High-precision atomic clocks with highly charged ions: nuclear spin-zero   $f^{12}$-shell ions
  Optical atomic clocks using highly-charged ions hold an intriguing promise of metrology at the 19th significant figure. Here we study transitions within the $4f^{12}$ ground-state electronic configuration of highly charged ions. We consider isotopes lacking hyperfine structure and show that the detrimental effects of coupling of electronic quadrupole moments to gradients of trapping electric field can be effectively reduced by using specially chosen virtual clock transitions. The estimated systematic fractional clock accuracy is shown to be below $10^{-19}$. 
V.A. Dzuba, A. Derevianko, and V. V. Flambaum
physics.atom-ph physics.optics quant-ph