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

使用金刚石-顶锤单元(DAC)对固体施加高外压可以导致材料结构的巨大变化，例如相变和原子间距离(键角和键长)、构象、分子轨道中的电子分布、氢键相互作用和分子轨道的相对能量的变化。这种结构变化通常会导致压电致变色，即由压力引起的颜色变化。通过使用商用DAC，在实验室中可以很容易地实现高达10 Gpa(~100,000 atm，即接近地质领域)的压力，因为材料被机械挤压在两个小宝石级钻石(小钻石，~0.6 mm)的平行面之间。钻石对大多数类型的电磁辐射都是透明的，随着压力的增加，任何结构的变化都可以通过钻石通过红外、拉曼、紫外可见和发光光谱等显微光谱技术以及粉末X射线衍射和差示扫描量热仪进行实时监测。我们的长期实验计划是使用常规和水热(温控)HDAC来促进高压下的固态化学反应，努力合成以前未知的材料和相，包括纳米材料，并扩大我们在超临界水条件(221bar，374°C)下的生物质转化工作，以生产氢气作为替代燃料来源。此外，由于球磨机中的固体研磨(机械力化学)涉及中等机械压力(~500兆帕)和由于摩擦而产生的局部加热(有时可能是实际熔化)，这种方法与熔体相中的化学密切相关。为此，在短期内，我们将使用特殊的热级来研究材料的红外和拉曼光谱以及温度高达~600°C的固态反应。未来5年，将在DAC和HDAC中以及在机械力化学和变温条件下检测的材料包括粘土矿物、药物多晶型、地质矿物、艺术家颜料、有机金属和聚合物材料。

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.