Phase transitions of NbHx (1 ≤ x ≤ 2) from first principles calculation

First-principles calculation reveals that the face-centered cubic (FCC) structure of NbH1 and NbH1.25 is energetically unstable and will automatically transform to the face-centered tetragonal (FCT) structure, and that the intrinsic composition range of the FCT → FCC transition of NbHx phases is 1.5 ≤ x < 2.0. Calculations also show that FCC is the most stable structure of NbHx under high pressure, and that the face-centered orthorhombic (FCO) and FCT structures of each NbH1 and NbH1.25 as well as FCT and FCC of each NbH1.5 and NbH1.75 are very close to each other in terms of the behaviors of phase transition and electronic structure. In addition, Poisson's ratio and density of states are discussed, respectively, to provide a fundamental understanding of various phase transitions in NbHx (1 ≤ x ≤ 2) phases.