Simulation of Structural Phase Transitions in Perovskite Methylhydrazinium Metal-Formate Frameworks: Coupled Ising and Potts Models

We present a theoretical model of methylhydrazinium CH3NH2NH2+ molecular cation ordering in perovskite [CH3NH2NH2][M(HCOO)3] (M = Mn, Mg, Fe, and Zn) formate frameworks, which exhibit two structural phase transitions. The proposed model is constructed by analyzing the available structural information and mapping the molecular cation states on a three-dimensional simple cubic lattice. The model includes the short-range Ising and Potts interactions between the dipolar CH3NH2NH2 + cations. We study the model using the Monte Carlo simulations as well as by the density functional theory calculations. The simulations indicate that our model accurately describes the methylhydrazinium cation arrangement in all three structural phases of the compounds. The calculated temperature dependences of the heat capacity and electric polarization are in good agreement with the experimental data. The simulations also allow us to obtain the characteristic energies of the molecular cation interactions for all members of the [CH3NH2NH2][M(HCOO)3] family. Copyright © 2019 American Chemical Society.