Journal of Alloys and Compounds 485 (2009) 565–568
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The family of Ln2 TeO6 compounds (Ln = Y, La, Sm and Gd): Characterization and synthesis by the Pechini sol–gel process Jaime Llanos a,b,∗ , Rodrigo Castillo a , Daniel Barrionuevo a , Darío Espinoza a , Sergio Conejeros a a b
Departamento de Química, Universidad Católica del Norte, Casilla 1280, Antofagasta, Chile Departamento de Química, Universidad de Antofagasta, Casilla 170, Antofagasta, Chile
a r t i c l e
i n f o
Article history: Received 4 May 2009 Received in revised form 2 June 2009 Accepted 3 June 2009 Available online 11 June 2009 Keywords: Nanostructures Sol–gel synthesis X-ray diffraction Thermal analysis
a b s t r a c t Nanocrystalline Ln2 TeO6 (Ln = Y, La, Sm, Gd) have been prepared by a Pechini sol–gel process using lanthanide nitrates and telluric acid as precursors. All samples were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), Raman and Fourier transform infrared spectroscopy (FT-IR), as well as thermogravimetric analysis (TG). The AFM study reveals that the samples consist of particles with average crystal size ranging from 70 to 110 nm. The results of X-ray diffraction indicate that Ln2 TeO6 (Ln = Y, La, Sm, Gd) crystallize isotypically with the orthorhombic La2 TeO6 in the space group P21 21 21 . The infrared and Raman spectra show that the TeO6 groups are independent vibrating groups and the results obtained were discussed on the basis of the site symmetry analysis derivates from the structural data. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In contrast to the large number of tellurates and hydrogen tellurates of the alkali metals, tellurates of transition metals, as well as tellurates minerals, a very few rare-earth metals tellurates have been reported [1–5]. Tellurates exists in two form, metatellurate ion, TeO4 2− and orthotellurate ion TeO6 2− [6]. For the lanthanide tellurates, only the orthotellurates of formula Ln2 TeO6 are known so far. The first rareearth orthotellurates were prepared by Direct reaction of rare-earth oxide and orthotelluric acid at high temperature. In 1987, Trömel et al. have reported the crystal structure, from single crystal data, of La2 TeO6 and Yb2 TeO6 and proved that both phases crystallize in the orthorhombic system with space group P21 21 21 (no. 19) [7]. In the last years Schleid and co-workers have described the synthesis and the crystal structure of Gd2 TeO6 and Y2 TeO6 . They crystallize isotypically with the orthorrombic La2 TeO6 -structure type [8,9]. All of the Ln2 TeO6 compounds were prepared by the solid-state reaction method. This kind of reactions requires high temperatures and lengthy heating process. The present paper, which belongs to the framework of our systematic study on the synthesis and characterization of rare-earth orthotellurates and their uses as host lattice in inorganic phosphors, describes a Pechini-type sol–gel synthesis of the nanocrystalline
∗ Corresponding author at: Dpto. Química, University Católica del Norte, Casilla 1280, Antofagasta, Chile. Tel.: +56 55 355624; fax: +56 55 355632. E-mail address:
[email protected] (J. Llanos). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.06.027
Ln2 TeO6 (Ln = Y, La, Sm, Gd) as well as their Raman and IR spectra. 2. Experimental 2.1. Synthesis The orthotellurates of formula Ln2 TeO6 (Ln = Y, La, Sm, Gd) were prepared by the Pechini sol–gel method [10,11]. According to the stoichiometric formula, 4.36 × 10−3 mol of Y2 O3 (Aldrich, 99.99% pure), La2 O3 (Aldrich, 99.99% pure), Sm2 O3 (Aldrich, 99.99% pure) and Gd2 O3 (Aldrich, 99.99+% pure) were dissolved in 30 ml of HNO3 (0.5 mol dm−3 ) under vigorous stirring. The pH of the solution was adjusted between 1 and 2. When the oxides were completed dissolved, they were mixed with a water–ethanol (v/v = 1:7) solution containing citric acid (Merck, A.R) as chelating agent for the metal ions and 4.36 × 10−3 mol of H6 TeO6 (Aldrich, 97.5–102.5% pure). The molar ratio of telluric acid to citric acid was 1:2. Afterward, c.a. 1.25 g de polyethylene glycol) (PEG, M.W. = 20,000, Fluka, A.R.) was added as a cross-linking agent. Transparent sols were obtained after stirring for 2 h. The sols were dried in a 374 K water bath. When the sols were completely dry, they were annealed at 673 K in a furnace. After annealing, the resulting powders were fired to 1073 K with a heating rate of 1 K/min and kept there for 2 h. Optical inspection of the products showed homogeneous powders of white color. 2.2. Characterization To check the phase’s purity, powder X-ray diffraction (PXD) patterns were collected with an Imaging Plate Guinier Camera HUBER G670 (Cu K␣1 radiation, = 0.15406 nm). FT-IR spectra were measured with PerkinElmer Spectrum BX spectrophotometer with the KBr pellet technique. Raman spectra were obtained with the Witec alpha 300 microscope equipped with Confocal Raman Spectroscopy using a HeNe laser ( = 633 nm). The accuracy of the peak position is typically 4 cm−1 . All measurements were carried out at room temperature. Thermogravimetric analyses were performed using a PerkinElmer, Pyris TGA-7 apparatus. The experiments were carried in an atmosphere of Ar, using sample masses of approximately 40–50 mg in platinum sample pans and heating at 10 K/min from room temperature to 1473 K. The
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Fig. 1. X-Ray diffraction patterns for nanocrystalline Ln2 TeO6 (Ln = Y, La, Sm, Gd).
