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+,3Ega) and (Cr3+,4A2g;Cr2+,3Egb) 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.