Synthesis and structure of BiFeO 3 :RE (RE=Gd 3+ , Dy 3+ , Nd 3+ ) ceramics

. In the present work the influence of rare earth elements concentration (0-10at-%) on BiFeO 3 :RE (RE=Gd 3+ , Dy 3+ , Nd 3+ ) ceramics were studied. All ceramic powders were synthesized by conventional ceramic method using high purity raw materials (>99,9%), and subsequently sintered by free sintering and cold pressing method. To analyze the powders and ceramics more the XRD, EDS, SEM, and DTA were performed.


Introduction
Multiferroic materials are the subject of intensive research during recent years, due to their interesting physical properties [1], as well as the great technological potential of the materials for microelectronics and spintronics [2,3].In these materials in which ferro/antiferromagnetic and ferroelectric properties occur simultaneously.Bismuth ferrite and its materials are attracting great attention owing to both great technological potential and the interesting physics behind their functional properties [4].BiFeO3 ceramic is an interesting candidate due to its high ferroelectric curie temperature (TC=850C) and antiferromagnetism below Neel temperature (TN=370C) [5,6].These characteristics are unrepeatable, taking into account that most magnetic ferroelectrics possess coexisting spin and dipole order only well below room temperature.At room temperature, BiFeO3 single crystal has distorted rhombohedral (R) structure with lattice parameter of (ar=3.96Åand ar=89.41Å) [7].The magnetic ordering is G-type with a weak canting moment with a 62 nm spin cycloid [8].When discussing the properties of bismuth irons, it should be noted that information on the BiFeO3 formations and temperature stability limits remains ambiguous.Despite the fact that bismuth ferrite is characterized by very good physical properties, attempts are made to improve the ferroelectric and ferromagnetic properties.One possible strategy for obtaining improved properties in BiFeO3 is partial ionic substitution.Additionally, the pure, single-phase bismuth ferrite is typically very difficult to obtain via solid-state reaction of Bi2O3 and Fe2O3 [9].During its preparation additional (impurity) phases such as Bi2Fe4O9, Bi36Fe24O57 or Bi25FeO39 appear very often [10,11].Lomanova et al [12] reported the possibility of Bi2Fe4O9 and Bi25FeO39 formation during BiFeO3 synthesis and it was shown to be dependent upon the quality of the initial reagents [19].They argue that not enough pure precursors can result in the formation of the above-mentioned phases and to their stable existence as impurities during BiFeO3 formation.Other authors postulate that it is difficult to synthesize the single-phase BiFeO3 because Bi2Fe4O9 and Bi25FeO39 are thermodynamically more stable than BiFeO3.It has been observed that the partial substitution of Bi atoms by elements like Eu, La, Nd, Sm, Tb [13,14,15] allows to eliminate the impurity phases.The short review of the results of experimental investigations of bismuth ferrite shows the necessity of performing more comprehensive and deeper research.Special attention should be paid to rare-earth substituted compounds, whose properties remain poorly known what can be stated after reading the available literature.Aim of this study was to synthesize and fabricate Bi1-xAxFeO3 ((A = Nd, Gd, Dy, for x = 0.03, 0.05, 0.07, 0.1) ceramics by solid state reaction.By means of simultaneous thermal analysis (TG / DTG, DTA) and X-ray diffraction analysis the process of synthesis of Bi1-xAxFeO3 ceramics has been studied.

The technology of the PFN material
The preparation and synthesis process of undoped and Gd 3+ , Dy 3+ and Nd 3+ co-doped BiFeO3 powders was performed through the solid state reaction, by the conventional ceramic method.Stoichiometric amounts of high purity oxdides powders, Bi2O3 (Sigma -Aldrich, 99,9%), Fe2O3 (Sigma -Aldrich, 99,9%), Nd2O3 (Sigma -Aldrich, 99,9%), Gd2O3 (Sigma -Aldrich, 99,9%), Dy2O3 (Sigma -Aldrich, 99,9%) were weighed according to the nominal composition of Bi1-xAxFeO3.The appropriate quantities of reagents were weighed according to the formula ( 1): (1) The high purity (99.9%) oxide powders were ground homogeneously in the mortar at room temperature for 1 h.Next, these mixtures were subjected to grind in a ball mill for 24 h using YTZ balls as grinding media in ethanol solution.The dried mixture of powders was compacted into pellets of 20 mm in diameter by pressing under pressure of p=60 MPa in a stainless -steel die.The synthesis was carried out at T =800 0 C in corundum crucible with air atmosphere for 3h.Calcined material were remilled for 24 h to reduce the particle size, and then cold pressed into pellets (d = 10 mm, p = 30 MPa).Then the material was sintered in air at the temperature of T = 860 0 C for 3h.The flowchart of the complete fabrication process is shown in Fig. 1.
Fig. 1 The flowchart of fabrication process of Bi1−xAxFeO3 ceramics.

