Determination of the Mössbauer parameters of rare-earth nitroprussides: Evidence for new light-induced magnetic excited state (LIMES) in nitroprussides

Nitroprussides of the rare-earth elements and some mixed rare-earth-sodium nitroprussides are studied by Mössbauer spectroscopy at ambient and lower temperatures. The high precision Mössbauer measurements reveal fine changes in the electronic configurations of the nitroprusside anions. A small increase of the quadrupole splitting reveals charge polarization effects in the nitroprusside anion caused by the oblate or prolate shape of the rare-earth ion and the lanthanide contraction. Despite the very large magnetic moment of holmium a magnetic phase transition is not observed down to 300 mK. The population of the metastable states SI and SII are evidenced in europium and scandium nitroprussides, and most likely they can be populated in all rare-earth nitroprussides. No distinct correlation between the Mössbauer parameters and the decay temperatures Tc of the metastable states are found. In a very thin surface layer strong color change, which remains stable at room temperature, is detected. A quadrupole doublet with Mössbauer parameters typical for Fe(III), low spin S=1/2 state is related to a new colored photoproduct. The photoproduct is called light-induced magnetic excited state (LIMES) and explained with a photochemical redox reaction, which changes the valence, spin, and magnetic state of 4f–3d bimetallic complexes.