Ab Initio Calculations of Carbon Impurities in Ferroelectric Lead Titanate

Lead titanate is of interest due to its room-temperature ferroelectric properties and high dielectric constant. Organic species present during growth make carbon as common contaminant, so various structural configurations of carbon in tetragonal PbTiO3 have been investigated. We have analyzed the electronic properties and vibrational modes using density functional calculations, from which we find that carbon contamination may be electrically active, such as substitution of Pb or O, or electrically passive, such as for iso-electronic substitution of Ti. The most stable site under most conditions is substitution of Ti, but due to the structural arrangement and chemical bonding it is a fixed electric dipole, with an activation energy for reorientation around 1.5 eV. This large barrier has implications for switching the direction of ferroelectric domains.