Structural Evolution and Electrical Properties of BaTiO₃ Doped with Gd³⁺

Abstract

BaTiO₃ doped with Gd³⁺ (Ba_1-xGd_xTi_1-x/4O₃) was synthesized using the solid-state reaction method with x = 0.001, 0.003, 0.005, 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, and 0.35 Gd³⁺ (wt. %). The powders were decarbonated at 900 ºC and sintered at 1400 ºC for 8 hours. The tetragonality of the synthesized Gd³⁺-doped BaTiO₃ particles was analyzed. XRD patterns and Raman spectra revealed that the crystal phase of the obtained particles was predominately tetragonal BaTiO₃; the intensity of the Raman bands at 205 cm⁻¹, 265 cm⁻¹, and 304 cm⁻¹ decreased when Gd³⁺ was increased. A secondary phase (Gd₂Ti₂O₇) was found when the Gd³⁺ content was higher than 0.15 wt. %. The capacitance of the sintering pellets was measured at 1 kHz; these values were used to calculate the relative permittivity, the maximum permittivity values were recorded for the samples with x = 0.001, 0.005, and 0.1.

Keywords
Gd³⁺; BaTiO₃; Dielectric

1. Introduction

BaTiO₃ (BT) is a ferroelectric material with a characteristic tetragonal distortion of the cubic perovskite structure. The ferroelectric (tetragonal) phase gets converted to paraelectric (cubic) phase at the Curie temperature (Tc) at ~ 120 ºC for single-crystals. However, Tc for polycrystalline BT will shift to lower values due to the metastable cubic phase at temperatures below 120ºC. The ferroelectric distortion is facilitated by the large size of the Ba cation. The ferroelectric property owes its origin to a displacement of the Ti atoms and the effect is very sensitive to interatomic distances (lattice parameter). Among the most important properties that can be presented to highlight both its chemical and mechanical stability is its ferroelectric properties at room temperature considering its use as a polycrystalline ceramic¹. BT is widely used in the manufacture of piezoelectric devices, electro-optical elements, ceramic capacitors, and resistors due to its high dielectric constant and semiconducting properties when they are modified using aliovalent or isovalent dopants². The high dielectric constant allows smaller capacitive components, thus offering an opportunity to reduce the size of electronic devices^3,4. The understanding of how these particular crystalline structures evolve from the tetragonal perovskite structure might provide a way to engineer more compounds with hexagonal structure.

It is known that an appropriate amount of doping into the BT can improve the structural, optical, and electrical properties of the system^4-7. Doping has been studied by several authors^7,8 and it was shown, for example, that the positive temperature coefficient of resistivity (PTCR) effect in BaTiO₃ doped with donors (when the substitution occurs at Ba sites) can be significantly increased by adding small amounts of dopants. This additional doping leads to an increase of the ρmax/ρmin ratio (ρ is the electric resistivity) which is the most important feature to be applied. Even more, the dopants can influence the temperature of the ferroelectric phase transition (Tc). The investigation of the dopants' effects plays a decisive role in understanding the nature and in optimizing the properties of BaTiO₃ ceramics. Manjit Borah⁹ report the structural, optical and dielectric characterization of solid state derived, pseudo-cubic nanoscale barium titanates with gadolinium (Gd³⁺) as substitutional dopant. It was found that the dielectric constant showed a decreasing tendency with doping content when the Gd-doping level was varied between 0 -7 %.

As a lanthanide element, Gd exhibits unusual metallurgical properties, apart from its significant use in magnetic resonance imaging (MRI). The inclusion of Gd into the pseudo-cubic host lattice of BT is rarely found in the literature. The main goal of this work was the investigation of structural evolution and electrical properties of BaTiO₃ ceramics doped with Gd³⁺ varied within 0.001, - 0.35 Gd³⁺ (wt. %) using the solid-state reaction method.

This method 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.

