Structural studies of evolution of solid solutions of BaTiO₃ doped with Er³⁺ (solid-state reaction method)

Miguel Pérez-Labra^a,*, Francisco R. Barrientos-Hernández^a, Juan P. Hernández-Lara^a, José A. Romero-Serrano^b, Martín Reyes-Pérez^a, Víctor E. Reyes-Cruz^a, Julio C. Juárez-Tapia^a, Gustavo Urbano-Reyes^a

^aAcademic Area of Earth Sciences and Materials, Autonomous University of Hidalgo State. Road Pachuca- Tulancingo Km 4.5 Mineral de la Reforma, Zip Code 42184, Hidalgo México

^bMetallurgy and Materials Department, ESIQIE-IPN. UPALM, Zacatenco, Zip Code 07738, Ciudad de México, México

(*Corresponding author: miguelabra@hotmail.com)

1. INTRODUCTIONTOP

Barium titanate (BaTiO₃) is a ferro-electric material that can be formulated in a large number of systems and solid solutions that provide a wide range of various applications. The ferro-electric (tetragonal) phase gets converted into para-electric (cubic) phase at the Curie temperature (Tc) at ∼ 120 °C for single-crystals. Its structure type perovskite has the capability to host ions of different sizes; so, an important number of distinct dopants can be accommodated in the BaTiO₃ lattice, which makes BaTiO₃ semi-conducting (Vijatovic´ et al., 2010). The doping process of BaTiO₃-based ceramics is of great value in the production of electric and electronic devices (multilayer capacitors, piezoelectric transducers sensors with positive temperature coefficient of resistivity etc.) (Jaffe, 1971; Moulson and Herbert, 2003). The BaTiO₃ doping process with rare earth ions has been of interest to many researchers, who have found significant improvements in its electrical properties (Pinceloup et al., 1999; Durán et al., 2001; Hwang et al., 2004; Zhao et al., 2006; Hao et al., 2011; Zhang et al., 2011; Yan-Xia et al., 2012; Zhang and Hao, 2013). Specifically, erbium, (trivalent lanthanide element) has been mainly researched as a dopant in optical fiber amplifiers (Tsur et al., 2001; Yongping et al., 2007; Markom et al., 2017), where it showed excellent properties. As a dopant in BaTiO_3, nanoparticles have been synthesized using the hydrothermal method, generating a stable cubic phase below 30 nm in size (Garrido-Hernández et al., 2014). The structure of BaTiO₃ is of great importance, because its properties depend on its crystallographic phase. The cubic phase has para-electric properties, and the tetragonal phase does not exhibit these properties (Kao, 2004; Carter and Norton, 2007).

In doping, the multiple occupation of ions in the sites A or B in the ABO₃ compounds affect the Curie’s temperature and other physical properties (Zhang et al., 2011). The incorporation of isovalent impurities it has no effect on the population of defects; however, the anisovalent impurities (valence different from that of those it replaces) require the formation of opposite charge compensation defects to maintain electrical neutrality. If the replacement cation has a lower valence than the original, electronic holes could free themselves and if the replacement cation has a valence greater than the original cation could release electrons (an electron hole or is the lack of an electron at a position where one could exist in an atom or atomic lattice) (Chan et al., 1986). It is noted that both the valence state and the radius of Er³⁺ ion (1.00 Å) are intermediate between those of Ba²⁺ ion (1.42 Å) and Ti⁴⁺ ion (0.61 Å). As a result, theoretically Er³⁺ can occupy either A or B site, depending on Ba/Ti mole ratio (Takada et al., 1987; Dunbar et al., 2004; Mitic et al., 2010; Zhang and Hao, 2013). Then, if Er³⁺ is added at the BaTiO₃ it would have (Chan et al., 1986) Eq. (1):

Based on the above, the structural behavior of BaTiO₃ may be affected; this is why this study addresses the effect of the addition of Er³⁺ on the structural characteristics of BaTiO₃. The amount of added erbium will be varied between 0.003 and 0.35 (wt. %) to know the limit of solubility of erbium in BaTiO₃. The method used is the solid state reaction, which is an important technique in the preparation of polycrystalline solids. A solid state reaction, also called dry reaction mixture of oxides, is a chemical reaction in which no solvents are used. The advantages of this method (compared to other techniques) are mainly economic and, hence large scale production is frequently based on solid-state reactions of mixed powders (Hernández Lara et al., 2017).

