ε-Fe 2 O 3, a metastable phase of iron oxide, is widely known as a room temperature multiferroic material or as a super-hard magnet. Element substitution into ε-Fe 2 O 3 has been reported in the literature; however, the substituted ions have a strong site preference depending on their ionic radii and valence. In this study, in order to characterize the crystal structure and magnetic properties of ε-Fe 2 O 3 in the Fe 2+ /Fe 3+ coexisting states, Li + was electrochemically insert into ε-Fe 2 O 3 to reduce Fe 3+ . The discharge and charge of Li + into/from ε-Fe 2 O 3 revealed that Li + insertion was successful. X-ray magnetic circular dichroism results indicated that the reduced Fe did not exhibit site preference. Increasing the Li + content in ε-Fe 2 O 3 resulted in decreased saturation magnetization and irregular variation of the coercive field. We present a comprehensive discussion of how magnetic properties are modified with increasing Li + content using transmission electron microscopy images and considering the Li + diffusion coefficient. The results suggest that inserting Li + into crystalline ε-Fe 2 O 3 is a useful tool for characterizing crystal structure, lithiation limit, and magnetic properties in the coexistence of Fe 2+ /Fe 3+ .

We synthesized polycrystalline perovskite-type Lii o n -c o n d u c t i n g o x i d e s ( g e n e r a l f o r m u l a : A B O 3 ) , Sr 0.5−x Li 0.3+2x Ti 0.3 Ta 0.7 O 3 (x = 0.030−0.100), and assessed their crystal structure, microstructure, ionic conductivity, and electrochemical stability. Based on first-principles calculations, local structure changes accompanied by Li-ion diffusion were discussed. It was found that the average structure of Sr 0.5−x Li 0.3+2x Ti 0.3 Ta 0.7 O 3 (x = 0.030−0.100) is a cubic perovskite-type one, and at x = 0.042, i.e., Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 , the highest bulk ionic conductivity and the total ionic conductivity at 300 K were observed to be 1.87 × 10 −3 and 1.05 × 10 −3 S cm −1 , respectively, which are greater than those of La 2/3−x Li 3x TiO 3 (LLTO). The first-principles calculations suggested that BO 6 octahedra are distorted, and the Li-ion diffusion is assisted by the dynamic distortion of BO 6 octahedra coupled with the second-order Jahn−Teller effect. The reduction potential of Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 was 1.6−1.7 V vs Li/Li + , which is comparable to that of LLTO. A cell using a Sr 0.458 Li 0.384 Ti 0.3 Ta 0.7 O 3 pellet with a deposited thin film LiCoO 2 cathode on one side was successfully operated as a secondary battery at room temperature, indicating that the compound can be applied as a solid electrolyte for Li-ion batteries.