Hydrogen-Vacancy-Induced Stable Superconducting Niobium Hydride at High Pressure

In recent years, the discovery of unconventional polyhydrides under high pressure, including notable instances like CaH6, YH9, and LaH10, with superconducting critical temperature (T c) above 200 K, has ignited considerable interest in the quest for high-temperature superconductivity in hydrogen-based materials. Recent studies have suggested the highly probable existence of hydrogen vacancies in these high-T c superconducting hydrides, although there is no conclusive evidence. In this study, taking niobium (Nb) hydride as a model, we showcase the observation of nonstoichiometric face-centered cubic (fcc) NbH4-delta (delta similar to 0.23-0.51) at pressures ranging from 113 to 175 GPa, employing in situ high-pressure X-ray diffraction experiments in conjunction with first-principles calculations. Remarkably, our further analyses indicate that the hydrogen vacancies, along with the resulting configurational entropy, play crucial roles in stabilizing this nonstoichiometric fcc NbH4-delta. Electrical transport measurements confirmed the superconductivity, as evidenced by zero resistance as well as suppression of T c with applying magnetic fields, with a T c reaching up to 34 K. Our current results not only confirm the presence of hydrogen vacancies in high-T c hydrides, but also provide key insights into the understanding of hydrogen-vacancy-induced stability for nonstoichiometric hydrides under high pressure.