Nb-H system at high pressures and temperatures

Abstract

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) \ensuremath{\rightarrow} hcp \ensuremath{\rightarrow} double hcp (dhcp) \ensuremath{\rightarrow} distorted body-centered cubic (bcc) as pressure increases. The appearance of fcc $\mathrm{Nb}{\mathrm{H}}_{2.5\ensuremath{-}3}$ 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 $\mathrm{Nb}{\mathrm{H}}_{xg2}$ 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.