La2Mo2O9 based materials (LAMOX) have been studied in last few years as potential solid electrolytes for intermediate temperature applications, such as oxygen sensor, dense membranes for oxygen separation and electrolyte for solid oxide fuel cells (SOFCs) (1,2). Non-substituted La2Mo2O9 phase presents two different polymorphs with a reversible structural phase transition at 560ºC, which is accompanied with a great improvement of the ionic conductivity above the phase transition temperature. The low temperature α-polymorph shows a 2×3×4 superstructure relative to the high temperature and better conductor cubic β-polymorph, which is usually associated to a rearrangement in the oxygen sublattice (3-6). The phase transition of non-substituted La2Mo2O9 is detrimental for practical applications due to a drastic drop in the conductivity below 560ºC and mechanical failure caused by the high thermal expansion between the high and low temperature polymorphs. A doping strategy has been investigated in order to stabilise