Exploration of Materials under Compression of Non-hydrostaticity and Shear
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Non-hydrostatic compression is in general a circumstance that material is subject to in the nature. Understanding the behaviors of materials under such a condition makes it possible to gain insights into the physics and chemistry beyond their natural phenomena. In this dissertation, we attempt to explore the behaviors of materials under non-hydrostatic conditions to seek their potential in engineering applications. Three topics are covered in this work: the search of new scintillators via non-hydrostatic compression, the synthesis of novel phases from known materials under shear load, and the reaction of super-hard materials to shear stress. A diamond anvil cell and a rotational anvil cell accompanied with synchrotron X-ray diffraction, Raman spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy were employed as diagnostic methods. MnWO4 was studied by synchrotron X-ray diffraction to 50.1 GPa in a diamond anvil cell. Comparison experiments under the hydrostatic and non-hydrostatic conditions were performed. A structural phase transformation is observed, of which the high-pressure phase is determined to be a triclinic structure. Under the non-hydrostatic condition, the transformation to a high-pressure phase of MnWO4 initiates at a far lower onset pressure. The low-pressure and the high-pressure phases are discovered to coexist in a wide range of pressure under both the hydrostatic and non-hydrostatic conditions, which indicates that the triclinic structure is energetically comparable to that of the wolframite one. Combined with previous reports, non-hydrostatic effect is believed to reveal the triclinic phase of wolframite tungstates at a far lower pressure. The discovery of this work suggests that the wolframite tungstates could be a new source of scintillating materials, and non-hydrostatic effects can be utilized to lower the condition required for their synthesis. Diamond synthesis from graphite is achieved at below 1 GPa and room temperature using a rotational anvil cell. By applying large plastic shear, graphite transformed into hexagonal and cubic diamonds at extremely low pressures of 0.4 and 0.7 GPa, respectively. The formation of a new orthorhombic diamond phase was also observed after pressure elevation to 3 GPa. It is discovered that shear, instead of pressure, plays the key role in this transformation. The discovery of this transformation suggests new mechanism of phase transformations with drastically reduced pressures by shear and is expected to new materials synthesis strategies. Furthermore, the formation of diamonds under unconventionally low pressures also opens up new thoughts in geophysics that the micro-diamonds at geological sites could have formed in the cold crust due to shear-related historical activities instead of the conventional subduction-exhumation process. Decomposition of B4C was observed at 1.0 GPa under large plastic shear using a rotational anvil cell. The products are determined to be a boron-very-rich compound, B50C2, and a pure carbon substance, nano-crystalline graphite. Amorphization of B4C is also observed in the quenched sample. The discovery of B4C’s decomposition and amorphization under large plastic shear suggests a new explanation, in addition to amorphization, to B4C’s mystery shear strength reduction over 20 GPa. It also reveals that shear combined with modest pressure is essential in initiating phase transformations and chemical reactions than hydrostatic compression. Furthermore, the discovery of such shear-induced decomposition of boron carbides may also open a new strategy of non-hydrostatic effects’ utilization in both engineering and chemistry.