Disorder–order transitions in the perovskite metal–organic frameworks [(CH3)2NH2][M(HCOO)3] at high pressure

Abstract

High-pressure single-crystal X-ray diffraction was performed on dimethylammonium metal formates (DMAMF), [(CH₃)₂NH₂][M(HCOO)₃] where M = Mn²⁺, Fe²⁺, and Cu²⁺, in order to compare the high-pressure phases with the known low-temperature ferroelectric (for DMAMnF) and multiferroic (for DMAFeF) phases. The ambient phases of dimethylammonium metal formates were stable up to 5.53(8), 5.7(3), and 7.3(2) GPa for DMAMnF, DMACuF, and DMAFeF, respectively. At higher pressures, phase transitions occurred that were initiated by the structural distortions of the metal formate framework. The distortions are accompanied by loss of dynamic disorder of dimethylammonium (DMA) in the DMAMnF and DMAFeF compounds, resulting in an antiferroelectric arrangement of the DMA cations. For DMACuF, the Jahn–Teller distortion is reduced upon compression up to ∼3.5 GPa; the inability to continue reducing the Cu–O bonding distances beyond this pressure could be responsible for triggering a distortive transition of the copper formate framework at 5.5 GPa. In the case of DMAFeF, the experiment was in addition conducted with a penetrating pressure-transmitting medium (PTM) that resulted in a disorder–order transition of the DMA cation at lower pressures with p = 4.0(6) GPa due to the PTM inclusion.