Density Functional Theory Study of Th-doped LiCAF and LiSAF for Nuclear Clock Applications

TL;DR

This study uses density functional theory to analyze the atomic structure and defect formation energies of Th-doped LiCAF and LiSAF crystals, providing insights into their suitability for nuclear clock applications.

Contribution

It systematically investigates charge compensation schemes and environmental effects, revealing differences in defect formation energies and clarifying previous experimental findings.

Findings

Th:LiSAF has lower defect formation energies than Th:LiCAF.

Results depend on specific environmental conditions and charge compensation schemes.

Electric field gradients were calculated for the most stable structures.

Abstract

Thorium-doped LiCaAlF$_6$ and LiSrAlF$_6$ (Th:LiCAF and Th:LiSAF) are promising crystals for a solid-state nuclear clock based on the 8 eV transition in $^{229}$Th; however, their complex crystal structures complicate understanding the atomic arrangement of the thorium defects. In this work, density functional theory simulations are employed to systematically investigate these systems, including temperature-dependent effects and environmental conditions of fluorine saturation and deficiency. We investigated 20 distinct charge compensation schemes for each material, revealing lower defect formation energies in Th:LiSAF than in Th:LiCAF. This suggests that the former may attain a higher concentration of thorium nuclei. Furthermore, we calculated the electric field gradient for the lowest energy structure per compensation pathway. Our investigation shows that results previously reported in the literature apply only to a subset of experimental conditions.