Effect of fluorination and high pressure on the structure and properties of the Hg-bearing superconducting Cu mixed oxides

Abstract

The T c variation of HgBa 2CuO 4+ δ (Hg-1201) and HgBa 2CuO 4F δ can be achieved by a change in the carrier concentration and by a compression of the structure under high pressure. Oxygenated and fluorinated series exhibit a cupola-shaped behavior for the T c vs. δ dependence, but the curves are shifted away from each other along the δ axis. NPD showed double amount of extra fluorine in comparison with extra oxygen for the oxygenated Hg-1201 phases with close T c's. An exchange of the extra oxygen by a double amount of fluorine causes a significant compression of the apical CuO bond distances, while the in-plane ones, as well as T c, do not vary. Fluorination of Hg-1223 resulted in a slight increase in T c in comparison with oxygenated material. The influence of the external pressure on the structure and T c of Hg-1201 strongly depends on the doping level. An increase in the extra oxygen content from underdoped to overdoped state results in the larger compression of the apical CuO and BaO Hg distances while the HgO 2 dumbbell as well as a distance between Ba and O from the (CuO 2) layers become practically pressure independent.