Crystal structure of ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ at high pressure (to 8.5 GPa) determined by powder neutron diffraction

The crystal structure of the high-temperature superconductor ${\mathrm{HgBa}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{8+\mathrm{\delta }}$ has been determined from powder neutron-diffraction experiments up to pressures of 8.5 GPa. Complete structural refinements were carried out on the experimental data taken at (approximate) pressures of <0.5, 3.5, 6.0, and 8.5 GPa. One of the most significant features to emerge from this study is the extreme sensitivity of the apical Cu-O bond to applied pressure. Between our ambient and high-pressure measurements the apical Cu-O bond length decreases by some 10.5%. In contrast, over the same pressure range, we have recorded significantly smaller changes in other crystal bond lengths. For example, the Cu-O in-plane bond distance changes by only around 2% as does the Hg-O apical bond. The availability of such structural data for this highest transition temperature superconductor may now allow an appraisal of the various structural and electronic factors responsible for superconductivity at such exceptionally high temperatures.