Anisotropic Nuclear Inelastic Scattering of an Iron(II) Molecular Crystal

Nuclear inelastic scattering (NIS) spectra were recorded for a monocrystal of the spin-crossover complex $\left[\mathrm{Fe}\right(\mathrm{tptMetame}\left)\right]\left(\mathrm{ClO}{}_4{)}_2\right(\mathrm{tptMetame}=1,1,1\mathrm{}-\mathrm{tris}\{\left[\mathrm{N}-\right(2-\mathrm{pyridylmethyl}\left)-\mathrm{N}-\mathrm{methylamino}\right]-\mathrm{methyl}\}\mathrm{ethane})$ at $T=30\mathrm{K}$ (low-spin state) and at room temperature (high-spin state) for different crystal orientations. The high energy resolution (0.65 meV) allowed us to resolve individual molecular vibrations which were unambiguously identified by density functional calculations. From the NIS spectra for the first time the angular-resolved iron-partial density of phonon states (PDOS) was extracted. The PDOS corroborates a vibrational entropy difference as driving force of the spin transition.