Optical studies of Cr3+-Cr2+ pair center in KZnF3 crystal

The optical absorption spectra of Cr3+-Cr2+ pair centers in KZnF3:Cr3+,Cr2+ crystals were investigated in a wide temperature range. A broad band at 30 800 cm-1 is attributed to a cation-cation eg-electron transfer transition. Narrow lines with maxima at 16 720 and 19 880 cm-1 have been assigned to purely electronic exchange-induced electric-dipole transitions from the ground (Cr3+,4A2g;Cr2+,5Eg) state to the excited (Cr3+,4A2g;Cr2+,3Eg a) and (Cr3+,4A2g;Cr2+,3Eg b) states, respectively. Its vibronic satellites corresponding to the a1g local mode of the Cr3+ fluorine octahedron of the pair are also observed. The energy of the local mode for the ground and the mentioned excited states are 580, 540 and 530 cm-1. The exchange integral J = -14.9±0.4 cm-1 and the Jahn-Teller splitting ΔJT = 340±40 cm-1 for the ground state of the pair were obtained by an analysis of the temperature dependence of the absorption lines. The important features of the crossover double exchange-ferromagnetic superexchange are discussed