The lone-pair-electron-driven phase transition and order-disorder processes in thermochromic (2-MIm)SbI4 organic-inorganic hybrid.

The easy to prepare and stable in air (2-methylimidazolium) tetraiodoantimoniate(iii) single-crystals with optical band gap of 2.17(1) eV at room temperature have been synthesized. The crystal structure features one-dimensional [SbI4]-n anionic chains, which are intercepted with stacks of 2MIm+ ions. At 294/295 K, it undergoes a structural phase transition to an incommensurately modulated phase as a result of subtle, lone-pair-electron-driven distortions of the anions. Separately from the anion displacements, the ordering of 2MIm+ countercations takes place over a wide temperature range of the modulated phase. The disorder changes from dynamic to static around 200 K, which affects the crystal structure leading to discontinuities and step-like contraction of the lattice parameters. The material is thermochromic with prominent color changes, from raspberry to yellow at low temperatures. The calculated electronic structures and observed optical properties signify its semiconducting character.