  Noble metals adopt close-packed structures at ambient pressure and rarely
undergo structural transformation at high pressures. Platinum (Pt), in
particular, is normally considered to be unreactive and is therefore not
expected to form hydrides under pressure. We predict that platinum hydride
(PtH) has a lower enthalpy than its constituents solid Pt and molecular
hydrogen at pressures above 21.5 GPa. We have calculated structural phase
transitions from tetragonal to hexagonal close-packed or face-centered cubic
(fcc) PtH between 70 and 80 GPa. Linear response calculations indicate that PtH
is a superconductor at these pressures with a critical temperature of about
10--25 K. These findings help to shed light on recent observations of
pressure-induced metallization and superconductivity in hydrogen-rich
materials. We show that formation of fcc metal hydrides under pressure is
common among noble metal hydrides and examine the possibility of
superconductivity in these materials.
