Structure of HgBa2CuO4+delta (0.06 <delta < 0.19) at ambient and high pressure

The results of a neutron-diffraction study of the HgBa2CuO4+delta structure at ambient pressure and under external pressure at different extra oxygen concentrations are presented. The results have been analyzed together with the data of previous investigations. It is shown that in the cation-stoichi ometric samples the O-delta oxygen is only present in the center of the mer cury layer, T-c is parabolically dependent on delta, and T-c,T-max is obtai ned at delta(opt) = 0.13 +/- 0.01. The influence of pressure on the structu re strongly depends on the doping level. At low oxygen content (delta appro ximate to 0.06), the compression of the structure is practically uniform. A n increase of the extra oxygen content to 0.19 (overdoped state) results in the larger compression of the apical Cu-O(2) and Ba-O delta distances, whi le the HgO2 dumb-bell as well as the distance between Ba and O belonging to the (CuO2) layer become practically pressure independent. These results ar e in agreement with models, in which the effect of the charge transfer from the reservoir to the (CuO2) layers does not play a dominant role in the T- c increase with pressure at low and optimal delta values, while in the over doped state the charge transfer is enhanced under pressure, thus inducing t he T-c decrease. [S0163-1829(99)03006-4].