Pressure-induced changes of the vibrational modes of spin-crossover   complexes studied by nuclear resonance scattering of synchrotron radiation
  Nuclear inelastic scattering (NIS) spectra were recorded for the spin-crossover complexes STP and ETP (STP = [Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg- ethane)](ClO4)2 and ETP = [Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg-butane)](ClO4)2) at 30 K and at room temperature and also at ambient pressure and applied pressure (up to 2.6 GPa). Spin transition from the high-spin (HS) to the low-spin (LS) state was observed by lowering temperature and also by applying pressure at room temperature and has been assigned to the hardening of iron-bond stretching modes due to the smaller volume in the LS isomer. 
A. X. Trautwein, H. Paulsen, H. Winkler, H. Giefers, G. Wortmann, H.   Toftlund, J. A. Wolny, A. I. Chumakov and O. Leupold
physics.chem-ph cond-mat.mtrl-sci