Investigation of structural, microstructural and magnetic properties of YbxY1-xF3 solid solutions
Introduction
Rare-earth inorganic materials have received significant research attention during the past because of their specific electronic, magnetic, optical, and chemical properties that arise from partially occupied 4f electronic shells [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. Recently, they have become more popular thanks to the advancement of synthesis of nanostructured materials and morphology-controlled synthesis [[14], [15], [16], [17], [18], [19], [20], [21]]. These materials have a decisive role in areas such as optoelectronic devices, biomedical imaging, solid-state lasers, and scintillators [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]]. Among many inorganic materials, yttrium trifluoride (YF3), pure and doped with rare-earth ions (RExY1-xF3, RE = Tb3+, Tm3+, Eu3+, Yb3+, Er3+, Dy3+, Ce3+, Pr3+, and Ho3+), has been thoroughly investigated as a promising host crystal for lanthanide-doped phosphor materials with interesting up/down conversion luminescent properties [[32], [33], [34], [35], [36]].
Yttrium trifluoride and trifluorides of the lanthanide elements from samarium to lutetium crystallize in orthorhombic β-YF3 structure type in space group Pnma (No. 62), with four molecules in a unit cell. In this structure, cations occupy 4c Wyckoff position and the fluorine ions occupy 4c and 8d general positions [37]. The three-dimensional structure consists of irregular nine-coordination YF9 polyhedra linked over mutual corners, edges, and faces. Around every polyhedron, there are eleven adjacent polyhedra. Every polyhedron shares the face with the two nearest polyhedra, the edge with the other two polyhedra, and the corner with the seven remaining adjacent polyhedra. Face-sharing polyhedra build chains aligned with the a-axis (Fig. 1), and every chain is related laterally with six neighboring chains by mutual edges and corners.
The β-YF3 structure of YF3 and REF3 (RE = Er, Tm, Yb, and Lu) is not stable in the whole temperature region up to the melting point [38,39]. As the temperature rises, a structural phase transition from orthogonal Pnma to a trigonal symmetry occurs, which is known as an α-YF3 structure. The appropriate transition temperatures are 1267 K and 1350 K for YbF3 and YF3, respectively [40]. The transition temperature of YbxY1-xF3 solid solutions, which are the topic of this work, is somewhere between these two temperatures, and it decreases with the increasing concentration of the ytterbium ions [41]. In the temperature range of concern in this paper, the obtained solid solutions exist in structure type β-YF3.
Owing to the similar ionic radius of Y3+ to lanthanides, trivalent rare-earth ions can easily replace the yttrium ions in orthorhombic YF3 without additional charge compensation. YF3 represents a suitable candidate for a host material, due to its high ionicity and coordination number, which results in a wide band gap (>10 eV) and low vibrational energies [42]. Rare-earth trifluorides also show a diversity of magnetic properties and are very attractive as model systems for theoretical investigations of magnetic ordering in rare-earth insulators with the competition of dipole-dipole and weak exchange interactions [[43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]]. The ground configuration of RE3+ ions is4fn5s25p6. The rare-earth trifluorides are paramagnetic in almost the whole temperature range due to weak exchange interactions between rare-earth ions and shielding effect of the 4f shell by the outer 5s and 5p shells. Among some rare earth trifluorides, magnetic ordering appears at temperature values of the order of 1 K. Several studies of the magnetic properties of heavy rare-earth trifluorides have been conducted in the past. According to the magnetization and specific heat measurements, ferromagnetic domains are induced in TbF3 and DyF3 at temperatures under TC = 3.95 K [43] and 2.53 K [44], respectively, due to magnetic dipole-dipole interactions among the rare-earth ions. The antiferromagnetically ordered state of Ho3+ magnetic moments occurs in HoF3 at temperature TN = 0.53 K [45]. The antiferromagnetic phase was also found in ErF3 from the neutron diffraction study at TN = 1.05 K [48].
The magnetism of YbxY1-xF3 compounds comes from the present Yb3+ ions. A free Yb3+ ion has electronic configuration 4f13 with one hole in almost filled 4f shell, ground multiplet 2F7/2 (L = 4, S = 1/2, J = 7/2) and the first exited multiplet 2F5/2. The energy splitting between 2F7/2 and 2F5/2 is about 10000 cm−1, which is the highest splitting among all rare-earth elements. Because of this, the excited multiplet 2F5/2 does not influence significantly to the magnetic properties. The free Yb3+ ion ground level 2F7/2 is an 8-fold degenerate term that is partially split by the ligand field. Under the effect of crystal fields that have symmetry lower than cubic, the Yb3+ ground level 2F7/2 splits into the four Kramer's doublets. Hence, we should include this splitting in the analysis of the magnetic properties of YbxY1-xF3 compounds for the entirely measured temperature range.
