Nb-H system at high pressures and temperatures

摘要

We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level ofhydrogen abundancewe could achieve from the resulting alloy.We probed the Nb-H system with laser heating andx-ray diffraction complemented by numerical density functional theory-based simulations. New quenched doublehexagonal close-packed (hcp) NbH_(2.5) appears under 46 Gpa, and above 56 Gpa cubic NbH3 is formed as theoreticallypredicted. 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 pressureincreases. The appearance of fcc NbH_(2.5−3) and dhcp NbH_(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 behaviorof NbH_(x>2) is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentrationincreases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution.