Phase evolution of Ba_1-xEu_xTi_1-x/4O₃ during the sintering process in air with high temperature in situ X-ray diffraction

Juan P. Hernández-Lara^a, Miguel Pérez-Labra^a,*, José A. Romero-Serrano^b, Aurelio Hernández-Ramírez^b, Francisco R. Barrientos-Hernández^a, Ricardo Martínez-López^a, Víctor E. Reyes-Cruz^a, José A. Cobos-Murcia^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

Undoped barium titanate (BaTiO₃) is a ferroelectric ceramic material with a perovskite lattice structure ABO₃, has a relative permittivity of approximately 7000 at room temperature, this property allows us to use it in a wide variety of electronic devices. The solid BaTiO₃ can exist in five phases, listing from high temperature to low temperature: hexagonal, cubic, tetragonal, orthorhombic, and rhombohedral crystal structure (Moulson and Herbert, 1990). The undoped BaTiO₃ powders are usually obtained by solid-state reactions among mixed oxides (Templeton and Pask, 1959; O’Bryan Jr. and Thomson Jr., 1974; Beauger et al., 1983a; Beauger et al., 1983b; Amin et al., 1983; Beauger et al., 1984; Mutin and Niepce, 1984; Hilton and Frost, 1992; Viviani et al., 1999; Veith et al., 2000), also called dry reaction mixture of oxides. BaTiO₃ is obtained (Beauger et al., 1983a; Veith et al., 2000) in the reaction process (sintering) between TiO₂ and BaCO₃ at high temperatures. The empirical model for the solid-state reaction is according to (Beauger et al., 1983b; Beauger et al., 1984):

It has been reported (Pavlovic et al., 2008) that the barium orthotitanate (Ba₂TiO₄) phase (secondary phase), obtained at 800 °C during the sintering process reacts with the TiO₂ nucleus to form the consolidated phase BaTiO₃. In the process, the sintering temperature plays an important role, has been observed (Brzozowski and Castro, 2003) that high sintering temperatures generate coarse BaTiO₃ particles that are unsuitable for manufacturing fine grained ceramics. Also, if the nominal ratio of BaCO₃/TiO₂ is 1, the intermediate phases (secondary phases) or BaCO₃ can persist as an end-product and can modify greatly the electrical properties. On the other hand, it is well known that doping BaTiO₃ in A- and B-site, where the radius ionic is the primary parameter that determines the substitution site, have been extensively employed to modify and improve the electrical properties of undoped BaTiO₃ for different applications (Vijatovic´ et al., 2010). Nevertheless, the effect of dopants on the structure of BaTiO₃ crystal and the determination of the phases formed at different temperatures during the heating and cooling process has not received much attention. For example, with dopants such as Nb⁵⁺, La³⁺ and Nd³⁺ the final structure becomes cubic when the dopant concentration exceeds ≈ 5 at% (Takada et al., 1987). Moreover, when Y³⁺ dopants are incorporated at the Ba site (up to 1.5 at%) the final crystal structure is tetragonal, if the incorporation happens at the Ti site (up to 12 at%) and the Y³⁺ fraction is more than 6 at% and the final crystal structure becomes cubic (Zhi et al., 1999).

Regarding grain size, the addition of donor dopants above the critical concentration (0.2 ± 0.5 at %) greatly inhibits grain growth (Buessem and Kahn, 1971; Belous et al., 1998). It has been proven then that, besides controlling the electrical properties, the presence of dopants considerably affects the final microstructure of polycrystalline BaTiO₃.

The present paper only has aims to determine and explain the structural and phase transformation changes that occur during the heating and cooling process (sintering) of the Ba_1-xEu_xTi_1-x/4O₃ solid solutions with x = 0.0054, 0.0384, 0.1920, 0.2689 mol% Eu₂O₃ as dopant using the high temperature in situ X-ray diffraction. This will be useful for the understanding of the relationship between the microstructure and doped BaTiO₃ properties, in this case with Eu³⁺. The inclusion of europium into the host lattice of BaTiO₃ is rarely found in the literature. Europium is the most reactive rare-earth element, exhibits unusual metallurgical properties and has been reported as an efficient sensitizer of photoluminescence in perovskites and related materials (Lefevre et al., 2018).

2. EXPERIMENTAL PROCEDURETOP

Four samples of solid solutions of Eu₂O₃ in BaTiO₃ were prepared by the solid state reaction route between BaCO₃ (Sigma-Aldrich, CAS No. 513-77-9, 99.9%), TiO₂ (Sigma-Aldrich, CAS No. 13463-67-7, 99.9%), and Eu₂O₃ (Sigma-Aldrich, CAS No. 1308-96-9 99.9%) using the equation Ba_1-xEu_xTi_1-x/4O₃ with x = 0.0054, 0.0384, 0.1920 and 0.2689 mol% Eu₂O₃. A detailed experimental procedure for the synthesis was given previously (Barrientos Hernández et al., 2014; Hernández Lara et al., 2017), so only its summary will be presented here. The precursor powders were dried at 200 °C, before weighing. The mixture of the powders was decarbonated at 900 °C for 12 h with a heating rate of 5 °C · min^-1. Subsequently the samples were analyzed in a Panalytical X-ray diffractometer model Empyrean (Malvern Panalytical, Royston, United Kingdom), coupling with temperature chamber Anton Paar (HTK1200N. Anton Paar GmbH, Graz, Austria) applying a heating and cooling rate of 5 °C · min^-1. The X-Ray patterns were acquired at 30, 300, 400, 500, 700, 900, 1100 and 1200 °C during heating. Before the acquiring each X-Ray patterns the temperature was held for 30 min. Also, X-Ray patterns were obtained during the cooling process of the samples at 900, 700, 500 and 25 °C. Finally, the morphological studies and distribution of Ti, Ba and Eu in the samples were performed in a high-resolution scanning electron microscope (HRSEM) JEOL 6701F (JEOL LTD, Tokyo, Japan).

3. RESULTS AND DISCUSSION TOP

3.1. High temperature in situ X-Ray DiffractionTOP

High temperature in situ X-ray diffraction is a method used in many research fields of materials science, chemical and pharmaceutical industries, metallurgy, geology, archeometry, or planetary science (Chung et al., 1994). In this study, the technique was applied to determine the structural and phase transformation changes that occur during the heating and cooling process of the samples Ba_1-xEu_xTi_1-x/4O₃ with x = 0.0054, 0.0384, 0.1920 and 0.2689 mol% Eu₂O₃. Figure 1 shows the X-Ray patterns acquired at 30, 300, 400, 500, 700, 900, 1100 and 1200 °C for the sample with x = 0.0054 mol% Eu₂O₃ during the heating (sintering process). This Figure also shows the results acquired at 900, 700, 500 and 25 °C during the cooling process. Before each acquisition, as mentioned already, the temperature was stabilized for 30 min. Similarly, Table 1 shows the identified events at each temperature. Figure 2 shows the results of high temperature in situ X-ray diffraction for Ba_1-xEu_xTi_1-x/4O₃ powders, with x = 0.0384 mol% Eu₂O₃during heating and cooling and Table 2 shows the events identified at each temperature for this composition.

Firstly, it should be noted in Table 1, equation 7, that BaTiO₃ is refers to the barium titanate phase doped with Eu³⁺. Owing to ionic radius of Eu³⁺ ion equal to 0.947 Å (for coordination number equal to 6), which is between those of Ba²⁺ ion equal to 1.61 Å (for coordination number equal to 12) and Ti⁴⁺ ion equal to 0.605 Å (for coordination number equal to 6), Eu³⁺ can occupy either A or B site in the ABO₃ compounds, depending on Ba/Ti mole ratio (Takada et al., 1987; Dunbar et al., 2004; Mitic et al., 2010; Zhang and Hao, 2013). Then, if Eu³⁺ is added to BaTiO₃ the following reaction would take place (Chan et al., 1986):

