Design of new thorium nuclear clock materials based on polyatomic ions

19 March 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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. However, ionicity of chemical bonding can be pushed to its limits by use of 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 materials 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.

Keywords

Thorium-229
Band gap
Nuclear clock
Superhalogen

Supplementary materials

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Description
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Supporting information and figures
Description
Additional electronic structure calculations (DOS and PDOS), details of cation disorder simulations, discussion of internal conversion
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Computational data - structures
Description
Optimized structures used in DFT calculations in VASP POSCAR format
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