2. EXPERIMENTAL PROCEDURETOP

Samples of BaTiO₃ doped with Er³⁺ were elaborated using the solid-state reaction method, by grinding stoichiometric amounts of BaCO₃ (Sigma-Aldrich CAS No. 513-77-9 99.0%), TiO₂ (Sigma Aldrich CAS No. 1317-80-2 99.99%) and Er₂O₃ (Sigma-Aldrich CAS Number: 12061-16-4 99.99%) in an agate mortar, with acetone as a control medium according to the equation Ba_1-xEr_xTi_1-x/4O₃ and x = 0, 0.003, 0.005, 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35 Er³⁺ (wt. %). The powders (BaCO₃, TiO₂ and Er₂O₃) were previously dried in a muffle-type oven for 24 h at 200 °C before weighting. The mixture was de-carbonated at 900 °C for 12 h using an Al₂O₃ crucible as a container, and subsequently ground again for 30 min in an agate mortar. The powder mixture obtained was sintered at 1400 °C for 12 h in a platinum crucible in air atmosphere with heating and cooling rates of 5 °C·min^-1, using a muffle furnace (Thermolyne model 46200). The powder mixes were ground again for 30 min in an agate mortar and then compacted using uniaxial pressing at 250 MPa in an 8-mm stainless steel die, to produce green pellets of approximately 3 mm thickness. The pellets were sintered at 1400°C for 5 h in air atmosphere with heating and cooling rates of 5 °C·min^-1. The structural evolution of the products was evaluated and monitored using X-ray diffraction (diffractometer Equinox 2000 Cu Kα). The analysis of the morphology in the pellets sintered was carried out using a JEOL 6300 SEM. Raman studies for each sample obtained after sintering were performed in a spectrophotometer (Perkin Elmer Spectrum Gx) over the range of 100–1200 wavelength (cm^-1). Additionally, to determine contamination of O-H group into the products, IR spectra were recorded for the samples with more significant results in the previous techniques using a Perkin Elmer 2000 FT-IR in the range 700–400 cm^–1.

3. RESULTS AND DISCUSSION TOP

3.1. X-Ray diffractionTOP

The X-ray diffraction patterns of BaTiO₃ doped with Er³⁺ are shown in Fig. 1. A combination of cubic (JCPDS 310174) and tetragonal phases (JCPDS 050626), with the latter being predominant, was obtained. It can be seen from the X-Ray patterns that when Er³⁺ content was 0.003 ≤ x ≤ 0.35, the presence of a double peak located at 2θ ≈ 46°, indicates the existence of the tetragonal phase (JCPDS 050626) (Hernández Lara et al., 2017), which, as mentioned above, exhibits ferroelectric properties. On the other hand, it can also be noted that when the concentration of Er³⁺ was x > 0.05, a secondary phase identified as Er₂Ti₂O₇ (JCPDS 731647) was detected in the peaks located at 2θ ≈ 28°, 2θ ≈ 29.6° and 2θ ≈ 35.36° (Fig. 2). It has been reported (Li et al., 2012), that pyrochlores A₂B₂O₇ belong to Fd3m space group, and are a superstructure of the fluorite structure (MO₂) but with two cations and one eighth of the oxygen anion absent. Pyrochlores A₂B₂O₇ are important candidates as ceramic waste forms for actinide immobilization and are among the principal host phases currently considered for the disposition of Pu from dismantled nuclear weapons and the ‘‘minor’’ actinides (Yashima et al., 1996; Dobal and Katiyar, 2002). The formation of the secondary phase (Er₂Ti₂O₇) indicates that the solubility limit of Er³⁺ in the BaTiO₃ crystal structure is reached when x = 0.05. Additionally, Fig. 3 shows the sizes of crystallites obtained for each composition estimated from the broadening of the diffraction deflections, by applying the Scherrer equation, (Eq. (2)):

Where: t = grain size, λ = wavelength, B = is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians. This quantity is also sometimes denoted as ∆(2θ), q = Bragg angle (in degrees), k = form factor (constant = 1.15).

The estimated average sizes were ~285.9 nm for (110), and 347.6 nm for (111) planes respectively. The absence of trend observed in Fig. 3 can be attributed to the radius of Er³⁺ ion being intermediate between those of Ba²⁺ ion and Ti⁴⁺ ion. Thereby, the Er³⁺ could have occupied the A or B site (Zhang et al., 2011).

