Photomagnetic CoFe Prussian Blue Analogues: Role of the Cyanide Ions as Active Electron Transfer Bridges Modulated by Cyanide−Alkali Metal Ion Interactions

Abstract

X-ray absorption spectra at the Co L(2,3)-edges were analyzed by means of ligand field multiplet calculations in different states of three photomagnetic CoFe Prussian blue analogues of chemical formula Cs(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O, Rb(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O and Na(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O. These simulations of the experimental spectra allowed the quantification of the crystal field parameter (10Dq). This determination led us (i) to evidence different behaviors of the Co(III)(LS) and Co(II)(HS) ions in the three-dimensional structure related to their electronic configurations, (ii) to propose an approach based on the electronic density distribution along the Co-NC-Fe linkage to account for the energy position of the states implied in the switching properties of the compounds, and (iii) to explain the different photomagnetic properties observed as a function of the size of the inserted alkali cation by competing interactions between the cyanide ion and the transition metal ions within the CoFe cyanide bimetallic network on the one hand and the cyanide ion and the alkali metal ions on the other hand.

Reversible mechanism for spin crossover in transition-metal cyanides

“…eV). 16,31,32 For the ferromagnetic (FM) high spin state, the band gap energy of 1.25 eV in the minority spin channel represents the Co II →Fe III CT excitation (Fig. 2).…”

“…1,2 The calculated Co crystal field splitting (∆ Co o =0.97 eV) also agrees with x-ray absorption spectra (1.1 eV). 16,31,32 There is a further splitting between the majority and minority spin channels due to intra-atomic exchange interaction. If this exchange splitting ∆ x is higher than ∆ o , the majority spin channel is occupied first, followed by minority spin channel.…”

“…1,2,12,13 However, neither the microscopic transition mechanisms nor the effects of Fe-vacancies, water, and alkali metals on these mechanisms are completely understood, despite considerable experimental and theoretical effort. [16][17][18][19] 6 ] does not influence the charge transfer excitation energy. 23 Here we investigate the microscopic spin transition mechanisms via an ab initio lattice model for CoFe-PBA.…”

Limitation of adsorptive penetration of cesium into Prussian blue crystallite

“…As a result, the vacancy is an important factor determining the amount of charge compensation cations, that is, potential adsorption sites; an increase in the vacancy leads to a decrease in the charge compensation cation sites. Considering this effect, the more general chemical formula of PB is written as M 4a-3 Fe(III)[Fe(II)(CN) 6 ] a h 1-a (0.75 £ a B 1), where h is the [Fe(II)(CN) 6 ] 4-vacancy (Cafun 2010 ] 3 from the elemental analysis and Mössbauer spectroscopy, whose value of a corresponds to 0.75 in the above formula (Buser et al 1977;Herren et al 1980;Ito et al 1968). This means that insoluble PB has no charge compensation cation.…”

“…Many reports have successfully determined the chemical formula of PB or PBA from the elemental analysis including ICP-MS, CHN, etc., using this simple principle of electrical neutrality(Bleuzen et al 2008;Cafun et al 2010;Her et al 2010;Kareis et al 2012;Ishizaki et al 2013;Matsuda et al 2012; …”