Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

As part of a search for enhanced superconductivity, we explore theoretically the ground-state structures and properties of some hydrides of niobium over a range of pressures and particularly those with significant hydrogen content. A primary motivation originates with the observation that under normal conditions niobium is the element with the highest superconducting transition temperature (${T}_{c}$), and moreover some of its compounds are metals again with very high ${T}_{c}$'s. Accordingly, combinations of niobium with hydrogen, with its high dynamic energy scale, are also of considerable interest. This is reinforced further by the suggestion that close to its insulator-metal transition, hydrogen may be induced to enter the metallic state somewhat prematurely by the addition of a relatively small concentration of a suitable transition metal. Here, the methods used correctly reproduce some ground-state structures of niobium hydrides at even higher concentrations of niobium. Interestingly, the particular stoichiometries represented by NbH${}_{4}$ and NbH${}_{6}$ are stabilized at fairly low pressures when proton zero-point energies are included. While no paired H${}_{2}$ units are found in any of the hydrides we have studied up to 400 GPa, we do find complex and interesting networks of hydrogens around the niobiums in high-pressure NbH${}_{6}$. The Nb-Nb separations in NbH${}_{n}$ are consistently larger than those found in Nb metal at the respective pressures. The structures found in the ground states of the high hydrides, many of them metallic, suggest that the coordination number of hydrogens around each niobium atom grows approximately as 4$n$ in NbH${}_{n}$ ($n$ $=$ 1--4), and is as high as 20 in NbH${}_{6}$. NbH${}_{4}$ is found to be a plausible candidate to become a superconductor at high pressure, with an estimated ${T}_{c}$ \ensuremath{\sim} 38 K at 300 GPa.