Nanoindentation Reveals Drastic Elastic Softening in a Spin Crossover Molecular Crystal

Spin-crossover molecular crystals exhibit a unique interplay between electronic and structural degrees of freedom, yet their mechanical response remains poorly understood. In this study, we employ variable-temperature nanoindentation to quantify elastic modulus and hardness of single crystals of the molecular complex [Fe0.89Zn0.11(bpp)2](BF4)2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine) across the thermally induced first-order spin transition. Our results reveal a sharp and reproducible elastic anomaly: within a narrow interval near the transition temperature (249 ± 1 K), the reduced modulus drops to a pronounced minimum (∼2.0 GPa), contrasting sharply with the values observed in the high spin (4.5 ± 0.3 GPa) and low spin (7.0 ± 0.5 GPa) states. This elastic softening is mirrored in the hardness, which otherwise remains comparable in the two spin states. At the transition point, the load–displacement curves display large hysteresis loops, revealing strong time-dependent deformation and enhanced energy dissipation. Crucially, the indentation does not induce a persistent spin-state change, suggesting the observed anelastic softening arises from molecular-scale spin-state relaxation rather than nucleation and growth mechanisms. On the other hand, the marked difference in modulus between the two spin states is not a relaxation effect, it is attributed to the substantial variation in crystal density accompanying the transition. These findings establish nanoindentation as a powerful method for probing the mechanical properties of spin crossover materials, offering valuable insights for their applications in mechanical sensors, actuators, mechanocaloric and energy harvesting devices.