Ab-initio cluster calculations of hole formation and trapping in PbF2 and PbF4

The authors have used ab-initio quantum chemistry computer codes to model the formation of holes and the energy barriers for their diffusion in two lead fluoride hosts of potential interest for scintillation--PbF2 and PbF4. The crystals were modeled by Pb24F48 and Pb14F56 atomic clusters embedded in arrays of several thousand point charges to reproduce the Madelung potential to an accuracy of several mV throughout the cluster. Cubic PbF2 has the same crystal structure as CaF2, however their electronic structures are different. It is known experimentally that in CaF2 holes travel easily along rows of F atoms which accounts for the high luminous efficiency of the scintillator CaF2:Eu. In contrast, these calculations show that in PbF2 holes are trapped on the Pb atoms by an energy barrier of approximately1 eV. This result is consistent with the failure of PbF2 as an activated scintillator. Similar calculations on the experimentally unexplored crystal PbF4 predict that the holes are trapped on F atoms with an energy barrier of approximately1 eV and is therefore not a promising host for an activated scintillator. These computational techniques can be applied to other crystals to find those with mobile holes for new heavy-atom scintillators and solid-state detectors