Structure,\nSynthetic Pathway, and Properties of Metastable\nPhosphorus Carbide, P<sub>4</sub>C<sub>3</sub>

Due to electronegativity (EN) differences, changing from\nC<sub>3</sub>N<sub>4</sub> to P<sub>4</sub>C<sub>3</sub> is not as\ntrivial\nas simply replacing nitrogen by phosphorus in the C<sub>3</sub>N<sub>4</sub> structure. Hence, the nonexistent P<sub>4</sub>C<sub>3</sub> phase is nominally the higher-homologue analogue of the well-known\nC<sub>3</sub>N<sub>4</sub>, but its structure and properties are practically\nunknown for fundamental reasons. Here we predict, by means of an extensive\nstructure search, three energetically favorable yet metastable P<sub>4</sub>C<sub>3</sub> phases adopting space groups <i>P</i>1̅, <i>Cm</i>, and <i>Cmmm</i>, followed\nby designing their synthetic routes. Different from the planar structural\nmotifs of C<sub>3</sub>N<sub>4</sub>, P<sub>4</sub>C<sub>3</sub> is\nlikely to adopt various “puckered” (as regards the P\natoms) forms, mainly related to the EN difference between nitrogen\nand phosphorus, the difference in atomic size, and also less favorable\nsp orbital mixing for phosphorus related to primogenic repulsion;\nhence, the P substructures are characterized by single and fractional\nP–P bonds with rectangular/linear motifs or those resembling\nsimple-cubic P, including multicenter bonding as a function of the\nlarge electron count. As regards the carbon substructures, infinite\npolyacetylene-like motifs, carbon dimers, and infinite chains of cyclopentadienyl-like\nand polyphenyl-like units are predicted, with C–C bond orders\nlarger than one. Band structure analyses indicate the metallic character\nof these three phases. The <i>P</i>1̅ polymorph exhibits\na Dirac cone, whereas the <i>Cmmm</i> phase, being composed\nof metallic “phosphorene” units and infinite polyphenyl\nunits, might show potential in applications, e.g., battery electrodes.\nMoreover, the monolayer of the <i>P</i>1̅ polymorph\nshould be easy to exfoliate and exhibit a large anisotropy with regard\nto mechanical and electronic properties. The synthesis of P<sub>4</sub>C<sub>3</sub> defining a new class of IV<sub>3</sub>V<sub>4</sub> compounds should be attempted due to its predicted structural and\nphysicochemical properties.