High-Pressure Synthesis and Characterization of the Alkali Diazenide Li2N2

Predicting Nitrogen-Based Families of Compounds: Transition-Metal Guanidinates TCN3 (T=V, Nb, Ta) and Ortho-Nitrido Carbonates T'2 CN4 (T'=Ti, Zr, Hf)

Due to its unsurpassed capability to engage in various sp hybridizations or orbital mixings, carbon may contribute in expanding solid-state nitrogen chemistry by allowing for different complex anions, such as the known NCN2- carbodiimide unit, the so far unknown CN3 5- guanidinate anion, and the likewise unknown CN4 8- ortho-nitrido carbonate (onc) entity. Because the latter two complex anions have never been observed before, we have chemically designed them using first-principles structural searches, and we here predict the first hydrogen-free guanidinates TCN3 (T=V, Nb, Ta) and ortho-nitrido carbonates T'2 CN4 (T'=Ti, Zr, Hf) being mechanically stable at normal pressure; the latter should coexist as solid solutions with the stoichiometrically identical nitride carbodiimides and nitride guanidinates. We also suggest favorable exothermic reactions as useful signposts for eventual synthesis, and we trust that the decay of the novel compounds is unlikely due to presumably large kinetic activation barriers (C-N bond breaking) and quite substantial Madelung energies stabilizing the highly charged complex anions. While chemical-bonding analysis reveals the novel CN4 8- to be more covalent compared to NCN2- and CN3 5- within related compounds, further electronic-structure data of onc phases hint at their physicochemical potential in terms of photoelectrochemical water splitting and nonlinear optics.

Penta-Pt2N4: an ideal two-dimensional material for nanoelectronics

Since the discovery of graphene, two-dimensional (2D) materials have paved new ways to design high-performance nanoelectronic devices. To facilitate applications of such devices, there are three key requirements that a material needs to fulfill: sizeable band gap, high carrier mobility, and robust environmental stability. However, among the most popular 2D materials studied in recent years, graphene is gapless, hexagonal boron nitride has a very large band gap, transition metal dichalcogenides have low carrier mobility, and black phosphorene is ambience-sensitive. Thus far, these three characteristics could seldom be satisfied by only a single material. Therefore, it is a great challenge to find an ideal 2D material that can overcome these limitations. In this study, we theoretically predicted a novel planar 2D material penta-Pt2N4, which was designed using the Cairo pentagonal tiling as well as the rare nitrogen double bonds. Most significantly, 2D penta-Pt2N4 exhibits excellent intrinsic properties, including large direct band gap (up to 1.51 eV), high carrier mobility (up to 105 cm2·V-1·s-1), very high Young's modulus (up to 0.70 TPa), and robust dynamic, thermal, and ambient stabilities. Moreover, penta-Pt2N4 is the global minimum structure among 2D materials with PtN2 stoichiometry. We also propose a CVD/MBE scheme to enable its experimental synthesis. We envision that 2D penta-Pt2N4 may find wide applications in the field of nanoelectronics.

Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with -Li-N-B-N- repeating units in α-Li(x)BN₂ (1 ⩽ x ⩽ 3)

Ultrahigh energy density batteries based on α-Li(x)BN2 (1 ⩽ x ⩽ 3) positive electrode materials are predicted using density functional theory calculations. The utilization of the reversible LiBN2 + 2 Li(+) + 2 e(-) ⇌ Li3BN2 electrochemical cell reaction leads to a voltage of 3.62 V (vs Li/Li(+)), theoretical energy densities of 3251 Wh/kg and 5927 Wh/l, with capacities of 899 mAh/g and 1638 mAh/cm(3), while the cell volume of α-Li3BN2 shrinks only 2.8% per two-electron transfer on charge. These values are far superior to the best existing or theoretically designed intercalation or conversion-based positive electrode materials. For comparison, the theoretical energy density of a Li-O2/peroxide battery is 3450 Wh/kg (including the weight of O2), that of a Li-S battery is 2600 Wh/kg, that of Li3Cr(BO3)(PO4) (one of the best designer intercalation materials) is 1700 Wh/kg, while already commercialized LiCoO2 allows for 568 Wh/kg. α-Li3BN2 is also known as a good Li-ion conductor with experimentally observed 3 mS/cm ionic conductivity and 78 kJ/mol (≈0.8 eV) activation energy of conduction. The attractive features of α-Li(x)BN2 (1 ⩽ x ⩽ 3) are based on a crystal lattice of 1D conjugated polymers with -Li-N-B-N- repeating units. When some of the Li is deintercalated from α-Li3BN2 the crystal becomes a metallic electron conductor, based on the underlying 1D conjugated π electron system. Thus, α-Li(x)BN2 (1 ⩽ x ⩽ 3) represents a new type of 1D conjugated polymers with significant potential for energy storage and other applications.