Neutron powder diffraction study at room temperature and at 10 K of the crystal structure of the 133 K superconductor HgBa2Ca2Cu3O8+δ

Abstract The crystal structure of the superconductor HgBa 2 Ca 2 Cu 3 O 8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a =3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO 2 )(Ca)(CuO 2 )(BaO)(HgO δ (BaO)(CuO 2 )(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba 2 Ca 2 Cu 3 O x at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ =0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is T c =133 K . No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO 2 ) are coplanar for the Z =0 (CuO 2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.