Raman scattering and X-ray diffraction study of the temperature- and pressure-induced multistep spin transition in the complex {FeII(AnPy)2[AgI(CN)2]2]}·NO2bz

Abstract

We acquired Raman spectra and X-ray diffraction (XRD) patterns across the temperature- and pressure-induced multistep spin transition in single crystals of the complex {FeII(AnPy)2[AgI(CN)2]2]}·NO2bz (AnPy = 4-anthracenepyridine, NO2bz = nitrobenzene). At the half transition plateau (ca. 200 – 220 K), previous work evidenced no superstructure or diffuse reflections in the XRD pattern, which could be indicative of spontaneous long- or short-range ordering of molecules in different spin states. Despite this lack of ordering, a distinct local symmetry breaking in the plateau is inferred in the present work from Raman spectroscopy, which revealed a complete extinction of several vibrational modes associated with the pure high spin and low spin states. We suggest that this local symmetry breaking does not transform into longer range order due to the positional disorder of NO2bz guest molecules. The investigation was extended also to the high pressure behavior of the complex revealing a similar, stepped variation of lattice parameters with a plateau near 2 - 4 kbar, corresponding to ca. 50 % transition, while the complete transformation to the LS form was achieved at ca. 6 kbar. The high pressure Raman spectra indicates local symmetry breaking in the plateau, which is closely comparable with the thermal transition behavior.