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)

Abstract

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 NCN^2- 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 NCN^2- 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.

Keywords: carbodiimide; guanidinate; non-linear optics; ortho-nitrido carbonate; water splitting.

Figure 1

Sketch of charge and dimensionality of four nitrogen‐based (complex) anions as a function of their shape.

Figure 2

a) Crystal structure of TCN₃ guanidinates (T=V, Nb, Ta) with space group P $\bar{6}$ c2 and b) bond‐length histogram of various interactions.

Figure 3

The crystal structures of the a) nitride carbodiimides, b) nitride guanidinates, and c) ortho‐nitrido carbonates of the T′ ₂CN₄ (T′=Ti, Zr, Hf) compositions using the Hf₂CN₄ example. The corresponding bond‐length histograms are given in d), e), and f).

Figure 4

(a) Bond lengths and (b) integrated crystal orbital Hamilton population (ICOHP) of (the sum of) various bonds in T′ ₂N₂(NCN), T′ ₂N(CN₃), and T′ ₂CN₄. For simplicity, we designate C−N1 as the shortest C−N bond whereas T′−N1 is the shortest T′−N bond. The inset in (b) is the electron localization function (ELF) of anionic groups, with an isosurface level of 0.8.

Figure 5

(a) Equation‐of‐state (EOS) fits for the total energies per atom as a function of volume for Hf₂N₂(NCN), Hf₂N(CN₃), and Hf₂CN₄; b) enthalpy‐pressure course of the three predicted compounds.

Figure 6

Calculated densities of states (DOS) for a) nitride carbodiimide Hf₂N₂(NCN), b) nitride guanidinate Hf₂N(CN₃), and c) ortho‐nitrido carbonate Hf₂CN₄. Orange, black and green lines represent Hf, C and N, respectively. Arrows on the right axis represent the atomic band centers below the Fermi level.

Figure 7

Calculated HSE06 band gaps for stable compounds of the composition TCN₃ (T=V, Nb, Ta) and T′ ₂CN₄ (T′=Ti, Zr, Hf), the latter grouped into nitride carbodiimides, nitride guanidinates, and ortho‐nitrido carbonates. The band‐edge potentials are referenced to the reversible hydrogen electrode. Grey and orange colors indicate the band gap itself whereas green and blue colors represent the conduction band minimum (CBM) and valence band maximum (VBM), respectively.