EPR and ENDOR investigation of the[FLi]0center in CaO

A new paramagnetic color center, denoted by ${\left[F_{\mathrm{Li}}\right]}^0$, is reported. It has been observed in single crystals of CaO grown with lithium impurities. The center consists of a single electron in a ${\mathrm{O}}^{2-}$ vacancy with a ${\mathrm{Li}}^{+}$ ion replacing an adjacent ${\mathrm{Ca}}^{2+}$ ion, i.e., an $F^{+}$ center next to a lithium substitutional impurity. Therefore, this is the first paramagnetic electron-defect center possessing local electrical neutrality to be seen in the alkaline-earth oxides. EPR and ENDOR measurements at both $X$ and $K$ band yield the following spin-Hamiltonian parameters at 4.2 K: $g_{\parallel }=1.999\mathrm{}3\left(2\right)\mathrm{}$; $g_{\perp }=2.000\mathrm{}1\left(2\right)\mathrm{}$; $A=\mp 9.01\left(1\right)\mathrm{}$ MHz; $B=\mp 5.35\left(1\right)\mathrm{}$ MHz; and $P<~0.005$ MHz. The hyperfine values are temperature dependent and are also reported for 77 and 150 K. The data suggest that there is an outward relaxation of the lithium ion and probably other ions in the lattice, but there is no electric-field gradient.