Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {Fe<sup>III</sup><sub>4</sub>Fe<sup>II</sup><sub>2</sub>} Compound
Hui‐Ying Sun, Yin‐Shan Meng, Liang Zhao, Nian‐Tao Yao, Pan‐Dong Mao, Qiang Liu, Fei‐Fei Yan, Hiroki Oshio, Tao Liu
Abstract
Abstract Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)Fe III (CN) 3 ] 4 [Fe II (Ppmp)] 2 ⋅2 CH 3 OH ( 1 , Tp*=hydrotris(3,5‐dimethyl‐pyrazol‐1‐yl)borate and Ppmp=2‐[3‐(2′‐pyridyl)pyrazol‐1‐ylmethyl]pyridine), which has a rhombic core structure abbreviated as {Fe III 2 Fe II 2 }. Magnetic susceptibility measurements and single‐crystal X‐ray diffraction analyses revealed that 1 underwent thermally‐induced spin transition with the thermal hysteresis. The Fe II site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the Fe II centers actuated anisotropic deformation of the rhombic {Fe III 2 Fe II 2 } core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.