Improving the compression environment in the toroidal DAC for equations of state studies above 4 Mbar

The toroidal diamond-anvil cell (tDAC) can achieve pressures under static compression above 400 GPa, but improving the compression environment for high-quality equation of state (EOS) measurements in the tDAC is difficult. The small tDAC sample chamber (~4 – 6 µm diameter) presents a great challenge for embedding micron-sized samples in soft media (i.e., noble gases). Here we will discuss our recent advancements in microfabricating a tDAC sample environment where target sample materials, including the refractory metals Mo and Ta, are fully encapsulated in softer metal media such as Cu and Bi. EoS measurements were conducted on Mo to 330 GPa and Ta to 430 GPa. We will additionally present density functional theory results for the elastic stiffness and compliance constants of these materials to the pressures reached in our experiments. By combining our experimental and computational results, we will compare the measured transition-metal EoSs with extrapolations of EoS calibrations in noble-gas media above 300 GPa along with the uniaxial-stress behavior for these differing metals at these pressures. Results from this work demonstrate that compressing a sample in a softer metal media in the tDAC can improve the compression environment and result in measured sample volumes comparable to those collected in noble-gas media at multi-megabar conditions.