Superconducting Hg-based mixed oxides and oxyfluorides

Syntheses under high pressure and under controlled mercury and oxygen partial pressures of different members of the HgBa2Can-1CunO2n+2+delta series have been developed. There are two main parameters influencing T-c in this family: width of a perovskite slab (n) and concentration (delta) of the extra oxygen located in the Hg layer. The increase of T-c with n occurs until the third member, while after that it decreases. All the members of the series exhibit similar cupola shaped dependencies of T-c vs. delta. Strongly overdoped high members of the series with n = 3-5 were prepared only using high pressure technique and BaO2 as an internal oxidizer. Neutron powder diffraction experiments were carried out for monophase oxygenated HgBa2CuO4+delta and fluorinated WgBa(2)CuO(4)F(delta) samples with different extra oxygen or fluorine content and T-c values. Fluorinated series also exhibits the cupola-like behavior for the T-c vs. delta dependence. NPD showed twice the amount of extra fluorine in comparison with those for the oxygenated Hg-1201 phases with close T-c's The exchange of the extra oxygen by double amount of fluorine causes shortening of the apical Cu-O distances, while the in-plane ones, as well as T-c, do not vary. The influence of the external pressure on the structure and T-c of Hg-1201 strongly depends on the doping level. The increase of the extra oxygen content on going from underdoped to overdoped state results in the larger compression of the apical Cu-O and Ba-O-Hg distances while the HgO2 dumbbell as well as the distance between Ba and O from the (CuO2) layers becomes practically pressure independent. These results together with the data for fluorinated materials allow to elucidate the crucial structural features responsible for the T-c variation under high pressure.