Electronic phase transitions under pressure in thermodynamically stable phases of SrC2: A route toward tunable semiconductor-metal materials

Abstract

We report a systematic first-principles investigation of the two most stable polymorphs of strontium dicarbide (${\text{SrC}}_2$): the high-symmetry tetragonal $I4/mmm$ phase and the low-symmetry monoclinic $C2/c$ phase. Free-energy calculations indicate a crossover at $\sim 600K$: below this temperature, $I4/mmm$ is thermodynamically favored, but above it, vibrational entropy stabilizes the $C2/c$ phase. Electronic-structure and optical-absorption spectra predict indirect band gaps of $2.91eV$ ($I4/mmm$) and $4.13eV$ ($C2/c$). The tetragonal phase exhibits nearly isotropic optical response, while the monoclinic phase shows moderate in-plane anisotropy, suggesting potential for polarization-sensitive photonic and ultraviolet-plasmonic devices. Under hydrostatic pressures up to $80GPa$, both polymorphs display tunable band-gap trends, highlighting opportunities for pressure-modulated optoelectronic applications. Taken together, our results reconcile earlier discrepancies, elucidate the interplay between symmetry, lattice dynamics, and electronic properties in ${\text{SrC}}_2$, and identify the monoclinic $C2/c$ polymorph as a promising platform for anisotropic photonics and UV-plasmonics.