Fluorescent lamps, ('energy-saving' lamps) are based on the electrical excitation of a mercury plasma which produces a UV radiation, that excite phosphors emitting in the visible range of the spectrum. The most current phosphors are the red (Y, Eu)2O3 ("YOX"), the green (La,Ce,Tb)PO4 ("LAP") and the blue (Ba,Eu)MgAl10O17 ("BAM"). The optimization of a phosphor layer in a lamp is a matter of empiric approach since the global mechanism of interaction between the UV light and the phosphors both depends on intrinsic parameters (such as optical absorption, quantum yield that depend on the crystal chemistry …) or extrinsic parameters (morphology, sintering state … that affect the light propagation). Aging problems also play a primary role in determining the quantum efficiency. These problems originate from defects caused by the interaction with the mercury atoms of the plasma, UV light, elevated temperature (around 150°C). The objectives of this project are triple: 1) First to optimize the preparation of these phosphors from well identified industrial precursors provided by Rhodia. This requires controlling the crystalline structure as well as the microstructure and the powder morphology. Usually, thermal treatments in the presence of a flux, added in minor proportions, are used to homogenise the powder, to optimize its surface states and to eliminate bulk defects. The mechanism of crystal reconstruction during this thermal treatment will be investigated though experiments (ICME, LCMCP) and numerical simulations (XPMC). Moreover, the numerical simulation of light interaction (UV, Visible) with a phosphor layer will allow to define the key parameters to improve the morphology (grain size, packing density, thickness) required for a better luminescence yield (XPMC, Rhodia). 2) A system will be designed to study the accelerated aging of phosphors. This device will simulate the combined action of UV light, temperature and mercury plasma and allow to compare the behaviour of different materials through a standardized and reproducible method. (task of the LOF, ISM) 3) Starting from the experimental results, a structural study will analyze the effects of the aging process on the bulk and the surface defects created by aging processes. chemical solutions will be proposed to stabilize the phosphors against the formation of defects, either by chemical substitutions (chemical trapping of defects, blocking the ionic conductivity …) or by coating with a passivating layer onto the phosphor. The objective of this work is to propose a methodology of quantitative evaluation of powder performances and a prediction of a luminescence yield of a powder coating, from the very first use to its end of life-cycle. Also, the results of this work should allow Rhodia to propose, at a pilot scale, a manufacturing plant optimized phosphors for fluorescent lamps. The numerical approach should make easier the extrapolation possible to other applications (Mercury-free lamps, plasma display panels …).