Novel magneto-lattice transition in the mixed-stack charge-transfer complexes with inter-columnar networks

We report on a novel spin–lattice phase transition in an ionic mixed-stack charge-transfer (CT) compound of (BEDO-TTF)(Cl2TCNQ). We found that the rotational modes for rigid-body displacements of molecules afforded an appreciable quantitative variation in the exchange interactions along the mixed stacks, which resulted in the symmetry-invariant first-order magneto-lattice phase transition at around 110 K. It was found in the electron paramagnetic resonance (EPR) spectra that the low-temperature strongly interacting spin phase was followed by a magnetic order at 16 K. We discuss the origin of the transition at 110 K in terms of the competition of free energy between the respective spin states. We also present neutral–ionic (NI) phase transition in the isomorphous compound of (BEDT-TTF)(ClMeTCNQ) under pressure.