Raman scattering and X-ray diffraction study of the temperature- and pressure-induced multistep spin transition in...

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 {Fe^II(AnPy)₂[Ag^I(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 from Raman spectroscopy in the present work, 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 indicate local symmetry breaking in the plateau, which is closely comparable with the thermal transition behavior.