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

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 BaTiO₃. The investigation combined several techniques, notably neutron powder diffraction (NPD), nuclear magnetic resonance (⁷Li-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 BaTiO₃ host lattice and another glassy phase which breaks the host lattice due to excessive Li accumulation. While the average structure of Li in BaTiO₃ could not be determined by NPD, ⁷Li-NMR revealed one broad “disordered” and multiple “ordered” peaks. Local structure models with different defect types involving Li⁺ were modeled and the corresponding chemical shifts (δ) were compared with experimental values. It is found that the closest defect model describing the ordered peaks, is with Ti⁴+ being replaced by four 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 Ti³⁺ as a charge-transfer defect. DFT calculations disclosed local antipolar displacements of Ti ions around both types of defect sites upon insertion of Li⁺. This is in accordance with the experimental observation of pinching effects of the EP in Li-doped BaTiO₃. These studies demonstrate the huge impact of the local structure of the doped smaller/lighter ions on the functional properties of oxides.