Gel-State Dependencies of Brown Patterns of Mn–Fe-Based Prussian Blue Analogues Studied by Combined X-ray Spectroscopies

Abstract

Abstract The gel-state dependencies of brown patterns of Mn–Fe-based Prussian blue analogues (PBAs) formed in water-glass gels by reaction–diffusion (RD) processes have been studied by using X-ray fluorescence and X-ray absorption near-edge structure spectroscopies. Three tubes containing admixtures of 0.25 M [Fe(CN)6]3− and 0.30 M acetic acid (A and B) or boric acid (C) in water-glass (9.1 (A) and 7.4 (B and C) mass%) were brought into contact with 0.20 M MnSO4 solution. Tube A formed periodic (but not genuine Liesegang) bands, whereas tubes B and C formed light-brown bands in the turbid zone (TZ). A lowered amount of water-glass (A → B) resulted in a more fluctuant distribution of the PBAs. [Mn(H2O)6]2+ ions were also present in the TZ of tube B and their content increased with distance from the gel-junction after stopping the RD process. Addition of boric acid (C) suppressed the RD processes in the gel, slowing down the formation of PBAs. It is likely that the areas of low PBA content in the TZ of tubes B and C generate the light-brown bands. FEFF calculations suggest that the most likely local structures of the predominant PBAs in tubes A/B and C are Mn(NCFe)2O4K2 and Mn(NCFe)2O4, respectively.