Optical transmission enhancement of ionic crystals via superionic fluoride transfer: Growing VUV-transparent radioactive crystals

The 8 -eV first nuclear excited state in Th-229 is a candidate for implementing a nuclear clock. Doping Th-229 into ionic crystals such as CaF2 is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of Th-229 prohibits the growth of high -quality single crystals with high Th-229 concentration; radiolysis causes fluoride loss, increasing absorption at 8 eV. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at 8 eV by annealing Th-229 -doped CaF2 at 1250 C-degrees in CF4. This technique allows to adjust the fluoride content without crystal melting, preserving its single -crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation -hard crystals. This approach enables control over the charge state of dopants, which can be used in deep -UV optics, laser crystals, scintillators, and nuclear clocks.