despite a large lattice mismatch of up to + 8.8%. We also determined the percentage of lithium excess in the target necessary to reach a maximum ionic conductivity. However, the presence of the blocking La2Ti2O7 phase strongly hinders the lithium ion migration and reduces the total conductivity compared to bulk properties.