Effects of anharmonic phonons on superconductivity in the NH4-intercalated boron-carbon clathrate NH₄B₂C₈

Achieving structural stability and high critical temperature (T_c) in hydrides under ambient conditions remains a major challenge. A feasible design strategy integrates the advantages of two material classes: (i) B-C–based clathrate superconductors, which exhibit excellent structural stability at ambient or relatively low pressures, and (ii) hydrogen-based unit, which possesses the potential for high T_c. Guided by this strategy, we theoretically predict a hypothetical compound NH₄B₂C₈ by embedding hydrogenated units into a B-C clathrate framework. Using the stochastic self-consistent harmonic approximation combined with machine-learning-accelerated structural sampling, we assess the impact of anharmonicity on its superconducting properties. Anharmonic effects harden the dominant phonon modes 13–15, reducing the electron-phonon coupling constant λ from 1.43 to 1.22 and lowering T_c from 102 to 91 K. Furthermore, hole doping realized by NH₄ vacancy engineering, which yields (NH₄ )³(B₂C₈ )⁴, is demonstrated to be ineffective in enhancing Tc under anharmonic conditions. Our results highlight the synergistic role between hydrogen units and B-C frameworks in enhancing superconductivity, and underscore the importance of anharmonicity in the design of such superconductors.