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 α-phase strontium
cyanurate as a prototype structure, we identify a group of metal–organic
frameworks (MOFs) that exhibit ultrawide 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 β-phase to a low-symmetry
ferroelectric α-phase when the temperature was reduced. In the
present study, utilizing group theory techniques, we unravel that
a zone-center Γ2– phonon mode modifies our structures from a high-symmetry
β-phase to a low-symmetry α-phase for A3(O3C3N3)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 the α-phase. The switching barriers between bistable polar
states are also estimated. Furthermore, to realize their feasibility,
we examine the dynamical and thermal stabilities for all of these
MOFs.