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) NbH2.5 appears under 46 GPa, and above 56 GPa cubic NbH₃ 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 NbH_2.5-3 and dhcp NbH2.5 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 NbHx>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.