Elastic Properties of the Iron(II)-Triazole Spin Crossover Complexes [Fe(Htrz)2trz]BF4 and [Fe(NH2trz)3]SO4

Two benchmark Fe(II)-triazole spin-crossover (SCO) compounds, [Fe(Htrz)2trz]BF4 (1) and [Fe(NH2trz)3]SO4 (2), have been investigated using variable temperature and pressure powder X-ray diffraction, dynamical mechanical analysis, and differential scanning calorimetry techniques. We assessed the transformation strain, the compressibility, and their anisotropy in both compounds. The measured bulk moduli are relatively high (20 and 13 GPa in 1 and 2, respectively), but against expectations, the Fe- triazole chain direction is the most compressible crystal axis in 1. We revealed considerable elastic softening and associated mechanical damping at the SCO in both compounds, which we interpreted, using the Landau theory, as a consequence of the coupling between the spin state and the lattice strain. The magnitudes of the elastic potential energy (similar to 15%), stored elastic work (<2-3%), and frictional work (<3-4%) with respect to the latent heat of the spin transition (26.8 and 19.5 kJ/mol in 1 and 2, respectively) have been also assessed. The knowledge of these properties is important both for a better understanding of the SCO behavior of this important family of complexes and for guiding their integration into mechanical transducers and other technological applications.