  The 8 eV first nuclear excited state in $^{229}$Th is a candidate for
implementing an nuclear clock. Doping $^{229}$Th into ionic crystals such as
CaF$_2$ is expected to suppress non-radiative decay, enabling nuclear
spectroscopy and the realization of a solid-state optical clock. Yet, the
inherent radioactivity of $^{229}$Th prohibits the growth of high-quality
single crystals with high $^{229}$Th concentration; radiolysis causes fluoride
loss, increasing absorption at 8 eV. We overcome this roadblock by annealing
$^{229}$Th doped CaF$_2$ at 1250$\unicode{x2103}$ in CF$_4$. The technique
presented here 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.
