Mössbauer Hyperfine Interactions in Sodium Nitroprusside Single Crystals

Abstract

The Mössbauer absorption spectrum of 57Fe in single crystals of Na2[Fe(II)(CN)5NO]·2H2O has been studied. The angular variation of the ratio of intensities of the quadrupole-splitted lines with the incident radiation has been measured at different crystal orientations. The results may be satisfactorily interpreted by considering the detailed symmetry of the crystal and the effects of the anisotropy of the recoilless absorption probability. It was found that an axially symmetric field gradient with its principal axis oriented along the Fe–NO directions, couples with the nuclear quadrupole moment of 57Fe giving a positive interaction with the value eq=+0.585×1018 V/cm2. This field gradient is assumed to originate from unbalanced d-electron density and the electronic population calculated for the molecule agrees with that obtained by molecular-orbital methods. The large total s-electron density at the 57Fe nucleus measured by the isomer shift is quantitatively related to the effective 3d-electron population and to the 4s partial occupation. Estimates of the 4s density also agree with that reported from molecular-orbital calculations.