Nonstoichiometry, point defects and magnetic properties in Sr(2)FeMoO(6-delta) double perovskites

The phase stability, nonstoichiometry and point defect chemistry of polycrystalline Sr(2)FeMoO(6-delta) (SFMO) was studied by thermogravimety at 1000, 1100, and 1200 degrees C. Single-phase SFMO exists between -10.2 <= log pO(2) <= -13.7 at 1200 degrees C. At lower oxygen partial pressure a mass loss signals reductive decomposition. At higher pO(2) a mass gain indicates oxidative decomposition into SrMoO(4) and SrFeO(3-x). The nonstoichiometry oat 1000, 1100, and 1200 degrees C was determined as function of pO(2). SFMO is almost stoichiometric at the upper phase boundary (e.g. delta=0.006 at 1200 degrees C and log pO(2)=-10.2) and becomes more defective with decreasing oxygen partial pressure (e.g. delta=0.085 at 1200 degrees C and log pO(2)=-13.5). Oxygen vacancies are shown to represent majority defects. From the temperature dependence of the oxygen vacancy concentration the defect formation enthalpy was estimated (Delta H(OV)=253 +/- 8 kJ/mol). Samples of different nonstoichiometry delta were prepared by quenching from 1200 degrees C at various pO(2). An increase of the unit cell volume with increasing defect concentration 0 was found. The saturation magnetization is reduced with increasing nonstoichiometry delta. This demonstrates that in addition to Fe/Mo site disorder, oxygen nonstoichiometry is another source of reduced magnetization values