Synthesis and structural characterization of Fe1-xMexBO3 (Me = Al, Sc) single crystals

Fe1-xMexBO3 (Me = Al, Sc) single crystals have been synthesized by flux growth technique using B2O3-PbO-PbF2 solvent and approach of identical synthesis conditions. The contents of the diamagnetic ions Me in the initial charge of both compositions were equal, xcharge=0.05. The exact contents and crystal lattice parameters of the synthesized crystals were determined by X-ray fluorescence analysis and X-ray diffraction, respectively.


Introduction
The creation of new materials with predetermined properties allows one to improve modern technologies, as well as to develop completely new scientific approaches for solving fundamental problems of solid-state physics. One of the most advanced ways to fine-tune different properties of existing materials is the isomorphous substitution method.
Iron borate, FeBO3, is a two-sublattice easy-plane trigonal antiferromagnet with weak ferromagnetism [1]. It possesses a calcite-type structure of 3 Rc space group and has several isostructural diamagnetic analogues MeBO3 (Me = Ga, Al, Sc, In), where Me 3+ ions occupy the same structural positions as Fe 3+ ions in FeBO3 [2][3][4]. Undoubtedly, using diamagnetically diluted Fe1-xMexBO3 single crystals, in which a part of paramagnetic iron ions is isomorphically substituted by diamagnetic ions, is suitable for studying the nature of the magnetic properties of pure FeBO3 [5,6]. This approach allows one to study different mechanisms that stand behind certain properties of the crystal, as well as to create materials with predetermined characteristics. For instance, the partial isomorphous substitution of Fe 3+ by Ga 3+ in FeBO3 crystals (with the formation of Fe1-xGaxBO3 crystals) allows one to study the transformation of magnetic properties under the transition from magnetically ordered to diamagnetic state, as well as modifies such magnetic characteristics of FeBO3 as the Dzyaloshinskii-Moriya field and the Néel temperature [6,7]. The major drawback of such isomorphous substitutions is that impurity ions significantly reduce the degree of the structural perfection of the crystals [8]. Thus, the search for new technological approaches for synthesizing Fe1-xMexBO3 single crystals with different diamagnetic ions that would have a negligible influence on structural perfection is of paramount importance.
In the present work, it is proposed to synthesize Fe1-xMexBO3 single crystals with low contents of Al 3+ and Sc 3+ ions that allow a slight decreasing the Néel temperature in comparison with pure FeBO3. Such diamagnetically diluted high-perfection crystals are of great importance for further practical applications in synchrotron technologies of a new generation [9]. The ionic radii of sixfoldcoordinated Fe 3+ , Al 3+ and Sc 3+ are 0.645, 0.535 and 0.745 Å, respectively [10]. Complete isomorphism is possible if the ionic radius of an impurity ion differs by no more than 15% from those of substituted ion [11]. In our case, the difference is up to 20% that probably indicates limited isomorphism. Moreover, in as much as the ionic radius of Al 3+ is smaller concerning the ionic radius of Fe 3+ , and Sc 3+ is larger; the probability of Fe/Al substitutions is bigger than that of Fe/Sc. In order to check this assumption, we propose (i) to synthesize Fe1-xMexBO3 (Me = Al, Sc) single crystals with a fixed contents of the diamagnetic ions in the charge, xcharge, under identical synthesis conditions; (ii) to determine and analyze the exact crystal composition and crystal structure.

Experimental
The synthesis of Fe1-xMexBO3 single crystals was carried out by the flux growth technique using a boron-lead solvent: B2O3 -PbO -PbF2 [12]. This technique allows one to obtain iron borate based single crystals in the form of hexagonal plates of high structural perfection.
It is supposed to obtain crystals with low x-values, thus, the charge composition and temperature mode of crystallization for the synthesis of pure FeBO3 were chosen as the initial [13].
The extraction of the synthesized crystals from the cooled crucibles was carried out by boiling in a 20% solution of nitric acid.
The exact contents of aluminium and scandium in the synthesized crystals have been determined using X-ray fluorescence analysis (XRF) analysis with a high-power benchtop sequential wavelength dispersive X-ray fluorescence spectrometer Rigaku Supermini200.
The crystal phase and parameters of the crystal lattice of the synthesized crystals were determined by X-ray diffraction (XRD) analysis with a diffractometer using cobalt radiation Co Kα in the angle range of 10° < 2θ < 130° with the step of 0.01°.

