CAREER: Understanding Local Stereochemistry of Anharmonic ns2 Lone Pairs in Heteroleptic Halides and Chalcohalides via Total Scattering

Part 1: NON-TECHNICAL SUMMARY The development of new semiconductors with outstanding properties is of critical importance to modern technology and energy security. With support from the Solid State and Materials Chemistry Program in NSF’s Mathematical and Physical Sciences Directorate, this CAREER project investigates "soft" metal halide and chalcohalide semiconductors, which have shown great promise as emerging materials for electronics and energy applications. These compounds exhibit unusual behavior due to local distortions of the atomic structure, but these distortions are challenging to characterize because they are invisible to most routine characterization techniques. This limits the ability of scientists and engineers to tailor properties critical to electronic performance. Through this award, Prof. Kyle McCall and his team will characterize the local structure of these materials using X-ray scattering techniques sensitive to the local atomic environment. These structural analyses will be complemented by optical and electronic property evaluation, linking the hidden local structure distortions to physical properties, and thereby advancing the understanding needed to engineer desirable semiconducting properties based on local atomic distortions. Additionally, graduate and undergraduate students will receive training in materials chemistry and electronic characterization, strengthening the semiconductor workforce in the Dallas-Fort Worth Metroplex. This CAREER project will expand materials research opportunities with a 4-semester undergraduate team research course and by establishing an Emerging Materials Workshop that provides lesson plans and demonstrations for educators, fostering interest in materials research for undergraduate and K-12 students in the DFW area. Part 2: TECHNICAL SUMMARY The development of emerging technologies including semiconductors and quantum materials requires in-depth understanding of the atomic structure underlying their performance. Heavy post-transition metal halides and chalcohalides host stereoactive lone pair electrons and large spin-orbit coupling, imparting anharmonicity that enables unusual properties and yields outstanding photovoltaics, ferroelectrics, and topological insulators. However, predicting and engineering such chemically "soft" anharmonic materials is an immense challenge due to limited understanding of local atomic structure deviations driven by the lone pair electrons. Long-range crystallography is blind to potentially active lone pairs, especially in materials with complex environments, and the resulting lack of quantitative information on local hidden lone pair stereochemistry is a critical knowledge gap. This CAREER project, with support from the Solid State and Materials Chemistry Program in NSF’s Mathematical and Physical Sciences Directorate, addresses this knowledge gap by experimentally characterizing the local structure of anharmonic lone pair electrons in heteroleptic halide and chalcohalides using total X-ray scattering. Analysis of select chemical series will elucidate the role of specific chemical features on the local structure, providing design principles for tuning the degree and dynamics of anharmonic distortions. These structure-property relationships will be tested through design of new chalcohalides targeting high symmetry environments. This CAREER project will yield fundamental understanding of the local chemistry underlying anharmonic dynamics in heteroleptic halides and chalcohalides, contributing to chemical control of complex semiconductors for optoelectronic and ferroic applications. Integrated with these research objectives is an educational program comprising two initiatives: 1) a team research course on Materials Chemistry and Crystallography, enabling undergraduates to work with national user facilities, and 2) an Emerging Materials Workshop that will provide experiments, demonstrations, and lesson plans to educators in the Dallas-Fort Worth area. These efforts will bolster research opportunities and foster interest in STEM careers, contributing to workforce development needs of the DFW semiconductor industry. 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.