Band Gap Calculations for Thorium‐Doped LiCAF

Abstract

The large band gap insulator LiCaAlF 6 (LiCAF) has been proposed as a possible host crystal for future realizations of a solid-state based thorium-229 nuclear clock, due to its excellent optical transmission in the vacuum ultraviolet range. To enable direct optical manipulation of the thorium isomeric state, the band gap has to remain larger than the nuclear excitation energy upon crystal doping. Here, a systematic search for possible charge compensation mechanisms, defect locations, and the emergence of other compounds, using density functional theory, is presented. Out of 535 optimized structures, the energetically most favorable arrangement is Th • Al + V ′ Li with an estimated band gap of 11.4 eV. Evaluating relevant uncertainties of the used methods suggests that the doped material remains transparent at the 229 Th isomer energy for all investigated configurations.

Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials

“…Presently, the reported research attempts on solid-state nuclear clock materials mainly include Th:LiF, Th:CaF 2 , Th:LiCaAlF 6 /LiSrAlF 6 , and Th:MgF 2 , − among which, Th:CaF 2 single crystal can easily achieve high concentration doping, because its cation radius and electronegativity are very close to Th ions. In addition, it has good VUV transmittance and low background luminescence in the nuclear transition band, making it a very promising solid nuclear clock material. − However, according to the latest report of Beeks et al, the VUV transmittance of their highly doped 232 Th:CaF 2 crystals with doping concentrations of 4.0 × 10 19 and 2.6 × 10 20 cm –3 are ∼35% and ∼5% at 150 nm, respectively.…”

Thorium‐Doped Lithium Fluoride Single Crystal: A Potential Promising Candidate for Solid State Nuclear Optical Clock Materials

“…[2] Currently, Th-doped or Th-based crystals reported experimentally or theoretically mainly include NaYF 4 , LiYF 4 , LiCaAlF 6 , Na 2 ThF 6 , LiSrAlF 6 , CaF 2 , and MgF 2 , most of which used 232 Th as a dopant, while only LiSrAlF 6 , MgF 2 , and CaF 2 crystals used 229 Th. [4,5,[9][10][11][12][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Because of extranuclear electron nature determines the chemical properties of elements, the 232 Th and 229 Th can be considered to have the same chemical properties; therefore, 232 Th, which is very low radioactive and easy to obtain, was used as a dopant in most experimental studies of host materials, while 229 Th, which is highly radioactive and difficult to obtain, was used more in the formal study of nuclear spectroscopy experiments. Certainly, if 232 Th can be converted to 229 Th by means of a 232 Th(p,p3n) 229 Th light-ion-induced fusion-evaporation reaction, it will greatly promote the research and development of 229Th -doped crystals.…”