  The structural properties under hydrostatic pressure up to 3 GPa of the
layered rare earth carbide halide superconductor Y2C2I2 are studied by neutron
powder diffraction at room temperature. The compressibilities are anisotropic,
such that the compressibility perpendicular to the layers being approximately
twice as large as within the layers. The atomic positional parameters
determined from the powder diffraction patterns as a function of pressure were
used as the basis for highly resolved electronic structure calculations. These
reveal the electronic density of states at E_F to increase with pressure. As is
shown quantitatively, this effect outweighs the pressure induced lattice
stiffening effects and is responsible for the pressure induced increase and
saturation of Tc towards 2 GPa.
