Supramolecular [Na(15-crown-5)]+ cations anchored to face-sharing octahedral lead bromide chains featuring a rotor-like one-dimensional perovskite with a reversible isostructural phase transition near room temperature

Abstract

Crystal engineering aims at a better understanding of the correlation between the components and crystal structures, so that the desired crystal structure and functionality will be acquired. In this study, a lead bromide perovskite, {[Na(15-crown-5)]PbBr₃}_∞ (1, 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane), has been obtained by assembly of equal molar quantities of PbBr₂, NaBr and 15-crown-5 in DMF, and 1 has been further characterized by microanalysis, IR spectroscopy, TG analysis, and single crystal and powder X-ray diffraction techniques. DSC measurement revealed that 1 experiences a reversible phase transition at ∼299/∼297 K on heating/cooling, which is an isostructural phase transition confirmed by single crystal structure analysis. 1 crystallizes in the orthorhombic space group Pnma in both low- and high-temperature phases, containing face-sharing coordination octahedral [PbBr₃]_∞ chains and supramolecular [Na(15-crown-5)]⁺ cations, which are bound to the [PbBr₃]_∞ chains via Na–Br coordination bonds to form rotor-like neutral chains. Variable-temperature crystal structure analysis revealed that the isostructural phase transition is driven by PbBr₆ coordination sphere distortion, which is related to the steric hindrance effect between the inorganic chain and 15-crown-5 molecules. Dielectric relaxation has been observed above 273 K, resulting from the distortion of PbBr₆ coordination octahedra and the change in polar Na–Br bonds under an AC electrical field.