Local structural perturbations in HgBa2CuO4+delta

The high-temperature superconductor, HgBa2CuO4+delta (delta > 0), is the fi rst and arguably the most simple member of the homologous series of mercury -based cuprates, of general formula HgBa2Can-1CunO2n+2+delta (n = 1...8), T he presence of interstitial oxygen in the HgOdelta rock salt layer is cruci al in controlling the electronic structure of HgBa2CuO4+delta. Thus, by car eful control of the concentration (delta) of interstitial oxygen, one may f ine-tune the electronic properties of this material from insulating, throug h superconducting, to metallic behavior. The local structure adjusts to the presence of interstitial oxygen by displacement of neighboring atoms from their "ideal" positions. We present here a combined powder X-ray and time-o f-flight neutron diffraction study of HgBa2CuO4+delta (space group P4/mmm, lattice parameters a = 3.8757(1) Angstrom and c = 9.4998(1) Angstrom, delta = 0.07), From this conjoint approach (powder X-ray and neutron diffraction ) we are able to identify the local structural perturbations and atom displ acements arising from the occupany (delta = 0.07) of the interstitial oxyge n site in HgBa2CuO4+delta. (C) 1999 Academic Press.