Pressure Dependence of the Mössbauer Spectra of Several Iron(III) Trisdithiocarbamate Complexes

The room-temperature pressure dependence of the Mössbauer spectra of the iron(III) trisdithiocarbamate complexes, Fe(R2dtc)3, where R represents the N,N'-disubstitution with methyl, ethyl, benzyl, n-hexyl and cyclohexyl groups, has been measured between zero and ca. 80 kbar. As expected, the Mössbauer-effect isomer shift decreases slightly with increasing pressure, whereas the quadrupole splitting, the linewidth and the recoil-free fraction increase with increasing pressure. The pressure dependence of the Mössbauer absorption area is modelled by using the Debye theory of lattice vibrations, this analysis being consistent with that of the temperature dependence of the Mössbauer absorption area and also with the mean square displacements of the iron(III) ions, as determined from single-crystal X-ray thermal parameters. The Mössbauer spectra do not show any changes with pressure that can be associated exclusively with changes in the electronic spin state with increasing pressure. The authors conclude that these compounds are completely low-spin even at small applied pressures of a few kilobars. Experimental techniques required to obtain reproducible Mössbauer spectra at high pressure, through the use of an opposed diamond-anvil cell, are critically evaluated.