Synthesis of SmLuO₃ and EuLuO₃ interlanthanides from hydrothermally-derived nanostructured precursors

1 Introduction

Interlanthanide oxides with general formula LnLn′O₃, also called ternary lanthanide oxides, have received special attention in the last years because of their remarkably complex structures that lead to potentially tunable optical (Blanusa et al., 2008), magnetic (Ito et al., 2001; Mele et al., 2009), and microwave-related properties (Moreira et al., 2005; Sharma et al., 2015). Many simple binary sesquioxides of trivalent lanthanides were investigated recently due to their intricate crystal chemistry, since they can be found in three different structures ranging from cubic fluorite-type (C-type), hexagonal A-type, to monoclinic B-type, with many scientific and technological applications (Mele et al., 2003; Peng and Zhang, 2007; Chung and Lee, 2004; Azimi et al., 2013; Yang et al., 2008; Hao et al., 2010). In particular, samarium and europium oxides are technologically important compounds due to their optical (Martel et al., 1998; Dakhel, 2004a), magnetic (Zhang and Zhu, 2009) and catalytic properties (Gao et al., 2002). For example, it is well known that europium oxide stands out for its light emissive and electrically insulating properties (Shionoya and Yen, 1999; Dakhel, 2004b; Singh et al., 2008). In the last decades, many works have investigated a large number of trivalent lanthanide combinations in order to obtain potentially ordered ternary mixed-lanthanide phases by systematically substituting smaller ions into lower coordination sites and larger ions into higher coordination sites (Ito et al., 2001; Mele et al., 2009; Moreira et al., 2005; Sharma et al., 2015; Bharathy et al., 2009; Dubok et al., 2003). Of course, problems related to order–disorder phenomena in these ternary compounds may occur, which can be advantageous from the perspective of doping (disordered materials) or from the point of view of the polymorphism (ordered materials). All combinations, however, deserve investigations toward a deep exploration of structure–property relationships where different lanthanide ions are structurally distinct.

This work focuses on the investigation of interlanthanides SmLuO₃ and EuLuO₃. Both compositions were firstly synthesized by Schneider and Roth (1960) at high temperatures. The authors studied various binary mixtures of lanthanide oxides in different proportions by solid-state reactions. In that study, Schneider and Roth reported a mixture of cubic and monoclinic phases for SmLuO₃, and a phase-pure cubic for EuLuO₃ after heat treatments at 1650 °C for 6 h, not presenting, however, any X-ray diffraction (XRD) data. Su et al. (1988) studied eighteen years later the interlanthanide EuLuO₃. This material was synthesized with a combination of high pressure and temperature (3 GPa and 1200 °C), obtaining phase-pure monoclinic samples under these extreme conditions. Bharathy et al. (2009) presented a structure map for all known interlanthanides in the literature, which shows that EuLuO₃ is known as belonging to a monoclinic structure. Another work of Su et al. (1989) was cited by Bharathy et al. (2009) as an example of previous investigation on EuLnO₃ materials. Nevertheless, Su et al. (1989) have indeed studied LnEuO₃ materials, with europium on B-sites in combination with large lanthanide ions, such as Sm³⁺, Nd³⁺, Pr³⁺, Ce³⁺, and La³⁺. Tamburini et al. (2009) have recently investigated the formation of lanthanide complexes starting from macrocyclic ligands. These authors verified that the thermal decomposition of heterodinuclear europium complexes leads to the formation of EuLuO₃, not showing however any XRD data or information regarding the crystal phase of this final product.

