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      Introduction

      Rare-earth multiferroic (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) is interesting because it is a potential candidate for use as a functional element in information storage devices, spintronics, magnetoelectric sensors, etc. Such applications are based on the presence of magnetic and ferroelectric sublattices in YbMF and the possibility of cross-infuence on them by electric and magnetic felds, respectively. The initial (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) component crystallizes in the hexagonal syngony (hereinafter h-YbMO), while the second YbFeO3 component crystallizes in the hexagonal (hereinafter h-YbFO) and orthorhombic (hereinafter o-YbFO) syngonies. Hexagonal h-YbMO is a multiferroic with a ferroelectric phase transition temperature Tc=993 K. The temperature of the antiferromagnetic phase transition difers in diferent works: According to, the antiferromagnetic transition in these manganites occurs at TN ≈ 70—130 K, according to other sources, TN is 81 K and 82 K, respectively. The stable orthorhombic phase o-YbMO can be obtained by heating the hexagonal h-YbMO under a pressure of 5 GPa and a temperature of 1100° C [14] or by quenching under high pressure. According to, the magnetic phase transition occurs at TN =43 K. Hexagonal h-YbFO is also multiferroic. At room temperature, its symmetry space group is P63cm, and it is characterized by two ferroelectric phase transitions. The transition temperature from the paraelectric to ferroelectric phase is Tc1=470 K, and the transition between the ferroelectric phases occurs at a temperature of Tc2=225 K. The Néel temperature is TN =120 K. The symmetry above 470 K is described by the space group P63cm. At temperatures below 1000 K, ferroelectricity in h-RMnO3 and h-RFeO3 is induced by lattice distortion with a tilt angle (θ) of the FeO5 (or MnO5) trigonal bipyramid. In the orthorhombic phase, o-YbFO is an antiferromagnet with a Néel temperature TN =627 K and is described by the symmetry space group Pbnm. According to, no ferroelectric properties were found in o- (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb). At room temperature, the structure and physical properties of rare-earth manganites (ReMnO3) and rare-earth orthoferrites (ReFeO3) change depending on the radius of Re3+ ions. ReMnO3 compositions with a greater Re3+ ionic radius than Dy (1.05 Å) have an orthorhombic distorted perovskite-type crystal structure, whereas those with a smaller Re3+ ionic radius than Dy have a hexagonal crystal structure. (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) can crystallize in diferent space symmetry groups depending on the Fe/Mn molar ratios; therefore, the physical properties of (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) can be purposefully changed over a wide range. It was discovered that the Néel temperature TN of hexagonal YbMnO3 manganites doped with Fe3+ ions increases from 81 to 116 K (for YbMn0.45Fe0.55O3), and YbMn0.45Fe0.55O3 crystals exhibit antiferromagnetic properties. In addition, its magnetization M (H) is more than 100 times higher than that of YbMnO3. The Mössbauer spectra of the hexagonal and orthorhombic phases of the YbMn1—xFexO3 compounds were studied in, and the magnetic susceptibility was studied during zero feld cooling (ZFC). However, YbMn1-xFexO3 compositions have not been studied by dielectric, FTIR and optical absorption spectroscopy over the entire concentration range; and there is practically no data on their magnetodielectric properties. There are no data on the morphotropic region (MR) of these compositions. In order to fll this gap, such measurements are carried out for the frst time. The aim of this work was to study the structural-phase state and physical properties of the YbMn1-xFexO3 compositions, as well as establish the «structure—property» relationship using complex physical methods [1—5].

