Electron-Nuclear-Double-Resonance and Electron-Spin-Resonance Evidence for a ${\mathrm{Fe}}^{3+}$ (${\mathrm{K}}^{+}$ Vacancy) Center in Iron-Doped KZn${\mathrm{F}}_3$

Crystals of KZn${\mathrm{F}}_3$ doped with ≥ 0.05-at.% iron exhibit ${\mathrm{Fe}}^{3+}$ electron-spin-resonance (ESR) and electron-nuclear-double-resonance (ENDOR) spectra which indicate the existence of a charge compensator along a $〈111〉$ direction. This is over and above the cubic spectrum observed at lower doping levels. The ESR spin-Hamiltonian parameters of the compensated center at 77°K are $a=+49.7\mathrm{}\left(4\right)\mathrm{}$, $D=+107.9\mathrm{}\left(3\right)\mathrm{}$, and $F=-2.7\mathrm{}\left(4\right)\mathrm{}$ (in units of ${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$) and $g=2.0029\mathrm{}\left(3\right)\mathrm{}$. The angular ENDOR data for the two sets of three nearest-neighbor fluorines were analyzed to determine the local distortion in some detail. The results are in good accord with a model which assumes that one of the eight nearest-neighbor ${\mathrm{K}}^{+}$ ions surrounding the ${\mathrm{Fe}}^{3+}$ is absent.