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 <i>P</i>1̅, <i>Cm</i>, and <i>Cmmm</i>, 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 <i>P</i>1̅ polymorph exhibits a Dirac cone, whereas the <i>Cmmm</i> 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 <i>P</i>1̅ 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.