Many synthetic methods were employed to produce binary and ternary lanthanide oxides as powders, single crystals or films. Although the standard solid-state reaction constitutes the main processing route for these materials (Ito et al., 2001; Moreira et al., 2005; Sharma et al., 2015), other non-conventional methods are also currently applied, such as reverse-strike coprecipitation (Tompsett et al., 1998), mechanochemical (Tsuzuki et al., 1998), thermal decomposition (Mele et al., 2003; Mele et al., 2009; Dubok et al., 2003; Artini et al., 2011; Ubaldini and Carnasciali, 2008; Antic et al., 2010), sol–gel processing (Heiba et al., 2003; Heiba et al., 2005; Heiba et al., 2012), combustion synthesis (Panitz et al., 2000), molecular beam epitaxy technique (Wang et al., 2010; Paranthaman et al., 1999), and hydroxide fluxes (Bharathy et al., 2009). Our research group currently works with the hydrothermal technology to synthesize nanostructured materials with practical interest (Dias and Ciminelli, 2003; Dias and Moreira, 2007; Dias et al., 2001; Dias et al., 2007; Dias et al., 2009; Siqueira et al., 2010; Siqueira and Dias, 2011). In these previous investigations, our goal was to show that phase behavior as well as morphology of the nanopowders can be tailored in selected systems by choosing the appropriate feed chemistry and processing conditions during hydrothermal syntheses. To achieve this purpose, we have used equipments and setups which allowed fully-controlled temperature–pressure–time–pH environments. Thus, potentially reactive materials could be produced in mild, reliable and reproducible conditions. As it is well known, these synthesis parameters have strong influence on the crystal structure, purity, ordering degree, particle size and morphology of the resulting compounds (Byrappa and Yoshimura, 2001; Yoshimura and Byrappa, 2008). Since all these aspects determine the properties and performance of the advanced materials, the investigation of their processing conditions becomes very important in this context.

In view of the aforesaid discussion, this work introduces the hydrothermal processing route for the production of highly reactive nanostructured precursors (in particular Sm(OH)₃, Eu(OH)₃ and LuO(OH)), aiming to the synthesis of phase-pure cubic SmLuO₃ and EuLuO₃ materials at high temperatures. The thermal behavior of nanostructured hydrothermally-derived precursors (samarium and europium hydroxides and lutetium oxy-hydroxides) was investigated in detail by X-ray diffraction, Raman spectroscopy, scanning and transmission electron microscopies. Our procedures allowed us to synthesize phase-pure cubic SmLuO₃ and EuLuO₃ materials belonging to the $\mathit{Ia}\overline{3}$ (#206) space-group. Crystal structures, morphological features and phonon modes were studied and assigned, thus contributing to the understanding of the role of the hydrothermal processing on the synthesis of these materials at high temperatures.

2 Experimental section

SmLuO₃ and EuLuO₃ were synthesized by using stoichiometric amounts of lanthanide nitrates, Sm(NO₃)₃·6H₂O and Eu(NO₃)₃·5H₂O (Sigma–Aldrich, purity >99.9%), and lutetium oxide (Lu₂O₃, Sigma–Aldrich, purity >99.9%) as starting materials. Because the lanthanide nitrates are highly hygroscopic, the amount of water was firstly checked prior to the experiments. Also, lutetium oxide was treated with HNO₃ 65% to produce lutetium nitrate, according to the chemical reaction:

Lanthanide nitrate hydrate was introduced and mixed to the mother solution followed by addition of NaOH (10 mol/L) until the pH reach values above 13 under vigorous stirring. The resulting solutions were loaded into stainless steel Parr® autoclaves equipped with pressure and temperature controllers. The reaction vessels were heated at 10 °C/min up to the processing temperature and saturated vapor pressure (250 ± 1 °C and 580 ± 2 psi), for 24 h. After synthesis, the produced powders were washed with distilled water in order to remove any remaining Na⁺ ion and dried. The general chemical reaction for the synthesis of the reactive precursors can be written as follows:

The nanostructured precursors were calcined in conventional high-temperature muffle furnaces (air atmosphere) in the temperature range 800–1600 °C for fixed times of 10 h in order to investigate their thermal behavior and reactivity.

The crystal structure of the samples was studied by X-ray diffraction (XRD) using a Shimadzu D-6000 diffractometer with graphite monochromator and a nickel filter in the range of 10–60° 2θ (15 s/step of 0.02 °2θ), operating with Fe Kα radiation (λ = 0.1936 nm), 40 kV and 20 mA (the results were automatically converted to Cu Kα radiation for data treatment and manipulation). The software MDI Jade 9.0 was employed to calculate the lattice parameters. Micro-Raman spectra were collected in back-scattering configuration by using an Olympus confocal microscope (100× objective) attached to a Horiba/Jobin-Yvon LABRAM-HR spectrometer, and equipped with 600 and 1800 grooves/mm diffraction gratings. The 632.8 nm line of a He–Ne laser (maximum effective power of 6 mW at the surface of each sample) was used as exciting line and a Peltier-cooled charge coupled device (CCD) detected the scattered light. An edge filter was employed to stray light rejection (Rayleigh line). In order to avoid any kind of crystallization due to laser exposition, very low intensities were employed, beside lens with low magnification and defocused laser, when possible. Accumulation times of typically 20 collections of 10 s were employed with spectral resolution better than 1 cm⁻¹.