morphology of the orthotellurates was inspected using an atomic force microscope (AFM, Witec) with contact mode.
3. Results and discussion The powder XRD patterns of the nanocrystalline La2 TeO6 , Sm2 TeO6 , Gd2 TeO6 and Y2 TeO6 in the region of 2 = 10–90◦ are shown in Fig. 1. The XRD results indicate that all samples crystal-
lized isostructurally with the orthorhombic La2 TeO6 -type structure in the space group P21 21 21 [7]. By using the Scherrer equation it is possible to estimate the crystallite size. The Scherrer equation, D = 0.90/ˇcos , predicts crystallite thickness if crystals are smaller than 1000 Å. Since small angular differences in angle are associated with large spatial distances (inverse space), broadening of a diffraction peak is expected to reflect some scale feature in the crystal. In this equation D is the average grain size, is the wavelength of the radiation used in the diffraction experiments, is the diffraction angle and ˇ is the full-width at half-maximum (FWHM) of the observed peak [12,13]. The strongest diffraction peaks at 2 used to calculate the grain sizes of the samples were 26.3◦ , 27.0◦ , 27.2◦ and 27.5◦ for La2 TeO6 , Sm2 TeO6 , Gd2 TeO6 and Y2 TeO6 , respectively. Our results show that the crystallite sizes of RE2 TeO6 are in the range from 100 to 88 nm (DLa2 TeO6 = 100 nm, DSm2 TeO6 = 93 nm, DGd2 TeO6 = 89 nm, DY2 TeO6 = 91 nm). The morphology of the crystallites of Ln2 TeO6 was inspected using an atomic force microscope (AFM). The AFM images of the powders annealed at 1073 K are shown in Fig. 2. It can be seen that the four samples consist of fine particles with an average crystallite size ranging from 70 to 110 nm. These results are in good agreements with crystallite sizes calculated from the XRD experiments. A description of the crystal structure of Ln2 TeO6 (Ln = Y, La, Sm, Gd) can be found in literature [7–9] and it is briefly recalled here. The rare-earth cations occupy two independent atomic positions and are coordinated sevenfold by oxygen in form of a square-triangle polyhedron and capped trigonal prism, respectively. Independent of each other, both coordination polyhedra build chains along [1 0 0]. Each of the chains order as hexagonal rod packing, which are interpenetrated and linked by common edges
Fig. 2. AFM images of Ln2 TeO6 annealed at 1073 K: (a) La2 TeO6 ; (b) Sm2 TeO6 ; (c) Gd2 TeO6 ; (d) Y2 TeO6 .
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Fig. 3. FT-infrared and Raman spectra of Ln2 TeO6 between 800 and 400 cm−1 : (a) La2 TeO6 ; (b) Sm2 TeO6 ; (c) Gd2 TeO6 ; (d) Y2 TeO6 .
