Determination of Lamb-Mössbauer factors and lattice dynamics in some nitroprusside single crystals

Mössbauer spectra have been recorded for sodium, guanidinium, barium and lithium nitroprussides single crystal cuts. The temperature dependence of the Lamb-Mössbauer fraction f, respectively, the mean-square nuclear displacement 〈x²〉 and the mean-square (of the total) velocity of the iron nucleus 〈v²〉 were analyzed on the basis of the Debye and Einstein lattice-vibration models. The characteristic temperatures of the two models, θ_D and θ_E, fitted to 〈x²〉 are considerably lower than those fitted to 〈v²〉. This effect seems to be typical for the iron complexes and was explained with the presence of low and high frequencies in the phonon vibration spectra and of low-temperature anharmonicity. The Lamb-Mössbauer fraction at 77 K in all principal crystal directions of sodium, guanidinium and barium nitroprussides has been determined. These values will be used for more precise studies of the population and the properties of the light-induced molecular states of the nitrosyl [Fe(CN)₅NO] ^2- anion.