Prediction of Superconductivity in Porous, Covalent Triazine Frameworks

Conventional superconductivity is a phenomenon where lattice vibrations, phonons, steer electrons through a solid with zero resistance. We theoretically predict such phonon-mediated superconductivity to occur in a covalent triazine framework, CTF-0, when it is intercalated with Li or Na. Porous CTF-0 is computed to possess several anticipated properties of interest, such as low-lying π*-bands that can be partially doped, as well as electrostatic anchoring points that improve the stability of the functionalized structures. However, it is the integral porosity that plays a surprisingly crucial role in driving superconductivity, by providing the space for donor atoms to engage in highly localized vibrational modes. These low-frequency rattling phonons couple strongly to the free π*-electrons, and they work in concert with in-plane aromatic-ring vibrations of the framework at a higher frequency. We compute a markedly improved critical temperature (Tc) for Li-decorated CTF-0 (6.2 K) over graphite-intercalation compound LiC6 (predicted at 0.9 K), an improvement that relates directly to the porosity of the material. We also predict a further enhancement of the Tc upon substitution with larger Na (9.1 K), which engages in more localized rattling phonons. This work proposes that CTFs and kindred porous frameworks can be a hub for an exciting new class of materials, for which tunable porosity gives control over superconducting properties.