High-pressure spectroscopic, structural and mechanochemical studies of molecular materials

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The application of high external pressures on solids using diamond-anvil cells (DACs) can lead to dramatic structural changes in materials, e.g., phase transitions and changes in interatomic distances (bond angles and bond lengths), conformations, electronic distributions in molecular orbitals, hydrogen bonding interactions, and relative energies of molecular orbitals. Such structural changes can often lead to piezochromism, i.e., color changes induced by pressure. By using commercially available DACs, pressures of up to 10 GPa (~100,000 atm, i.e., approaching the geological realm) can readily be achieved in the laboratory as materials are squeezed mechanically between the parallel faces of two small gemstone quality diamonds (culets, ~0.6 mm). Diamonds are transparent to most kinds of electromagnetic radiation and any structural changes that occur with increasing pressure can be monitored in real time through the diamonds by microspectroscopic techniques, such as infrared, Raman, UV-visible and luminescence spectroscopy, as well as by powder X-ray diffraction and differential scanning calorimetry. Our long-range experimental plan is to employ conventional and hydrothermal (temperature controlled) HDACs to facilitate solid-state chemical reactions under high pressures in an effort to synthesize previously unknown materials and phases, including nanomaterials and to extend our work on biomass conversion under supercritical water conditions (221 bar, 374 °C) to produce hydrogen gas as an alternative fuel source. Moreover, since grinding solids in ball mills (mechanochemistry) involves moderate mechanical pressures (~500 MPa) and also some local heating (possibly sometimes actual melting) due to friction, this approach is closely related to chemistry in the melt phase. For this reason, in the short term we will examine the infrared and Raman spectra of materials and solid-state reactions at temperatures up to ~600 °C using a special thermal stage. Examples of the materials that will be examined in DACs and HDACs, and under mechanochemical and variable-temperature conditions over the next 5 years include clay minerals, drug polymorphs, geological minerals,artists' pigments, organometallics and polymeric materials.