The stability of the $\epsilon$ phase of zirconium hydride is studied using the pseudopotential plane wave method. The fluorite structure is mechanically unstable with respect to a tetragonal distortion. However, the minimum corresponding to $c>a$ is almost degenerate in energy with that for $c<a$. The distortion explains the uniaxial misfit strain responsible for delayed hydride cracking in zirconium alloys.

• Received 29 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2233