La2Mo2O9 exhibits two different crystallographic polymorphs,
including a high temperature cubic β phase and a
low temperature monoclinic α phase. The α phase of ordered
oxygen reversibly transforms into the β phase of disordered
oxygen around 580 °C. The ion conductivity of β phase has
been reported to be 0.06–0.08 S cm−1 at 800 °C, higher than
that of YSZ [6,7]. Although the β phase may be preserved for a
long period via quenching in single crystal experiments [8,9],
yet a sufficient phase stability is the prerequisite for electrolyte
applications