Ferroelectric metal-organic frameworks as wide band gap materials

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.