Direct mechanosynthesis of pure BiFeO3 perovskite nanoparticles: reaction mechanism. Antonio Perejón a, Nataliya Murafab, Pedro E. Sánchez-Jiméneza, José M. Criadoa, Jan Subrtb, María J. Diáneza, Luis A. Pérez-Maqueda*a 5 Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this work, a mechanochemical procedure is proposed as a simple and fast method to synthesize pure BiFeO3 perovskite phase as a nanostructured material without the need for purification treatments, while the mechanochemical reaction mechanism has been investigated and correlated with that of the 10 conventional solid-state reaction. Thus, different milling conditions have been used as a tool for tailoring the crystallite size of the resulting BiFeO3 nanoparticles. The materials prepared by mechanochemical reaction could be annealed or sintered without the formation of unwilling phases. Both the ferroelectric and ferromagnetic transitions were observed by DSC. Finally, the dielectric constants at different frequencies as a function of the temperature for the prepared material have been measured, showing that 15 the material is clearly an isolator below 200ºC, characteristic of a high quality BiFeO3 material. Introduction 45 Many attempts have been made in order to obtain a pure phase in Multiferroics, by the original definition, are materials which bulk of BiFeO by means of numerous different processes, i.e. combine two or more “ferroic” properties in the same phase: 3 solid state reactions,9-14 wet chemical synthesis including sol-gel ferroelectricity, ferromagnetism and ferroelasticity. However, the methods with the use of polymeric precursors,15-20 solution 20 possibility of ferrotoroidic order is included and the classification 21, 22 23 50 combustion methods, mechanical activation, of a multiferroic has been broadened to include antiferroic order.1 mechanochemical synthesis,24-26 hydrothermal synthesis,27-30 Among them, magnetoelectric materials are those which present using molten salts,31, 32 and others, including combinations of an induction of a magnetization by an electric field or a these methods. However, pure BiFeO in bulk has been obtained polarization by a magnetic field. This compounds have attracted 3 only in special synthesis conditions because it is difficult to avoid 25 great attention in recent years, because of their physical 55 the formation of small amounts of impurity phases, mainly properties, that becomes these compounds potential candidates Bi FeO and Bi Fe O ,11, 21 although other phases could appear for technical applications such as data storage, spintronics, 25 39 2 4 9 due to non-suitable preparation methods or an improper control sensors, etc.2, 3 of the experimental conditions. For instance, the Bi Fe O phase Perovskites have been considered as the most promising 36 2 57 has been found in samples prepared by conventional solid state 30 multiferroic materials because many of them are magnetic or 33-35 60 synthesis and in fibers prepared by sol-gel methods. The ferroelectric. However, only a few of stoichiometric perovskites Bi Fe O phase was detected in samples prepared by present both properties at the same time.4 BiFeO is one of these 46 2 72 3 conventional solid state synthesis and in nanoparticles obtained compounds, which presents multiferroic properties at room by sol-gel.34, 36 Both phases were also observed by Yuan et al.37 temperature, making it one of the most studied multiferroics. The phase diagram proposed for the system Bi2O3-Fe2O3 shows 35 BiFeO3 is reported to present a rhombohedrally distorted 65 that BiFeO is formed in a narrow range of compositions and, perovskite structure at room temperature with space group R3c. It 3 even, it has been argued that BiFeO is thermodynamically shows a G-type antiferromagnetic behavior below its Néel 3 metastable in air12, 38, 39 and that its dissociation temperature is temperature (T ~ 360 ºC) and is ferroelectric below its Curie N lower than its formation temperature.40 Researchers have temperature (Tc ~ 830ºC). 5-8 suggested different reasons for the formation of secondary 40 70 phases, but how these phases are formed during the synthesis or a Instituto de Ciencia de Materiales de Sevilla (C.S.I.C. – Univ. Sevilla), during thermal treatments are still under debate. Américo Vespucio 49, Sevilla 41092, Spain Several authors have reported that oxygen vacancies play an b Institute of Inorganic Chemistry AS CR, 250 60 Řež, Czech Republic. important role in the formation of deoxygenated impurities and † Electronic supplementary information (ESI) available. Table 1. Experimental conditions used for the different milling experiments. Rotational Corrected Experimental Jar radius Ball diameter Ball Ball-to powder Power speed impact energy conditions (mm) (mm) material mass ratio (W) (rpm) (mJ hit-1) Hardened 1 700 19.8 15 20:1 60 12.6 Steel Hardened 2 650 23.2 15 20:1 71 20.7 Steel Hardened 3 700 23.2 15 20:1 82 25.8 Steel Hardened 4 700 26.5 15 20:1 109 34.3 Steel Tungsten 5 700 26.5 16 40:1 230 72.6 Carbide