Transmutation in 90SrF2: A density functional theory study of phase stability in ZrF2

The stability of multiple possible phases of ZrF2 is computed using density-functional theory. Motivated by radioactive samples of fluorite 90SrF2 stored at the Hanford site, we consider β− radioactive decay as the route by which the 90ZrF2 is generated. To find suitable structures for the ZrF2 compound two methodologies are used. The first follows imaginary phonon modes from the fluorite ZrF2 while the second employs random structure searching. Six possible ZrF2 phases are identified; however, none of the structures resemble the lone experimentally reported orthorhombic structure for ZrF2. Although we predict these phases to be less stable (~0.3 eV/f.u.) than a phase-decomposed mixture of β-ZrF4 and Zr metal, they still may be relevant due to the kinetics of formation via radioactive decay and raise questions as to the nature of the ZrF2 structure and the state of the samples at Hanford