Ferroelectric metal-organic frameworks as wide band gap materials

TL;DR

This study identifies ferroelectric metal-organic frameworks with ultra-wide band gaps (5.5-5.7 eV), explores their phase transition mechanisms, and assesses their structural, ferroelectric, optical, and stability properties for high-temperature applications.

Contribution

The paper reveals ferroelectric MOFs with ultra-wide band gaps and elucidates their phase transition mechanisms using group theory and first-principles calculations.

Findings

Identified MOFs with 5.5-5.7 eV band gaps.

Demonstrated phase transition from high-symmetry to ferroelectric phase.

Assessed stability and switching barriers of the MOFs.

Abstract

Wide band gap materials are particularly relevant at high temperatures. The band gap shrinkage at higher temperatures prevents device applications with narrow band gap semiconductors. Considering $\alpha$-phase strontium cyanurate as a prototype structure, we identify a group of metal-organic frameworks (MOFs) that exhibit ultra-wide band gaps ranging from 5.5 to 5.7 eV. Recently, a strontium cyanurate compound was found to undergo a phase transition from a high-symmetry $\beta$-phase to a low-symmetry ferroelectric $\alpha$-phase when the temperature was reduced. In the present study, utilizing group theory techniques, we unravel that a zone-center $\Gamma_2^-$ phonon mode modifies our structures from high-symmetry $\beta$-phase to a low-symmetry $\alpha$-phase for A$_3$(O$_3$C$_3$N$_3$)$_2$ MOFs with A = Mg, Ca, Sr, and Ba. We implement first-principles calculations to investigate structural, ferroelectric, and optical properties of these compounds in $\alpha$-phase. The switching barriers between bistable polar states are also estimated. Further, to realize their feasibility, we examine the dynamical and thermal stabilities for all of these MOFs.