Tungsten 6 650 37.7 16 40:1 377 355 Carbide . have found that they are the main cause of low electrical processes, as barium titanate and barium titante composites,49, 50 51 52 resistivity and high porosity in the multi-phase synthesized 45 lead titanate and lead lanthanum zirconate titanate. The 37, 41 37 42 24-26 5 samples. In this sense, Yuan et al and Selbach el al have mechanochemical synthesis of BiFeO3 has been also reported studied the influence of the partial pressure of oxygen in the in literature, but the study of the factors influencing the stabilization of the BiFeO3 phase. They have proposed that the kinematic of the mill on the reaction kinetic as well as the study phase relations near BiFeO3 depends strongly on oxygen pressure mechanism of the reaction induced by mechanical treatment are and by DTA have determined that BiFeO3 is more stable in O2 50 still missing. 10 than in N2 atmosphere, thus a higher partial pressure of oxygen The objective of this work is to explore the mechanochemical could stabilize BiFeO3. synthesis as a general, simple and reproducible method to Recently, it has been reported that nanostructured (0D and 1D) synthesize pure nanostructured BiFeO3 paying special attention to multiferroic materials show improved properties as compared the mechanism of the solid-state reactions involved, the presence with bulk ones. Thus, Chen et al have found that the dielectric 55 of intermediates and the formation of the final BiFeO3 15 constant of BiFeO3 pellets prepared from nanoparticles is much nanoparticles. Additionally, the control of the crystallite size of higher than the values previously reported for ceramics and the products by means of selecting milling conditions will be films.31 Moreover, for this material, it has been described a size- investigated. dependent magnetic behavior. Thus, Park et al have observed increased magnetization values at the nanoscale43 while Experimental section 20 Mazumder et al have reported that nanoscale BiFeO3 depicts not 60 Sample preparation only high saturation magnetization but genuine ferromagnetic behavior with finite coercivity at room temperature.44 Da Silva et The samples were prepared from commercially available oxides: al have shown that BiFeO3 nanoparticles exhibit enhanced Bi2O3 (Sigma-Aldrich 223891-500G, 10 µm, 99.9% in purity) 26 magnetization, enhanced coercivity, and shifted hysteresis loop. and Fe2O3 (Sigma-Aldrich 310050-500G, <5 µm, ≥99% in 25 Additionally, it has been described that the Néel temperature purity). This purity refers to maximum deviation from 19, 45 decreases with decreasing the crystallite size. Finally, it has 65 stoichiometry provided that both iron and bismuth have variable been confirmed the presence of strong multiferroic coupling in valence, but the starting materials used only contain traces of nanometric BiFeO3 and thus those nanoparticles are expected to other metals. Stoichiometric amounts of the oxides were 45 be more useful for magnetoelectric device applications. mechanically treated under different milling conditions, i.e. 30 Mechanochemical processes produce nanostructured materials milling energy, reaction time and grinding atmosphere. Two by using mechanical energy to activate chemical reactions and 70 different planetary mills were used, i.e. Pulverisette 7 (Fritsch) structural changes as well as particle reduction.46 Thus, this has and PM100 (Retsch). Both mills were modified by incorporating become a popular method to make nanocrystalline materials a rotary valve that allows connecting the stainless steel jar (sealed because of its simplicity, the relative inexpensive equipment with a Viton o-ring and equipped with a male taper straight 35 needed and the applicability to essentially all classes of materials, adaptor) and the gas cylinder during milling experiment. Thus, existing the possibility to scale up to tonnage quantities of 75 the pressure inside the jar could be maintained constant during material for various applications.47 It has been shown that, by the entire treatment even if the gas is consumed in the reaction. In high energy mechanochemical milling process, some ferroelectric all experiments, the jars were purged several times with the materials can be synthesized directly from their oxide precursors selected gas and then the desired pressure was selected and 40 in the form of nano-sized powders, without the need for maintained during milling. The influence of the atmosphere in the calcination at intermediate temperatures, thus making the process 80 grinding products was investigated by performing experiments very simple and cost-effective.48 Various ferroelectric compounds under static air, nitrogen and oxygen. have been produced via the high-energy mechanical milling To study the influence of milling conditions on the reaction of formation of BiFeO3, different experiments were performed using has been avoided because it is well known that platinum and 55, either stainless steel balls or tungsten carbide balls (table 1).