Collaborative research: experimental and computational study of structure and thermodynamics of rare earth oxides above 2000 C

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NON-TECHNICAL DESCRIPTION: Materials that can withstand ultrahigh temperatures are important to ceramic processing, aerospace, and nuclear applications. Oxides containing rare earth elements have high melting points, do not evaporate easily, and are generally chemically unreactive. This refractory nature favors high temperature applications, but at the same time makes the determination of their heat and energy relationships (thermodynamic) properties challenging, and current data are incomplete. Such data are essential for prediction of material stability or degradation during use in both simple and complex systems in a variety of extreme environments (such as high-temperature or high-pressure environments). New experimental and computational approaches to determine thermodynamics above 2000 degrees C form the primary focus of the project, which represents collaboration between Navrotsky's group at University of California at Davis performing thermochemical measurements and structural studies, and van de Walle's group at Brown University undertaking calculations and phase diagram assessments. The project provides university-level students an opportunity to participate in state-of-the-art experimental and computational research applied to high temperature processes. TECHNICAL DETAILS: The goals are to develop, test and apply new methodology (i) to determine temperatures and heats of phase transitions (both solid-solid and melting) of rare earth oxides and oxycarbides above 2000 degrees C, (ii) to measure structural parameters, including thermal expansion and volume change upon phase transition by synchrotron X-ray diffraction on levitated samples above 1500 degrees C, (iii) to calculate high-temperature thermodynamic properties using ab initio density functional theory (with hybrid functionals) and molecular dynamics in conjunction with statistical mechanical techniques, and (iv) to generate a unified picture, based on both theory and experiment, of thermodynamic properties, to be made publicly available for use in calculation of phase diagrams (CalPhaD) modeling and other applications. The combination of calorimetry, X-ray diffraction, and computation provides both needed thermodynamic data and physical insight into the properties of refractory rare earth oxides, with extensions into more complex systems. In addition to international journals and meetings on ceramics, dissemination is taking place through smaller focused conferences and lectures on their websites. Newly developed ab initio computational techniques are being integrated within the widely used and freely distributed ATAT software.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.