Characterization of the thorium-229 defect structure in CaF2 crystals

Recent advancements in laser excitation of the low-energy thorium-229 (229Th) nuclear isomeric state in calcium fluoride (CaF2) single crystals render this system a promising candidate for a solid-state nuclear clock. Nonetheless, the precise experimental determination of the microscopic ion configuration surrounding the doped 229Th and its electronic charge state remains a critical challenge. Such characterization is essential for precisely controlling the clock transition and evaluating the performance of this solid-state nuclear clock system. In this study, we use x-ray absorption fine structure spectroscopy of 229Th:CaF2 to investigate the charge state and coordination environment of doped 229Th. The results indicate that 229Th displays a 4+ oxidation state at the substitutional site of a Ca2+ ion, with charge compensated provided by two F− ions positioned at interstitial sites adjacent to 229Th.