Chemically and temperature-induced phase transformations of metal vanadates
Patridge, Christopher James
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Metal vanadates contain a diverse family of compounds due to the facile accessibility of different vanadium oxidation states and local coordination environments. Though these systems present a number of applications in catalysis and electronics, there may exist untapped physical phenomena that only reveal themselves when scaling these materials to nanoscale dimensions. Finite-size effects result from a number of factors including surface energy structural instabilities, nanostructure "self-purification," and physical constraints on mechanistic or conductive pathways. The M x V 2 O 5 bronze materials possess non-stoichiometry and this interesting property has hindered synthetic techniques to procure perfect crystalline material which is needed to expose the true physical properties. Through hydrothermal synthesis methods, pseudo one—dimensional nanostructures of M x V 2 O 5 display fascinating new properties and may be model systems for studying fundamentals associated with correlated electron dynamics in solid-state physics. Electron microscopy and powder X-ray diffraction reveal the near-perfect crystalline nanostructures. X-ray absorption spectroscopy studies show strong evidence for the localization of electron density and long-range crystal structure alignment of the nanowires. Single-nanowire electron transport measurements for the β'-Cu x V 2 O 5 and the δ-K x V 2 O 5 data shows novel temperature-induced reversible metal—insulator transition (MIT) near room temperature. The unprecedented magnitude (∼10 5 ) and discontinuous nature of the MIT suggests a mechanism closely associated with correlated electron motion. Additionally, the MIT can be induced by voltage ramping. The simultaneous temperature/voltage studies of single-nanowire transport support the existence of a critical threshold to overcome in order to facilitate instability in the insulating phase and transition to a metallic phase for the δ-K x V 2 O 5 bronze. The MIT transition magnitudes of several different individual β'-Cu x V 2 O 5 nanowires vary widely. Using scanning transmission X-ray microspectroscopy of individual β'-Cu x V 2 O 5 nanowires, correlations appear to exist between MIT characteristics and the markedly different orbital hybridization of vanadium and oxygen at the O K and V L absorption edges. These comprehensive nanostructure studies hint at the possibility of approaching the incredibly important realm of single-domain measurements which are needed to understand and exploit the intrinsic physical properties of materials. In addition to the bronze MIT studies, the classical MIT material vanadium dioxide, VO 2 , also shows new properties when scaling down to nanoscale dimensions as well as incorporation of substitutional dopants such as tungsten. X-ray absorption spectroscopy of the dopant local structure suggests an increased symmetry and depairing of V 4+ -V 4+ , which is critical for transition to the lower temperature insulating phase thereby super-cooling the metallic phase to temperatures as low as 254 K. Mechanistic insight and structural changes associated with the intercalation of Li + are key aspects in understanding and designing useful secondary Li ion batteries. In similarity to the M x V 2 O 5 studies, another metal vanadate, Ag 2 VO 2 PO 4 , undergoes phase transformations due to introduction of Li and the vacancy of Ag ions. Employing a comprehensive study on Ag 2 VO 2 PO 4 using X-ray absorption spectroscopy, information about chemical state changes and rehybridization of frontier orbitals allows for a more precise understanding of how the material discharges, what, if any, intermediate phases exist during the process, and provides evidence for the posited structural stability at high depths of discharge.