A Quanta FEG 3D (FEI) field-emission scanning electron microscope (FESEM) equipped with energy-dispersive spectrometer (EDS) was employed to analyze the morphology and chemical features of the SmLuO₃ and EuLuO₃-derived materials (applied voltages varied from 5 to 30 kV). Images were taken in both detection modes, i.e., secondary (SE) and backscattered (BSE) electrons, in order to differentiate the precursors and final compounds. High-resolution transmission electron microscopy (HRTEM) was carried out to investigate the crystalline aspects of the nanopowders in a Tecnai G2-20 (FEI) transmission microscope (applied voltage of 200 kV). We apply also nanobeam diffraction (NBD) mode in addition to HRTEM for acquiring diffraction patterns from selected areas of samples calcined at higher temperatures. A beam diameter of about 2 nm was used with the smallest condenser aperture. The resulting diffraction patterns were interpreted like a selected-area diffraction pattern.

3 Results and discussion

Fig. 1 presents XRD patterns and Raman spectra (Sm and Eu are displayed in black and red lines, respectively) obtained for the precursors hydrothermally synthesized at 250 °C. The XRD results (Fig. 1a) showed that Sm(OH)₃, Eu(OH)₃ and LuO(OH) were produced and could be indexed according to the ICDD (International Committee for Diffraction Data) cards #01-083-2036, #01-083-2305 and #01-072-0928, respectively. Fig. 1b presents the Raman spectra for the precursors hydrothermally synthesized at 250 °C. As it can be seen, a set of relatively broad Raman modes in the region 100–1050 cm⁻¹ correspond to the Sm(OH)₃, Eu(OH)₃ and LuO(OH) vibrations. The bands related to LuO(OH) remain unchanged in both systems, such as below 200 cm⁻¹ and those centered at 410, 660 and 830 cm⁻¹. For samarium and europium hydroxides, there are bands that do not change significantly their frequencies, like the modes at 302/305 and 380/382 cm⁻¹ (Sm/Eu) (Kang et al., 2014; Swanson et al., 1978). On the other hand, the bands centered at around 480 and 595 cm⁻¹ present very different intensities, being stronger in Eu(OH)₃ precursors in comparison with Sm(OH)₃ (Kang et al., 2014; Swanson et al., 1978).

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Figure 1

(a) XRD patterns and (b) Raman spectra for the hydrothermally-synthesized [Sm(OH)₃ + LuO(OH)] and [Eu(OH)₃ + LuO(OH)] precursors at 250 °C.

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Following, the thermal evolution aiming to the production of phase-pure cubic SmLuO₃ and EuLuO₃ materials from hydrothermal-derived precursors was studied in the temperature range 800–1600 °C. Fig. 2 shows the XRD results for the sample heat treated for 10 h at selected temperatures (800, 1000, 1200, 1400 and 1600 °C). Samples calcined at 800 °C have converted to lanthanide oxides and could be indexed according to the ICDD cards #01-076-0153 (Sm₂O₃), #01-073-6441 (Eu₂O₃) and #01-072-6366 (Lu₂O₃). Samarium and europium oxides belong to the $I{2}_{1}3$ space group (#199), while lutetium oxide belongs to the $\mathit{Ia}\overline{3}$ (#206) space group (cubic fluorite-type phase). The lattice parameters were calculated for the two systems: a = 10.81 Å (Sm₂O₃) and a = 10.34 Å (Lu₂O₃), for the SmLuO₃ system, while a = 10.79 Å (Eu₂O₃) and a = 10.38 Å (Lu₂O₃), for the EuLuO₃ system. These lattice parameters changed for increasing temperatures, as expected, reaching values at 1200 °C of a = 10.80 Å (Sm₂O₃) and a = 10.37 Å (Lu₂O₃), for the SmLuO₃ system, and values of a = 10.81 Å (Eu₂O₃) and 10.35 Å (Lu₂O₃), for the EuLuO₃ system. The phase evolution with temperature reaches 1400 °C, now exhibiting mixed cubic $\mathit{Ia}\overline{3}$ space group and monoclinic C2/m SmLuO₃ or EuLuO₃ materials (see Fig. 2b, which focuses on the range 25–35° 2Θ). At 1400 °C, the beginning of the chemical reactions between samarium or europium and lutetium oxides toward the formation of cubic solid solutions must be noted (Fig. 2b). For comparison, Fig. S1 (Supporting Information) presents XRD patterns for monoclinic Sm and Eu oxides along with our experimental data at 1400 °C. As it can be seen, these monoclinic polymorphs do not match with the secondary phase detected, which proves that monoclinic SmLuO₃ and EuLuO₃ materials are present at 1400 °C. Finally, at 1600 °C, solid solution could be formed between the lanthanide oxides and both SmLuO₃ and EuLuO₃ become visible as phase-pure, high crystallinity cubic materials, belonging to the $\mathit{Ia}\overline{3}$ space group (#206). The mean crystallite sizes were calculated by using the Scherrer’s equation as 41 nm, for EuLuO₃, and 54 nm, for SmLuO₃.

