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

Abstract

A detailed structural disorder investigation of Prussian blue analogues M_1.5[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 M^II—N≡C—Cr^III sequence, confirms the formation of Prussian blue analogues, M_1.5[Cr(CN)₆]·zH₂O. The IR study also infers the presence of cyanide flipping in the Fe_1.5[Cr(CN)₆]·zH₂O compound. The Mössbauer study on the Fe_1.5[Cr(CN)₆]·zH₂O 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]·zH₂O and Co_1.5[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.