  We studied Tc of YBa2Cu3Ox (Y123), Y0.89Ca0.11Ba2Cu3Ox (YCa123) and
NdBa2Cu3Ox (Nd123) single crystals with various oxygen contents x. Compared to
Tc(x) of Y123 the Tc(x) curves of YCa123 are shifted to lower oxygen contents
and the maximum transition temperature Tc,max decreases with increasing Ca
content whereas in Nd123 Tc(x) is shifted to higher oxygen contents and Tc,max
is increased. According to the universal parabolic Tc(nh) behavior the
differences in Tc(x)of Y123, YCa123 and Nd123 can be ascribed to different hole
concentrations nh in the CuO2 planes caused by doping via changes in chemistry
or structure. In order to study the influence of structural changes on Tc we
examined the hydrostatic pressure effect dTc/dp (p up to 0.6 GPa). In the
underdoped region, at nh=0.11, the examined compounds show a peak in dTc/dp
which is very pronounced for systems with well ordered CuO chains. As this peak
occurs at the same nh in all investigated systems it is not caused by oxygen
ordering, but its origins might be found in a strong influence of lattice
deformations on the electronic structure.
