EVIDENCE FOR CA SUBSTITUTING BA IN YBA2-XCAXCU4O8+/-Y - A NUCLEAR-QUADRUPOLE RESONANCE INVESTIGATION

For the understanding of the T(c) enhancement in Ca doped YBa2Cu4O8+/-y it is crucial to know which crystallographic site the Ca+2 ion is occupying. We investigated this problem by measuring the Cu nuclear quadrupole resonance (NQR) spectrum at 100 K in YBa2Cu4O8+/-y doped with 10% Ca (T(c) = 91 K). The NQR lines of the plane and chain Cu sites appear at the same frequencies v(q) as in the pure compound, however, both lines are broadened by static defects. Within the linewidths no temperature dependence of v(q) was observed and no indication for magnetic ordering at the Cu sites was found. At 20 600 MHz, near to the Cu(1) line, a resonance line has been detected which is absent in the pure YBa2Cu4O8.04 spectrum. Based on NQR arguments we assign this new line to Cu(1) nuclei whose electric field gradients are disturbed by Ca+2 ions occupying Ba+2 sites. We conclude that a major fraction of Ca ions occupy Ba sites, therefore the increase of T(c) by Ca doping cannot be explained by an increase of the hole concentration.