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

Abstract

The 8-eV first nuclear excited state in ²²⁹Th is a candidate for implementing a nuclear clock. Doping ²²⁹Th 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 ²²⁹Th prohibits the growth of high-quality single crystals with high ²²⁹Th concentration; radiolysis causes fluoride loss, increasing absorption at 8eV. 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 8eV by annealing ²²⁹Th-doped CaF₂ at 1250∘C 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.