Charge compensation mechanism for Th-229 in CaF2

Determine the exact charge compensation mechanism for thorium-229 substituting Ca2+ in calcium fluoride (229:CaF2) crystals, specifying the nature and configuration of the compensating defects created by Th4+ incorporation into the CaF2 lattice.

Background

In the 229:CaF2 system, Th4+ substitutes for Ca2+, creating a local defect within the crystal lattice. Charge neutrality must be maintained via a compensation mechanism, but the specific defect species and their arrangement are presently undetermined. Understanding this mechanism is critical because the local defect environment sets the electric field gradient and electron charge density at the thorium nucleus, which in turn influences quadrupole splitting and temperature-dependent frequency shifts of the nuclear clock transition.

Clarifying the charge compensation mechanism will aid in modeling and controlling inhomogeneous broadening and frequency reproducibility across different doping concentrations and preparation methods. Recent studies have begun probing the defect structure in Th-doped CaF2, but the precise compensation pathway remains unresolved and is an active area of investigation.