Luminescence of tetrahedrally coordinated Co 2+ in zirconia

Co 2+ enters the lattice of zirconia stabilized with a 46% of yttria in fourfold coordinated cation sites. The absorption spectrum consists mainly of two triplets, one of which is rather weak at around 5500 cm -1 (corresponding to the 4 A 2 to 4 T 1 ( 4 F) transition), while the other is very intense at around 15175 cm -1 corresponding to the 4 A 2 to 4 T 1 ( 4 P) transition. Excitation in the visible region produces two strong broad emission bands at 13330 and 10420 cm -1 due to the 4 T 1 ( 4 P) to 4 A 2 and 4 T 1 ( 4 P) to 4 T 2 ( 4 F) transitions respectively. The lifetime of the emitting level is approximately=0.75 mu s at 10 K. The structure of the triplet states is caused by the combined effect of spin-orbit and Jahn-Teller interactions which, besides the splitting of the cubic levels, produce a strong admixture of some spin doublets with the 4 T 1 ( 4 P) level. The authors have calculated the energy levels using the Eisenstein matrices thus taking into account both Coulomb, crystal field and spin-orbit interactions. The best fit with the experimental measurements is obtained for the following values of the parameters. 10Dq=3025 cm -1 , B=720 cm -1 , C/B=4.0 and spin-orbit constant zeta 3d =-515 cm -1 . The 4 T 1 ( 4 P) emission suffers a strong thermal quenching at temperatures above 100 K. The evolution with temperature of the intensity of the photoluminescence and its decay curves are discussed with reference to the energy level diagram of the ion and the defective character of the sample.