Defect structure and property consequence when small Li+ ions meet BaTiO3

Abstract

In the present work the longstanding issue of the structure and dynamics of smaller ions in oxides and its impact on the properties was investigated on 7% Li-doped $\mathrm{BaTi}{\mathrm{O}}_{3}$. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance ($^{7}\mathrm{Li}$-NMR), electron paramagnetic resonance (EPR), electron microprobe, electric polarization (EP) measurement, and electronic structure calculations based on density-functional theory (DFT). Electron microprobe confirmed multiple phases, one containing incorporated Li in the $\mathrm{BaTi}{\mathrm{O}}_{3}$ host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in $\mathrm{BaTi}{\mathrm{O}}_{3}$ could not be determined by NPD, $^{7}\mathrm{Li}$-NMR revealed one broad ``disordered'' and multiple ``ordered'' peaks. Local structure models with different defect types involving ${\mathrm{Li}}^{+}$ were modeled and the corresponding chemical shifts $(\ensuremath{\delta})$ were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with $\mathrm{T}{\mathrm{i}}^{4+}$ being replaced by four ${\mathrm{Li}}^{+}$ ions. The biexponential behavior of the spin-lattice relaxation of the ordered peaks each with a short and a long relaxation discloses the existence of paramagnetic ions. Finally, EPR revealed the existence of the paramagnetic ion $\mathrm{T}{\mathrm{i}}^{3+}$ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of ${\mathrm{Li}}^{+}$. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped $\mathrm{BaTi}{\mathrm{O}}_{3}$. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides.