Symmetry-Resolved Study of Lattice Vibration and Libration Modes in [Fe(phen)(2)(NCS)(2)] Crystal

In the family of spin-crossover materials, which undergo thermal conversion between low-spin (LS) and high-spin (HS) phases, it is of great interest to study vibrational modes. On the one hand, vibration modes are characteristic of the spin state, and vibrational spectroscopies are often used for monitoring spin-state switching driven by temperature, pressure, or light. On the other hand, spin-state thermal conversion is an entropy-driven process, and the vibrational entropy change represents the main contribution to the total entropy difference between LS and HS phases at solid state. However, the discussion of vibrations in spin-crossover materials is often limited at the molecular scale. Here we study vibration modes in the [Fe(phen)(2)(NCS)(2)] crystal, and we compare symmetry-resolved vibrational spectroscopy data performed on single crystal to density functional theory calculations performed in a periodic three-dimensional crystal. We discuss the complex nature of vibrational modes in crystals, including the vibration of molecules within the crystalline lattice, with different symmetries and frequencies. We also highlight the presence of many low-frequency libration modes of different symmetries. The contribution of vibrational entropy, added to the electronic entropy, provides a total entropy difference in the solid state, which is in very good agreement with calorimetric measurements.