SOME CHEMICAL PROBLEMS ON LUMINESCENCE STUDIES OF RARE EARTH IONS IN SOLID STATE HOSTS
Shi Chunshan Ye Zeren (Changchun Institute of Applied Chemistry, Academia Sinica)
Some chemical problems on luminescence studies of rare earth ions in solid state hosts are reviewed in this paper.First, the influence of the chemical composition and crystal structure on luminescence properties of rare earth ion activated phosphors is discussed.lt involves the influence of the surroundings of rare earth ion on transition probabilities, luminescence efficiency, thermal quenching and spectral nature.The influence of the surroundings of a rare earth ion on its 4f levels is very weak. This results in a very weak crystal-field splitting of these levels. The transition probabilities for f-f transitions can be strongly influenced by the surroundings. Energy levels and transitions involving a charge transfer or a 5d state are more strongly influenced by the rare earth ion surroundings in the lattice.△r is the defference between the equilibrium configuration of the excited state and that of the ground state. The △r is negative when the cation is excited and is positive when the anion is excited. Luminescence centres are divided into two groups. △r0 (excitation of anion electrons) and △r 0 (excitation of cation electrons).The spectral nature is different when the symmetry at the crystal-lographic site of the rare earth ion is different. The relation between the spectral nature and the symmetry can be studied with 32 point groups.Second,the influence of the activator concentration on luminescence nature is discussed. The concentration quenching may be explained in a number of cases as follows. If the concentration of the activator becomes so high that the probability of energy transfer exceeds that of emission, the excitation energy will be transferred and lost through the lattice.The influence of concentration on chromaticity is marked.Third, the influence of impurities on luminescence is discussed. Impurities can affect brightness of rare earth ion activated phosphors. Cerium is the most dramatic quencher in the red light emitting rare earth phosphors. Calcium is generally detrimental in rare earth raw materials.Finally, the influence of the anion on luminescence is discussed too.