Instability of monovalent ions of transitional metals in alkaline earth fluorides: CaF2 and SrF2 activated by Co2+ or Ni2+
The ground state of monovalent ions of transitional metals in alkaline-earth fluorides is localized in the conduction band. It is shown by the example of monovalent Co and Ni ions in CaF2 and SrF2 crystals that this is responsible for the unstable state of these ions in cubic environments. A stable state is achieved by the monovalent ions shifting to off-center positions.