FORMATION OF COMPOUNDS BASED ON BISMUTH FERRITE WITH PARTIAL SUBSTITUTION OF BISMUTH IONS BY LANTHA NUM AND PRASEODYMIUM

Using the solid-phase reaction method we obtained b ismuth ferrite based compounds generated by partial substitution of bismuth ions with rare earth metal ions (lanthanum and praseodymium). We used th ermogravimetric analysis and X-ray analysis at the temperature range from 29 7 to 1123 K to study the peculiarities of the processes of phase formation of reaction-products synthesis in the systems which contain oxides of iron, bismuth, lanthanum, and praseodymium; their compositions were also determined. For t he final synthesis temperature of 1123 K we determined a homogeneous concentr a ion area, where bismuth ferrite solid solutions with the structure of distorted perovskite exist. We studied the influence of lanthanum and praseodymium oxides on phase composition of bismuth ferrite compounds in the process of isovale nt alloying of a part of bismuth ions by lanthanum and praseodymium ions. In the frequency range of 20 Hz – 1 MHz we determined the key parameters, whi ch characterize electrical and magnetic properties of perovskite-like phases. We found that for the considered samples the frequency dependencies of complex magnetic permeability and electrical parameters (real and alleged part of per mittivity) are of almost the same type. We showed that for the given frequency r ange both electrical (complex permittivity) and magnetic (complex magnetic permeability) parameters decrease with the frequency increase.


Introduction
Multiferroic materials are inorganic compounds, which have both ferromagnetic and ferroelectric features. Such compounds can be used to create magnetoelectric materials, which can operate in various fields of microelectronics and microwave technology (e. g. extra-high density storage devices: magnetic memory, logical components, magnetic field sensors [1][2][3]). Therefore, the synthesis of new multiferroic materials attracts the great interest of researchers.
Bismuth ferrite (BiFeO 3 ) is a well-known multiferroic compound, in which dipole ordering occurs near 1100 K and antiferromagnetic ordering -near 640 K. Hence, it can be used for the room temperature applications [4]. Besides, on the base of bismuth ferrite it is possible to create, using sol-gel processing and solid-state synthesis, solid solutions with the broad homogeneous region by double isovalent alloying of a part of Bi 3+ ions by rare earth metal ions (Ln 3+ ), and of the same amount of Fe 3+ by Со 3+ ions. This modification can increase the possible number of application of such compounds [5]. However, the synthesis of monophase ceramic BiFeO 3 in the system Bi 2 O 3 -Fe 2 O 3 is hindered by the formation of transitional crystalline phases Bi 2 Fe 4 O 9 and Bi 25 FeO 39 .
So, the intent of our work was to investigate the formation of bismuth ferrite solid solution in systems Bi 2 O 3 -La 2 O 3 -Fe 2 O 3 and Bi 2 O 3 -La 2 O 3 -Pr 6 O 11 -Fe 2 O 3 under heating in air. We considered the influence of lanthanum and praseodymium oxides on the phase composition of bismuth ferrite compounds in the process of isovalent alloying of a part of Bi 3+ ions by La 3+ and Pr 3+ ions. We aimed to determine main parameters, which characterize magnetic and electrical properties of the obtained compounds.

Experiments
We synthesized polycrystalline samples of solid solutions Bi 1-x La x FeO 3 (х = 0; 0,05; 0,1; 1,0) and Bi 0,9-x La 0,1 Pr x FeO 3 (х = 0; 0,05; 0,1; 0,7) by solid-phase reaction from chemically pure oxides Bi 2 O 3 , Fe 2 O 3 , La 2 O 3 , and Pr 6 O 11 . Oxides of rare earth metals (La 2 O 3 and Pr 6 O 11 ) were thermally annealed at 1273 K for one hour. Mixtures, prepared according to preset quantities of starting reagents, were mixed and thoroughly ground in the agate mortar for 30 minutes with the addition of ethanol. The obtained alcohol-containing load was pressed under the pressure of 50 MPa to cylinders with the diameter of 8 mm and thickness of 1-4 mm. The cylinders were thermally annealed at 1073 K in air for 3 hours. The synthesis of investigated compounds in air was made in the incinerator at Т 1 = 650 K (3 hours), Т 2 = 833 K (3 hours), Т 3 = 1093 K (3 hours) и Т 4 = 1123 K (6 hours) until the mass of the samples did not change.
We calculated the content of the obtained reaction products by weighting on the analytic balance of 0,05 mg sensitivity.
Thermogravimetric measurements were done dynamically in air for broad temperature range (297-1123 K), using the thermogravimetric analyzer "Derivatograph Q-1000" (Paulic-Erdey system) at the heating rate of 10 degrees in one minute.
After every isothermal exposition we examined the qualitative phase composition of the reaction products by the diffractometer DRON-3, using the standard procedure for polycrystalline powder: monochromatic CuKα 1 radiation, diffraction angle 2θ from 13° to 80°. The synthesized compounds were identified from the International Center for Diffraction Data database (JCPDS-ICDD).
Complex permittivity was calculated using the values of capacity frequency profile and loss angle (alternating current), which were measured in air at room temperature and the frequency range from 20 Hz to 1 MHz. Measurements were done using the meter RLC AKTAKOM AM-3028 (double contact method). Before that we coated the end faces of the cylinders with silver electrodes by baking of a silver conductive paste. We did not take into account errors which stem from the edge electric field and the field irregularities on the electrode surfaces because they are small.
To investigate a frequency profile of complex permeability we used the technique of a solenoid partial filling. We measured the inductance and the Q-factor of the coil with the sample inside. The instrument error was estimated taking into account the coil inductance (without the samples) and losses connected with the coil resistance and eddy currents in it.