2. Experimental

Samples of BT doped with Gd³⁺ were prepared according to the formula Ba_1-xGd_xTi_1-x/4O₃ using the solid-state reaction method by grinding BaCO₃ (Sigma-Aldrich, CAS No. 513-77-9, 99.9%), TiO₂ (Sigma-Aldrich, CAS No. 13463-67-7, 99.9%), and Gd₂O₃ (Sigma-Aldrich, CAS No. 278513-25G, 99.9%) in an agate mortar, with acetone as a control medium and x = 0.001, 0.003, 0.005, 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, 0.35 Gd³⁺ (wt. %)

The precursor powders (BaCO₃, TiO₂ and Gd₂O₃) were dried at 200 ºC, before weighing. The powder mixture was placed in an alumina boat then decarbonated at 900 ºC overnight, and later was reground for 10 min in an agate mortar. After that, the powder mixtures were sintered in a platinum crucible at 1400 ºC for 8 hours using a muffle furnace (Thermolyne model 46200). The purity of the products was monitored by X-ray diffraction (Diffractometer Inel Equinox 2000 Cu Kα, radiation of λ =0.15418nm).

Once the powders were obtained for each composition pellets were manufactured for each composition. The powder mixtures were regrounded again and later compacted using uniaxial pressing at 250 MPa in an 8-mm stainless steel die to produce green pellets of 3 mm thickness. The pellets were sintered at 1400 ºC for 5 hours in air atmosphere with heating and cooling rates of 5 ºC/min. They were grounded with silicon carbide abrasive paper, polished with slurry of alumina and later cleaned in an ultrasonic bath. The morphology studies were performed in a JEOL 6300 SEM, 15kV, WD 8.1mm. Electrical measurements were performed on an LC Meter Analyzer (ELC-3133A LCM Escort), at 1 KHz. Additionally, Raman studies were carried out for each powder samples using a Perkin Elmer Spectrum Gx spectrophotometer, (Überlingen, Germany), with neodymium laser (1064 nm) excitation.

3. Results and discussion

3.1 X-Ray diffraction

Owing to the difference in effective ionic radii between gadolinium and titanium (r(Gd³⁺) = 1.00 Å, r(Ti⁴⁺) = 0.605 Å)¹⁰ the lattice will expand to some extent.

This may be one of the reasons why the XRD reflections shift to lower angles (Figure 1 and Figure 2) at 2θ = 65.7 and x ≥ 0.15 after sintering at 1400 ºC, this is similar to reported by Barrientos et al.¹¹.

Figure 1
XRD diffractograms for Ba_1-xGd_xT_i1-x/4O₃ powders sintered at 1400 ºC for 8 h for different values of x.

Figure 2
XRD diffractograms for Ba_1-xGd_xTi_1-x/4O₃ powders sintered at 1400 ºC for 8 h for different values of x. Zoom at 64–67º

It can be seen from Figure 1 that the diffractograms show a double peak at 2θ ≈ 45, indicating the presence of tetragonal ferroelectric phase (JCPDS 050626) for the diffractograms when Gd³⁺ content 0.001 ≤ x ≤ 0.3. This phase disappears for the sample with Gd³⁺ content x = 0.35, however, a secondary phase (Gd₂Ti₂O₇) (JCPDS 731698) was found when the Gd³⁺ content was higher than 0.15, as observed in the peak 2θ ≈ 32. The Gd₂Ti₂O₇ phase was identified for 0.001 ≤ x ≤ 0.35. Parida et al.¹² studied a series of Gd₂Ti₂O₇/GdCrO₃ composites prepared by the solid state combustion method using Gd(NO₃)₃, TiO₂, Cr₂O₃ as metal sources and urea as a fuel. Parida et al. have found the Gd₂Ti₂O₇ phase, whereby the main peaks matched the secondary phase found in this work.

