  Thermodynamic, structural and electronic properties of isolated copper and
iron atoms as well as their complexes with oxygen vacancies in tetragonal lead
titanate are investigated by means of first principles calculations. Both
dopants exhibit a strong chemical driving force for the formation of M_Ti-V_O
(M = Cu, Fe) defect associates. The most stable configurations corresponds to a
local dipole aligned along the tetragonal axis parallel to the spontaneous
polarization. Local spin moments are obtained and the calculated spin densities
are discussed. The calculations provide a simple and consistent explanation for
the experimental findings. The results are discussed in the context of models
for degradation of ferroelectric materials.
