Compressibility of the HgBa2Can-1CunO2n+2+δ (n=1,2,3) high-temperature superconductors

The transition temperatures (Tc) of the homologous series HgBa2Can-1CunO2n+2+δ of high-temperature superconductors show very large (∼30 K) pressure enhancements, saturating at 118, 154, and 164 K for Hg1201, Hg1212, and Hg1223, respectively. We have measured the compressibility of the first three mercury superconductors in a diamond-anvil cell using energy-dispersive synchrotron x-ray diffraction. We report the tetragonal-cell parameters a and c up to 30 GPa. We find ambient-pressure bulk moduli, KV0==-V0(P/V)0, of 65.4(3.0), 91.6(4.7), and 83.8(10.5) GPa for Hg1201, Hg1212, and Hg1223, respectively. The Hg superconductors are highly compressible compared to other high-temperature superconductors which generally show bulk moduli in the range of 100-200 GPa. The relation c=A+B(n-1) holds for the c-axis cell parameter at ambient pressure, however, the relation breaks down at high pressures.