3.2 Raman Spectroscopy

Raman spectroscopy (RS) was used to study the phase transition of Gd³⁺-doped BaTiO₃ ceramics. RS is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system⁷. Figure 3 shows the Raman spectra for BaTiO₃ doped with several Gd³⁺ concentrations prepared by the solid-state reaction. The graphs show the characteristic Raman peaks at room temperature of BaTiO₃^8,13,14 located at 205 cm^-1 (E(TO + LO), A1(LO)), 265 cm^-1 (A1(TO)), 304 cm^-1 (B1, E(TO + LO)), 513 cm^-1 (A1(TO), E(TO)) and 717 cm^-1 (A1(LO), E(LO)). Similar bands were observed in BaTiO₃ based ceramics by Suchanicz et al.¹³ and Pokorny et al.¹⁴. An extra band was observed at 833 cm^-1 whose intensity was proportional to the Gd³⁺ concentration. This high-frequency mode (above 700 cm^-1) can be caused by the vibrations resulting from the shift of oxygen and also correlated to present of oxygen vacancies, Suchanicz et al.¹⁵. On the other hand, was observed that this extra band occurs when Gd³⁺ content was Gd 0.01≤ x ≤ 0.35, which was coherent with the appearance of the secondary phase Gd₂Ti₂O₇ (see Figure 1). Barrientos et al.¹¹, in a study of the structural evolution of hexagonal phase Ba₈Ti₃Nb₄O₂₄ by adding Nb₂O₅ to perovskite BaTiO₃, associated the appearance of an extra band around 743 cm^-1 and 830 cm^-1 with the presence of the secondary phase Ba₈Ti₃Nb₄O₂₄, equally identified by XRD. When the dopant concentration in BaTiO₃ is enough to cross through the tetragonal-cubic phase transition point, all the active Raman modes in the tetragonal phase (P4 mm) are inactive in the perfect cubic phase (Pm3 m) according to forbidden Raman selection rules. However, the broad A1 (TO) band at 513 cm^-1 persisted into the cubic phase above Tc, which was attributed to intrinsic disorder in the cubic phase, and it becomes broad and weak with increasing Gd³⁺ content¹⁶. A dramatic change at low frequencies was found with increasing dopant content. The Raman peak around 304 cm^-1 showed a decrease in intensity. This diminution was attributed to the decrease of tetragonality of BT ceramics. The phase transformation was observed in DRX results when Gd³⁺ content was x ≥ 0.35 (Figure 1) and dielectric measurements.

Figure 3
Raman spectra of powders sintered at 1400 ºC, 0 ≤ x ≤ 0.3.

3.3 Electric Properties

Platinum paste was painted on opposite surfaces of the pellets to form electrodes; a thin platinum strip was applied to the platinum electrodes to act as a contact. Figure 4 shows the results of relative permittivity (k) obtained at 1 kHz for the compositions prepared in the range of 0 ≤ x ≤ 0.35. The maximum permittivity values were recorded for the samples with x = 0.001, 0.005, and 0.1. It has been reported⁹ that the dielectric constant showed a decreasing tendency with doping content and with increasing frequency for pseudo-cubic nanometric barium titanate doped with gadolinium (Gd³⁺) where the doping level was varied between 0-7 %. However, it was also determined that in the low-frequency region, the loss tangent (tan δ), which is the combined result of orientational polarization and electrical conduction, was found to be quite high in the doped samples as compared to their un-doped counterpart. In this study only results obtained at 1 kHz were reported.

Figure 4
Relative permittivity at 1 kHz as a function of temperature capacitors prepared from Ba_1-xGd_xTi_1-x/4O3 with 0 ≤ x ≤ 0.35

The maximum value of k was registered for the sample with x = 0.001 (10474.35). The main Curie peak shift from 112 ºC to 76 ºC indicated that the cubic phase was becoming more stable. Furthermore, the compositions x = 0.25, x = 0.2, x = 0.3 and x = 0.35 showed Curie temperature values of 83 to 65 ºC with permittivity values of 5319, 4364, 3529, and 3174 respectively. However, the low intensity and width of the peaks indicated a mixture of phases.

3.4 Morphology and microstructure

Figure 5 shows SEM- EDS images of Gd³⁺-doped BaTiO₃ sintered at 1400 ºC for the samples with x = 0.15 and x = 0.35, which consisted of rounded grains with a wide grain-size distribution visually observed from SEM images; grain sizes of 1.8- 8.4 mm for x = 0.15 (Figure 5a), and 3.2-11.2 mm for x = 0.35 (Figure 5b). The presence of Gd can be observed in the EDS spectrum. The SEM images of the samples indicated that Gd³⁺ does not drastically influence the microstructure.

Figure 5
SEM-EDS micrographs of BaTiO₃, doped with different Gd³⁺. a) x= 0.15, and b) x= 0.35.

