Chemical pressure and other effects of strontium substitution in YBa 2Cu 3O 9−δ

Abstract

High quality samples of the Y(Ba 1− y Sr y ) 2Cu 3O 6.948(6) solid solution phase were prepared via citrate gels by mixing the components on an atomic level. The range of solid solubility is 0.00 ≤ y ≤ 0.35(3) for a 910°C firing temperature and constant oxygen content, as determined by iodometry. At standardized conditions, the limiting phase with y = 0.35(3) is in equilibrium with Y 2(Ba 0.9Sr 0.1)CuO 5 and (Sr, Y) 14Cu 24O 41, and the situation is described in terms of a tetrahedral four-component phase diagram. The slightly enhanced (anisotropic) contraction along the c axis of the unit cell, observed upon introduction of the smaller Sr atoms, is associated with apical compression of the coordination square pyramids of copper. Both the volume contraction and its anisotropy can be compared with the effects of applied pressure. In terms of chemical pressure P∗ = ε · y, coefficients of ε V = 6.8(2) GPa or ε D t = 2.4(4) GPa are obtained, depending on whether ε is deduced from unit cell volume V or its (tetragonal) distortion D t . The responses of the superconducting state upon the applied and chemical pressure (the latter exerted at the Ba site) are, however, quite different, with ΔT c ΔP = 0.7 K/GPa and ΔT c Δy = −20(2) K . A specific feature must hence either be introduced by the real pressure (e.g., increase in carrier concentration) or by the chemical pressure [e.g., local strain effects and/or redistribution of oxygen around Cu(1) site].