Order Under Pressure: Structural and Magnetic Characterization at Extreme Stresses

Abstract

Mechanical stress is an exquisitely versatile tool for controlling chemical bonding. This multi-dimensional synthetic lever tunes the electronic structure of elements and changes the way that atoms arrange and coordinate to one another. These unique electronic configurations and coordination environments have profound impacts on the properties of materials giving rise to functionality ranging from high-temperature superconductivity to diverse magnetism. Despite over a century of research on solid-state materials over one gigapascal (GPa), experimental and theoretical obstacles remain for structural and physical characterization of complex phases which only persist at these conditions. We begin to address the wide-reaching challenge of structural characterization in complex, bulky sample environments by employing recent advancements in generative artificial intelligence to develop a generalized approach to solving the structure of crystalline solid-state materials. We demonstrate that our model achieves a 42% match rate on a curated set of experimental powder diffraction patterns, and we then use our model to solve the structure of several previously unsolved structures at high pressure. We proceed to focus on a different structural characterization problem: defects which arise exclusively under mechanical stress. We demonstrate that site-disorder is unlikely to occur at room temperature and high pressure in InBi and instead propose a set of defects which explain the X-ray spectra and scattering patterns equally well. Progressing to properties characterization and magnetic ordering at high pressure, we experimentally demonstrate that MnBi2, a compound which does not persist to ambient pressure, is a permanent magnet. Comparing the orbital and spin contributions to the total moment across compounds in the Mn–Bi system, we build up design principles for permanent magnets using heavy main-group elements. The combination of our work in structural and physical characterization at extreme stresses charts a path towards the discovery of functional high-pressure bulk materials and defects.