Si- and Ge-mullite single crystals doped with foreign cations

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Intensive research on the properties of mullite was mainly performed on mullite powders and polycrystalline mullite ceramics within the last decades. Little work was documented on synthesis and characterization of mullite single crystals. There is no information on single crystals of mullites doped with rare-earth (RE) elements, although these materials display interesting luminescence properties as shown on corresponding powder materials. The first aim of this project is to study the incorporation mechanism of RE elements (Eu, Tb, Gd etc.) into the mullite structure and to determine their photo-luminescent properties with potential applications. For this aim RE-doped mullite single crystals (2:1 mullite, 2Al2O3 . SiO2) of centimeter size will be synthesized by the Czochralski method. They will be characterized by microchemical, spectroscopic and crystal-chemical methods. A second aim of the project focusses on the basic crystal-chemical study of the incorporation of RE elements combined with "lone-pair-elements" (Pb, Bi) in the germanium analog of mullite, 'germanium-mullite' (3 Al2O3 . 2GeO2). The synthesis will be performed in a series of flux experiments of RE-doped germanium mullites out of Pb- or Bi-containing high-temperature melts. This basic research study intends to shed light on the suggested complex and novel incorporation mechanism of foreign cations with lone electron pairs in the mullite crystal structure. For this purpose, for the first time large RE-doped Si-mullite crystals and doped single crystals of Ge-mullite are synthesized and fundamentally characterized. Finally, with respect to mullite in a general sense, the contributions may be of great importance for the development of mullite materials.