Mössbauer study of KFe Cl 3 . I. Fine and hyperfine structure of Fe 2

M\"ossbauer measurements on KFe${\mathrm{Cl}}_{3}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K and in the paramagnetic region above 25\ifmmode^\circ\else\textdegree\fi{}K have been used for the determination of the fine structure of ${\mathrm{Fe}}^{2+}$. A least-squares fit of the spectrum at 4.2\ifmmode^\circ\else\textdegree\fi{}K yields ${e}^{2}qQ=5.06\ifmmode\pm\else\textpm\fi{}0.02$ mm/sec, $\ensuremath{\eta}=0.41\ifmmode\pm\else\textpm\fi{}0.1$, and an effective field ${H}_{\mathrm{eff}}=168\ifmmode\pm\else\textpm\fi{}1$ kOe along the direction of the crystallographic $b$ axis. These results uniquely determine the orbital ground state. An analysis of the temperature dependence of the quadrupole splitting yields ${\ensuremath{\Delta}}_{1}=920\ifmmode\pm\else\textpm\fi{}100$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\Delta}}_{2}=515\ifmmode\pm\else\textpm\fi{}15$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for the crystal-field splitting of the orbital $^{5}D$ levels, and $\ensuremath{\lambda}=\ensuremath{-}58\ifmmode\pm\else\textpm\fi{}5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for the spin-orbit coupling constant. The bare quadrupole coupling constant is determined also as $\ensuremath{\Delta}{E}_{0}=3.2\ifmmode\pm\else\textpm\fi{}0.3$ mm/sec. The reduction of the values of $\ensuremath{\lambda}$ and $\ensuremath{\Delta}{E}_{0}$ relative to the free ion is indicative of strong covalency effects. This conclusion is substantiated also by the decrease in isomer shift and the low value of the contact hyperfine field constant ${H}_{c}=300\ifmmode\pm\else\textpm\fi{}20$ kOe. These results are discussed in comparison with previously reported values of ${\mathrm{Fe}}^{2+}$ compounds.