Local Iron Environment in Prussian Blue Analogues Probed by Mössbauer Spectroscopy

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 Ni1.5[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.