Atomic ordering of the fluorine dopant in theHgBa2CuO4+δhigh-Tcsuperconductor

Abstract

Lattice sites and collective ordering of fluorine atoms in oxygen-reduced samples of HgBa2CuO4 were investigated with the perturbed angular correlation technique by measuring the electric field gradients induced at 199m Hg nuclei. The experimental data were interpreted with the help of ab initio calculations of charge distributions for different fluorine configurations in HgmBa2mCumO4mFn, supercells. Internal parameters were allowed to relax, to cancel residual atomic forces due to the dopant. The experimental results show clearly that fluorine occupies only the center of the mercury mesh. For a fluorine content F 0.35 the best agreement with theoretical data is obtained under the assumption that fluorine shows a tendency toward ordering along interstitial rows parallel to a ,b. In conformity with experimental data from diffraction techniques the calculations show elongations of the O2-Hg-O2 dumbbell and barium shifts towards the fluorine atoms as effects of the dopant. No strong ordering of the transferred charge at the copper planes is present in the computed charge density, even when the fluorine atoms order at high concentrations.