The properties of crystalline materials are controlled by their composition and by their structure, however, the structure of a crystal is only partly controlled by its composition. Development of specifically directed inorganic syntheses will require an understanding of the dynamics of crystal phase forming processes, especially those processes involved in generating specific symmetry features. One successful strategy, intended to exert synthetic control over the symmetry of crystal structures involves incorporating polar, anionic species, called basic building units (BBUs). Polar BBUs tend to adopt ordered configurations, increasing the probability that discovered structures will have polar symmetry. Discovery of eight new compounds with general formula K10(M 2OnF11-n)X (M = VV, NbV , n = 2, M = MoVI, n = 4; X- = (F 2Cl)1/3, Cl, ([Br][Br3])1/2, and ([I][I 3])1/2) is reported. Post-synthetic structure analysis these compounds which crystallize in the space groups P3¯m1, Pmn21, and C2/m, is used to analyze and expand on the Λ-shaped anionic unit strategy.
The atom scale processes involved in the phase transition are examined through computational Molecular Dynamics (MD) simulations. Atomistic MD simulations present the technical challenge of developing models for calculating interatomic interactions that can reliably reproduce experimental results. A strategy for improving the accuracy of empirical interatomic interaction models is outlined. Cryogenic temperature (~15K) and variable-pressure (1-10 GPa) diamond anvil cell single crystal diffraction experiments on the Pna2 1 phase of KNaNbOF5 are used to develop a set of empirical pairwise interatomic interaction functions of the complex five element system. The atom scale dynamics of a temperature driven reconstructive phase transition in KNaNbOF5 are examined, leading to the discovery of a dynamically disordered high temperature crystal structure, and the origin of the NCS phase stabilization upon cooling. The reconstructive transition going from the P4/nmm phase to the high temperature Cmcm phase is believed to result from a loss of O/F site ordering caused by rigid rotations of octahedral [NbOF5]2- BBUs. The high temperature phase is found to be a dynamically disordered state involving two locally stable phases in the potential energy landscape with Pbcm and Pnma space group symmetries.
|Advisor:||Poeppelmeier, Kenneth R.|
|Commitee:||Freedman, Danna, Kanatzidis, Mercouri|
|School Location:||United States -- Illinois|
|Source:||DAI-B 78/05(E), Dissertation Abstracts International|
|Keywords:||Crystal, Noncentrosymmetry, Oxyfluoride, Reconstructive transition|
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