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

The T-c variation of HgBa2CuO4+delta (Hg-1201) and HgBa2CuO4Fdelta can be a chieved by a change in the carrier concentration and by a compression of th e structure under high pressure. Oxygenated and fluorinated series exhibit a cupola-shaped behavior for the T-c vs. delta dependence, but the curves a re shifted away from each other along the delta axis. NPD showed double amo unt 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. Fluorinat ion of Hg-1223 resulted in a slight increase in T-c in comparison with oxyg enated material. The influence of the external pressure on the structure an d T-c of Hg-1201 strongly depends on the doping level. An increase in the e xtra oxygen content from underdoped to overdoped state results in the large r compression of the apical Cu-O and Ba-O-Hg distances while the HgO2 dumbb ell as well as a distance between Ba and O from the (CuO2) layers become pr actically pressure independent. (C) 2000 Elsevier Science B.V. All rights r eserved.