In the following study, we present the synthesis of YbxY1-xF3 (x = 0, 0.01, 0.03, 0.05, 0.1, 0.25, 0.5, 0.7, 0.9, and 1) solid solutions. We have synthesized Yb3+ doped YF3 samples by the fluorination of an appropriate mixture of oxides with ammonium hydrogen difluoride (NH4HF2). Fluorination of oxides with NH4HF2 is one of the most suitable methods to produce pure fluorides (oxygen-free) [54]. It is suitable due to its low price, easy usage, and the fact that the reaction with this agent does not require complex equipment. Fluorination of yttrium sesquioxides (or other rare-earth sesquioxides) with NH4HF2 in the air is taking place at temperatures near 300 °C according to the overall reaction [54]:where H2O, NH4F, and remaining NH4HF2 evaporate and only YF3 lefts [43]. Different intermediate compounds could appear during the fluorination reaction, and there is no established agreement about the actual flow of the reaction. During heating, fluorination of Y2O3 in the air probably takes place in two steps by forming two intermediate compounds, (NH4)3Y2F9 and NH4Y2F7 [55].
The following work presents research on structural, morphological, and magnetic properties of YbxY1-xF3 solid solutions, systems that have not yet been thoroughly investigated, as far as we know. Structural and microstructural analysis of obtained final samples was performed using X-ray powder diffraction (XRD), which is one of the most used methods due to its simplicity, speed and the fact that it can be applied to different classes of materials. Today's most used technique for refining crystal structure was presented by Hugo M. Rietveld in 1969 [56]. This method becomes widely used not only for structure refinement but also for different kinds of analyses, such as quantitative phase analysis, measurements of microstrain and crystallite size, stacking and twin faults. Refinement of the crystal structure of YbxY1-xF3 solid solutions is necessary to show that single-phase samples are obtained and to monitor the changes in structural and microstructural parameters with changes in the concentration of the ytterbium.
Section snippets
Experimental
All starting chemicals were from Sigma−Aldrich (purity of ≥98.5–99.99%). The appropriate amounts of commercial Y2O3 and Yb2O3 were mixed with NH4HF2 according to the overall reaction:
The obtained mixtures (for x = 0, 0.01, 0.03, 0.05, 0.1, 0.25, 0.5, 0.7, 0.9, and 1) were heated, first in the air at 170 °C for 20 h and then at 500 °C for 3 h in a reducing atmosphere (Ar-10% H2).
We have checked the synthesized intermediate and final powder samples
Synthesis, structural and microstructural properties
The samples obtained from the first stage of synthesis in the air at 170 °C were analyzed by XRD (Fig. 2), and that products correspond to a complete phase formation of (NH4)3(YbxY1-x)2F9 (PCPDF: 43–0840). The SEM image of the representative (NH4)3Yb0.02Y1.98F9 sample synthesized at 170 °C is presented in Fig. 3. Examined powders are composed of irregularly formed particles with average sizes of about 160 nm. Additionally, particles strongly aggregate and form larger structures.
In the second
Conclusion
The polycrystalline YbxY1-xF3 samples analyzed in this work were synthesized by the fluorination of a mixture of Y2O3 and Yb2O3 with NH4HF2. The fluorination takes place in stages, first in the air at 170 °C by the formation of (NH4)3(YbxY1-x)2F9 and then at 500 °C in a reducing atmosphere by the formation of final solid solutions. XRD measurements show that all final synthesized samples crystallize in the same orthorhombic crystal structure of a β-YF3 type. Increasing the concentration x of Yb
CRediT authorship contribution statement
Jelena Aleksić: Conceptualization, Formal analysis, Writing - original draft. Tanja Barudžija: Writing - review & editing. Dragana Jugović: Visualization, Investigation. Miodrag Mitrić: Methodology, Validation. Marko Bošković: Investigation. Zvonko Jagličić: Investigation. Darja Lisjak: Investigation. Ljiljana Kostić: Supervision.
Acknowledgments
The authors acknowledge the financial support from the Ministry of Education, Science and Technological Development of the Republic of Serbia (projects III 45015, III 45004, OI 171025 and TR 32026), Bilateral Serbia-Slovenia (projects No 06-00-118/2018-09/32/02 and BI-RS/18-19-031) and Research Agency of the Republic of Slovenia (core funding P2-0089).
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