Table 1 Site-symmetry analysis for the TeO6 -octahedra in the Ln2 TeO6 lattice (Ln = La, Sm, Gd, Y). Free TeO6
Ln2 TeO6
Mode 1 2 3 4 5 6
Oh A1g Eg F1u F1u F2g F2u
Site-symm. C1 A 2A 3A 3A 3A 3A
Activity
IR: F1u Raman: Ag , Eg , F2g Inactive: F2u
IR: A Raman: A
and vertices. The resulting three-dimensional framework has channel in which the Te6+ cations are reside. The Te6+ ions occupy in this structure the 4(a) position with site symmetry C1 . The Te6+ cations in Ln2 TeO6 (Ln = Y, La, Sm, Gd), are surrounded by six oxygen atoms in the shape of a distorted octahedron. As a consequence of the lack of symmetry all vibrational modes may be found in the Raman, as well as in the I.R. spectra. Table 1 shows the five
modes of vibration of an octahedral molecule (symmetry Oh ). Vibration 1 , 2 and 5 are Raman active, whereas only 3 and 4 are infrared active. In general, the order of the stretching frequencies is 1 > 3 > 2 . The order of the bending frequencies is 4 > 5 [14]. The Raman and infrared spectra of Ln2 TeO6 are shown in Fig. 3. As can be seen, the 1 is stronger than those for other frequencies (vide supra). The peak positions are listed in Table 2. The assignments of the vibrational absorptions were made starting from the results of Siebert for the TeO6 6− anion in PbMnTeO6 (kuranakhite) [15]. The results show that the 1 of Y2 TeO6 (744 cm−1 ) is larger than those for other three compounds (Gd2 TeO6 : 1 = 738 cm−1 ; Sm2 TeO6 : 1 = 729 cm−1 ; La2 TeO6 : 1 = 716 cm−1 ). The Te–O bond reinforcement is visualized mainly by the 1 and 3 values. These differences are related to the disparity in ionic radii of the lanthanide cations. The reinforcement of the Te–O bonds is close related with the size of the ionic radii of the lanthanide cations. According to Shannon, the ionic radii are 110, 102, 100 and 96 pm for La3+ , Sm3+ , Gd3+ and Y3+ , respectively [16]. The Fig. 4 shows the TG curves for Ln2 TeO6 (Ln = Y, La, Sm, Gd). All samples have shown thermal stability up the temperature range 1250–1400 K. The loss in the mass is related to the vaporization of TeO3 [17]. The lanthanide sesquioxides should be obtained as decomposition products under our experimental conditions.
Table 2 Assignment of the IR and Raman spectra of La2 TeO6 , Sm2 TeO6 , Gd2 TeO6 and Y2 TeO6 (band positions in cm−1 ). Vibration
1 2 3 4 5
La2 TeO6
Sm2 TeO6
Gd2 TeO6
Y2 TeO6
IR
Raman
IR
Raman
IR
Raman
IR
Raman
720 w 636 vs 670 m 548 m, 572 w 472 m, 486 w
716 vs 636 w 667 m 542 w, 573 w 465 m, 476 m
734 w 652 vs 688 m 566 w, 584 w 476m, 484 w
729 vs 652 w 686 m 558 w, 590 w 486 w, 498 w
742 w 660 vs 702 m 580 m, 594 w 488 m, 498 w
738 vs 661 w 695 m 570w, 602 w 495 w, 507 m
754 w 667 vs 732 m 590 w, 606 w 490 m, 506 w
744 vs 669 w 702 m 579 w, 610 w 505 w, 518 w
vs–very strong; s–strong; m–medium; w–weak.
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perature of decomposition. The next step will be to prepare by the sol–gel method, nanoscale phosphors using nanocrystalline Ln2 TeO6 (Ln = Y, La, Sm, Gd) as host lattice and Eu3+ or Tb3+ as activator cations. Acknowledgments This work was supported by FONDECYT-CHILE (grant 1090327). Authors thank also Dr. Raul Cardoso-Gil (Max Planck Institut fuer Chemische Physik fester Stoffe, Dresden, Germany) for the X-ray measurements, and Dr. Antonio Zárate (UCN) for RAMAN measurements. References [1] [2] [3] [4] [5] Fig. 4. Thermogravimetric curves for the Ln2 TeO6 (Ln = Y, La, Sm, Gd) samples.
4. Conclusions Nanocrystalline Ln2 TeO6 (Ln = Y, La, Sm, Gd) were successfully prepared by the Pechini sol–gel process using as starting products the lanthanide sesquioxides and telluric acid. AFM study revealed that the compounds consisted of crystallites with average grain size ranging from 70 to 110 pm. According to the results of the vibrational spectra, it is possible to conclude that the stretching force constant of the TeO6 group is reinforced with the decreasing of the ionic radii of the rare-earth cations. The TG measurements exclude the changes in the mass of the samples in the interval from room temperature to the tem-
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