Evidence for the Existence of Oxygen Clustering and Understanding of Structural Disorder in Prussian Blue Analogues Molecular Magnet M_1.5[Cr(CN)₆]·<i>z</i>H₂O (M = Fe and Co): Reverse Monte Carlo Simulation and Neutron Diffraction Study

Abstract

A detailed structural disorder investigation of Prussian blue analogues M1.5[Cr(CN)6]·zH2O (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, Mössbauer 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 MII—N≡C—CrIII sequence, confirms the formation of Prussian blue analogues, M1.5[Cr(CN)6]·zH2O. The IR study also infers the presence of cyanide flipping in the Fe1.5[Cr(CN)6]·zH2O compound. The Mössbauer study on the Fe1.5[Cr(CN)6]·zH2O compound confirms the presence of high as well as low spin FeII ions due to isomerization of some CrIII—C≡N—FeII linkages to the CrIII—N≡C—FeII form. The magnetization data show a soft ferromagnetic nature of both compounds with a Curie temperature of ∼17 and ∼22 K for Fe1.5[Cr(CN)6]·zH2O and Co1.5[Cr(CN)6]·zH2O, 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.