  We experimentally study two-dimensional (2D) Coulomb crystals in the
"radial-2D" phase of a linear Paul trap. This phase is identified by a 2D ion
lattice aligned entirely with the radial plane and is created by imposing a
large ratio of axial to radial trapping potentials. Using arrays of up to 19
$^{171}$Yb$^+$ ions, we demonstrate that the structural phase boundaries and
vibrational mode frequencies of such crystals are well-described by the
pseudopotential approximation, despite the time-dependent ion positions driven
by intrinsic micromotion. We further observe that micromotion-induced heating
of the radial-2D crystal is confined to the radial plane. Finally, we verify
that the transverse motional modes, which are used in most ion-trap quantum
simulation schemes, remain decoupled and cold in this geometry. Our results
establish radial-2D ion crystals as a robust experimental platform for
realizing a variety of theoretical proposals in quantum simulation and
computation.
