Cubic to hexagonal iron phase transition promoted by interstitial   hydrogen
  Using ab-initio density functional theory we study the role of interstitial hydrogen on the energetics of the phase transformation of iron from bcc to hcp along Bain's pathway. The impurity creates an internal stress field that can be released through a tetragonal distortion of the lattice, promoting the bcc (ferromagnetic) $\rightarrow$ fcc (frustrated antiferromagnetic) $\rightarrow$ hcp (ferromagnetic) transition. The transformation between crystal systems is accompanied by a drastic magnetic reorganization and sudden variations of the unit cell volume, that can be one of the reasons for embrittlement and mechanical failure of iron upon hydrogen adsorption. 
A. Castedo, J. Sanchez, J. Fullea, M.C. Andrade, P.L. de Andres
cond-mat.mtrl-sci