Pressure Calibration Method of 28 GPa for Large-Volume Press

In the large-volume press (LVP), the pressure calibration for the sample chamber is generally carried out by using phase transitions of specific materials with resistance changes. But there are no suitable materials for pressure calibration in the range of 22.5−34.5 GPa. As we know, the mineral 50 mol%MgSiO₃-50 mol%Al₂O₃ (En₅₀Cor₅₀) systems in deep earth undergo structural transition to perovskite phase under high pressure and high temperature (HPHT) conditions. Moreover, the content of Al₂O₃ dissolved in perovskite MgSiO₃ increases gradually with raised pressure above 27 GPa. Therefore, En₅₀Cor₅₀ was selected as the pressure calibration material for further calibrating 28 GPa in the LVP in this study, and this is an indirect pressure calibration method. First, the low-pressure calibration curve (6.0−22.5 GPa) of system oil pressure versus chamber pressure was obtained by using the phase transitions of different pressure calibration materials. Then, based on the low-pressure calibration curve and previous research results En₅₀Cor₅₀, the estimated system oil pressure corresponding to the 28 GPa of chamber pressure is 65 MPa. At the estimated oil pressure, the En₅₀Cor₅₀ samples were heated to 2000 K and maintained for 3−7 h. The results of X-ray diffraction, Raman, and electron probe measurements indicate that bridgmanite has been successfully synthesized, and the dissolved Al₂O₃ molar fraction is greater than 13.7%. According to the previous research results, the corresponding sample chamber pressure is about 29 GPa. This method successfully calibrated the sample chamber pressure around 28 GPa in the LVP, which fill in the blanks for calibrating this pressure range.