Evidence for the Existence of Oxygen Clustering and Understanding of Structural Disorder in Prussian Blue Analogues Molecular Magnet M_1.5[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 ]· z H 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, 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 Fm 3 m ). 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≡C—Cr III sequence, confirms the formation of Prussian blue analogues, M 1.5 [Cr(CN) 6 ]· z H 2 O. The IR study also infers the presence of cyanide flipping in the Fe 1.5 [Cr(CN) 6 ]· z H 2 O compound. The Mössbauer study on the Fe 1.5 [Cr(CN) 6 ]· z H 2 O compound confirms the presence of high as well as low spin Fe II ions due to isomerization of some Cr III —C≡N—Fe II linkages to the Cr III —N≡C—Fe II form. The magnetization data show a soft ferromagnetic nature of both compounds with a Curie temperature of ∼17 and ∼22 K for Fe 1.5 [Cr(CN) 6 ]· z H 2 O and Co 1.5 [Cr(CN) 6 ]· z H 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.