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 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 $β$-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 $Γ_2^-$ phonon mode modifies our structures from high-symmetry $β$-phase to a low-symmetry $α$-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 $α$-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.

💡 Research Summary

The authors present a systematic first‑principles investigation of a family of metal‑organic frameworks (MOFs) with the formula A₃(O₃C₃N₃)₂ (A = Mg, Ca, Sr, Ba) that combine ultra‑wide electronic band gaps (5.5–5.7 eV) with robust ferroelectricity. The study is motivated by recent experimental observations that strontium cyanurate (Sr₃(O₃C₃N₃)₂) undergoes a temperature‑driven phase transition from a high‑symmetry trigonal β‑phase (R3̅c) to a low‑symmetry monoclinic α‑phase (Cc), acquiring a spontaneous polarization. Using the ISODISTORT group‑theory package, the authors identify a zone‑center Γ₂⁻ soft phonon as the primary order parameter that drives the β→α transformation by displacing the A‑site cations.

Density‑functional theory calculations are performed with the PAW‑GGA method in VASP, employing a 4 × 4 × 4 k‑point mesh, a 600 eV plane‑wave cutoff, and tight convergence criteria (1 µeV energy, 0.01 eV Å⁻¹ forces). Geometry optimizations reveal that all four compounds crystallize in the polar monoclinic Cc space group, with lattice parameters expanding systematically from Mg to Ba due to the increasing ionic radius. The Berry‑phase approach yields spontaneous polarizations of 1.15 µC cm⁻² (Mg), 2.20 µC cm⁻² (Ca), 2.39 µC cm⁻² (Sr), and 3.13 µC cm⁻² (Ba), confirming that larger A‑site cations enhance the ferroelectric dipole.

To assess the reversibility of the polarization, the authors construct a centrosymmetric C2/c reference structure and trace the energy profile along the symmetry‑adapted polar distortion that connects C2/c to Cc. The resulting double‑well potentials give energy differences (ΔE = E_Cc − E_C2/c) of –152 meV/f.u. (Mg), –138 meV/f.u. (Ca), –70 meV/f.u. (Sr), and –82 meV/f.u. (Ba). These barriers are lower than those reported for many ferroelectric MOFs, indicating that electric‑field‑induced switching should be feasible at modest fields.

Electronic structure is refined with the modified Becke‑Johnson (MBJ) potential, which predicts direct band gaps of 5.5–5.7 eV for all members. Such ultra‑wide gaps place the absorption edge well into the deep‑ultraviolet (DUV) region, making the materials attractive for DUV photodetectors or high‑voltage optoelectronic components where leakage currents must be suppressed. Optical spectra calculated within the independent‑particle approximation show a sharp onset at the band edge and high transparency beyond 7 eV.

Dynamic stability is verified by phonon calculations using a dense 8 × 8 × 8 q‑mesh and a 2 × 2 × 2 supercell. No imaginary frequencies appear in any of the α‑phase phonon dispersions, confirming that the structures are locally stable. Thermal robustness is examined via ab‑initio molecular dynamics (AIMD) in the NVT ensemble at 300 K, 500 K, and 700 K for 10 ps with a 1 fs timestep. Throughout the simulations the lattice parameters and bond lengths remain essentially unchanged, indicating that the frameworks retain their integrity well above typical operating temperatures.

The combined analysis establishes clear design rules: (i) the Γ₂⁻ soft mode governs the ferroelectric transition; (ii) larger A‑site cations increase spontaneous polarization but modestly raise the switching barrier; (iii) the cyanurate (C₃N₃O₃)³⁻ ligand provides a rigid, planar electronic scaffold that yields the ultra‑wide band gap. Consequently, the Mg‑based MOF offers the lowest switching barrier for memory‑type applications, whereas the Ba‑based MOF delivers the highest polarization for high‑field devices.

In summary, this work introduces a new class of ferroelectric MOFs that simultaneously possess ultra‑wide band gaps, sizable switchable polarizations, low switching barriers, and confirmed dynamical and thermal stability. These attributes make them promising candidates for next‑generation high‑temperature, high‑voltage, and deep‑UV optoelectronic technologies, as well as for non‑volatile ferroelectric memory devices where leakage suppression is critical.