Pressure-induced charge transfer and dT_c/dP in YBa₂Cu₃O_7-x

Subtle pressure-induced structural changes in YBa₂Cu₃O_6.93 and YBa₂Cu₃O_6.60 have been measured by neutron powder diffraction for samples in a hydrostatic helium-gas pressure cell. Small, but significant, differences in the compression of particular Cu-O bonds (notably Cu(2)-O(4)) are observed. However, when the charges on the two copper sites are calculated, requiring overall charge conservation versus pressure, it is found that the net pressure-induced charge transfer of holes from Cu(1) to Cu(2) is essentially the same for both systems. We conclude that the much smaller value of dT_c/dP for YBa₂Cu₃O_6.93 results from the fact that, in the 90 K superconductor, the T_c has already reached its optimum value and the introduction of additional hole carriers cannot further increase T_c.