  We study experimentally and theoretically structural defects which are formed
during the transition from a laser cooled cloud to a Coulomb crystal,
consisting of tens of ions in a linear radio frequency trap. We demonstrate the
creation of predicted topological defects (`kinks') in purely two-dimensional
crystals, and also find kinks which show novel dynamical features in a regime
of parameters not considered before. The kinks are always observed at the
centre of the trap, showing a large nonlinear localized excitation, and the
probability of their occurrence surprisingly saturates at ~0.5. Simulations
reveal a strong anharmonicity of the kink's internal mode of vibration, due to
the kink's extension into three dimensions. As a consequence, the periodic
Peierls-Nabarro potential experienced by a discrete kink becomes a globally
confining potential, capable of trapping one cooled defect at the center of the
crystal.
