Short-range order of H in the Nb-H solid solution

The effect of the short-range order (SRO) of H on the structural properties of the Nb-H solid solution is studied in this work using an accurate description of the configurational entropy. For that purpose, a general analytical expression to compute the configurational entropy of interstitial solid solutions is deduced. The expression, suitable for the treatment of interstitial clustering and SRO, can be applied to tetrahedral or octahedral interstitial solutions in any crystal lattice. The structural model for the Nb-H solid solution presented in this work is based on the existence of H pairs in the α-phase and double pairs in the α′-phase. The model explains unsolved controversies in this system as the length of the H-H interaction and the relation between the disordered and the ordered phases.