Evidence for the Existence of Oxygen Clustering and Understanding of Structural Disorder in Prussian Blue Analogues Molecular Magnet M1.5[Cr(CN)6)•zH2O (M = Fe and Co): Reverse Monte Carlo Simulation and Neutron Diffraction Study

A detailed structural disorder investigation of Prussian blue analogues M-1.5[Cr(CN)(6)]center dot zH(2)O (M = Fe and Co) has been done by carrying out a reverse Monte Carlo (RMC) simulation on the powder neutron diffraction data. X-ray diffraction, infrared spectroscopy, Mossbauer spectroscopy, and dc magnetization measurements have also been employed to investigate the structural and magnetic properties of the compounds. The Rietveld refinement of the X-ray and neutron diffraction patterns reveals that both compounds are in a single phase with a face-centered cubic crystal structure (space group Fm3m). The observation of characteristic absorption bands in the range 1900-2200 cm(-1) of infrared (IR) spectra, which corresponds to the CN stretching frequency of M-II-N equivalent to C-Cr-III sequence, confirms the formation of Prussian blue analogues, M-1.5[Cr(CN)(6)]center dot zH(2)O. The IR study also infers the presence of cyanide flipping in the Fe-1.5[Cr(CN)(6)]center dot zH(2)O compound. The Mossbauer study on the Fe-1.5[Cr(CN)(6)]center dot zH(2)O compound confirms the presence of high as well as low spin Fe-II ions due to isomerization of some Cr-III-C equivalent to N-Fe-II linkages to the Cr-III-N equivalent to C-Fe-II form. The magnetization data show a soft ferromagnetic nature of both compounds with a Curie temperature of similar to 17 and similar to 22 K for Fe-1.5[Cr(CN)(6)]center dot zH(2)O and Co-1.5[Cr(CN)(6)]center dot zH(2)O, respectively. A large amount of structural disorder is present in both compounds, which is manifested in the form of a diffuse scattering in neutron diffraction patterns. The RMC results, obtained after the modeling, simulation, and analysis of the neutron diffraction data, propose that the water molecules and the [Cr(CN)(6)] vacancies are mainly responsible for the structural disorder. Moreover, a clustering of the non-coordinated oxygen atoms around the coordinated oxygen atoms is also ascertained by the RMC analysis. The correlation of structural disorder with the water content and [Cr(CN)(6)] vacancies is also discussed.