Defects in the new oxide-fluoride Ba₂PdO₂F₂: the search for fluoride needles in an oxide haystack

The defect chemistry and fluorine insertion properties of the new Ba_2−xSr_xPdO₂F₂ system have been investigated using atomistic simulation techniques. The interatomic potential model produces good agreement between simulated and observed structures for the “parent” oxides Ba₂PdO₃ and Sr₂PdO₃, and the oxide-fluorides Ba₂PdO₂F₂ and BaSrPdO₂F₂. The perfect lattice simulations confirm the most favourable structure type is the T′ (Nd₂CuO₄) structure for the oxide-fluoride phases comprised of square planar Pd, which accords with recent EXAFS studies. The fluorination reaction in the precursor oxide Ba₂PdO₃, involving substitution of two fluorine ions for one oxygen ion, is highly favourable. The formation of fluorine interstitials and holes in Ba₂PdO₂F₂ is an unfavourable process in accord with the observed resistance to excess fluorine, and in contrast to the related cuprate superconductor Sr₂CuO₂F_2+δ.