Structure, Synthetic Pathway, and Properties of Metastable Phosphorus Carbide, P₄C₃

Due to electronegativity (EN) differences, changing from C₃N₄ to P₄C₃ is not as trivial as simply replacing nitrogen by phosphorus in the C₃N₄ structure. Hence, the nonexistent P₄C₃ phase is nominally the higher-homologue analogue of the well-known C₃N₄, but its structure and properties are practically unknown for fundamental reasons. Here we predict, by means of an extensive structure search, three energetically favorable yet metastable P₄C₃ phases adopting space groups P1̅, Cm, and Cmmm, followed by designing their synthetic routes. Different from the planar structural motifs of C₃N₄, P₄C₃ is likely to adopt various "puckered" (as regards the P atoms) forms, mainly related to the EN difference between nitrogen and phosphorus, the difference in atomic size, and also less favorable sp orbital mixing for phosphorus related to primogenic repulsion; hence, the P substructures are characterized by single and fractional P-P bonds with rectangular/linear motifs or those resembling simple-cubic P, including multicenter bonding as a function of the large electron count. As regards the carbon substructures, infinite polyacetylene-like motifs, carbon dimers, and infinite chains of cyclopentadienyl-like and polyphenyl-like units are predicted, with C-C bond orders larger than one. Band structure analyses indicate the metallic character of these three phases. The P1̅ polymorph exhibits a Dirac cone, whereas the Cmmm phase, being composed of metallic "phosphorene" units and infinite polyphenyl units, might show potential in applications, e.g., battery electrodes. Moreover, the monolayer of the P1̅ polymorph should be easy to exfoliate and exhibit a large anisotropy with regard to mechanical and electronic properties. The synthesis of P₄C₃ defining a new class of IV₃V₄ compounds should be attempted due to its predicted structural and physicochemical properties.