Design of new thorium nuclear clock materials based on polyatomic ions

Compounds of polyatomic anions are investigated theoretically as hosts for thorium in nuclear clock devices. The ²²⁹Th nucleus has an excited state at 8.355 eV which can be reached using a VUV laser as demonstrated in recent experiments. Incorporating ²²⁹Th into a host crystal is an essential step towards developing an ultra-stable nuclear clock. To be a suitable host, a material must have a band gap larger than the nuclear transition frequency. Thus, most research to date has focused on fluorides like LiSrAlF₆ and CaF₂, for they feature ionic bonding and large band gaps. Ionicity of chemical bonding can be pushed to its limits by superhalogens - polyatomic anions whose electron affinity can be greater than that of the halogens. An additional concern for fluorides is the presence of ¹⁹F nuclear spins in the material that can couple to the spin of the ²²⁹Th, detrimental to clock performance. In this work we investigate salts containing superhalogen anions, with the goal of identifying promising new hosts for ²²⁹Th clocks. Specifically, we investigate compounds of Ca, Sr, and Ba with three anions - BF₄^-, ClO₄^-, and SO₄^2-. By computing the electronic properties of the pure and thorium-doped materials, we predict that M(BF₄)₂ have the widest band gaps, making them a promising material class. On the other hand, MSO₄ could produce the most accurate clocks by eliminating inhomogeneous broadening due to nuclear spin interactions. These results may guide experimental searches for new ²²⁹Th clock materials and offer new opportunities to study this unique nuclear transition in the solid state.