Effect of Pressure on the Mössbauer Resonance in Ionic Compounds of Iron

Abstract

The effect of pressure to 200 kbar has been measured on the Mössbauer resonance in a number of high-spin ionic ferrous and ferric compounds. The isomer shift and quadrupole splitting characterize the ionic state and spin state of iron. In general, there is a decrease in the iosmer shift with increasing pressure correspinding to an increase of electron density at the iron nucleus. This can amount to 8%—20% of the difference in isomer shift between typical ferrous and ferric compounds. The pressure effect is usually larger in ferrous than in ferric compounds, and is associated with changes in the 3d−3s shielding. The quadrupole splitting usually increases with pressure. A quantitative interpretation depends on knowledge of changes in local symmetry and of spin—orbital coupling factors, as well as local compressibility. For six of the eight ferric compounds, we observed a significant amount of reduction to the ferrous state at high pressure. This phenomenon was definitely reversible. It appears to be associated with a general tendency for the ground state of the ferrous ion to decrease in energy relative to the ligands with increasing compression. Thus, electron transfer is facilitated. Some evidence exists that strong light also reduces iron compounds. These results tend to broaden the analogy previously noted between photochemical and high-pressure reactions.