Results and discussion
Charge compositions, see Table 1, were calculated in such a way that the contents of the diamagnetic ions in the charge, xcharge = 0.05 and is the same for synthesizing Fe1-xMexBO3 (Me = Al, Sc) single crystals. The conditions of the preparation of the solution-melts -dehydration of the reagents, homogenization -were identical for both compositions; and both crucibles had the same shape and volume. In order to reduce the influence of such external factors as evaporation of the solvent and contamination of the solution-melt, the growth crucibles were covered with lids. The uniformity of the temperature modes of crystallization was ensured by placing crucibles into a uniform temperature zone of the growth furnace close to each other.
The main functional element of the laboratory-developed crystallization setting is a resistance furnace СШОЛ 1.3/12-И1. Its construction provides a gradient-free zone inside the furnace shaft about 5-7 cm in height. Software control of the temperature mode of crystallization was provided by a precision controller ТЕРМОДАТ-19Е5. The temperature mode of crystallization used in synthesizing Fe1-xMexBO3 single crystals is shown in Figure 1   The existence of concentration and convection flows in the solution-melt during crystallization leads to the fact that the contents of the diamagnetic ions in the synthesized crystals, xcrystal, differ from those in the initial charge, xcharge. Moreover, xcrystal differs for crystals extracted from the same  [5]. XRF studies of the synthesized crystals have shown that the contents of the diamagnetic ions are within the following limits: 0.06 ≤ xcrystal ≤ 0.22 for Fe1-xAlxBO3 and 0.05 ≤ xcrystal ≤ 0.07 for Fe1-xScxBO3 single crystals. It can be explained by the fact that Al 3+ ion possesses smaller with respect to Fe 3+ radius, thus, enters the overall crystal structure FeBO3 more easily than Sc 3+ that possesses a larger one.
Fe0.94Al0.06BO3 and Fe0.94Sc0.06BO3 crystals with the same values of xcrystal, were selected for detailed structural studies. Powder XRD patterns of these crystals are shown in Figure 3. XRD analysis has shown that the synthesized crystals have a rhombohedral calcite-type structure.
Isomorphous substitution of iron with aluminium or scandium produces a slight shift of the XRD peaks from their positions in pure FeBO3. The hexagonal lattice parameters a and c for Fe0.94Al0.06BO3 and Fe0.94Sc0.06BO3 single crystals, as well as the percentage of the lattice parameter shifts, ɛa and ɛc are shown in Table 2. As one can see, the diamagnetic ion with a larger ionic radius with respect to Fe 3+ leads to increasing lattice parameters, and vice versa, smaller ionic radius leads to decreasing lattice parameters.

Conclusion
We have succeeded in obtaining Fe1-xMexBO3 (Me = Al, Sc) single crystals with xcharge=0.05 using the flux growth technique. We have applied identical synthesis conditions for both compositions.
The XRF analysis has revealed a certain dispersion of the contents of diamagnetic ions in the crystals extracted from the same crucible. The aluminium content in the synthesized Fe1-xAlxBO3 crystals is much higher than in the charge, while the scandium content in the Fe1-xScxBO3 crystals almost coincides with the initial one in the charge.
Crystal structure and lattice parameters of Fe0.94Al0.06BO3 and Fe0.94Sc0.06BO3 single crystals have been determined using XRD analysis. It was shown that the synthesized crystals have a rhombohedral calcite-type structure. It was found that the lattice parameters of the Fe0.94Al0.06BO3 crystal are smaller, and the lattice parameters of the Fe0.94Sc0.06BO3 crystal are larger than those of pure FeBO3. X-ray diffraction and topography studies of the crystal structure perfection are in progress and will be published elsewhere.