Ab-initio cluster calculations of hole formation and trapping in PbF/sub 2/ and PbF/sub 4/ [scintillation]

We 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-PbF/sub 2/ and PbF/sub 4/. The crystals were modeled by Pb/sub 24/F/sub 48/ and Pb/sub 14/F/sub 56/ 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 PbF/sub 2/ has the same crystal structure as CaF/sub 2/, however their electronic structures are different. It is known experimentally that in CaF/sub 2/ holes travel easily along rows of F atoms which accounts for the high luminous efficiency of the scintillator CaF/sub 2/:Eu. In contrast, these calculations show that in PbF/sub 2/ holes are trapped on the Pb atoms by an energy barrier of /spl sim/1 eV. This result is consistent with the failure of PbF/sub 2/ as an activated scintillator. Similar calculations on the experimentally unexplored crystal PbF/sub 4/ predict that the holes are trapped on F atoms with an energy barrier of /spl sim/1 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.