3.2. Raman spectroscopyTOP

Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. This has the ability to be very sensitive to transitions involving oxygen displacements and can detect the local lattice distortions and crystallographic defects at the molecular level (Yashima et al., 1996; Dobal and Katiyar, 2002). Hence, this technique is appropriate to reveal change from the tetragonal ferroelectric phase to a cubic para-electric phase. Fig. 4 shows the evolution of Raman spectra obtained for powders of BaTiO₃ added to different concentrations of Er³⁺, for 0.003 ≤ x ≤ 0.35 prepared by the solid state reaction. It shows the characteristic depolarized scattering profiles for single and polycrystalline BaTiO₃ (Venkateswaran et al., 1998).

The plots present the typical BaTiO₃ tetragonal phase Raman scattering bands at room temperature at about 250 (A₁(TO)), 515 (E(TO) , A₁(TO)) and 716 cm^-1 (E(LO) , A₁(LO)) and a sharp peak at around 305 cm^-1(B₁, E(TO + LO)) (Asiaie et al., 1996). An extra band was observed about 834 cm^-1, and it was noted that this occurs when Er³⁺ content was Er 0.10≤ x ≤ 0.35. This result can be associated to the formation of the secondary phase (Er₂Ti₂O₇)identified in the Fig. 1 in the peaks located at 2θ ≈ 28°, 2θ ≈ 29.6° and 2θ ≈ 35.36°.

On the other hand, according to the nuclear site group analysis, (Rousseau et al., 1981) Raman active phonons of the tetragonal P4mm (C_4v¹) crystal symmetry are represented by 3A₁+ B₁+ 4E. Long-range electrostatic forces induce the splitting of transverse and longitudinal phonons, which results in split Raman active phonons represented by 3[A₁(TO)+A₁(LO)]+B₁+ 4[E(TO)+ E(LO)]. It has been reported (Venkateswaran et al., 1998) that while the cubic phase theoretically does not reveal any Raman active modes, this polymorph generally shows broad bands at around 250 and 520 cm^-1, which may be caused by local disorder associated with the position of Ti atoms.

3.3. Infrared spectroscopyTOP

Infrared spectroscopy is the analysis of infrared light interacting with a molecule. It is used to determine functional groups in molecules. IR spectroscopy measures the vibrations of atoms, and based on this, it is possible to determine the functional groups.

Figure 5 shows the IR spectra for Ba_1-xEr_xTi_1-x/4O₃ powders sintered at 1400 °C for 5 h for x = 0 (BaTiO₃), 0.10 and 0.35. From the results, the characteristic absorption bands relating to BaTiO₃, which are located at ~545 and ~410 cm^–1 (Asiaie et al., 1996), can be clearly seen.

No bending vibrations of O-H corresponding to coordinated H₂O were observed, which validates the assumption that the synthesis method used generate pure phases of BaTiO₃. Garrido-Hernández et al. (2014) reported the presence of OH^– groups caused by the hydrolysis method used, as well as the presence of CO₃^2– group.

The BaTiO₃ is formed of Ti–O₆ octahedrons, and the Ba²⁺ is located at the center of eight Ti–O₆ octahedrons. On the other hand, the Ti–O₆ octahedron is the most stable form of Ti⁴⁺ and is the basic structural element in perovskite BaTiO₃.

3.4. Morphology and microstructureTOP

The selected images obtained from SEM-EDS of Er³⁺ doped BaTiO₃ are shown in Fig. 6 for the samples sintered at 1400 °C for 5 h in air atmosphere, with heating and cooling rates of 5 °C·min^-1 and x = 0 (a) and x = 0.25 (b). The micrographs consist of rounded grains with a wide grain-size distribution. For the undoped sample (Fig. 5 a), grain sizes average 11 μm and 6 μm for the sample with 0.25 Er³⁺ (wt. %). This figures shows that grain size diminishes when the concentration of erbium is increased. The SEM images of the sintered pellets shows that Er³⁺ did not drastically modify the microstructure. Hernández Lara et al. (2017) reported similar sintering structures for Gd³⁺ doped BaTiO₃; this can be attributed to the fact that both of them belonging to the lanthanide group. The incorporation of erbium in the crystalline structure of BaTiO₃ was corroborated in the EDS spectra obtained for each of the samples.

The detailed image acquired for the sample of Er³⁺ doped BaTiO₃ with x = 0.005 (Fig. 7) showed higher amounts of inter-granular porosity. In the same way, necks generated in the sintering process of the particles were observed.

4. CONCLUSIONSTOP

ACKNOWLEDGMENTTOP

The author is grateful to PRODEP and CONACyT-México for the financial support.

REFERENCESTOP