Showcase of Undergraduate Research and Creative Endeavors (SOURCE)

Thermodynamics-Based Discovery of New K-La-Zr-O Compounds via Hydrothermal Synthetic Methods

This project investigates aqueous modeling coupled with mild hydrothermal methods (200 °C, 16 atm) for discovery of new compounds, goals for advanced materials development outlined in SC Vision 2025 and NSF Big Ideas. Innovative luminescent materials, such as scintillators, are needed for opto-electronics and other optical technologies. Hydrothermal methods were performed, with thermodynamic guidance from aqueous speciation calculations in OLI Studio, to look for compounds in the K-La-Zr-O quaternary system. This system choice was inspired by several Na-Y-Si-O compounds previously synthesized by supercritical hydrothermal methods. By altering compositions of reactants, it is possible to generate trace amounts of novel crystals of new stoichiometries. In the previous discovery of Zn₂EDTA·2H₂O, optimum hydrothermal conditions were just outside of the thermodynamic stability region for ZnO, suggesting that the edges of such stability regions are potential places for discovery work. With OLI Studio, yield diagrams were constructed for the K-La-Zr-O system, with water-soluble metal salts, chelating agent, and base as reactants. Chemical systems readily form thermodynamically stable binary/ternary compounds: in this case, zirconia (ZrO₂) and lanthanum hydroxide (La(OH)₃). Within Zr and La subsystem yield diagrams, where the concentration ratio of metals is plotted against base concentration, locations just outside of the ZrO₂ stability region were targeted for Zr:La ratios of 1:1 and 4:1. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) revealed polycrystalline morphology with some single crystals (≈50 microns) of hexagonal and greater (6+ sides) geometry containing significant amounts of oxygen, lanthanum, and zirconium, suggesting formation of a lanthanum zirconate compound.