Synthesis and characterization of La1+xSr2−xCoMnO7−δ (x=0,0.2; δ=0,1)

The n=2 Ruddlesden–Popper phases LaSr2CoMnO7 and La1.2Sr1.8CoMnO7 have been synthesized by a sol–gel method. The O6-type phases LaSr2CoMnO6 and La1.2Sr1.8CoMnO6 were produced by reduction of the O7 phases under a hydrogen atmosphere. The materials crystallize in the tetragonal I4/mmm space group with no evidence of long-range cation order in the neutron and electron diffraction data. Oxygen vacancies in the reduced materials are located primarily at the common apex of the double perovskite layers giving rise to square pyramidal coordination around cobalt and manganese ions. The oxidation states Co3+/Mn4+ and Co2+/Mn3+ predominate in the as-prepared and reduced materials, respectively. The materials are spin glasses at low temperature and the dominant magnetic interactions change from ferro- to antiferromagnetic following reduction.