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

TL;DR

This study investigates the lattice phonon modes of the spin crossover crystal [Fe(phen)2(NCS)2] using combined spectroscopic techniques and DFT calculations to understand vibrational contributions to spin-state transitions.

Contribution

It provides a detailed analysis of crystal phonon modes and their relation to molecular vibrations, enhancing understanding of vibrational entropy in spin crossover phenomena.

Findings

Identification of specific phonon modes associated with spin states

Correlation between experimental spectra and DFT calculations

Insights into vibrational contributions to spin transition mechanisms

Abstract

[Fe(phen)2(NCS)2] is a prototype transition metal complex material, which 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 photo-induced 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 different symmetries. We compare the experimental results to DFT calculations performed in a periodic 3D crystal for understanding the phonon modes in the crystal, compared to molecular vibrations. Indeed, each vibrational mode of the isolated molecule combines into several modes of different symmetry and frequency 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.