Results and discussion
Solid solutions of BiFeO 3 -LnFeO 3 are broadly investigated binary systems, where phases of BiFeO 3 and LnFeO 3 crystallize in the rhombohedral structure and the orthorhombic structure of perovskite type, respectively [5]. Isovalent alloying of a part of Bi 3+ ions by La 3+ and Pr 3+ ions leads to the formation of solid solution Bi 0,9-x La 0,1 Pr x FeO 3 , in which there is an orthorhombic distortion of perovskite unit cell. The unit cell parameters are determined for the BiFeO 3 phase (а = 3,959 Å and α = 89°46′).
From the analysis of X-ray patterns of Bi 1-x La x FeO 3 samples, synthesized by solid-phase reaction, the authors [5] noted that the quantitative content of mullite (Bi 2 Fe 4 O 9 ) and sillenite (Bi 25 FeO 39 ) phases increased with the increase of concentration x, starting from the molecular ratio x > 0.1. This indicates the thermal instability of solid solutions based on bismuth ferrite. However, by X-ray diffraction analysis we showed that solid solutions Bi 1-x La x FeO 3 (х = 0; 0,05; 0,1; 1,0) and Bi 0,9-x La 0,1 Pr x FeO 3 (х = 0; 0,05; 0,1; 0,7) had the crystal structure of perovskite with an orthorhombic distortion (see Fig. 1). Moreover, the percentage of impurity phases Bi 2 Fe 4 O 9 and Bi 25 FeO 39 did not exceed 3 % at all investigated concentrations x.
Using thermogravimetric analysis we showed that the formation of bismuth ferrite and its derivatives was accompanied by a multistage decrease of samples masses in the broad temperature range (297 -1123 K), which indicates that physical and chemical processes in the considered systems were staged (see Fig. 2). The differential thermogram (DT) of the base mix [Bi 2 O 3 -La 2 O 3 -Fe 2 O 3 ]·nH 2 O had several peaks in low-temperature (520-710 K) and high-temperature regions (780-1123 K), which could be the evidence of change in the oxidation state of the metals in forming compounds (Fig. 2, b). Fig. 1. X-ray patterns of samples, obtained using solid-phase reaction (T = 1123 K): a -BiFeO3; b -Bi0,8La0,2FeO3; c -Bi0,8La0,1Pr0,1FeO3; d -LaFeO3. All samples have the crystal structure of perovskite with an orthorhombic distortion

Fig. 2. Thermogram (a) and differential thermogram (b) of the base mix [Bi2O3-La2O3-Fe2O3]·nH2O
Low-temperature processing of compounds (at 650 K) did not change the phase composition of samples. It can be seen from the XRD patterns with reflexes of the initial reagents of the corresponding oxide systems (see Fig. 3, a). The rise of temperature up to 833 K contributed toward the formation of crystalline phases in the synthesized powders, which can be seen from the appearance of fuzzy diffraction peaks (see Fig. 3, b). These peaks correspond to the reflexes of BiFeO 3 phase. Further increase of fusion temperature (in the range 833-1093 K) resulted in the recrystallization process and formation of additional small peaks, which correspond to phases of Bi 2 Fe 4 O 9 and Bi 25 FeO 39 (Fig. 3, c).
So, by the experimental thermogravimetric and qualitative X-ray diffraction analysis we showed that the formation of bismuth ferrite and its derivatives starts at temperature ~ 833 K, and at T > 833 K BiFeO 3 decomposes with the formation of impurity phases. The number of these impurity phases did not rise with the increase of degree of substitution of lanthanum and praseodymium in the synthesized compounds.
Complex permeability of the considered samples had the similar type of the dependence on frequency as electric parameters (the real and the imaginary parts of permittivity). In the frequency range 20 Hz -1 MHz the real part of permeability µ' decreased with the frequency increase. The maximum value of µ' (~ 496) was obtained in the compound Bi 0,9 La 0,1 FeO 3 .

Conclusion
We determined homogeneous concentration region, where solid solutions Bi 1-x La x FeO 3 and Bi 0,9-x La 0,1 Pr x FeO 3 with the crystal structure of perovskite with an orthorhombic distortion exists. We found that the formation of bismuth ferrite and its derivatives starts at temperature ~ 833 K, and at T > 833 K BiFeO 3 decomposes with the formation of impurity phases of mullite (Bi 2 Fe 4 O 9 ) and sillenite (Bi 25 FeO 39 ). The number of these impurity phases does not rise with the increase of degree of substitution of lanthanum and praseodymium in the base compound BiFeO 3 . We showed that in perovskite-like phases in the frequency range 20 Hz -1 MHz both electrical (complex permittivity) and magnetic (complex permeability) parameters decrease with the frequency increase. The maximum values of permittivity ε' and permeability µ' were obtained in the compounds Bi 0,7 La 0,1 Pr 0,2 FeO 3 and Bi 0,9 La 0,1 FeO 3 , respectively.