Professor Paolo G. Radaelli OSI

Abstract:

The crystal structures of superconducting HgBa2CuO4+δ and HgBa2CaCu2O6+δ have been investigated with a pressure up to 0.6 GPa and HgBa2Ca2Cu3O8+δ to 9.2 GPa by neutron powder diffraction. The compressibility along the c-axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a-axis. The one-layer compound, HgBa2CuO4+δ, shows the largest a-axis compressibility, while HgBa2Ca2Cu3O4+δ shows the smallest compressibility. The bond compressibilities of HgBa2CuO4+δ and HgBa2CaCu2O6+δ are significantly different from HgBa2Ca2Cu3O8+δ bond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance. © 1994.

Abstract:

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La2CuO4+x and HgBa2CuO4+x has provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)3Cu2O6+x with various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO2 layers in this structure systematically lower Tc and eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO2 planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO2, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO2 planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr0.9La0.1CuO2 (Tc = 42 K) have not found any interstitial oxygen. © 1994 Plenum Publishing Corporation.

Abstract:

High-pressure high temperature synthesis has allowed us to prepare members of the new superconducting mercury cuprates series with n = 1-6. The variation of the structural and magnetic properties, oxygen content and Tc across the series, which appears to be the richest among the different superconducting cuprates, will be presented and discussed. Tc and the oxygen content δ increases with n from Hg-1201 (Tc=95K, δ=0.06) to a maximum of 135 K with δ=0.40 for Hg-1223. Structural refinement from neutron diffraction data on Hg-1245 showed a further decrease of δ = 0.30 in the high pressure as prepared compound (Tc= 101K). Tentatives to increase δ by a treatment under 200 bar of oxygen in Hg-1234, Hg-1245 and Hg-1256 showed instead a loss of oxygen from the overdoped as prepared samples. © 1994.