Local Iron Environment in Prussian Blue Analogues Probed by Mössbauer Spectroscopy

Abstract

Prussian  blue  analogues  are  a  versatile  class  of  cyanide-bridged  materials  whose  properties  can  be  tailored  through  compositional  engineering  and  control  of  the  local  coordination  environment.  Here,  we  investigate  the  mixed-metal  series Ni_1.5[Fe(CN)₆]_1-x[Cr(CN)₆]_x  · nH₂O (where x = 0–0.75) by Mössbauer spectroscopy at 300 and 80 K. Low-spin Fe(III) dominates the room-temperature spectra, while a minor Fe(II) contribution is attributed to trace surface species. Chromium substitution induces subtle changes in the hyperfine parameters, indicating modifications of the electronic density and local symmetry around the iron sites. At 80 K, two distinct low-spin Fe(III) environments emerge, revealing increased local structural heterogeneity. The observed behaviour is attributed to temperature-driven changes in the hydration network and suppression of water dynamics within the framework. The relatively weak dependence of the Mössbauer parameters on Cr content indicates efficient electronic isolation of Fe and Cr centres through the cyanide-bridged network. These findings provide new insight into the interplay between chemical substitution, hydration, and local electronic structure in mixed-metal PBAs, highlighting the unique capability of Mössbauer spectroscopy to probe subtle local effects in cyanide-based coordination frameworks.