Expanding known binary technetium nitrides and sulfides: A computationally-led synthesis program

University Of Nevada Las Vegas, Las Vegas NV

Investigators

Abstract

PART I: NON-TECHNICAL SUMMARY Technetium is a radioactive element, and its fundamental solid state chemistry is not well established in comparison to neighboring elements. This lack of understanding of technetium limits what may be understood regarding the transition metals that surround it on the periodic table. Solids composed of only technetium and either nitrogen or sulfur are nearly unexplored. However, such compounds are both important in understanding the fundamental chemistry of the element and of importance to developing materials with exceptional hardness or useful electronic properties. This investigation, supported by the Solid State and Materials Chemistry program in the Division of Materials Research and the Established Program to Stimulate Competitive Research (EPSCoR) uses high pressure/high temperature synthesis methods to prepare and characterize a range of new examples of technetium nitrides and sulfides. Synthesis productivity is enhanced by using cutting-edge computational approaches to guide synthesis efforts and to assist in understanding the new solids produced. More detailed calculations using quantum mechanical methods will provide predicted spectra and other physical properties to the experimental team before they attempt to synthesize these phases, enabling quick identification and providing insights into how to begin characterizing new materials. Outreach has been established through the National Atomic Testing Museum for school students and the general public, designed not only to inform them of the science behind radiation and nuclear processes, but to further excite their imagination about science. PART II: TECHNICAL SUMMARY This project fills in critical gaps in fundamental knowledge of the solid state chemistry of binary technetium compounds. Targeted synthesis of new solid-state nitrides and sulfides relies upon diamond anvil cells (DACs), coupled with resistive heating (fully uniform up to 1200 K) and laser heating (in excess of 5000 K). Reactions carried out in DACs are monitored in situ during synthesis through X-ray diffraction (XRD) and spectroscopic (XAS) techniques and in-house optical spectroscopy. Crystal structure prediction searches identify which compounds are sufficiently stable that they might be synthesized. By comparing free energies across pressure ranges for competing phases computationally, very specific experimental conditions where desired compounds are likely to form can be targeted. Machine learning approaches will be used to extract empirical, reactive potentials from this library of data which can be applied to the prediction of more compositionally and structurally complicated phases than will be initially accessible with first principles-based structure search techniques. These tools will also enable the team to accurately simulate systems at length and time scales inaccessible to conventional electronic structure methods. As simulation and experiment provide an improved picture of the kinetic and thermodynamic aspects of stability, a detailed understanding of adjusting structure and composition may be exploited to rationally design materials. Outreach has been established through the National Atomic Testing Museum for school students and the general public, designed not only to inform them of the science behind radiation and nuclear processes, but to further excite their imagination about science. This project is supported by the Solid State and Materials Chemistry program in the Division of Materials Research and the Established Program to Stimulate Competitive Research (EPSCoR). 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.