posted on 2021-12-13, 20:04authored byTeng Lu, David Cortie, Zuo-Xi Li, Narendirakumar Narayanan, Zhen Liu, Qingbo Sun, Terry J. Frankcombe, Garry J. McIntyre, Dehong Yu, Yun Liu
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.