Electron spin resonance investigation of impurity and intrinsic defects in Nb-doped $BaTiO3$ single crystal and ceramics Available to Purchase

Electron spin resonance (ESR) spectra of impurity and intrinsic defects have been measured at 4.2–295 K in Nb-doped $BaTiO3$ single crystals and ceramics to clarify their role in the conductivity and positive temperature coefficient of resistivity (PTCR effect). The measurements revealed a small amount of $Fe3+$⁠, $Cr3+$⁠, and $Mn2+$ impurities, which change their valence state (for example, $Cr5+→Cr3+$⁠, $Mn4+→Mn2+$⁠) with increased Nb concentration due to the compensation of the excess charge of $Nb5+$ ions. Besides the $3d$-metal impurities, several types of $Ti3+$ polaronic and possible fluctuon states, where electrons can be localized near ferroelectric domain boundaries, have been revealed as well. All of them are associated with $Ti3+$ lattice ions and not with $Nb5+∕4+$ impurity, which apparently represents rather a very shallow donor level. The data obtained strongly support the polaronic origin of Nb-doped $BaTiO3$ conductivity at $T<300K$⁠. Comparative investigations of ESR spectra in single crystals and ceramics of the same kind of $BaTiO3$ together with computer simulation allowed us unambiguously to ascribe complex ESR signals observed in ceramic samples to $Cr3+$⁠, $Mn2+$⁠, and $Fe3+$ ions and $Ti3+$ polarons and/or fluctuons. The role of manganese ions at grain boundaries in the PTCR effect is discussed as well.