Modified Chesnut Model for Spin-Crossover Semiconductors [Fe(acpa) 2 ](TCNQ) n

Embedding of bistable molecules into an organic conductor is expected to yield interesting hysteretic conductive properties. As a typical bistable species switchable under a temperature change, a spin-crossover iron(III) complex was utilized to prepare two charge-transfer salts containing an organic acceptor molecule, [Fe(acpa) 2 ](TCNQ) n (Hacpa = N -(1-acetyl-2-propylidene)-2-pyridyl-methylamine; TCNQ = 2,2'-(2,5-cyclohexadien-1,4-diylidene)-bispropanedinitrile), and their magnetic and electrical properties were studied. While magnetic behavior of dilute spin-crossover complex molecules is well described by an entropy-driven spin equilibrium between low-spin and high-spin states across an energy gap, intermolecular interactions should be taken into account and often abrupt phase transitions are found in densely-packed complex molecules like crystalline solids. In order to delineate these systems in a unified manner, a phenomenological two-state model based on a mean-field approximation was examined. It worked well to reproduce the temperature dependence of electrical conductivities associating with a spin-crossover transition assuming a Slater-type distance dependence for the band gap. spin-crossover phenomena iron(III) complex TCNQ Chesnut model first-order phase transition band gap