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Figure 2

XRD patterns for samples produced in the temperature range 800 °C, 1000 °C, 1200 °C, 1400 °C and 1600 °C (from bottom to top). (a) General XRD patterns in the 10–60° 2Θ range, and (b) particular view with peak identification of different phases (in color) in the region 25–35° 2Θ.

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According to the literature discussed above, perovskite-like SmLuO₃ and EuLuO₃ were not previously obtained (Schneider and Roth, 1960; Su et al., 1988). In fact, the structural stability and formability of perovskite compounds are based on the so-called tolerance factor (t), a criterion proposed by Goldschmidt and discussed by Zhang et al., 2007. In an ideal cubic perovskite structure, the tolerance factor is equal to the unity. Lower t values may occur, which means that distorted structures are usually present. It has been accepted that almost all perovskites have t values ranging from 0.75 to 1.00. However, the tolerance factor is a necessary but not a sufficient condition for the formation of perovskite structures. The criterion proposed by Goldschmidt very often neglects the influence of the coordination number on the effective ionic radii, and was debated by the literature by years without a well defined criterion. For SmLuO₃ and EuLuO₃, the tolerance factors could be calculated as 0.7753 and 0.7712, respectively, which indicates that these compounds could be produced in distorted perovskite-like phases even if one considers these values near to the border for the traditional perovskite atomic arrangement. That is the case for some orthorhombic LnLn′O₃ (e.g. LaLuO₃, LaYbO₃, PrLuO₃) interlanthanides. In this work, the phase behavior followed by our SmLuO₃ and EuLuO₃ materials was quite different, since the hydrothermal precursors started forming cubic solid solutions between the lanthanide oxides at around 1400 °C. Beside, secondary monoclinic phase at this temperature was detected only as an impurity phase. Further increase in the temperature did destabilize the monoclinic phase, allowing the production of phase-pure cubic ( $\mathit{Ia}\overline{3}$ , #206) compounds at 1600 °C. Perovskite-like phases (orthorhombic) were not detected in our experiments for all conditions employed in this study. Fig. S2 (Supporting Information) presents XRD data for the phase-pure cubic SmLuO₃ and EuLuO₃ materials together with their Miller indexes (Alonso et al., 2004). The lattice parameters were determined for the cubic $\mathit{Ia}\overline{3}$ space group (Cullity, 1978) as a = 10.64 Å (V = 1204.8 ų), for SmLuO₃, and a = 10.61 Å (V = 1193.7 ų) for EuLuO₃.

The thermal behavior of hydrothermally treated precursors for the synthesis of SmLuO₃ and EuLuO₃ was also monitored by Raman scattering. Fig. 3 presents spectra for the samples sequentially treated at 800 °C, 1000 °C, 1200 °C, 1400 °C and 1600 °C. The spectra for the samples heat treated at 800 °C exhibit bands related to Lu₂O₃ (Ubaldini and Carnasciali, 2008), Sm₂O₃ (Martel et al., 1998) and Eu₂O₃ (Kang et al., 2014). The strongest bands for these compounds are located, respectively, at 390, 350 and 425 cm⁻¹. For the samples prepared at 1000 °C and 1200 °C, we can observe the same bands as for the sample produced at 800 °C, but with different relative intensities. At 1400 °C, monoclinic SmLuO₃ and EuLuO₃ can now be observed, in accordance with Mele et al. (2009), Tompsett et al. (1998) and Heiba et al. (2012), particularly in the frequency range 350–500 cm⁻¹ for the same crystal structure (C2/m space group). The Raman spectra for the samples produced at 1400 °C also present bands related to the cubic phase, for example at 370 cm⁻¹, for SmLuO₃, and 362 cm⁻¹, for EuLuO₃. The most characteristic bands for each of the observed phases are indicated by symbols in the figures.

