  The lattice parameters and Tc of Y1-yCayBa2Cu3Ox have been determined under
pressure up to 10 GPa using a diamond anvil cell. The results are used to test
the extended charge transfer model that connects the pressure effect on Tc to
an intrinsic dTc,max/dp and a pressure induced hole doping dnh/dp, the latter
being caused by charge transfer from the CuO chains to the CuO2 planes. dnh/dp
and dTc,max/dp are usually assumed to be constant with respect to pressure p.
However, our experiments show that the usage of this model gives a poor
description of the experimental Tc(p) values. We connected the extended charge
transfer model to the pressure induced changes of the compressibilities. With
this ansatz the calculated Tc(p) values show an excellent agreement with the
experimentally determined values.
