Evolutionary search for superconducting phases in the lanthanum-nitrogen-hydrogen system with universal neural network potential

Recently, Grockowiak et al. reported "hot superconductivity" in ternary or multinary compounds based on lanthanum hydride [A. D. Grockowiak et al., Front. Electron. Mater. 2, 837651 (2022)]. In this paper, we explored thermodynamically stable phases and superconducting phases in the lanthanum-nitrogen-hydrogen system (LaxNyH1−x−y, 0≤x≤1, 0≤y≤1) at pressure of 20GP. We rapidly and accurately constructed the formation-enthalpy convex hull using an evolutionary construction scheme based on density functional theory calculations, extracting the candidates for stable and moderately metastable compounds by the universal neural network potential calculations. The convex hull diagram shows that more than fifty compounds emerge as stable and moderately metastable phases in the region of ΔH≤4.4mRy/atom. In particular, the compounds are concentrated on the line of x=0.5 connecting between LaH and LaN. We found that the superconductivity is gradually enhanced due to N doping for LaH and the superconducting critical temperature Tc reaches 8.77K in La2NH with y=0.25. In addition, we predicted that metastable La2NH2 shows the highest Tc value, 14.41K, of all the ternary compounds predicted in this study. These results suggest that it is difficult to obtain the hot superconductivity in the La-H compounds with N at 20GPa.