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Figure 3

Raman spectra (50–1050 cm⁻¹ range) for samples produced in the temperature range 800 °C, 1000 °C, 1200 °C, 1400 °C and 1600 °C (from bottom to top) from hydrothermally-processed precursors at 250 °C, aiming to the production of phase-pure cubic (a) SmLuO₃ and (b) EuLuO₃.

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The spectra for the samples calcined at 1600 °C (Fig. 3a and b, top) show the Raman-active modes for the phase-pure SmLuO₃ and EuLuO₃. For the corresponding cubic $\mathit{Ia}\overline{3}$ space group, there are sixteen formula units per unitary cell (Z = 16). The cations are distributed into two special Wyckoff positions: 8b (C_3i local symmetry, for Sm or Eu) and 24d (C₂ symmetry, for Lu), while the oxygen ions are located into 48e general Wyckoff positions with C₁ symmetry (Marezio, 1966). Based on these occupation sites, the site-group method of Rousseau et al. (1981) was applied to obtain the following Raman-active modes:

The results depicted in Fig. 3 show that all the 22 predicted bands are not easily visualized for either SmLuO₃ or EuLuO₃ treated at 1600 °C, which indicates that additional data procedures are necessary for analyzing the spectra. Therefore, the Raman spectra for these samples were fitted by Lorentzian curves and the results are presented in Fig. 4. The results showed that 18 Raman lines could be depicted (green curves) for both samples, in very good agreement with the theoretical predictions. Based on these fitted spectra, Table 1 presents the frequencies and full-width at half maxima (FWHM) for all observed bands and their proposed assignment (Schaac and Koningstein, 1970; Repelin et al., 1995; Laversenne et al., 2001; Jinqiu et al., 2014). In accordance with the previous literature, the Raman spectra depicted in Fig. 4 could represent a fingerprint of the cubic $\mathit{Ia}\overline{3}$ structure of SmLuO₃ and EuLuO₃ materials. We notice that accidental (or quasi-accidental) degeneracy between the orthogonal modes T_g ⊕ A_g (modes 21,22 in Table 1) or T_g ⊕ E_g (modes 9,10; 14,15; and 19,20 in Table 1) prevents the observation of all the 22 gerade modes for these materials, due to the polycrystalline nature of the samples. This only would be possible by using polarized light in appropriately oriented crystals or micro-crystals, which is not the present case. It is important to note the occurrence of very narrow Raman modes (see Table 1), which indicates that our samples exhibit high crystallinity (as also verified by XRD data) and superior long-range ordering degree. These characteristics could designate the phase-pure cubic materials obtained in the present work as promising candidates for advanced optical applications.

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Figure 4

Micro-Raman spectra for SmLuO₃ and EuLuO₃ samples in the spectral regions (a) 50–150 cm⁻¹ and (b) 250–650 cm⁻¹. Experimental data are in open circles, whereas the fitting curves are represented by red lines. Green lines represent the phonon modes adjusted by Lorentzian curves.

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The thermal behavior of the resulting materials after heating was also investigated by FESEM and HRTEM. Typical samples, starting from the hydrothermally-treated precursors as well as the produced nanostructured interlanthanide oxides were examined in their morphological, chemical and structural aspects. Due to the similarity between the results for SmLuO₃ and EuLuO₃, the data for the SmLuO₃ materials were omitted. Fig. 5 presents a sequence of FESEM images (secondary electrons or SE, and backscattering electrons or BSE) for EuLuO₃ samples obtained under different processing conditions. First, Fig. 5a (SE) shows nanosized, agglomerated particles observed for the precursors synthesized by hydrothermal processing at 250 °C, for 24 h. As it can be seen, two different morphologies are present and could be associated after chemical EDS analyses with the precursors obtained after synthesis. LuO(OH) exhibits tabular, faceted particles (identified in Fig. 5a), while Eu(OH)₃ shows nanostructured, heavily agglomerated particles. Fig. 5b–e shows representative FESEM images (BSE) for our EuLuO₃ calcined in the temperature range 800–1600 °C. It was observed a generalized increase in their particle sizes; in particular, the morphologies due to LuO(OH) and Eu(OH)₃ changed above 800 °C (Fig. 5b and c) because of the conversion to the lanthanide oxides. EDS analyses showed that Lu and Eu are still in opposite corners, which means that a chemical reaction between these precursors was not in progress. Fig. 5d and e presents the morphology of the samples obtained at 1400 °C and 1600 °C, which exhibit features of materials where some solid-state reaction could not be neglected. Hard, compacted agglomerates can be seen, which indicate that a chemical reaction between the hydrothermally-derived precursors has now occurred. In fact, EDS analyses indicate that it is not possible to discern between lutetium and europium along the materials anymore.

