Effect of Pressure on the Mössbauer Resonance in Potassium Ferrocyanide, Potassium Ferricyanide, and Insoluble Prussian Blue

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Abstract
The effect of pressure to 200 kbar has been measured on the Mössbauer resonance in potassium ferrocyanide, potassium ferricyanide, and insoluble Prussian blue. The ferrocyanide exhibits a decrease of isomer shift with pressure large compared with typical high-spin ionic ferrous compounds. The ferricyanide exhibits an initial shift with pressure twice as large as the ferrous material. These large changes can be associated with changes in ``back donation'' and in 4s admixture in the binding. The ferricyanide reduces to ferrous compound with increasing pressure, paralleling to behavior of many high-spin ferric compounds. As in the case of the high-spin material, the phenomenon reverses with decreasing pressure. Near 50 kbar a first-order phase transition occurs which apperantly relieves the internal compression of the ferricyanide ion. It is accompanied by an increase in the isomer shift of the ferric ion and the pressure-induced ferrous ion as well as a decrease in the quadrupole splitting of the former. It is also accompanied by a large decrease in the percentage of Fe2+ ion present. This definitely relates the reversible reduction of ferric iron to overlap of the wavefunctions of metal and ligand. Insoluble Prussion blue contains high-spin ferric ion and low-spin ferrous ion. This compound also shows a reversible reduction of high-spin ferric ion with increasing pressure.

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