Characterization
Microstructure and chemical composition of the final ceramics were investigated by scanning electron microscope (SEM) JSM -7100F equipped with an energy dispersive spectrometer (EDS) NORAN Vantage.The microscope was operating at 15 kV acceleration voltage.Structure of the samples was investigated using PANAlytical X-Pert Pro difractometer with Cu lamp (λ = 0.154 nm).X'Pert HighScore Plus computer program equipped with the ICDD PDF2 data base was used to phase analysis and Rietveld refinement of the structure.The densities of the sintered pellets were measured by Archimedes' principles.Parameters of the thermal treatment were determined by simultaneous thermal analysis (DTA/TG/DTG).Simultaneous measurements were executed by heating the dried powders in air at 10°C/min.The test specimens were prepared in powder form, the reference material was aluminum oxide Al2O3.The simultaneous thermal analysis method enables to determine the optimum synthesis temperature.

Density
Fig. 2 shows the density of obtained ceramic samples Bi1−xAxFeO3.Density of the samples is closely related to the type of dopant and its content in the material.For the dopant neodymium and dysprosium it can be concluded that with increase of the dopant content in the studied ceramics its density rises too.Vice versa in the case of dopant gadolinium: with increase of the dopant content the density of the ceramic decreases.Fig. 3 Differential thermal analysis (DTA) curves for stoichiometric mixtures of oxides used for synthesis of Bi0.9Nd0.1FeO3.

X-ray diffraction analysis
The results of XRD measurements are presented in Fig. 5. Diffraction lines corresponding to impurity phases (Bi2Fe4O9) were marked by symbols whereas all other lines belong to the main phase, i.e.Bi0,9A0,1FeO3 solid solution with a suitable dopant marked on the image.Analysis of the X-ray diffraction patterns of the ceramic powders was carried out using a computer program PowderCell [16].Refinement of the structural parameters of Bi0.9A0.1FeO3solid solutions was performed with the Rietveld method e.g.[17,18].The angular position of diffraction lines agree well with positions for rhombohedral BiFeO3 phase given in ICDD 01-082-1254 card.A small content of Bi2Fe4O9 was also noticed (99-100-8872).
On the basis of analysis of diffractograms it was concluded that the structure of the obtained solid materials was rhombohedral (R3c space group).Also the weight fraction of phases from diffractograms was estimated.Diffraction lines coming from impurity phases are rather small.The occurrence of weak lines for 2θ = 27 o isn't connected with the BiFeO3-type structure and rather caused by the presence of a vestigial quantity of Bi2Fe4O9.For the content of neodymium x=0.1 the concentration of the main phase, i.e.Bi0.9Nd0.1FeO3, is about 93 wt.%.The rest are impurity phases, namely Bi2Fe4O9 in amount 7 wt.%.For x = 0.1 of dysprosium the concentration of the main phase, i.e.Bi0.9Dy0.1FeO3, is about 97 wt.%, whereas the impurity phases, namely Bi2Fe4O9 in amount 3 wt.%.The weight fraction of phases from diffractograms for the x=0.1 gadolinium are the same like a Bi0.9Dy0.1FeO3(as shown in Fig. ).In another author's work on the influence of rare earth ions on BiFeO3, it is shown that a higher neodymium dopant concentration results in a single phase material [19].
Results of calculations of elementary cell parameters as well as agreement indices are given in Table 1.The determined lattice parameters from the number of atomic (dopants) are presented in Fig. .The linear drop in the values of a lattice constant may be observed while c parameter increases with the increase of the atomic number.

Microstructural and EDS tests
Using scanning electron microscopy analysis microstructure was performed and the chemical compositions of obtained materials were determined.SEM pictures of Bi0.97A0.03FeO3ceramics obtained by pressureless sintering from stoichiometric mixtures of oxides are shown in Fig. 6.Analyzing the SEM image it can be noted, that addition of the individual dopants did not affect the packing of the beans.The shape of the grains changed.The distribution of all elements was investigated with Energy Dispersion X-ray spectrometer (EDS) and carried out for randomly selected areas.Obtained results are very close to the calculated stoichiometric ratio for each prepared material.The small deviations from the theoretical composition have occurred but they do not exceed a value of 2.7%, what is consistent with the resolution of the utilized method of investigation.

Conclusion
By means of the mixed oxide method followed by pressureless sintering Bi1−xAxFeO3 (A = Nd, Gd, Dy, for x = 0, 0.03, 0.05, 0.07, 0,1) ceramics was successfully fabricated from stoichiometric mixture of Bi203, Fe2O3 and Nd2O3/Gd2O3/Dy2O3 powders, via the solid state reaction route.The thermal analysis method enables to determine the optimum synthesis temperature.The temperature of synthesis was chosen as T=800C.The density of ceramic samples depends on the type and content of dopant.Increase of the content of neodymium and dysprosium increases the density of the obtained materials, but with increasing of gadolinium dopant the density of the ceramics decreases.Analyzing the SEM image it can be noted, that addition of the individual dopants did not affect the packing of the beans.The shape of the grains changed.It was found that Bi1−xAxFeO3 ceramics exhibited rhombohedral symmetry with R3c (36) space group.

Fig. 2 3 . 2
Fig. 2 Effect of dopant content on the density of ceramic samples.