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Figure 5

SE–FESEM and BSE–FESEM images for samples obtained at different conditions. (a) Precursors obtained by hydrothermal synthesis at 250 °C; samples calcined at (b) 800 °C, (c) 1000 °C, (d) 1400 °C and (e) 1600 °C.

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Fig. 6 shows representative HRTEM images obtained for EuLuO₃ samples produced at 1400 °C and 1600 °C. Also, electron nanobeam diffraction (NBD) was used to improve the characterization of the polycrystalline samples obtained at these higher temperatures (Cowley, 2004). Strong electromagnetic lenses were used to focus on intense electron beams (200 kV) to diameters of 1 nm or less. In this configuration, the scattered electrons are so strong that, when a bright source such as a field emission gun (FEG) is used, a diffraction pattern can be observed on a plane of observation distant from the sample and recorded using a CCD camera (Cowley, 2004). For our samples produced at 1400 °C, it was observed the presence of monoclinic EuLuO₃ phase, corroborating the results from XRD and Raman scattering. Fig. 6a presents a HRTEM image for this secondary phase, which exhibits interplanar spacing of about 2.86 Å corresponding to the (0 0 3) planes of the monoclinic C2/m structure. The inset in Fig. 6a shows the respective NBD pattern, which agrees with a monoclinic space group. Fig. 6b presents a HRTEM image for the phase-pure cubic EuLuO₃ obtained at 1600 °C. Interplanar spacing was computed to be about 4.33 Å and 3.06 Å, which correspond to the (1 1 2) and (2 2 2) planes, respectively, for the cubic $\mathit{Ia}\overline{3}$ space group. The inset in Fig. 6b exhibits its NBD pattern, which evidenced the nice crystallinity of the phase-pure sample produced at 1600 °C. Also, the diffraction recorded from this area gives a very high resolution and is consistent with cubic close-packed stacking.

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Figure 6

HRTEM images and NBD patterns for the EuLuO₃ materials produced at 1400 °C and 1600 °C. (a) Secondary monoclinic phase detected in samples obtained at 1400 °C, with interplanar spacing corresponding to the (0 0 3) planes of the C2/m space group. (b) Phase-pure EuLuO₃ samples obtained at 1600 °C, with interplanar spacings corresponding to the (1 1 2) and (2 2 2) planes of the cubic $\mathit{Ia}\overline{3}$ space group. Insets exhibit NBD patterns for the samples with different crystal structures, observed by XRD and HRTEM.

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4 Conclusions

In this work, phase-pure cubic SmLuO₃ and EuLuO₃ interlanthanides were synthesized for the first time from nanostructured hydrothermally-derived precursors. Aqueous solutions of lutetium and samarium or europium nitrates were hydrothermally treated at 250 °C for 24 h in order to produce nanostructured LuO(OH), Sm(OH)₃ and Eu(OH)₃ precursors. The thermal behavior of the mixtures of these precursors was investigated by XRD, micro-Raman spectroscopy and electron microscopy (FESEM and HRTEM). It was observed that the precursors were converted to cubic lanthanide oxides at 800 °C, whose crystal structure did not change up to 1200 °C. Monoclinic and cubic SmLuO₃ and EuLuO₃ materials were observed for samples thermally treated at 1400 °C. Phase-pure cubic ( $\mathit{Ia}\overline{3}$ space group, #206) materials were obtained only at 1600 °C, as verified by XRD, Raman spectroscopy and transmission electron microscopy. In order to check these findings, micro-Raman scattering was used to probe the phonon modes in cubic samples, which were determined and assigned. After fitting procedures, it was observed a total of 18 bands among the 22 modes predicted by group theory calculations for the $\mathit{Ia}\overline{3}$ structure, confirming the production of phase-pure cubic interlanthanides at 1600 °C.