We investigate the electronic and atomic structure of interstitial hydrogen
impurities in PbTiO3 and investigate their effect on ferroelectric polariz
ation through first-principles total-energy calculations. Interstitial H in
PbTiO3 is found to bond to oxygen and to act as a shallow donor impurity.
At equilibrium, H does not much affect nearby Ti-O bonds and the H-O dipole
increases the polarization. The barrier for reversing the defect dipole ca
n give rise to imprint (extra stabilization of one of the two polarization
modes).