Existence of BeCN2 and Its First-Principles Phase Diagram: Be and C Introducing Structural Diversity.

The existence and structure of BeCN2, the lightest representative of II-IV-V2 compounds, have for long remained unsolved, although previous theoretical studies have relied on assuming chemical similarity toward the known wurtzite-type BeSiN2. To solve the BeCN2 puzzle, we have now explored its potential-energy surface and here predict two additional polymorphs with space groups Cmc21 (porous phase) and Pmc21 (graphitic phase) in addition to another I4̅m2 type (carbodiimide-like), which is only slightly higher in energy than the wurtzite type. The phase diagram constructed from density-functional theory shows the Cmc21-type to be the ground state, stable in terms of the Gibbs energy under standard conditions, whereas the Pmc21- and I4̅m2-types are high-temperature phases; the wurtzite type, however, is the high-pressure phase. The kinetic barrier between the porous and graphitic phases is small, about 4 kJ mol-1, but larger toward the carbodiimide type, 25 kJ mol-1, and the wurtzite type, 28 kJ mol-1. Chemical-bonding analysis further reveals how beryllium and carbon induce structural diversity. As regards the second-lowest Pmc21-type, a monolayer of such graphitic BeCN2 shows the potential of photoelectrochemical water splitting, while a bilayer configuration should exhibit ferroelectricity with a polarization of 0.75 pC m-1. Further electronic-structure data of the four polymorphs indicate their potential for nonlinear optics.

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.

Structural evolution of CrN nanocube electrocatalysts during nitrogen reduction reaction.

Metal nitrides have been suggested as prospective catalysts for the electrochemical nitrogen reduction reaction (NRR) in order to obtain ammonia at room temperature under ambient pressure. Herein, we report that templated chromium nitride porous microspheres built up by nanocubes (NCs) are an efficient noble-metal-free electrocatalyst for NRR. The CrN NCs catalyst exhibits both a high stability and NH3 yield of 31.11 µg h-1 mgcat.-1 with a Faradaic efficiency (FE) of 16.6% in 0.1 M HCl electrolyte. Complementary physical characterization techniques demonstrate partial oxidation of the pristine CrN NCs during reaction. Structural characterization by means of scanning transmission electron microscopy (STEM) combining electron energy loss spectrum (EELS) and energy dispersive X-ray spectroscopy (EDX) analysis reveals the NC structure to consist of an O-rich core and N-rich shell after NRR. This gradient distribution of nitrogen within the CrN NCs upon completed NRR is distinct to previously reported metal nitride NRR catalysts, because no significant loss of nitrogen occurs at the catalyst surface.

Metathetic synthesis of lead cyanamide as a p-type semiconductor.

Lead cyanamide PbNCN was synthesized by solid-state metathesis between PbCl2 and Na2NCN in a 1 : 1 molar ratio, and its structure was confirmed from Rietveld refinement of X-ray data. Electronic-structure calculations of HSE06 density-functional type reveal PbNCN to be an indirect semiconductor with a band gap of 2.4 eV, in remarkable quantitative agreement with the measured value. Mott-Schottky experiments demonstrate PbNCN to be a p-type semiconductor with a flat-band potential of 2.3 eV vs. the reversible hydrogen electrode (RHE) which is commonly used to estimate the value of the valence band edge position. Moreover, thin films of powderous PbNCN were assembled into a photoelectrode for photoelectrochemical water splitting. On the example of p-type PbNCN, this study provides the first experimental evidence that MNCN compounds can be applied as photocathodes for reductive reactions in photoelectrochemical cells.

A hypervalent and cubically coordinated molecular phase of IF8 predicted at high pressure.

Up to now, the maximum coordination number of iodine is seven in neutral iodine heptafluoride (IF7) and eight in anionic octafluoride (IF8 -). Here, we explore pressure as a method for realizing new hypercoordinated iodine compounds. First-principles swarm structure calculations have been used to predict the high-pressure and T → 0 K phase diagram of binary iodine fluorides. The investigated compounds are predicted to undergo complex structural phase transitions under high pressure, accompanied by various semiconductor to metal transitions. The pressure induced formation of a neutral octafluoride compound, IF8, consisting of eight-coordinated iodine is one of several unprecedented predicted structures. In sharp contrast to the square antiprismatic structure in IF8 -, IF8, which is dynamically unstable under atmospheric conditions, is stable and adopts a quasi-cube molecular configuration with R3[combining macron] symmetry at 300 GPa. The metallicity of IF8 originates from a hole in the fluorine 2p-bands that dominate the Fermi surface. The highly unusual coordination sphere in IF8 at 300 GPa is a consequence of the 5d levels of iodine coming down and becoming part of the valence, where they mix with iodine's 5s and 5p levels and engage in chemical bonding. The valence expansion of iodine under pressure effectively makes IF8 not only hypercoordinated, but also hypervalent.