Evolution of lattice parameters during oxygen disordering for underdopedNdBa2Cu3O

Abstract

Strongly underdoped NdBa 2 Cu 3 O 6.67 ceramics is known to become superconducting under pressure only. The appearance of superconductivity in the sample originates from the oxygen ordering process, whereas its disappearance, after pressure release, comes from oxygen disordering. The time evolution of the a, b, and c lattice parameters in the relaxation process which occurs after pressure release from 1.3 GPa was studied by x-ray diffraction at two temperatures of 26 and 15°C. The anisotropic expansion of the unit cell with different time dependence for each parameter was found at both temperatures. The largest change was observed along the a axis, which increased by 0.4%, while the b axis contracted by 0.08% and the c axis increased by 0.05% only. The corresponding orthorhombic strain bϪa was reduced by 45%, while the unit-cell volume increased by about 0.4%. The rate of the structural relaxation process depends strongly on temperature. An increase in the relaxation time constant ͑from Ϸ0.67 h to Ϸ1.75 h͒ was observed with lowering temperature, as expected for oxygen motion with an activation energy of 0.91 eV. The experimental results are evidence that the disappearance of superconductivity after pressure release is related to the shortening of Cu-O chains due to O͑1͒→O͑5͒ oxygen hopping, causing the hole transfer from CuO 2 planes. The possible mechanisms of specific changes in the basal plane are discussed. We report also related results obtained for a more underdoped NdBa 2 Cu 3 O 6.64 sample, which exhibits no superconductivity under the same pressure. The change in symmetry, from orthorhombic to tetragonal, in the disordering process occurring after pressure release at constant temperature, was observed for a 123 sample with fixed oxygen content.