Spin transition in a four-coordinate iron oxide
Raimund Podloucky2009, Nature Chemistry
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Abstract
Spin transition has attracted interests of researchers in various fields since early 1930's, with thousands of materials being realized including minerals and biomolecules. However, so far the metal centers in which it occurs have been almost always octahedral sixcoordinate 3d 4-7 metals, such as Fe(II). A five-coordination is only rarely seen. Here we report that SrFe 2+ O 2 , featuring a four-fold square-planar coordination, exhibits a high-spin (S = 2) to intermediate-spin (S = 1) transition on pressurization. A transition from antiferromagnetic insulator to ferromagnetic metal transition takes place at the same time. The ferromagnetic S = 1 state is found to be a half metal due to the inception of halfoccupied spin-down (d xy , d yz) states. These results highlight the square-planar coordinated iron oxides as a new class of magnetic/electric materials. Since the early 1930's the phenomenon of spin transition has attracted researchers of various fields, finding thousands of materials including minerals and biomolecules. In spite of all these efforts up to now, the metal centers, which undergo the spin transition, are nearly exclusively in an octahedral six-fold coordination such as Fe(II). A five-fold coordination is only rarely seen. Here we report that SrFe 2+ O 2 , featuring a four-fold square-planar coordination, exhibits a high-spin (S = 2) to intermediate-spin (S = 1) transition on pressurization. This transition is accompanied by a transition from antiferromagnetic insulating to a ferromagnetic so-called half-metallic states: only half of the spin-down (d xy , d yz) are filled. These results highlight square-planar coordinated iron oxides as new class of magnetic/electric materials. Spin transition, or spin crossover, generally occurs in compounds of octahedrally coordinated 3d transition metal ions with d 4 , d 5 , d 6 and d 7 electronic configurations, and they are driven by the competition between the intra-atomic exchange energy and the crystal field energy. The former stabilizes a high-spin state, a spin state with a maximum spin multiplicity as for the free ion, while the latter stabilizes a low-spin state where the electrons occupy low energy orbitals only at the expense of increasing the exchange energy 1. This transition, which can be induced by external perturbations such as heating 2, 3 , pressurization 4-8 , lightning 9 , magnetic field 10 , chemical substitution 11 , and gas adsorption 12 , were found in a wide range of materials like oxides (FeBO 3 , CaFeO 3) 4, 5, 7, 10 , metal-organic complexes 1 , porous materials 12 ,
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