Thermodynamic nature of and spontaneous strain below the cubic-monoclinic phase transition in La₂Mo₂O₉

The pseudocubic lattice parameter of lanthanum molybdate, La ₂Mo₂O₉, has been measured as a function of temperature between 13 and 1100 K. These data show an anomaly at 842 K, associated with the alpha (pseudocubic monoclinic, non-conducting) to beta (cubic, conducting) phase transition. For a sample which has been quenched from high temperatures, the a(T) data are continuous across this transition, indicating that the transition is smeared or thermodynamically second order. When the same material is cooled slowly, subsequent measurements show the transition to be first order, with a similar transition temperature. The difference between these two behaviours is attributed to changes in oxygen ordering resulting from different thermal histories.