  We report on a comparative study of the electronic structure, phonon spectra,
and superconducting properties for recently discovered superconducting
hydrides, H3S and H3P. While the electronic structures of these two materials
are similar, there are notable changes in the phonon spectra and
electron-phonon coupling. The low-frequency bond-bending modes are softened in
H3P and their coupling to the electrons at the Fermi surface is enhanced
relative to H3S. Nevertheless, coupling to the high-frequency modes is reduced
so the resulting calculated superconducting transition temperature is reduced
from ~166 K in H3S to ~76 K in H3P.