All of the samples revealed comparable grain sizes. A relatively homogeneous microstructure with higher amounts of inter-granular porosity was also observed; this may due to the behavior of Gd³⁺ in the BaTiO₃ ceramics. In the detailed image (Figure 6), this characteristic can be observed¹¹.

Figure 6
SEM micrograph detail of Gd³⁺ doped BaTiO₃, x = 0.15.

A summary of semiquantitative compositions obtained by SEM-EDS for all samples is shown in Table 1. The EDS investigations showed that the ceramics BaTiO₃ + x wt. % Gd³⁺ (0.001 ≤ x ≤ 0.35) contained Ba, Ti, O, and Gd elements near their surfaces. No other elements were detected.

Table 1
Semiquantitative chemical compositions (SEM-EDS) of powders sintered at 1400 ºC, 0 ≤ x ≤ 0.35 (wt.%).

Sample	Wt. (%)
	Ba	Ti	O	Gd
BaTiO3	68.46	25.31	6.22	0
x = 0.001	65.64	23.98	10.31	0.07
x = 0.003	61.41	21.93	16.41	0.25
x = 0.005	61.87	20.13	17.66	0.34
x = 0.010	58.03	20.51	20.74	0.72
x = 0.050	54.38	21.15	21.75	2.72
x = 0.100	50.81	22.74	13.03	13.42
x = 0.150	48.68	19.33	15.85	16.14
x = 0.200	46.64	19.22	9.83	24.31
x = 0.250	41.01	19.14	12.83	27.02
x = 0.300	38.88	19.01	12.37	29.74
x = 0.350	36.02	19.11	10.92	33.95

It was noted that the content of Gd increased as x increased. On the contrary, with x content increasing, the Ba and Ti contents decreased. This can be explained by the amphoteric behavior of BaTiO₃¹⁷, e.g. BaTiO₃ behaves as an acceptor when the Ti site is substituted or as donor when the substitution occurs at the Ba site.

Yongping et al.¹⁷ reported that the thermodynamic conditions, such as the partial pressure of oxygen in the sintering atmosphere and temperature, play an important part in the distribution of rare earth elements with amphoteric behavior in the A or B sites of BaTiO₃^18,19.

4. Conclusions

BaTiO₃ doped with Gd³⁺ (Ba_1-xGd_xTi_1-x/4O₃) with x = 0.001, 0.003, 0.005, 0.01, 0.05, 0.1, 0.15, 0.20, 0.25, 0.30, and 0.35 Gd³⁺ (wt. %) was investigated by XRD, RS and electric measurements. A double peak at 2θ ≈ 45 was confirmed by XRD, indicating the presence of a tetragonal ferroelectric phase for the diffractograms when the Gd³⁺ content was 0.001 ≤ x ≤ 0.3. This phase disappeared for the sample with Gd³⁺ content x = 0.35. A secondary phase (Gd₂Ti₂O₇) was found when the Gd³⁺ content was higher than 0.15, as was observed in XRD at 2θ ≈ 32. Gd₂Ti₂O₇ phase was identified for 0.001 ≤ x ≤ 0.35. The Raman results showed the characteristic Raman peaks of the tetragonal ferroelectric phase of BaTiO₃ located at 205 cm^-1 (E(TO + LO), A1(LO)), 265 cm^-1 (A1(TO)), 304 cm^-1 (B1, E(TO + LO)), 513 cm^-1 (A1(TO), E(TO)), and 717 cm^-1 (A1(LO), E(LO)). The maximum permittivity values were recorded for the samples with x = 0.001, 0.005, and 0.1. The extra band observed 0.01≤ x ≤ 0.35 (833 cm^-1) was coherent with the appearance of the secondary phase (Gd₂Ti₂O₇) observed equally by DRX.

The incorporation of Gd³⁺ ions into the BT system could greatly manifest dielectric properties and can find immense scope in electronic elements including ceramic capacitors.

5. Acknowledgment

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

6. References

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Publication Dates

• Publication in this collection
  02 Mar 2017
• Date of issue
  Mar-Apr 2017

History

• Received
  19 Aug 2016
• Reviewed
  10 Jan 2017
• Accepted
  02 Feb 2017