IONIC RELAXATION AROUND THE JAHN-TELLER DISTORTED CR2-CR STUDIED BY ESEEM( ION IN SRF2)

Electron Spin Echo Envelope Modulation (ESEEM) experiments have been p erformed on the orthorhombically distorted Cr2+ in SrF2. The pulsed EP R results allowed us to determine the superhyperfine interaction tenso rs of the Cr2+ electrons with the nearest neighbouring (nn) fluoride i ons (A(parallel to)(1) = +40 +/- 2 MHz, A(perpendicular to)(1) = +17 /- 1 MHz, theta(1) = 39 +/- 5 degrees), the next nn fluoride ions (A(p arallel to)(2) = +12.5 +/- 2.5 MHz, A(perpendicular to)(2) = +2.0 +/- 0.5 MHz, theta(2) = 35 +/- 10 degrees) and the third shell neighbourin g fluoride ions (A(parallel to)(3) = 1.35 MHz, A(perpendicular to)(3) = -0.7 MHz, theta(3) = 65 degrees). Analyzing the shf parameters with the orthogonalized envelope function method we have estimated the Cr2-F- distances, proposing a defect model consisting of four fluoride io ns in the yz defect plane relaxing inwards to touch Cr2+ ion whereas t he remaining four F-ions relax outwards until touching the Sr2+ cation . The defect model is compatible with a strongly stabilized Jahn-Telle r distorted cent-e due to a coupling T(2g)x(e(g) + t(2g)).