It can be observed from Fig. 1 and Fig. 2 that during the sintering process for Ba_1-xEu_xTi_1-x/4O₃ powders, with x = 0.0054 and x = 0.0384 mol % Eu₂O₃ in the temperature range 30 – 700 °C, the precursor powders did not react, (equation 6, Table 1). At 900 °C, a change in the crystal structure of the BaCO₃ precursor from orthorhombic (JCPDS 451471) to hexagonal (JCPDS 460611) could be identified. This change in crystal structure observed during heating process is characterized by an increase in cell volume of 304.42 Å to 496.27 Å (Antao and Hassan, 2007).

The formation of cubic BaTiO₃ (JCPDS 310174) was identified at 900 ºC at the angles in the X-Ray pattern 2θ ≈ 22.03°, 31.42°, 38.72°, 44.89°, 50.61°, 55.87°, 65.52°, 69.93°, 74.39°, 78.60° and 82.94°, together with the unreacted precursor powders, namely: hexagonal BaCO₃ (JCPDS 460611), TiO₂ (JCPDS 211272) for x = 0.0054 mol% Eu₂O₃ and Eu₂O₃ (JCPDS 862476) for x = 0.0384 mol% Eu₂O₃. It was observed that the cubic phase BaTiO₃ remained stable during heating up to 1200 °C and during the cooling process up to 500 °C. Down from 500 °C the tetragonal ferroelectric phase BaTiO₃ (JCPDS 050626) was identified. Subsequently this phase remained stable until room temperature was reached for both compositions of Eu₂O₃.

It can also be observed in Figs. 1 and 2that the formation of the secondary phase, barium orthotitanate (Ba₂TiO₄) JCPDS 381481, during the heating process was identified at 1100 °C with the angles in the X-Ray pattern at 2θ ≈ 25.18°, 28.86°, 29.12°, 34.79°, 36.09°, 40.69°, 41.54°, 47.32°, and 51.99°, this phase remained stable during the heating process up to 1200°C and during cooling to room temperature (2θ ≈ 29.12°).

The final structure observed consisted of the phases BaTiO₃ (JCPDS 050626) and Ba₂TiO₄ (JCPDS 381481) for the sample with x = 0.0054 mol% Eu₂O₃(equation 10, Table 1) and BaTiO₃ (JCPDS 050626), Ba₂TiO₄ (JCPDS 381481) and Eu₂O₃ (JCPD 862476) (equation 15, Table 2) for the sample with x = 0.0384 mol% Eu₂O₃.

It has been recently reported (Sahmi et al., 2019) that the phase barium orthotitanate (Ba₂TiO₄) crystallizes in a monoclinic perovskite structure and it finds applications in environmental protection. Further, Ba₂TiO₄ exhibits electrochemical stability when it is subjected to high electrical current values., its easy synthesis by chemical route increases the active surface area, resulting in enhanced photo activity with a high mineralization degree.

Figure 3 and 4 show the result corresponding to samples with x = 0.1920 and x = 0.2689 mol% Eu₂O₃, respectively, and in Tables 3 and 4,the events identified in each temperature. In the heating process, in the temperature range of 30 to 700 °C, similar to the results obtained for compositions with x = 0.0054 and x = 0.384 mol% Eu₂O₃, no changes in the composition of the precursor powders were observed. At 900 °C the phases present in both samples were similar to those identified in the sample with x = 0.0384 mol% Eu₂O₃. Cubic BaTiO₃ as well as unreacted precursor Eu₂O₃ were observed in the sample with x = 0.1920 mol% Eu₂O₃ at 1100 °C during the heating process, at this same temperature, for the sample with x = 0.2689 mol% Eu₂O₃ (Fig. 4, Table 4 equation 23), and due to the excess of the dopant precursor Eu₂O₃ the presence of secondary phases: pyrochlore Eu₂Ti₂O₇ (JCPD 231072) and orthorhombic Eu₂TiO₅ (JCPD 221100) together with phase Ba₂TiO₄ were observed, besides remnants of Eu₂O₃. For the sample with x = 0.1920 mol% Eu₂O₃ (Fig. 3) the formation of the secondary phases Eu₂Ti₂O₇, Eu₂TiO₅ and Ba₂TiO₄ together with the unreacted precursor Eu₂O₃ and the cubic BaTiO₃ phase were identified during the cooling process in the temperature range from 1200 °C to room temperature. This last event (Equation 19, Table 3) is similar to that identified for the sample with x = 0.2689 mol% Eu₂O₃ in the same temperature range, but the surplus precursor identified in this case was TiO₂. (Equation 24-25, Table 4).

