Stabilizing MgB2-like superconductivity in LiB2C2-derived borocarbides through isovalent metal cosubstitution

Abstract

Recent first-principles studies have shown that LiB2C2 can reach a higher superconducting transition temperature Tc than its parent material MgB2, yet its structural instability hinders experimental realization. Here, through a systematic screening of 594 XM2B6C6 candidate systems with different metal elements X and M, we propose a design strategy based on isovalent metal substitution to improve structural stability while maintaining excellent superconducting performance. Using a rapid screening approach based on the zone-center electron-phonon coupling (EPC) descriptor, we screen all possible XM2B6C6 variants derived from the LiB2C2 network. We demonstrate that isovalent ns1X−M metal pairs, such as K-Li, Rb-Li, and Ag-Na, synergistically optimize the interlayer spacing and metal-boron/carbon hybridization to retain strong electron-phonon coupling while hardening low-frequency phonon modes to enhance lattice stability. Representative quaternary systems such as KLi2B6C6, RbLi2B6C6, and AgNa2B6C6 are predicted to possess Tc values significantly exceeding that of MgB2 within the same computational framework, together with enhanced stability. In contrast, aliovalent combinations (e.g., Tc-Ca) can dramatically enhance Tc, but at the expense of stability. These results establish isovalent cosubstitution as a stabilization-enabled route for realizing and optimizing MgB2-like superconductivity in borocarbides by improving stability while preserving the well-known B/C σ-band-driven EPC channel.

Source-reported materials — not catalogue approval

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