Scientists predicted metallic aluminum oxides in the interior of giant planets

Aluminum is the most abundant metal element in the earth's crust, and most of the aluminum in the earth's crust exists in the form of aluminum oxide. Among many aluminum oxides, aluminum oxide (Al2O3) is used as a raw material for the bauxite industry and refractory materials, and is of great significance in industrial applications such as aerospace and construction, as well as basic research in materials and planetary science. Previous astronomical studies have found that gaseous planets such as Neptune and Uranus have a rocky core inside, which indicates that aluminum oxides are likely to exist. All the previous theoretical calculations show that there is no material with metallic properties in aluminum oxides under the pressure and temperature conditions on Earth. However, is it the same for aluminum oxides under the temperature and pressure conditions inside giant planets?

In this study, Professor Jian Sun at Nanjing University and collaborators searched the crystal structures of the Al-O system using their own computer code called MAGUS (machine learning and graph theory assisted universal structure searcher) and predicted several new stable aluminum oxides under the pressure condition in the interior of giant planets. They found that under high pressure (above 750 GPa), a variety of new aluminum oxides will become the lowest enthalpy structures, including the Al2O with a symmetry of P4/mbm, the Al2O3 with symmetry P4/nmm, the AlO with symmetry P63/mmc, and AlO3 with symmetry I-43d. It can be seen under extreme pressure, aluminum can exhibit a variety of different valences. Surprisingly, the newly discovered P4/nmm phase Al2O and P63/mmc phase AlO have metallic properties and electride features. Moreover, under the pressure environment of 800GPa, the calculated conductivity of Al2O is as high as 6.18×107Ω-1m-1, which is almost an order of magnitude higher than that of iron at the same condition.

Figure. The theoretical Pressure-Temperature phase diagram of Aluminum oxides.

As we all know, the convection of liquid iron in the outer core is the direct cause of the formation of the earth's magnetic field. This work indicates that, if there is enough content of aluminum in some planets, the distribution of these newly discovered aluminum oxides might influence the total conductivity of these giant planets and have an indirect impact on the planets’ magnetic field. This work provides new insights on the matters in the interior of planets and helps to promote people's understanding of the multi-dipole magnetic fields of giant planets, which has been a topic in the field of astrophysics for decades.

More details can be found in their publication:

T. Huang, C. Liu, J. Wang, S. Pan, Y. Han, C. J. Pickard, R. Helled, H.-T. Wang, D. Xing, and J. Sun, Metallic Aluminum Suboxides with Ultrahigh Electrical Conductivity at High Pressure, Research 2022, 9798758 (2022).

DOI: https://doi.org/10.34133/2022/9798758

https://spj.science.org/doi/10.34133/2022/9798758