Preparation of Al2O3–Cr2O3 Solid Solutions as Buoyancy Markers and Their High Pressure Synchrotron X-ray Diffraction Analysis

Understanding the density of silicate melts under high pressures is of great significance for establishing the differentiation and dynamic modeling of materials in the interior of the Earth. The sink/float method is widely used in the density measurement of silicate melts under high pressures. However, there are significant limitations in using natural minerals as buoyancy markers in this method. In this study, the Al2O3–Cr2O3 solid solution, which is a buoyancy marker, was prepared using the optical floating zone melting method. The crystalline structures of Al2O3–Cr2O3 solid solutions with different Cr2O3 contents at ambient pressure were studied. It was found that lattice parameters a and c increase linearly with the increasing Cr2O3 content, but the doping with Cr3+ ions leads to a more severe lattice strain in the c-axis direction. As the Cr2O3 content increases, the diffraction angle of the diffraction peak decreases linearly. In addition, it was found that the relationship between the theoretical density of each solid solution and its Cr2O3 content can be fitted using a second-order polynomial. Al95Cr05, Al90Cr10, and Al30Cr70 were analyzed using synchrotron radiation X-ray diffraction analysis under different pressures in order to study the stability and compressibility of the Al2O3–Cr2O3 solid solutions under high pressure. The results show that the cell parameters of the three solid solutions vary with pressure. The bulk elastic moduli of these three solid solutions were obtained, which provides experimental data for the design and preparation of different buoyancy markers in the future.