Three-Dimensional Network-Structured Cyanide-Based Magnets
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Three-Dimensional
Network-Structured Cyanide-Based Magnets Joel S. Miller
Introduction Magnets based on metal oxides have been important for hundreds of years. Magnetite, Fe3O4, Co-doped -Fe2O3, and CrO2 are important examples. The oxide (O 2) bridge between the magnetic metal ions has filled p orbitals (Figure 1a) that provide the pathway for strong spin coupling. Albeit with twice as many atoms, cyanide (CN) can bridge between two metal ions via its pair of empty antibonding orbitals (Figure 1b) and filled nonbonding orbitals. Even prior to a detailed understanding of either their composition or structure, magnetic ordering of several cyanide complexes, although at low temperature, was noted.1 The differing atoms at each end of the cyanide ion have different binding affinities to metal ions, and
Figure 1. Orbital overlap diagram for (a) an M–O–M bridge with filled oxygen p orbitals and for (b) an M–CN–M bridge with empty orbitals.
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simple coordination compounds, for example, [FeII(CN)6]2 (ferrocyanide), with alkali cations can easily be made. Replacement of the alkali cations with transitionmetal cations affords insoluble materials, for example FeIII4[FeII(CN)6]3 (Prussian blue). Prussian blue has been used as a pigment and as an electrochromic and electrocatalyst material.2 The structure of Prussian blue was elucidated3 to be cubic (isotropic) with lFeII kCNlFeIII kNClFeII k linkages along all three crystallographic directions (Figure 2). The FeII · · · FeIII separation is 5 Å. However, based on the composition, this is an idealized structure, as one FeII site per unit cell is missing. Water fills the vacant sites as well as the channels present in the structure. Due to the structural defects, it has been a challenge to grow single crystals. The metal ion, its oxidation state, and, consequently, its number of spins per site can be altered with relative ease, leading to a variety of magnetic behaviors.4 Depending on the charges, the modified Prussian
Figure 2. Idealized structure of Prussian blue type material with lMkCNlMkCNlMk linkages along all three crystallographic directions. For Prussian blue, M Fe II and M Fe III.
MRS BULLETIN/NOVEMBER 2000
Three-Dimensional Network-Structured Cyanide-Based Magnets
blue may have more or fewer structural defects. For example, NiII3[CrIII(CN)6]2 has 4/3 [CrIII(CN)6]3 sites per unit cell missing and filled with water, while CsNiII[CrIII(CN)6] has no defects in the metal-ion sites, but has an interstitial Cs ion. Neglecting defects, the Prussian blue structure type is composed of My[M(CN)6], with M always being C-bound and M being N-bound. Due to the strong ligand field character of the cyanide ligand, the C-bound M is always low spin. In contrast, the N-bound M is high spin. Thus, for FeIII4[FeII(CN)6]3, the d6 FeII’s are low spin (S 0) (Figure 3a), while the d5 FeIII’s are high spin (S 5/2) (Figure 3b). Consequently, the only spin coupling is between distant FeIII sites. Nonetheless, ferromagnetic ordering occurs at 5.6 K.3 Both ferr
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