Finally, the crystal structure change of cubic BaTiO₃ to tetragonal ferroelectric phase BaTiO₃ was identified at temperatures ≥ 500 °C during the cooling process. The excess dopant precursor Eu₂O₃ caused the formation of secondary phases consolidated with europium in its structure and remnants of TiO₂ (Fig. 4).

Regarding the secondary phases that were identified, it has been found that lanthanides or their oxide compounds have relatively large thermal neutron absorption cross sections, and hence compounds of europium, dysprosium, and gadolinium, including Eu₂Ti₂O₇ and Eu₂TiO₅, have been considered as potential candidates for use as control rod materials in nuclear reactors (Syamala et al., 2008; Glerup et al., 2001). The secondary phase Eu₂Ti₂O₇, which crystallize in the face-centered cubic pyrochlore structure with the general formula RE₂Ti₂O₇ (RE = rare earth element) occupies an exceptional position in the field of materials for spintronic devices (Glerup et al., 2001).

3.2. Morphology and microstructureTOP

The samples obtained after the heating and cooling process were analyzed by scanning electron microscopy (SEM-EDS). Figures 5 and 6 shows the X-ray mapping images of O, Ti, Ba and Eu elements for the samples with x = 0.0054 mol% Eu₂O₃ and with x = 0.2689 mol% Eu₂O₃, in the figure are also shown the compositions obtained by SEM-EDS semi-quantitative method. The X-ray mapping evidence a contrast along the element concentration gradient. So, in the X-ray image for europium, the bright areas correspond to europium-rich zones and the dark areas correspond to europium-poor zones. The X-ray mapping images show a uniform distribution of Ti, Ba and Eu in the sample. The incorporation of Eu³⁺ ions into the BaTiO₃ system could greatly manifest dielectric properties and can find immense scope in electronic elements including ceramic capacitors (Hernández Lara et al., 2017).

The microstructures of samples with x = 0.0054, 0.0384, 0.1920, and 0.2689mol% Eu₂O₃ are in Fig. 7, all the analyzed samples reveal similar grain sizes with a partially homogeneous microstructure and higher amounts of inter-granular porosity which was attributed to the behavior of Eu³⁺ in the BaTiO₃ phase. It could be observed that the amount of Eu³⁺ at the sample slightly affect the microstructures, which were characterized as subrounded grains with a wide grain-size distribution, between 0.28 and 2.13 μm for x = 0.0054 mol% Eu₂O₃ (Fig. 7a), 0.25 and 2.33 μm for x = 0.0384 mol% Eu₂O₃ (Fig. 7b), 0.58 and 3.66 mm for x = 0.1920 mol% Eu₂O₃ (Fig. 7c) and 0.23 and 4.03 μm for x = 0.2689 mol% Eu₂O₃ (Fig. 7d). Similar structures were reported in the study of the structural evolution BaTiO₃ doped with Gd³⁺(Hernández Lara et al., 2017).

4. CONCLUSIONSTOP

ACKNOWLEDGMENTSTOP

The authors are grateful to CONACyT-México for financial support.

REFERENCESTOP