Computational Design and Theoretical Properties of WC₃N₆, an H-Free Melaminate and Potential Multifunctional Material

Abstract

By means of first-principles theory, existence, synthetic conditions, and structural as well as physicochemical properties have been predicted for the first hydrogen-free melaminate salt of the composition WC3N6. We find at least two energetically favorable polymorphs adopting space groups P1 and P3, both of which are layer-like porous materials. In addition to sizable Madelung fields stabilizing saltlike WC3N6, the complex C3N6 6– anions are connected via perfectly optimized W–N bonds, forming WN5 in the P1 and WN6 coordination polyhedra in the P3 polymorphs. The band gaps of the P1 and P3 phases are HSE-predicted as 2.25 and 1.21 eV, respectively, significantly smaller than those of g-C3N4 and WO3. Moreover, both phases have suitable band-edge potentials that may provide sufficient driving force for photocatalytic water splitting; at least for the P1 phase, there is also a reasonable chance for reduced electron–hole recombination. In addition, the polymorphs’s large optical absorption coefficients should greatly enhance the photocatalytic performance. WC3N6 defines a new class of compounds and has unique structural characteristics, mirrored from its electrical and optical properties, and it should provide another chemical path for preparing efficient photocatalysts and optoelectronic devices.

Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases

“…This distance is extremely close to the expected W−N bond length of 2.06 Å for a W 6+ ion coordinated to N 3− in a 6-fold coordination state. 57 Phonon calculations confirmed that this structure was a local minimum. The previously reported P3 and P1 structures were found as the 101st and 150th phases in the constrained search (since they were employed as seeds), as illustrated in the energy versus structure index plot in Figure 5(d), and the newly predicted polymorph was found as the 515th structure.…”

“…49 Additionally, we have predicted a new metastable melaminate salt, P1̅ WC 3 N 6 , which is found to be lower in energy compared to the compounds reported recently by Chen, Wang, and Dronskowski. 57 Our results demonstrate that a constrained search is more efficient in identifying metastable phases with user-defined structural features compared with a traditional CSP search.…”

“…(a–d) The crystal structures of multiple phases of WC 3 N 6 . (a, b) The previously proposed P 1 and P 3 polymorphs with (C 3 N 6 ) 6– units . (c) Im 3̅ m phase, which is the ground state, found in a traditional search.…”

“…This polymorph is calculated to be lower in energy than the previously reported potential multifunctional materials based upon the (C 3 N 6 ) 6− unit. 57 Constraining Coordination Numbers for Prediction of 2D Polymeric Nitrogen. The prediction and discovery of novel polymeric nitrogen or nitrogen-rich compounds is of great interest in contemporary times because of their potential as environmentally friendly high-energy-density materials (HEDMs).…”

“…Since the melaminate anion cannot be represented by a VSEPR geometry, it is highly unlikely that the structures generated via this method or using the RANDSPG algorithm would possess suitable chemical environments. Therefore, in addition to utilizing the aforementioned constraints to filter the breeding pool, the initial generation was populated with 100 random structures generated by RANDSPG, the previously predicted P1 and P3 configurations, 57 and with 98 molecular crystals of WC 3 N 6 generated by the PYXTAL code. 61 The initial generation was created in this way for both constrained and traditional searches.…”