Stability-superconductivity map for compressed Na-intercalated graphite

TL;DR

This study explores the stability and superconducting properties of Na-intercalated graphite compounds under pressure, identifying new stable stoichiometries and confirming high-temperature superconductivity potential with critical temperatures up to 48K.

Contribution

It provides a detailed analysis of the thermodynamic stability and superconductivity of various Na-C compounds, revealing new stable phases and refining the understanding of their electron-phonon interactions.

Findings

NaC4 can reach a Tc of 48K at 10 GPa.

New stable stoichiometries like Na3C10, NaC8, NaC10, and NaC12 are identified.

Na3C10 is also a promising superconductor.

Abstract

A recent ab initio investigation of Na-C binary compounds under moderate pressures has uncovered a possible stable NaC$_4$ superconductor with an estimated critical temperature up to 41K. We revisit this promising binary system by performing a more focused exploration of Na-intercalated graphite configurations, assessing the sensitivity of their thermodynamic stability to density functional approximations at different (T,P) conditions, and examining their superconducting properties with the anisotropic Migdal-Eliashberg formalism. The combinatorial screening of possible Na arrangements reveals additional stable stoichiometries, i.e., Na$_3$C$_{10}$, NaC$_8$, NaC$_{10}$, and NaC$_{12}$, that redefine the previously proposed convex hulls for pressures up to 10 GPa. The evaluation of formation enthalpies with different van der Waals functionals indicates that the proposed compounds might not be thermodynamically stable at zero temperature but some of them could stabilize due to the vibrational entropy or form via cold compression if graphite is used as a starting material. Our more rigorous modeling of the electron-phonon coupling in NaC$_4$ confirms the material's potential for high-temperature superconductivity, with a critical temperature reaching 48 K at 10 GPa, and reveals a well-defined two-gap structure unusual for an electron-doped compound. By tracking the position of the intercalant nearly free electron states with respect to the Fermi level in viable Na-C compounds, we map out the range of pressures and compositions needed for strong electron-phonon coupling and identify Na$_3$C$_{10}$ as an equally promising superconductor.