  The identification of new materials with superconducting properties is the
pursuit in the realm of superconductivity research. Here, excitedly, we show
that the simplest salt daily used can be made a superconductor at normal
pressure only by adjusting its stoichiometry of Na and Cl as Na3Cl at normal
pressure based on first-principles calculations. This bulk stable abnormal
Na-Cl stoichiometric crystal of 3:1, the first 'magic' ratio, includes metallic
(Na) atoms in the core as well as hybridization of ionic and metallic bonding,
facilitating the electron-phonon-coupling for superconductivity with a critical
temperature Tc of 0.13 K. The flat bands and van Hove singularities near the
Fermi level produce large densities of states, similar to H3S and LaH10, which
is beneficial for the emergence of superconductivity. The crystal composed of
with abnormal Na-Cl magic stoichiometry is a precisely tunable, purely sodium
and chloride-based, three-dimensional bulk superconductor, which is therefore
an ideal material for designing and understanding abnormal stoichiometric
crystals. The methodology of constructing this bulk abnormal crystal may be
general to almost all elements, which could lead to insights into the physics
of other conventional superconductors and even high-critical-temperature
superconductors.
