Effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-d (part 1): a neutron powder diffraction study

The effect of Sr chemical pressure on superconductivity was investigated in Hg2(Ba1-ySry)2YCu2O8-d. The samples were synthesized at high pressure-high temperature from y = 0.0 to full substitution, y = 1.0. These Sr-substituted compounds are superconducting, without Ca doping on the Y site, and show an increasing Tc with Sr, reaching 42 K for y = 1.0. A detailed neutron powder diffraction study compares the structural changes induced by this chemical Sr/Ba substitution and the mechanical pressure effects in the Hg-2212 system. It shows the strong decrease of the three Ba/Sr-O distances and consequently the shrinkage of Cu-O1(in-plane) and Cu-O2(apical) bonds. These structural changes, by affecting the charge transfer which occurs between the charge reservoir and the superconducting block, are responsible of the Tc enhancement with Sr content.