Altered Mode of Structural Changes in Solid Solutions Leading to Dual Modulation: Spin Transition Temperatures and Steps

The development of high-density information storage materials requires precise control of electron spin states. Multistep spin-crossover (SCO) materials with multiple stable spin states are prime candidates for this purpose. However, accurate control of the dynamic SCO behavior, including the dual dynamic modulation of the spin transition temperature (T_c) and transition steps, has been a significant hurdle. In this study, we propose a new two-dimensional SCO solid solution system, [Fe^III(H_0.5L^I)_2-2x(H_0.5L^Cl)_2x]·H₂O, where H_0.5L^X (L^X for short) denotes 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, with X being I or Cl and x ranging from 0 to 1. The proposed system exhibits a unique nonmonotonic variation in T_c with x, obtaining a minimum at x = 0.7, allowing fine-tuning of T_c across a 66 K range and control over transition steps from two steps to one step, accomplishing dual modulation. Single-crystal diffraction analysis and periodic density functional theory (DFT) calculations demonstrate that the doped ligand L^Cl modulates the Fe^IIIN₂O₂S₂ ligand field with increasing doped L^Cl ligands during the HS → LS transition in solid solutions, enabling dual dynamic modulation of the T_c and the number of transition steps from two steps to one step through dynamic variation in the structural contraction modes (from b- and c-axis contraction to a-axis contraction). This study motivates the synthetic control of dynamic SCO solid solutions through the altered mode of structural contraction as a complementary route to adjust their SCO behavior.