On the theory of the polarization effects and its experimental test by angle-dependent Moessbauer spectroscopy with guanidinium nitroprusside (CN_3H_6)_2[Fe(CN)_5NO] (GNP) single crystals

摘要

The theory of the angle-dependent Moessbauer spectroscopy on nitroprusside-like single crystals has been complemented with expressions for the nuclear resonant cross-sections in case of the wave vector of the γ-radiation lying in the low symmetric non-mirror planes. They have been tested by rotation experiments in these planes using guanidinium nitroprusside (CN_3H_6)_2[Fe(CN)_5NO], (GNP) single crystal. As a result both the electric field gradient (EFG) and the mean square displacement (MSD) tensor parameters at the iron sites in GNP, have been determined at room temperature with high precision. The EFG asymmetry parameter is η = 0.01(1) and the principal z-axes of the two EFG tensors are oriented at α = +-8(1)° with respect to the c-axis in the mirror ac-plane. The corresponding orientation angle for the MSD tensor is ω = +-11(4)° and lies also in the ac-plane. The crystal shows strong vibrational anisotropy with different Lamb-Moessbauer factors f_(LM)~((i)) for the principal crystal directions: f_(LM)~(a) = 0.118(3), f_(LM)~((b)) = 0.174(2) and f_(LM)~((c)) = 0.198(2). These values are in very good agreement with the results obtained by Mossbauer line broadening method and nuclear forward scattering of synchrotron radiation. In our analysis the saturation and polarization effects, which are very important for the case of single crystal absorbers with quadrupole split Mossbauer spectra and finite thickness, are taken into account. A total of 51 high quality spectra, recorded and calibrated with the sodium nitroprusside standard Na_2[Fe(CN)_5NO] ·2H_2O (SNP), have yielded for the quadrupole splitting the value ΔE_Q = +1.854(1) mm/s and for the isomer shift relative to SNP the value IS = +0.006(1) mm/s (-0.251 mm/s relative to the α-Fe standard at room temperature). GNP has been confirmed as a very promising material for the investigation of long-lived metastable states observed in nitroprussides.