F. Nakada

Magnetic and Electronic Properties of Valence-Controlled Ni–Fe Cyanide

Magnetics and electronic properties has been investigated for the Prussian blue type cyano-bridged transition metal compound, Na 0.72-δ Ni[Fe(CN) 6 ] 0.68 ·5.1H 2 O (0.0 ≤δ≤0.58), as a function of the hole concentration δ of the d -electron system. Mother compound (δ= 0) takes Ni 2+ ( t 2g 6 e g 2 : S = 1)–Fe 2+ ( t 2g 6 : S = 0) configuration, and is paramagnetic down to zero temperature. Holes are selectively introduced on the Fe site. Ferromagnetic transition appears at δ= 0.26, and the transition temperature T C increases from 11 K at δ= 0.26 to 21 K at δ=0.58. We further systematically investigated the absorption spectra in the infrared–violet region against δ. We compared these magnetic and electronic properties of the valence-controlled Ni–Fe compound with those of an isostructural valence-controlled Co–Fe compound.

Valence-differential spectroscopy of Co–Fe cyanide films

Electrochromism is extensively investigated for practical application of display and memory devices. To develop the material, reliable information on the optical and electronic properties of the solid film is indispensable. Here, we propose valence-differential spectroscopy that can selectively extract the spectral components related to the oxidized/reduced metal site. We applied the spectroscopy to Co2+–Fe2+δ and Co2+δ–Fe2+ cyanide films with finely control of averaged valence (δ) of the transition metal by external electric pulses. The spectroscopy revealed transition energy E, width Γ, and oscillator strength f of the spectral components related to the transition metal.

Oxidization/Reduction Process of Prussian Blue Film as Investigated by Valence-Differential Spectroscopy

The oxidization/reduction of a Prussian blue (FeIII[FeII(CN)6]3/44.2H2O) film was investigated by valence-differential spectroscopy, which selectively extracts spectral components related to the oxidized/reduced metal. The spectroscopy results revealed weak absorption components hidden in a structureless spectrum above ~2.6 eV. We found a significant red shift of the charge-transfer (CT) transition of an electron from the 3d t2g orbital of [FeII(CN)6] to the 3d t2g orbital of FeIII. We interpret the red shift in terms of the inhomogeneous broadening of the CT transition and selective oxidization/reduction.

Desorption-erasure of the Spin State Transition in Prussian-blue Type Cyanide

Desorption/adsorption of guest molecules in a nano-porous material is one of the sensitive tools to control the nature of the host framework via the strong guest-host interaction. A Co–Fe cyanide film, Na0.20Co[Fe(CN)6]0.71zH2O, shows a spin state transition from the high-spin (HS) phase [Co2+ (S = 3/2)–Fe3+ (S = 1/2)] to the low-spin (LS) phase [Co3+ (S = 0)–Fe2+ (S = 0)] at Tc = 220 K. Here, we found that the existence (Tc = 220–263 K) and nonexistence (Tc≤0 K) of the phase transition reversibly switched by the site-selective desorption of the guest waters. Such a gigantic controllability of Tc is ascribed to subtle competition between the HS phase and the LS phase together with the cooperative nature of the transition.

Desorption-induced first-order phase transition in a cyano-bridged compound

The guest-host interaction is one of the promising tools to control the material state. Here, we found that a cyano-bridged compound Na0.50Co[Fe(CN)6]0.723.8H2O shows a first-order structural phase transition below a critical pressure Pc (150Pa) at 300K. Judging from suppression of the OH stretching mode in the infrared spectra, we ascribed the phase transition to desorption of the ligand waters. The phase transition accompanies a significant change of the visible absorption spectra, reflecting the strong hybridization between the Coeg state and the CNσ states.

Transient photo-induced phenomena in vacancy-controlled Co-Fe cyanides

Time-resolved absorption spectra have been investigated for vacancy-controlled films of Na1.23Co[Fe(CN)6]0.92.9H2O and Na0.46Co[Fe(CN)6]0.713.8H2O, whose electronic configuration is CoIII - FeII and CoII - FeIII, respectively. In both the films, we observed absorption saturation at 2.3 eV, and ascribed it to formation of anti-state. We found that the charge dynamics is qualitatively different between two films.

Photoinduced dynamics of prussian blue type cyanide

Charge and lattice dynamics has been investigated for Na0.79Co[Fe(CN)6]0.902.9H2O film at 300 K by means of the time-resolved x-ray diffraction apparatus coupled with the in situ time-resolved absorption measurement. We observed an uniform lattice expansion whose relaxation time is ~ 40 ns. Based on the close correlation between the charge and lattice dynamics, the lattice expansion is ascribed to the photo-created charge-transferred Co2+ - Fe3+ state.