Significant Impact of Quantum and Anharmonic Effects on the Structural Stability and Superconductivity of NbH3 at High Pressures
First-principles calculations combined with the stochastic self-consistent harmonic approximation reveal significant effects of the quantum ionic fluctuations and lattice anharmonicity on the dynamical stability of NbH3 under high pressures. Previous theoretical predictions, which ignored ionic fluctuations and relied on the harmonic approximation, suggested that the I43d phase is the most thermodynamically favorable structure between 33 and 400 GPa, with the Fm3m phase considered thermodynamically metastable. However, recent experiments at 187 GPa identified the Fm3m phase, conflicting with the prediction. In contrast, the present study indicates that the Fm3m phase remains dynamically stable down to at least 145 GPa, approximately 145 GPa lower than harmonic estimates, while the I43d phase is dynamically unstable at 187 GPa, consistent with the experimental findings. Furthermore, systematic calculations are performed on the structural, vibrational and superconducting properties of Fm3m NbH3 under pressures ranging from 100 to 300 GPa, revealing dramatic modifications due to the quantum and anharmonic effects. The calculated superconducting critical temperature (Tc) from the McMillan equation for Fm3m NbH3 at 187 GPa is 44 K, with mu set at 0.15, close to the measured value. These findings highlight the crucial role of quantum anharmonic effects in stabilizing the Fm3m phase.