Onset of structural transformation in thorium metal at high pressures

Abstract

This paper reports that novel phases of metals are formed under high pressures as a result of the rearrangement of electrons in different energy bands. Diamond anvil cells can produce calibrated static pressures of several million atmospheres and materials can be studied in-situ with synchrotron x-ray diffraction sources. Metals, in particular, show a wide variety of phase transformations driven by changes in the electronic structure with pressure. Electron transfer from one band to another is believed to be the cause of several high pressure transformations in rare earth, actinide and transition metals. At one atmosphere, the actinide metal thorium (Th), has an unoccupied 5f band approximately 4 eV wide located 1.5 eV above the Fermi energy. At high pressures, due to the relative movement of various energy bands, the 5f band should cross the Fermi energy. As a result, the 5f band becomes occupied at high pressures due to electron transfer from the sd-band. This electronic transition is expected to cause structural transformation. Th crystallizes into the face centered cubic (FCC) phase at ambient conditions. X-ray diffraction studies have been carried out on Th up to pressures of 30 GPa(6), 65 GPa(7) and 100 GPa(8).