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.