(La3+ Mg2+) codoped BiFeO3 nanopowders: Synthesis, characterizations, and giant dielectric relaxations

摘要

A new strategy to improve the dielectric properties of BiFeO3聽is proposed by codoping with La3+聽and Mg2+聽to control the ceramic microstructure and increase the dielectric permittivity (e垄), respectively. The main phase of BiFeO3聽is obtained in nanocrystalline powders of LaxBi1-xFe1-xMgxO3聽(x聽= 0, 0.05 and 0.1), which are prepared by a chemical co-precipitation method. The particle size of the codoped LaxBi1-xFe1-xMgxO3聽is smaller than that of the BiFeO3. A dense ceramic microstructure without porosity is obtained by sintering at 800 掳C for 3 h. The mean grain size of the BiFeO3聽ceramics decreases with increasing codoping (La3+-Mg2+) concentration. The primary roles of La3+聽and Mg2+聽are to suppress the grain growth and enhance the densification rate, respectively. At 1 kHz, the e垄 of the LaxBi1-xFe1-xMgxO3聽with聽x聽= 0.1 increased significantly compared to that of the BiFeO3, while the loss tangent (tand) was lower than that of the BiFeO3. In addition, another role of Mg2+聽is to increase the e垄 without any effect on the tand. Two dielectric relaxations are observed in low-frequency (150-250 K) and high-temperature (250-400 K) ranges. An X-ray photoelectron spectroscopy shows that the Fe2+/Fe3+聽ration in the codoped LaxBi1-xFe1-xMgxO3聽increased compared to that of the BiFeO3, corresponding to the increase in e垄. Thus, a low-temperature dielectric relaxation is attributed to the electron hopping between Fe2+-O-Fe3+. On the other hand, a high-temperature dielectric relaxation is caused by interfacial polarization relaxation.