The Effect of Ca Doping on the Structures of YBa2Cu4O8 and Y2Ba4Cu7O14+x Single Crystals

✍ Scribed by H. Schwer; E. Kaldis; J. Karpinski; C. Rossel

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 455 KB
• Volume: 111
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1994.1203

✦ Synopsis

The transition temperature of (\mathrm{YBa}{2} \mathrm{Cu}{4} \mathrm{O}{8}(124)) is increased by about (10 \mathrm{~K}) by doping with calcium, whereas Ca substitution in (\mathrm{Y}{2} \mathrm{Ba}{4} \mathrm{Cu}{7} \mathrm{O}{14+x}) (247) has no effect on (T{\mathrm{c}}). X-ray structure refinements of (13 \mathrm{YBa}{2} \mathrm{Cu}{4} \mathrm{O}{8}) and (12 \mathrm{Y}{2} \mathrm{Ba}{4} \mathrm{Cu}{7} \mathrm{O}{14+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 Bal (single-chain 123 unit) and (Y) are substituted by (\mathrm{Ca}) in the ratio (3: 2). The changes of the charge distribution caused by the substitution of trivalent (\mathrm{Y}^{3+}) by divalent (\mathrm{Ca}^{2+}) and ion size effects lead to structural changes in Ca-doped crystals. One structural consequence of the replacement of the small (\mathrm{Y}(r=1.02 \AA)) by the larger (\mathrm{Ca}(r=1.12 \AA)) 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 (\mathrm{Ca}). Ca doping diminishes the buckling of the (\mathrm{CuO}{2}) superconducting layers. The (\mathrm{Ba}-\mathrm{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.