Effet JAHN-TELLER cooperatif et spectre optique de l'ion Cu2+ dans quelques chromites

Phase diagrams of Cu1−xMxCr2O4 (M2+ = Ni2+, Zn2+, 0,5(Li++Ga3+)) are established by X-ray diffraction at high temperature and some cationic distributions deduced from neutron diffraction. The ligand field reflectance spectra of tetrahedrally coordinated Cu2+ in different oxidic host structures (I41/amd and Fd3m) are analysed at different temperatures (300 – 600 K). The evolution of electronic spectra may be correlated with the disparition of cooperative JAHN-TELLER effect. The quadratic distortion (ca < 1) in mixed nickel-copper chromites (0,5 < x ⩽ 0,8) is mainly due to Ni2+; but, electronic spectra at high temperature in the range 4000 – 12000 cm−1 discloses a certain contribution of Cu2+ ions to JAHN-TELLER distortion.