Nb-H system at high pressures and temperatures

We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level of hydrogen abundance we could achieve from the resulting alloy. We probed the Nb-H system with laser heating and x-ray diffraction complemented by numerical density functional theory-based simulations. New quenched double hexagonal close-packed (hcp) $\mathrm{Nb}{\mathrm{H}}_{2.5}$ appears under 46 GPa, and above 56 GPa cubic $\mathrm{Nb}{\mathrm{H}}_3$ is formed as theoretically predicted. Nb atoms are arranged in close-packed lattices which are martensitically transformed in the sequence: face-centered cubic (fcc) → hcp → double hcp (dhcp) → distorted body-centered cubic (bcc) as pressure increases. The appearance of fcc $\mathrm{Nb}{\mathrm{H}}_{2.5-3}$ and dhcp $\mathrm{Nb}{\mathrm{H}}_{2.5}$ cannot be understood in terms of enthalpic stability, but can be rationalized when finite temperatures are taken into account. The structural and compressional behavior of $\mathrm{Nb}{\mathrm{H}}_{x>2}$ is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentration increases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution.

Figure 1

($\lambda =0.308245\AA$) Diffraction patterns of the samples after heating under different pressure in (down) cell 1 and (up) cell 2. The downmost black pattern at 22 GPa is from the indirectly heated sample. The asterisks denote the new peaks of $\mathrm{Nb}{\mathrm{H}}_x$.

Figure 2

The diffraction profile of mixed $\mathrm{Nb}{\mathrm{H}}_{2.5}$ and $\mathrm{Nb}{\mathrm{H}}_3$ at 81 GPa. The black open circles and red solid line represent the Rietveld fits and observed data, respectively, and the blue solid line is the residual intensity.

Figure 3

The structures of (a) $\mathrm{Nb}{\mathrm{H}}_2$ ($Fm\text{−}3m$) at 20 GPa, (b) $\mathrm{Nb}{\mathrm{H}}_{2.5}$ (Ibam) at 20 GPa, (c) $\mathrm{Nb}{\mathrm{H}}_{2.5}$ ($P{6}_{3}mc$) at 80 GPa, (d) $\mathrm{Nb}{\mathrm{H}}_2$ (Pnma) at 50 GPa, and (e) $\mathrm{Nb}{\mathrm{H}}_3$ ($I\text{−}43d$) at 80 GPa. Large and small spheres represent Nb and H atoms, respectively. The green cells represent the niobium sublattice.

Figure 4

Volumes as a function of pressure for the different $\mathrm{Nb}{\mathrm{H}}_x$ phases, including a comparison of the element Nb with the former experimental paper [37]. The solid lines and symbols represent the simulations and experimental data, respectively.

Figure 5

(a) Enthalpy curves for various structures as a function of pressure. (b) Ground-state and static enthalpy of formation per atom of the $\mathrm{N}{\mathrm{b}}_{1-x}{\mathrm{H}}_x$ phases with respect to their separated counterparts; the hydrogen molar content ($x=0$ corresponds to pure niobium; $x=1$ to pure hydrogen) for the ground state and $P=1\mathrm{atm}$, 20, 40, 100 GPa. Only related stoichiometries are plotted; others are not considered here. The symbols on the solid lines denote that the hydrides are stable at the corresponding pressures, while those on the dashed lines represent that the hydrides are unstable with respect to their decomposition into hydrogen and other hydride or niobium.