Nouvelle approche de synthèse d'oxysulfures de terres rares luminescents au départ de précurseurs moléculaires

Rare-earth doped oxysulfide compounds are important materials in the field of luminescence. The Eu3+-doped Y2O2S phase has widely been used in TV screens for instance. Such phases are commonly synthesized by solid state methods, requiring high temperature and strictly controlled conditions. The aim of this work was to develop a widely applicable, softer and more convenient method for the synthesis of rare-earth oxysulfide compounds, allowing us to avoid the use of sulfur-containing gases or elemental sulfur. We chose the molecular precursor pathway for this purpose. Homoleptic and heteroleptic dithiocarbamate complexes of rare earth elements have been selected as molecular precursors due to their remarkable stability. We designed a new rapid, easy and uniform synthesis for [Ln(Et2dtc)3(phen)], [Ln(Et2dtc)3(bipy)] and Et2NH2[Ln(Et2dtc)4] (with Ln = Y, La, Pr, Nd, Sm-Lu ; Et2dtc = diethyldithiocarbamate ; phen = 1,10 phenanthroline and bipy = 2,2-bipyridine) starting from rare earth trifluoromethanesulfonates. We developed then a new continuous two-step process under moderate temperature to synthesize rare earth oxysulfides. Rather than trying to achieve direct lanthanide oxysulfide synthesis by a one-step molecular precursor combustion process, we aimed for the readily obtained oxysulfate phase beforehand, this phase being easily reduced afterwards by using diluted H2 atmosphere at moderate temperature. This protocol proved to be successful for a large series of rare-earth oxysulfides. Significant differences were nonetheless observed depending on the molecular precursors involved, especially about the thermolysis temperature required for the first step, which affects the purity of the final compounds. This continuous two-step process was adapted afterwards to study the formation of homogeneous mixed phases. Solid solutions were obtained for La2 xGdxO2S, Gd2 xYxO2S, La2 xYxO2S, La2 xDyxO2S and La2 xEuxO2S for 0 ≤ x ≤ 2 starting from [Ln(Et2dtc)3(phen)]. Rietveld refinement was performed in order to prove the solid solution character of the obtained phases. Luminescent doped phases Ln2O2S:Ln3+ were obtained too for Ln = Gd or La and Ln3+ = Eu3+, Tb3+, Tm3+, Dy3+, Sm3+. Those doped phases were studied by fluorospectrocopy. Luminescent thin films were obtained by a similar method involving a spin coating step. Co-doping was finally investigated by the synthesis of La2O2S:Eu3+,Tb3+ and Gd2O2S:Eu3+,Tb3+. Those materials show tunable photoluminescent properties, as their emission wavelengths depend on the excitation wavelength applied. The last part of this work was devoted to bismuth compounds, as we intended to investigate mixed lanthanide – bismuth phases. The complex [Bi(Et2dtc)3] proved to be successful to synthesize various pure bismuth phases by a molecular pathway. This study led us to discover unexpected excellent luminescent properties for a specific bismuth phase: Bi28O32(SO4)10:Eu3+. XRD analyses, fluorospectroscopy, and Raman spectroscopy allowed us to evidence the critical role of the doping agent percentage on the Bi28O32(SO4)10 phase homogeneity.(SC - Sciences) -- UCL, 201