Driving piezochromism and metallicity in van der Waals materials under compression

Complex chalcogenides provide an important platform to explore the interplay between structure, charge, and spin across pressure-induced phase transitions. Where much of the previous research has been focused on tuning these materials towards the single-layer limit, we instead explore the modification of bond lengths and bond angles under compression. In the first project we revealed piezochromism in MnPS₃. We combined high pressure optical spectroscopy and first-principles calculations to analyze the dramatic color change (green → yellow → red → black) that takes place as the charge gap shifts across the visible and into the near infrared region, moving systematically toward closure at a rate of approximately -50 meV/GPa. The discovery of deterministically controlled piezochromism at room temperature provides an exciting opportunity to seek out this functionality in other complex chalcogenides. NiPS₃ is the second platform of investigation in this dissertation. By combining a variety of high pressure experimental techniques including synchrotron-based infrared spectroscopy, Raman scattering, x-ray diffraction, and an in-depth symmetry analysis with first-principles calculations we revealed five different states of matter up to 39 GPa. Bringing together the appearance of a polar high pressure phase from symmetry analysis and the insulator-metal transition with the appearance of a Drude, we suggest the development of a room temperature polar metal above 23 GPa. By providing a platform to access this uncommon state of matter, this research will further the development and understanding of polar metals more broadly. In our final project, we compared the prototypical parent compound FePS₃ with chemically-similar CrPS₄. We found that these materials displayed markedly different symmetry progressions and high pressure states of matter under pressure. CrPS₄ in fact drives toward P2/m - a symmetry not observed in any of the other systems. These differences are attributed to the structural differences including the van der Waals gap size, layer corrugation, and character of the P-P linkage, as well as the orbital occupation of the transition metals. The structure-property relations in these compounds demonstrate the importance of structural and electronic contributions in the determination of the different symmetry breaking within these materials under compression.