to Megabar Pressures*

Despite years of intense study, the superconducting mechanism seen in the cuprates is not yet fully understood. In recent years pressure has been used as a tuning parameter to probe the unusual electronic properties of cuprate superconductors, such as anomalous changes in T_c observed in the bismuth-based cuprates. We report the effects of quasi-hydrostatic megabar pressures (>100 GPa) on the structure of bismuth-based high-Tc cuprate superconductors - Bi₂Sr₂Ca_n-1Cu_nO_2n+4+δ (n=1,2,3) - using synchrotron x-ray diffraction and diamond anvil cells. To ensure nearly hydrostatic conditions during compression, neon was used as a pressure transmitting medium. A stiffening of the c axis occurs for n=1 and 2 at 10 GPa and 20 GPa, respectively, but no such incompressibility effect was observed in the n=3 composition. Below the stiffening structural properties agreed with previously reported data, but differ from earlier work as a result of non-hydrostatic stress in those experiments, as confirmed by our own measurements without a pressure transmitting medium. In all three compositions the changes in lattice parameters do not correlate with the changes in T_c including its monotonic rise seen in all compositions above a critical pressure.