Mechanosynthesis and characterization of Bi1-xLaxFeO3 with 0 ≤ x ≤ 0,6

A. A. CRISTÓBAL; PAULA G. BERCOFF; CINTHIA P. RAMOS; SUSANA CONCONI; PABLO M. BOTTA

Simposio; ISMANAM 2013; 2013

Universita di Torino

This work deals with the synthesis of Bi1-xLaxFeO3 (0 ≤ x ≤ 0,6) induced by a high energy ball-milling of Bi, La and Fe chlorides (Lewis? acids) with sodium hydroxide. This method was originally developed to prepare a variety of single oxides [1]. However its application to the synthesis of mixed oxides is very uncommon. BiFeO3 is a well-known compound where both ferroelectric and ferromagnetic orderings combine. The continuous replacement of Bi by La has shown to improve the electrical properties of BiFeO3 [2]. The mechanochemical solid-state reaction occurred at room temperature, yielding nanocrystalline Bi1-xLaxFeO3 (BLFO) and NaCl as a byproduct. After mechanical activation for 9h, the solids are heated at 600ºC in order to enhance the crystallinity of the formed mixed-oxide. An ulterior washing and filtering operation removes the saline byproduct, leaving Bi1-xLaxFeO3 particles free of impurities. The samples were characterized by XRD (Rietveld refinement), Raman spectroscopy, HRTEM, dynamic scanning calorimetry (DSC), ifferential thermal analysis (DTA), Mössbauer spectroscopy (MS) and vibrating sample magnetometry (VSM). XRD patterns showed a shift of the main BLFO peaks to higher angles, as a consequence of the unit cell contraction produced by the replacement of Bi by La. For x=0.6 the diagrams evidenced additional peaks corresponding to secondary phases, indicating that the formation of BFLO was not complete. TEM images showed a nanocrystalline system with particles? agglomerates of micrometric size. DSC analyses revealed a thermal event attributed to an antiferromagnetic-paramagnetic transition (Néel temperature). For the undoped compound this transition took place at 372ºC, and it gradually increases as La content grows. At higher temperatures (from 800ºC on), DTA curves showed a wide endothermic band, which was assigned to the ferroelectric-paraelectric transition (Curie temperature) associated with a structural change in BLFO. Raman spectroscopy seems to confirm this hypothesis, since a remarkable change in the vibrational modes pattern was observed for samples with x ≥ 0,1 suggesting that the structural transition occurs at low levels of La. The relationship between composition, crystalline structure and magneto-electric behavior of Bi1-xLaxFeO3 materials is discussed.