Mechanism of the order–disorder phase transition, and glassy behavior in the metal-organic framework [(CH 3 ) 2 NH 2 ]Zn(HCOO) 3

Abstract

Transitions associated with orientational order–disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and 1 H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH 3 ) 2 NH 2 ]Zn(HCOO) 3 . This compound, which possesses a perovskite-type architecture, undergoes a remarkable order–disorder phase transition at 156 K. The (DMA + ) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─H⋯O ∼ 2.9 Å ) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA + is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65–150 K, as a result of a “memory effect.” This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.