Role of A-Site Molecular Ions in the Polar Functionality of Metal-Organic Framework Perovskites

Abstract

Recent studies on organic–inorganic hybrid perovskites (OIHPs) and ferroelectric metal–organic framework perovskites (MOFPs) reveal their superb performance as highly efficient materials for photovoltaics and ferroelectrics. This has enabled the development of a new generation of optic-electronic-mechanical devices based on green chemistry. However, the fundamental understanding of these polarization-related functionalities is not yet clear, which has hindered the progress in further designing and developing materials with expected properties. In this work, we investigate three MOFPs that have the same Mg(HCOO)3– frameworks with different molecular ions: [CH3NH3][Mg(HCOO)3] (MA-MOF), [(CH3)2NH2][Mg(HCOO)3] (DMA-MOF), and [C(NH2)3][Mg(HCOO)3] (GUA-MOF). Single-crystal and powder X-ray diffraction, inelastic neutron spectroscopy, and ab initio molecular dynamics simulations are combined to achieve a detailed description of the three MOFPs’ static and dynamic structures as a function of temperature. Intriguingly, our study reveals that the alignments and motions of the guest molecular ions are highly dependent on the directional hydrogen bonds that link N–H units to the surrounding MgO6 octahedra through the O acceptor from the frameworks. At the same time, the size, dynamic behavior, and alignments of the A-site molecular ions influence the distortive framework structures and their temperature-dependent deformation. Therefore, the mutual interaction between the guest and the framework determines the overall functionalities of the MOFPs. This study indicates that the configuration of the A-site molecular ions and the potential hydrogen bonds are critical to design the polar functionalities in both MOFPs and OIHPs. Show more