Lattice phonon modes of the spin crossover crystal [Fe(phen)2(NCS)2] studied by THz, IR, Raman spectroscopies and DFT calculations

[Fe(phen)2(NCS)2] is a prototype transition metal complex material that undergoes a phase transition between low-spin (LS) and high-spin (HS) phases, induced by temperature, pressure or light. Vibrational modes play a key role for spin-state switching both in thermal and photoinduced cases, by contributing to vibrational entropy for thermal equilibrium transitions or driving the fast structural trapping of the photoinduced high-spin state. Here we study the crystal phonon modes of [Fe(phen)2(NCS)2] by combining THz, IR, and Raman spectroscopies sensitive to modes in different frequency ranges and symmetries. We compare the experimental results with DFT calculations performed in a periodic 3D crystal for understanding the phonon modes in the crystal. Indeed, each vibrational mode of the isolated molecule combines into several modes of different symmetries and frequencies in the crystal, as the unit cell contains four molecules. We focus our attention on the HS symmetric and anti-symmetric breathing modes in the crystal as well as on the N-CS stretching modes. Graphical abstract