Levshov S. Synthesis and luminescent properties of the alkaline-earth silicates, activated with Eu2+ ions

The thesis is devoted to the study of the influence of preparative conditions and crystal structure of alkaline-earth silicates on the luminescent properties of Eu2+ ions. Alkaline-earth silicates MLi2SiO4 (M= Ca,Sr,Ba), Ca3Si2O7, LT- and HT-Ca3SiO4Cl2, activated with Eu2+ ions, were prepared via solid state reactions and sol-gel method. It was shown that the sol-gel process for the SrLi2SiO4 and Ca3Si2O7 synthesis allows one to shorten the duration of heat treatment, to avoid the use of fluxes and to obtain materials with an average particle size less than 1.5 ?m. The reasons for the differences in luminescent properties of Eu2+ ions in the row of MLi2SiO4 (M= Ca,Sr,Ba) were found. In particular, it is shown that the long-wave position of Eu2+ emission in SrLi2SiO4 (Lambda max= 578 nm) is caused by the large crystal-field splitting of the Eu2+ 4f65d1-configuration and relatively high degree of covalency of the Eu–O bond. It was found that the stabilization of Eu2+ in MLi2SiO4 during the synthesis process requires the use of a strong reducing agent (H2). Two phenomenological approaches to explain the low stability of Eu2+ ions in MLi2SiO4 were employed. They appeared to agree with general experimental observations. By means of IR- and luminescent spectroscopy it is shown that Ca3SiO4Cl2 (LT>HT) phase transition is accompanied by substantial changes in both the symmetry of [SiO4]4- groups and the structure of calcium polyhedra, which are basic building blocks of the both modifications. Due to their favorable luminescent characteristics SrLi2SiO4:Eu2+ and Ca3Si2O7:Eu2+ are considered as promising phosphors for white light-emitting diodes.