Properties of transition metal oxides with layered perovskite structure

The manganates of La_n-nxSr_1+nxMn_nO_3n+1(x = 0.4; n = 1, 2, ∞) were prepared to study the relationship between their electronic properties and their crystal structure; two-dimensional structure with n = 1, pseudo-two-dimensional with n = 2 and three-dimensional with n = ∞. The n = 1 manganate exhibits semiconducting behavior below room temperature. When the manganate has a higher dimensional structure with an increase in the value of n, the electric conduction changes to a more itinerant one. The n = 2 manganate exhibits an electric transition from semiconductor to metal at 127 K when the temperature is reduced below room temperature. The n = ∞ manganate exhibits metallic behavior below room temperature. Ferromagnetism was found when the metallic conduction occurred, i.e., in the n = 2 manganate below the electric transition temperature, and in the n = ∞ manganate below room temperature. From photoelectron spectroscopy, the valence band structure was found to consist of two features, due to the photoemission from the Mn 3d orbital (∼ 3 eV) and the O 2p orbital (∼5.5 eV). The change in the dimension of the crystal structure affects the electronic structure, that is, the ratio of the intensity of the photoemission from the Mn 3d orbital to that of the O 2p orbital increases with an increase in n.