Structural and electronic effects of Sr substitution for Ba in Y(Ba1-xSrx)(2)Cu3Ow at varying w

To unveil the structural mechanisms associated with T-c variations induced by either mechanical or chemical pressure, samples of the Y(Ba1-xSrx)(2)Cu3 Ow system with x less than or equal to 0,0.02,0.1,0.25,0.35,0.5,0.625,0.75, 1, as well as those with x = 0.5 and w = 6.685,6.80,6.96,6.98, have been pr epared and characterized. Characterization includes crystal structural refi nements based on powder neutron diffraction data taken at room temperature, electron microscopy for detecting a possible Ba/Sr ordering, and resistive and magnetic measurements of the superconducting transition. The effects o f Sr substitution on the structural parameters are equivalent to those of a pressure of approximately 10 GPa/x. The main difference is the thickness o f the superconducting block CuO2-Y-CuO2, which increases with increasing x and decreases with increasing pressure. As a consequence of the displacemen t of O4 from the (0, 1/2, 0) to the (x, 1/2, 0) position, the Ba/Sr-O4 dist ance decreases with increasing Sr content. At constant w, T-c decreases at the rate of 20 K/x. For x>0, the maximum of T-c. occurs at a value of w hig her than for x = 0. For x = 0.5 the thickness of the superconducting block increases with increasing w and consequently with increasing T-c. This indi cates that, at least in this system, the thickness of the superconducting b lock is not the key parameter controlling T-c. The stress of the Ba/Sr and the Y sites, estimated from the bond valence sums (BVSs), decreases with in creasing x. For x = 0.5, the BVS values are almost equal to the formal valu es of 2+ and 3+, respectively. The average BVS of Cu1 and Cu2 increases wit h increasing x. However, this does not correspond to a real increase becaus e the total charge of the Cu cations, as determined by iodometric titration or neutron diffraction, remains constant with x. Our analysis of the struc tural data suggests that the relaxation of the Ba/Sr layer hinders the char ge transfer from Cu1 to Cu2, which accounts for the decrease of T-c with in creasing x. [S0163-1829(98)05645-8].