Structural and magnetic properties of double perovskites AA ' FeMoO6 (AA '= Ba-2, BaSr, Sr-2 and Ca-2)

Room temperature x-ray diffraction, magnetization and neutron diffraction m easurements up to 500 K have been carried out on polycrystalline bulk doubl e perovskites AA'FeMoO6 (AA' = Ba-2, BaSr, Sr-2, Ca-2) in order to determin e and correlate their structural and magnetic properties. As the average io nic radius is diminished, the crystallographic structure evolves from cubic (for AA' = Bat, BaSr) to tetragonal (Sr-2) and finally to monoclinic (Ca-2 ). In the case of AA' = Sr-2, a novel crossover from a high temperature par amagnetic cubic state to a low temperature ferrimagnetic tetragonal state h as been observed. For all the studied compounds, neutron diffraction patter ns and magnetization measurements are consistent with a ferrimagnetic order ing of the Fe and Mo sublattices. A remarkable correlation is found between the Curie temperature and the electronic bandwidth, which is controlled by structural parameters.