Structural investigations of elemental metals at multi-megabar pressures using toroidal diamond anvil cells

Abstract

Research at high pressures has revealed that the elemental metals, most of which have simple crystal structures at ambient conditions such as face-centred cubic, body-centred cubic, or hexagonal close-packed, transition to more complex structures at high pressure. X-ray diffraction methods are well-suited to exploring these complexities, providing detailed data on the structure of elements at high pressure. In particular, this work focuses on studying the alkali metals K and Rb with x-ray diffraction methods. These elements have been found to exhibit a wealth of structural complexity such as the incommensurate host-guest structures K-III and Rb-VI, or the orthorhombic Cmca phases of K-VI and Rb-VI. To study these and other elements at high pressures, diamond anvil cells are frequently used to compress the sample, as they allow for accurate structure identification using x-ray diffraction and can reliably reach pressures of around 300 GPa. However, the study of elements at even higher pressures becomes difficult using conventional diamond anvil cells which are prone to diamond failure at these conditions. This has motivated the development of new DAC designs. In particular, toroidal diamond anvil cells are a promising modification wherein the diamond anvils are ‘sculpted’ to create a geometry able to achieve pressures above 500 GPa. This work initially investigates the behaviour of the light metals Mg and Al and the alkali metals K and Rb, using conventional diamond anvil cell techniques. The phase transitions of Mg and Al are discussed, with compression data up to 301 GPa and 236 GPa presented for Mg and Al, respectively. Equations of state are fitted for each metal and the data are compared to other studies in the field. In K and Rb, the static phase diagrams are extended up to 321 GPa and 264 GPa, respectively. These studies observe significant changes in the compression curves occurring between 0-100 GPa, where the various phase transitions of these metals display a great variety of compressive behaviour. The predicted high-pressure oC16→hP4 phase transition is observed and presented for the first time in Rb. However, no analogous transition is seen in K in spite of theoretical predictions. The design, manufacture, and implementation of toroidal diamond anvil cells are subsequently detailed, including the specifics of focused ion beam milling. Experiments on the strongly scattering elements W and Ce are then presented. In W, the body-centred cubic phase remains stable up to the highest pressure of 381 GPa, as expected. However, unanticipated changes in the sample pressure environment demonstrate the complexity of performing toroidal diamond anvil cell experiments. In Ce, an apparent shift in the compression curve was observed between 200-250 GPa, accompanied by decreasing c/a ratio in the body-centred tetragonal phase above 250 GPa. The cause of the change in compressive behaviour is unknown, but the c/a ratio decline is found to agree with theoretical predictions. Finally, results from toroidal diamond anvil cell investigations of Rb are presented along with the challenges of studying light and highly reactive elements using toroidal diamond anvil cells. A highest pressure of 272 GPa is recorded from Raman spectroscopy measurements of the diamond.