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Abstract
The ALaMgMnO6-x (A = Ca, Sr, Ba) oxides with cubic perovskite structure have been prepared under high oxygen pressure. From the chemical and thermogravimetric analyses, the extent of oxygen deficiency and the average oxidation state of manganese were estimated. The content of Mn(V) increases with a cation going from Ca to Sr, and to Ba. The magnetic measurements indicated that CaLaMgMnO6-x follows the curie-Weiss law with μeff = 3.80 B.M., whereas SrLaMgMnO6-x and BaLaMgMnO6-x are ferromagnetic. Through EPR study, Mn(VI) in a tetrahedral site and Mn(IV) in an octahedral site have been detected in CaLaMgMnO6-x while weak ferromagnetic couplings were observed in SrLaMgMnO6-x and BaLaMgMnO6-x due to the superexchange interactions between the t 2g electrons of Mn(IV) and Mn(V) ions. The evolution of all the physical properties could be explained by the variation of covalent bonding character between Mn—O bond in three compounds; the π(Mn—O) bond competing with σ[(A, La)—O] bond becomes stronger with A cation going from Ca to Sr, and to Ba, which enhances the formation of Mn(V) in the perovskite lattice.