Nominal Effect of Mg Intercalation on the Superconducting Properties of 2H-NbSe₂

2H-NbSe₂ is a phonon-mediated, Fermi-surface topology-dependent multiband superconductor with an incommensurate charge-density wave (CDW) that coexists at a local level with superconductivity. Usually, the intercalation in 2H-NbSe₂ enriches the CDW, enhances the c-axis lattice parameter, and distorts the Fermi surface, which result in a decrease in the superconducting transition temperature (T_c). The rate of decrease of T_c depends on the electronic structure, size, valence, magnetic nature, and electronegativity of the intercalating species. Herein, we report an unusual effect of Mg intercalation on the superconductivity of 2H-MgNbSe₂ (x = 0.0, 0.02, 0.06, 0.08, 0.10, and 0.12) synthesized by a high-temperature solid-state reaction method. Unlike other s- and p-block elements/species as intercalants (Rb, Sn, Ga, and Al) that have a sharp detrimental effect on the T_c of 2H-NbSe₂ within 1-5% of intercalation, Mg is found to be an exception. Upon Mg intercalation up to x = 0.06, no remarkable changes in T_c as compared to the parent 2H-NbSe₂ (T_c ∼ 6.7 K) are observed, and further intercalation results in a small decrease in T_c (for x = 0.12, T_c = 6.2 K). From heat-capacity measurements, it is inferred that superconducting Mg-intercalated 2H-NbSe₂ exhibits strong electron-phonon coupling. Electronic structure calculations on two s-block element intercalated compounds of formula M_0.125NbSe₂ (M = Mg, Rb) show that Rb s-, p-, and d-states completely overlap with the Nb d states, while the Mg s states lie in a low-energy region as compared to Nb d states, indicating a weak interaction between the intercalant and the Nb sublattice in Mg_0.125NbSe₂ as compared to Rb_0.125NbSe₂. These results suggest that the electronic states of the Nb network in 2H-NbSe₂ are least altered with Mg intercalation, which could be one of the reasons for the minimal effect on the T_c with intercalation.