Phase Transition and Ferroelectricity of Two Perovskite-Like Mn(II) Metal–Organic Frameworks Tuned by Phosphonium Cations and Dicyanamide Ligand

Two metal–organic frameworks, [Me(n-Bu)3P][Mn(dca)3] (1) and (Et4P)[Mn(dca)3] (2) (dca: N(CN)2–, dicyanamide), exhibit perovskite-like (ABX3) three-dimensional structures. Two phosphonium cations occupy the cavities of [Mn(dca)3]− anionic frameworks. Their ferroelectric hysteresis loops, DSC curves, in situ variable-temperature PXRD, and magnetic properties were investigated. Compound 1 crystallizes in centrosymmetric monoclinic space group P21/n, while compound 2 belongs to chiral orthorhombic P212121 space group at room temperature. Thermogravimetry, differential scanning calorimetry, and variable-temperature PXRD measurements reveal several phase transitions occurred in compounds 1 and 2. Guest phosphonium cations affect their crystal space groups, structural phase transitions, and ferroelectric properties. Upon heating, compound 1 undergoes an irreversible order–disorder ferroelectric phase transition to the monoclinic chiral C2 space group at 348 K and has a ferroelectric spontaneous polarization (8.2 μC·cm–2) determined at 361 K. Compound 2 undergoes two ferroelectric phase transitions to the Pnn2 space group at 313 K with a spontaneous polarization (2.2 μC·cm–2) and to the Cmc21 space group at 407 K with spontaneous polarization (1.95 μC·cm–2). Two manganese compounds show weak antiferromagnetic couplings.