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2022
These
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Haytem Bazzaoui, 'Synthesis and Complete Characterisation of new Oxide Materials by full and Congruent Crystallisation from Glass or the Molten liquid', These 18 Nov (2022)
The development of new materials with high performing properties is a major theme in today's research. In this thesis, new out-of-equilibrium materials were developed by unique synthesis approaches: full and congruent crystallisation from glass or from an under-cooled melt. High temperatures were reached by aerodynamic levitation coupled to two CO₂ lasers. The synthesised materials target optical and/or ionic conductivity properties.The structural study of the SrREGa₃O₇ RE= [Dy - Lu, Y] melilites, synthesised by full and congruent crystallisation from glass precursors, characterised the ordering of Sr²⁺ and RE³⁺ cations over three distinct sites, inducing a tripling of the a cell parameter of the parent tetragonal melilite, leading to the stabilisation of a new orthorhombic cell. Moreover, the stability of this orthorhombic polymorph studied in situ against temperature, showed the occurrence of a phase transition to the parent fully disordered tetragonal melilite. In contrast, the cationic ordering in the Ca₃Ga₂₋₂ₓZnₓGe₄₊ₓO₁₄ langasite solid solution, a melilite-related mineral crystallising in a trigonal cell, could not be probed directly by diffraction methods (X-rays and neutrons) due to the presence of three isoelectronic cations (Ga³⁺, Zn²⁺, Ge⁴⁺). In order to fulfil this task, a unique analysis approach was performed: refinement of atomic resolution 2D STEM-EDS maps for each cation. Last, new ionic conductor materials were investigated in the melilite La₁₊ₓBa₁₋ₓGa₃O₇₊ₓ/₂ and melilite-related LaₓCa₅₋ₓGa₆O₁₄₊ₓ/₂ solid solutions. The La₁₊ₓBa₁₋ₓGa₃O₇₊ₓ/₂ series was synthesised by direct crystallisation of their under-cooled melts. The series retained its parent tetragonal structure up to La1.725Ba0.275Ga3O7.3625 (x= 0.725), which is the highest interstitial oxide concentration ever observed in this polymorph. Despite its lower carrier concentration, tetragonal La₁.₆Ba₀.₃Ga₃O₇.₁₅ (x= 0.6) member was found to exhibit the highest conductivity values at 500°C, matching the best known melilite conductors La₁.₆₄Ca₀.₃₆Ga₃O₇.₃₂ and La₁.₅₄Sr₀.₄₆Ga₃O₇.₂₇. In contrast, the LaₓCa₅₋ₓGa₆O₁₄₊ₓ/₂ series revealed a much lower substitution limit of x= 0.25 attained by solid-state reaction, with a 100x enhancement in conductivity upon La doping. The low substitution rate posed major challenges for the structural characterisation of these compositions, which were addressed by combining maximum-entropy analysis of neutron diffractograms with DFT-based structure prediction.
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