Glass-to-crystal transition of sodium(Na) super ionic conductors (NASICON)

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The glass-to-crystal transition of sodium (Na) super ionic conductors (NASICON) will be investigated by total scattering and Bragg-diffraction techniques. Two substitution series will be studied: Na_(1+x)Al_xGe_(2-x)P_3O_12 and Na_(1+x)Ti_2P_(3-x)Si_xO_12 (x = 0 to 2). These materials are relevant as solid-electrolytes or as electrode materials in sodium-based batteries which are discussed as a replacement for lithium-based ones. NASICON materials benefit strongly from a preparation by a glass-ceramic synthesis. Nevertheless, the glass structures are hardly investigated. Crystal structures were only studied for sintered materials. Thus, the glass-ceramics are structurally not well characterized as well. Amorphous residues are not determined but will lead to compositional changes in the crystalline phases. The proposed work aims to determine the atomic structure of NASICON-glasses by scattering techniques for the first time. Structural ordering on different length scales of the glass phases can be determined from reciprocal-space data, i.e., from the behavior of the first sharp diffraction peak. Bond lengths and coordination numbers will be extracted from the pair-distribution function (real-space) obtained by Fourier transformation of the scattering data. 3-D atomic models will be refined to experimental data by the reverse Monte-Carlo method. Additional constraints, e.g., on coordination numbers, will be provided by complementary experiments, e.g. solid-state NMR. This will lead to detailed knowledge of the atomic structure which is a prerequisite to interpret the strength of ionic bonding and, therefore, ionic mobility. The ceramic products will be characterized regarding exact compositions of the crystalline phases and amorphous residues by the Rietveld technique. A Re-determination of crystal structures, especially regarding substitutions and coordination environments, will provide a reference point for the structural work on the glass phases. These materials show a homogeneous nucleation. Such a mechanism in was not investigated for non-siliceous systems, yet. A total scattering approach on differently annealed samples at the threshold of crystallization will provide insights into structural reorganization of the glasses during the nucleation process. The understanding of changes of the local environments from glass to crystalline state will help to guide the development of new fast ion-conducting glasses and ceramics. Furthermore, experimental data will help to develop and validate predictive models that are still inadequate for non-crystalline materials. Finally, a high temperature total scattering experiment will be done to monitor crystallization in situ. Synchrotron based rapid acquisition PDF (RAPDF) is the method of choice to study changes in the glassy phase as well as the formation and composition of crystalline phases. This will give us a comprehensive understanding of the formation of the glass-ceramics.