Effect of nonstoichiometry on the stability of Fe3+-V0 defect centres in PbTiO3

Abstract

Dielectric and electromechanical properties of ceramics based on $PbTiO_3$ are greatly influenced by nonstoichiometry and lattice defects prevailing in the solid. Nonstoichiometry in $PbTiO_3$ can be expressed as $Pb_{l-x} TiO_{3-x-y}$ where x is the deviation with respect to PbO content and y represents the additional oxygen nonstoichiometry. The variation in y is < 3 x $10^{-4}$ in the range of 1 to $10^5$Pa of oxygen pressure while the value of x \simeq 0.1 at 1375K [1]. Besides nonstoichiometry, impurities also affect the electrical properties; there are hard (acceptor type) and soft (donor type) impurities. $Fe^{3+}$ is a commonly encountered native impurity in $PbTiO_3$. The concentration of $Fe^{3+}$ ions has to be closely monitored for high quality ceramics, and this can be carried out by electron paramagnetic resonance (EPR) spectroscopy even at low concentrations (\leq 2p.p.m.). This is because $Fe^{3+}$ ions substituting for $Ti^{4+}$ in $PbTiO_3$ localize themselves nearest to oxygen vacancies and the resulting defect centre $Fe^{3+}-V_o$ has a fairly long spin-lattice relaxation time around room temperature to produce a strong EPR signal. We have observed that the $Fe^{3+}-V_o$ EPR signal shows intensity variations with respect to PbO nonstoichiometry. Therefore the EPR spectrum can be used for monitoring the $Fe^{3+}$ impurity content as well as the nonstoichiometry in $PbTiO_3$ ceramics.