  To understand the pressure-induced changes in the electronic structure and
the electron-phonon interaction in yttrium, we have studied hexagonal
close-packed (hcp) yttrium, stable at ambient pressure and double hexagonal
close-packed (dhcp) yttrium, stable up to around 44 GPa, using
density-functional-based methods. Our results show that as one goes from hcp
yttrium to dhcp yttrium, there is (i) a substantial charge-transfer from s->d
with extensive modifications of the d-band and a sizable reduction in the
density of states at the Fermi energy, (ii) a substantial stiffening of phonon
modes with the electron-phonon coupling covering the entire frequency range,
(iii) an increase in the electron-phonon coupling constant \lambda from 0.55 to
1.24, leading to a change in the superconducting transition temperature T_c
from 0.3 K to 15.3 K for \mu*=0.2.
