  Noble-metal nano-particles have been the industry-standard for plasmonic
applications due to their highly populated plasmon generations. Despite their
remarkable plasmonic performance, their widespread use in plasmonic
applications is commonly hindered due to limitations on the available laser
sources and relatively low operating temperatures needed to retain mechanical
strength in these materials. Motivated by recent experimental works, in which
exotic hexagonal-closed-packed (HCP) phases have been identified in Gold (Au),
and Silver (Ag), we present the plasmonic performance of two HCP polytypes in
these materials using high-accuracy first-principles simulations. HCP phases
commonly reach thermal and mechanical stability at high temperatures due to
monotonically decreasing Gibbs free energy differences with respect to the
corresponding face-centered-cubic (FCC) phases. We find that several of these
polytypes are harder and produce bulk plasmons at the lower energies with
comparable life-times than their conventional FCC counterparts. We also show
that surface plasmons are generated at substantially lower energies for
perfectly spherical nano-grains embedded on face-centered cubic matrices
compared to their bulk counterparts. Furthermore, the increasing grain-size
slightly shifts the surface-plasmon peaks to higher energies and increases the
plasmon intensity. Our work suggests that noble metal nano-particles can be
tailored to develop exotic HCP phases to obtain novel plasmonic properties.
