The effect of ca doping on the structures of yba₂cu₄o₈ and y₂ba₄cu₇o_14+x single crystals

The transition temperature of YBa2Cu4O8 (124) is increased by about 10 K by doping with calcium, whereas Ca substitution in Y2Ba4Cu7O14+x (247) has no effect on Tc. X-ray structure refinements of 13 YBa2Cu4O8 and 12 Y2Ba4Cu7O14+x Ca-doped and undoped single crystals have been performed in order to determine structural changes due to Ca doping. In 124 a major fraction of Ca replaces Y; in 247 Ba1 (single-chain 123 unit) and Y are substituted by Ca in the ratio 3:2. The changes of the charge distribution caused by the substitution of trivalent Y3+ by divalent Ca2+ and ion size effects lead to structural changes in Ca-doped crystals. One structural consequence of the replacement of the small Y (r = 1.02 Å) by the larger Ca (r = 1.12 Å) is the increase of the distance between the adjacent oxygen planes in the c direction. In 247 the 123 unit is compressed due to the substitution of Ba1 by the smaller Ca. Ca doping diminishes the buckling of the CuO2 superconducting layers. The Ba-O sublattices in the double-chain 124 units of 247 and 124 are identical and change in the same way upon Ca doping; differences are observed in the Ba-Y sublattice. © 1994 Academic Press, Inc.