Optical studies of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$ crystal
  Optical absorption spectra of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$:Cr$^{3+}$,Cr$^{2+}$ crystal were investigated in wide temperature range. Broad band at 30800 cm$^{-1}$ is attributed to cation-cation (e_g)-electron transfer transition. Narrow lines with maxima at 16720 cm$^{-1}$ and 19880 cm$^{-1}$ have been assigned to purely electronic exchange-induced electric-dipole transitions from the ground (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^5E_g)) state to excited (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^a)) and (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^b)) states, respectively. It's vibronic satellites corresponding to (a_{1g}) local mode of Cr$^{3+}$ fluorine octahedron of the pair are also observed. Energy of the local mode for the ground and mentioned excited states are 580, 540 and 530 cm$^{-1}$. Instead of expected double exchange for mixed valence pair ferromagnetic superexchange for Cr$^{3+}$-Cr$^{2+}$ pair in KZnF$_{3}$ crystal is realized. Exchange integral (J=-14.9\pm0.4) cm$^{-1}$ and Jahn-Teller splitting (\Delta_{JT}=340\pm40) cm$^{-1}$ for the ground state of the pair were obtained by analysis of the temperature dependence of absorption lines. Important features of the crossover double exchange - ferromagnetic superexchange are discussed. 
M.V. Eremin, S.I. Nikitin, S.Yu. Prosvirnin, N.I. Silkin, R.V. Yusupov