  The domain structure of uniaxial ferroelectric lithium niobate single
crystals is investigated using Raman spectroscopy mapping. The influence of
doping with magnesium and poling at room temperature is studied by analysing
frequency shifts at domain walls and their variations with dopant concentration
and annealing conditions. It is shown that defects are stabilized at domain
walls and that changes in the defect structures with Mg concentration can be
probed by the shift of Raman modes. We show that the signatures of polar
defects in the bulk and at the domain walls differ.
