Synthesis and electronic structure of proton-type partially substituted birnessite by period-four transition metal

Partially substituted proton-type birnessite were prepared by solid state reaction and their structures were refined. The formed birnessite with no substitution is identified to rhombohedral phase. In the case of substitution treatment by V and Cr for Mn, birnessite phase was not formed. On substituting Fe, hexagonal phase increased with increase of the amount of the Fe. For Co and Ni-substitution, monoclinic phase emerged at substitution ratio of around 0.37 and 0.02, respectively. For the substitution of Cu, only the monoclinic birnessite formed irrespective of the ratio. The electric conductivity of the partially substituted birnessites was examined at room temperature. The general trend is lower conductivity with increasing ratio of contained substituents. On several mol% of the substitution by Ni and Cu, the conductivity slightly increased. From DOS calculation of these compounds, the partially substitution for Mn by Fe, Co and Ni in the birnessite poses splitting of crystal field to emerge new bands at around −1 and +1 eV by Mn(IV) 3d orbital.

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► Partial metal substitutions of birnessite result in three types of crystal structure.
► Rhombohedral, monoclinic and hexagonal phase emerged in the substituted birnessite.
► The electrical conductivity decreased by substitution of transition metal for Mn.
► Partially substitution by Fe, Co and Ni poses splitting of crystal field for MnO6.