Phase stability and properties of Ln(2)Mn(2/3)Mo(4/3)O(7) (Ln=rare earth)

The high temperature phase stability of the complex oxide with a pyrochlore-related structure Ln(2)Mn(2/3)Mo(4/3)O(7) was investigated by thermogravimetric analysis at 1273 K under various oxygen partial pressures. The Ln(2)Mn(2/3)Mo(4/3)O(7) phase for Ln = Sm and Eu decomposes in two steps below 10(-15) atm oxygen partial pressure, while when Ln = Gd and Er the Ln(2)Mn(2/3)Mo(4/3)O(7) phase is stable only in a limited oxygen partial pressure range near 10(-11) atm and this phase decomposes in three steps with decreasing oxygen partial pressure. In both cases, when the oxygen partial pressure is higher than 10(-5) atm, Ln(2)Mn(2/3)Mo(4/3)O(7) is easily oxidized and changes into other substances. From the relationship of thermodynamic equilibrium, the free energy changes of each decomposing reaction and standard free energy of formation of the Ln(2)Mn(2/3)MO(4/3)O(7) phase at 1273 K were calculated. The measurement of magnetic susceptibility shows that Er2Mn2/3Mo4/3O7 is paramagnetic in the temperature range 77-600 K. Semiconductivity with a small activation energy was observed by measuring electrical conductivity above 135 K, and a smaller temperature dependence was seen below 135 K.