Structural and superconducting properties of phases in the underdoped Y 2(Ba 1 − xSr x) 4Cu 7O 14 + δ system: The evidence for a chemical pressure effect

Abstract

An attempt was made to investigate the chemical pressure effect (i.e., smaller isovalent cation substitution) on the superconductivity of phases in the Y 2(Ba 1 − x Sr x ) 4Cu 7O 14 + δ (Sr-doped 247) system. A series of Sr-doped 247 phases with x = 0, 0.05, 0.10, 0.15, 0.175, 0.20 and 0.30 were prepared under 20 bar of oxygen at 900°C, followed by thermal annealing under 100 bar of oxygen at 300°C and slowly cooled down to ambient temperature. The solubility limit of Sr in the 247 phase was found to be ca. 0.10 to 0.15 and the orthorhombic lattice distortion for the Sr-doped 247 phases increased with increasing Sr content, as determined from X-ray diffraction (XRD). Rietveld refinement based on the XRD data obtained for Y 2(Ba 0.9Sr 0.1) 4Cu 7O 14.74 indicated that 92.5% of Sr atoms substituted for the Ba sites in the YBa 2Cu 4O 8 subcell. The T c of the Sr-doped 247 phases increased from 72 K ( x = 0) to 75 K ( x = 0.10) monotonically upon Sr substitution, as indicated by both dc resistivity and magnetic susceptibility data. The dopant composition coefficient of T c, d T c/d x, for the series of Sr-doped 247 phases with 0 ≦ x ≦ 0.10 was olso determined to be + 7.5 K per Sr atom per formula unit based on magnetic susceptibility data. We have demonstrated that the principle of chemical pressure effect can be applied to enhance T c in a series of Sr-doped 247 phases without applying high physical pressure.