Enhanced Curie Temperature of Two-Dimensional Cr(II) Aromatic Heterocyclic Metal–Organic Framework Magnets via Strengthened Orbital Hybridization

Abstract

Two-dimensional (2D) metal–organic frameworks (MOFs) with room-temperature magnetism are highly desirable but challenging due to the weak superexchange interaction between metal atoms. For this purpose, strengthening the hybridization between metal ion and organic linkage presents an experiment-feasible chemical solution to enhance the Curie temperature. Here, we report three 2D Cr­(II) aromatic heterocyclic MOF magnets with enhanced Curie temperature by bridging Cr­(II) ions with pyrazine, 1,4-diphosphinine, and 1,4-diarsenin linkers, i.e., Cr­(pyz)2, Cr­(diphos)2, and Cr­(diarse)2, and using first-principles calculations. Our results show that Cr­(pyz)2, Cr­(diphos)2, and Cr­(diarse)2 are ferrimagnetic semiconductors. In particular, the Curie temperature of Cr­(pyz)2 is estimated to be about 344 K and could be enhanced to 512 and 437 K in Cr­(diphos)2 and Cr­(diarse)2 by strengthening the hybridization between Cr ions and organic linkers via d−π* direct exchange interaction. This study presents a prototype to obtain room-temperature magnetism in 2D Cr­(II)-based MOF magnets for nanoscale spintronics applications.

Quintuple Function Integration in Two-Dimensional Cr(II) Five-Membered Heterocyclic Metal Organic Frameworks via Tuning Ligand Spin and Lattice Symmetry

“…To treat the partially filled 3 d orbitals of transition metal atoms, the strongly correlated correction is considered with the PBE + U method . The values of effective exchange interaction parameter ( J ) and onsite Coulomb interaction parameter ( U ) are, respectively, set as 1.0 and 3.0 eV, which are the same as those of the previously calculated Cr(pyz) 2 sheet. , In addition, we also test the magnetic exchange energies under different U values, and find that when U = 3 eV, the calculated relative energies Δ E of the FM state to the FiM1 state for the Cr(TDZ) 2 sheet is closest when using the hybrid HSE06 functional (Table S8). The energy cutoff for the plane-wave basis set is 520 eV.…”

“…Considering the structural rigidity and limited tunability of inorganic compounds, we turn our attention to organometallic materials with structural variability and rich functionalization possibilities. , Organometallic frameworks are hybrid porous materials composed of abundant metal nodes and inexpensive organic linkers . By tuning metal nodes or organic linkers or the connectivity between them, they can possess functional properties with potential applications in traditional fields of gas separation, sensing, and optoelectronics − or emerging fields of electromechanical, magnetoelectronic, and topological quantum technologies. − For instance, by selecting benzimidazole as an organic linker, ultrathin poly­[Zn 2 (benzimidazole) 4 ] sheets exhibit excellent performance in H 2 /CO 2 gas separation . By using dicyanoquinonediimine as a rotatory unit, the Cr­(dicyanoquinonediimine) 2 sheet has been predicted to be an auxetic magnet .…”

“…By using dicyanoquinonediimine as a rotatory unit, the Cr­(dicyanoquinonediimine) 2 sheet has been predicted to be an auxetic magnet . By changing the spin state of organic linkers from singlet to doublet and introducing a strong d - p direct ferrimagnetic (FiM) exchange interaction, high Curie temperature ( T C ) magnetic semiconductors (SCs) Cr­(pentalene) 2 ( T C = 560 K), Cr­(diketopyrrolopyrrole) 2 ( T C = 316 K), and Cr­(pyrazine) 2 ( T C = 342 K) , have been designed theoretically. Via distorting the out-of-plane K + counterions, ferroelectric 2D magnetic K 3 M 2 [PcMO 8 ] (M = Cr-Co) sheets have been forecasted .…”

“…Among numerous organometallic frameworks, a transition metal Cr atom as a common metal node has been widely used, − and its related planar tetracoordinate molecules or crystals have been extensively synthesized. − Although divalent Cr compounds are uncommon in chemistry, Perlepe et al have prepared a layered Li 0.7 [Cr­(pyrazine) 2 ]­Cl 0.7 ·0.25­(THF) (THF = tetrahydrofuran) crystal with room-temperature ferrimagnetism under a dinitrogen or argon atmosphere in which each Cr­(II) is coordinated to four pyrazine organic linkers within the layers forming a square planar lattice . If the pyrazine rings are replaced by inversion symmetry-breaking organic linkers, the functional properties can be further enriched as extra tunable degrees of freedom are introduced into the crystal structure.…”

Two-Dimensional Multifunctional Metal–Organic Framework with Intrinsic Bipolar Magnetic Semiconductivity and Negative Poisson’s Ratio

“…In addition, as a charming type of magnetic semiconductors, bipolar magnetic semiconductors (BMS) − could provide 100% spin-polarized currents with the spin-polarization direction manipulated by the applied gate voltage, which originates from their unique electronic band structure: i.e., the valence band maximum (VBM) and conduction band minimum (CBM) occupy opposite spin channels. Up to now, the search for 2D BMS materials is still limited and remains a pending task. − …”