METAL-TO-METAL CHARGE-TRANSFER TRANSITION IN LI0.5FE2.5-XCRXO4 SPINELS

The structural and optical properties of polycrystalline lithium chrom iferrites Li0.5Fe2.5-xCrxO4 have been investigated. The order-disorder transitions P4(3)32 --> Fd3m and Fd3m --> <F(4)over bar 3m> studied b y X-ray diffraction and IR spectrometry are respectively observed for x = 0.6 and x = 1.6. The influence of antiferromagnetic coupling betwe en tetrahedral Fe3+ and octahedral Cr3+ ions induces a large electroni c transition assigned to Cr3+ + Fe3+ --> Fe2+ + Cr4+ intervalence char ge transfer at 14500 cm(-1). This conclusion is unambiguously demonstr ated by the study of the diffuse reflectance spectra of the ferrimagne tic samples of the two systems: Li0.5Fe2.5-xGaxO4 and Li0.5Fe2.5-xCrxO 4.