Mechanical properties of pyrophyllite under the coupling of high temperature-pressure: A first-principle study

Pyrophyllite [Al2Si4O10(OH)2] is known to be the pressure transmission medium in the high temperature-pressure synthesis industry of super-hard materials, which is attributed to its exceptional support properties, heat resistance, and pressure transmission. Therefore, it is of utmost importance to characterize the mechanical behavior of pyrophyllite under the coupling action of high temperature-pressure (the combined effect of temperature-pressure is called T-P coupling in this work). In this work, the optimal structures, elastic constant, elastic modulus, and anisotropy index of pyrophyllite under T-P coupling are investigated systematically based on the first principle calculation for the first time. Several structure parameters, including lattice constants and layer spacing, indicate that the effect of T-P coupling on the structure mainly acts on internal layer spacing (dI). The elastic modulus (B: Bulk modulus, E: Young's modulus, G: Shear modulus) of pyrophyllite increase gradually with augmented pressure and decreases with the process of heating. Under T-P coupling, high pressure weakens the degree of influence caused by temperature on the elastic modulus of pyrophyllite. This phenomenon is mainly attributed to the structural compactness of pyrophyllite caused by pressure, which weakens the thermal expansion effect caused by heating. As a result, the impact of temperature on the stiffness of pyrophyllite was weakened with rising pressure. At the same time, the changes in crystal structure and atomic distribution density under different T-P conditions also change the toughness and elastic anisotropy of pyrophyllite. The 3D spatial distribution diagrams of anisotropy reflect that temperature increases the elastic mechanical anisotropy of pyrophyllite, in contrast to pressure which tends to decrease it. The sealed pressure transmission medium should not only have good mechanical and thermodynamic properties, but also have insulation. The change of electronic structure under T-P coupling shows that pyrophyllite always maintains good insulation characteristics. These results reveal new insight into the application of pyrophyllite in the synthesis of super-hard materials. And all of these studies are expected to be a guideline for others in their experimental investigations.