They grew CaF2 crystals with a small amount of thorium, which take the place of some of the Ca atoms in the crystal lattice. There’s an illustration in fig 1 of

They grew CaF2 crystals with a small amount of thorium, which take the place of some of the Ca atoms in the crystal lattice. There’s an illustration in fig 1 of the preprint of what the substitutional defects look like Like the other poster mentions, CaF2 is an ionic crystal, but I don’t think that’s an important detail because you wouldn’t expect a nuclear transition to be affected by the bonding state of the electrons. My guess is it’s just a convenient way to get a very dilute collection of thorium atoms without using an ion trap