      Synthesis, sample preparation and measurement apparatus

      The (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) compositions studied in this work were obtained by the solid-phase reaction method using the oxides Yb2O3, Mn2O3 and Fe2O3 (Yb2O3≥ 99.9%, Mn2O3≥ 99.9%, Fe2O3≥ 99.9%, Sigma-Aldrich). Eleven diferent molar fractions (x=0—1 with a step of 0.1) were mixed and ground in an agate mortar for 3 h in the presence of ethanol. After that, each mixture was made into a cylinder of 6 mm in diameter and 1 mm in thickness under a pressure of 120 MPa. The compositions were then placed in a closed platinum crucible. The synthesis was performed in a high-temperature furnace with thermal stabilization at 1250° C for 4 h in an air atmosphere, after which the furnace was switched of and cooled by inertia to room temperature. To obtain a ceramic sample, each pre-synthesized sample was ground in an agate mortar for 0.5 h in the presence of ethanol, then pressed into a cylinder 6 mm in diameter and 1 mm thick under a pressure of 200 MPa. The sintering of ceramics was carried out in the furnace with thermal stabilization at 1150° C for 4 h in an air atmosphere. For electrophysical measurements, silver paste electrodes were applied to both sides of the sample and dried at 750° C for 10 min. Phase analysis of the as-synthesized samples was performed on an X-ray difractometer (D2 Phaser, Bruker, Germany) using Cu Kα radiation with a step Δ2θ=0.01º and a data acquisition time τ=1 s. The microstructure of the samples was observed and examined by a scanning electron microscope (LEO EVO 40 XVP; Carl Zeiss AG, Germany). Optical absorption spectra were studied on a Shimadzu UV-2600 two-beam spectrophotometer at room temperature. The magnetic properties were studied on a vibrating sample magnetometer (VSM, LakeShore 7404, USA) at room temperature. The dielectric properties were analyzed using a High-Performance Dielectric Analyzer (Alpha-A, Novocontrol Technologies, Germany). The magnetodielectric and magnetoresistive properties were studied under a current magnetic induction feld up to 2 T and a laboratory attachment for samples with temperature control from the boiling point of nitrogen to 900 K.

      Structural characterization

      Electron micrographs of the cleavages of (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) (x=0.0—1.0) ceramics are shown in Fig. 1. As shown in Fig. 1, the ceramics are rather porous, and there are no visual diferences in the habits of crystallites of diferent compositions; they have rounded shapes. The average size distribution of crystallites is in the range of 1—2 μm. The density of the ceramics was evaluated by hydrostatic weighing, and the pore concentration was determined from the difference between the x-ray density and the density obtained by hydrostatic weighing. Were obtained at room temperature for each concentration of the (1-х) АFe₁₂O₁₉ – xBTiO₃ (A-Ba, Pb, Cd; B-Ba, Pb) ceramics. Rietveld full-profle analysis was used to process the difraction profles. As shown in Fig. 1 (a), increasing the molar concentration of Fe3+ to x=0.5 leads to a decrease in the parameter ah, and the parameter ch increases in the concentration range x=0.0—0.8. X-ray difraction patterns show that Yb (Mn, Fe) O3 solid solutions of hexagonal syngony form in the range x = 0.1—0.5, with a general trend in D and Δd/d (see Fig. 3 (b)). At the point x=0.5, the crystal lattice is deformed due to the formation of an additional phase of a diferent system. Starting from x=0.5, not only ah increases, but also the parameters of the orthorhombic phase. Apparently, the concentration of the second phase at the point x=0.5 is low, and it is not X-ray resolved under our conditions. Therefore, we called the interval x=0.6—0.8 the morphotropic region (MR), as in ferroelectrics. Although it would be more correct to start this interval with x=0.5. In the MR region, there is a coordinated change in the parameters of coexisting phases. As can be seen, in the region where solid solutions are formed, D and Δd/d decrease. This situation is possible if the ionic radii of the substituted ion and the substituent are approximately equal.

      Fig. 1. Dependencies of the unit cell parameters of the hexagonal and orthorhombic phases (a), the sizes D and the microstrains Δd/d (b) of the

      In our case, this condition is met. The linear parameters of unit cells in the two phases increasing with x is a characteristic of the two phases coexistence region (x=0.6—0.8). In the orthorhombic phase, the parameters bo and ao that decrease as x→1.0 and increase as x increases. In this phase, the changes in D and Δd/d have a general trend.

      References

      1. S.L. Samal,

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