Electron-spin-resonance study ofPb−6p3in KC1: A possible Jahn-Teller system

Four $S=\frac{1}{2}$ electron-spin-resonance spectra exhibiting orthorhombic symmetry ($\left[1\mathrm{}\overline{1}0\right]$, [001], [110]) and observed in KC1: ${\mathrm{Pb}}^{2+}$ after x irradiation at temperatures above 220 K are identified as originating from ${\mathrm{Pb}}^{-}$ centers. The mobility of the anion and cation vacancies above this temperature is shown to be essential in the production of these defects. The ${\mathrm{Pb}}^{-}$ spectra are characterized by strongly anisotropic $g$ values ranging from ∼0.7 to ∼4 and their explanation requires a coupling scheme for ${\mathrm{Pb}}^{-}$ intermediate between $\mathrm{LS}$ and $j-j$ and a full diagonalization of the crystal field, the spin-orbit interaction, and the Coulomb repulsion between the 20 states of the $p^3$ configuration. A comparison is made between the crystal fields experienced by ${\mathrm{Pb}}^{-}6p^3$ and by the previously studied $\mathrm{LS}$-coupled system ${\mathrm{Sn}}^{-}5p^3$. Because of the sizable presence of orbital angular momentum in its ${\Gamma }_8$ ground state, the ${\mathrm{Pb}}^{-}$ is shown to induce a static Jahn-Teller distortion when occupying an unperturbed octahedral anion site. The Jahn-Teller energies are estimated to be ∼${10}^3$ ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\mathrm{Pb}}^{-}$, and ∼${10\mathrm{c}\mathrm{m}}^{-1\mathrm{}}$ for ${\mathrm{Sn}}^{-}$. It is concluded that the orthorhombic symmetry in all four ${\mathrm{Pb}}^{-}$ defects is caused by a static Jahn-Teller distortion of mixed $E_g$ and $T_{2g}$ character and that vacancies, if present, merely exert an additional perturbation. This viewpoint is shown to be a fruitful one: It allows us (i) to explain the near constancy throughout the four defects of the dominant crystal-field parameter (∼ -2 × ${10}^4$ ${\mathrm{cm}}^{-1\mathrm{}}$) and (ii) to propose specific models for two of the ${\mathrm{Pb}}^{-}$ centers.