Optical studies of Cr 3+-Cr 2+ pair center in KZnF 3 crystal

The optical absorption spectra of Cr ³⁺-Cr ²⁺ pair centers in KZnF ₃:Cr ³⁺,Cr ²⁺ crystals were investigated in a wide temperature range. A broad band at 30 800 cm ^-1 is attributed to a cation-cation e _g-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 (Cr ³⁺, ⁴A _2g;Cr ²⁺, ⁵E _g) state to the excited (Cr ³⁺, ⁴A _2g;Cr ²⁺, ³E _g^a) and (Cr ³⁺, ⁴A _2g;Cr ²⁺, ³E _g^b) states, respectively. Its vibronic satellites corresponding to the a _1g local mode of the Cr ³⁺ 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 Δ_JT=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.