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Chemistry Africa

, Volume 2, Issue 1, pp 57–66 | Cite as

Structural, Electrical and Morphological Properties of Materials Type Sillenite Phase Bi12TiO20

  • Hajar AitOulahyaneEmail author
  • Leila Loubbidi
  • Abdeslam Chagraoui
  • Lamia Bourja
  • Sylvie Villain
  • Omar Ait Sidi Ahmed
  • Abdennajib Moussaoui
  • Aziz Menichi
Original Article
  • 166 Downloads

Abstract

This work reports the investigation of the ternary system Bi2O3–TiO2–MgO. Some compositions have been synthesized by solid state reaction at 800 °C and characterized by powder X-ray diffraction. The doping of (α-Bi2O3) allowed us to stabilize three compositions isotype of sillenite structure phase with formulas Bi0.9 Ti0.1 O1.55, Bi0,9 Ti0,05Mg0,05 O1,5, and Bi0.8 Mg0.1 Ti0.1 O1.5. The structural resolution of the synthesized materials was performed using Rietveld method by means of FullProf program. It crystallizes in the cubic system. The space group I23 and lattice parameter a = 10.1723(2) Å. The morphological proprieties of the synthesized compositions have been investigated by means of scanning electron microscopy (SEM). Sintered samples showed a dense and uniform microstructure. The density of sintered samples obtained is nearly 92% of the theoretical density. Various impedance model including capacity and Warburg impedances have been used to interpret the Nyquist representations of electrical analyses. The highest conductivity is observed for Bi0.9 Ti0.1 O1.55 (σ = 1.39E−07 at 600 °C).

Keywords

Sillenite X-ray diffraction Electrical Impedance Spectroscopy SEM 

1 Introduction

The sillenite structure with chemical composition Bi12MO20 where M represents a tetravalent ion (M = Si4+, Ge4+, Ti4+) or a combination of ions, which gives an average charge of 4 + . It belongs to the body centered cubic with space group I23. This material exhibits a number of interesting properties, including piezoelectric, electro-optical, elasto-optical and photoconductive properties [1, 2]. Due to these features, sillenite phases are useful for many advanced and promising applications, such as image processing applications, coherent light amplification and many optical techniques [3, 4]. Many recent studies have reported that Bi12TiO20 could be used as photocatalyst due to its high photocatalyst activity for decoloration of methyl orange [5, 6].

Bismuth titanium oxide crystal Bi12TiO20 presents some practical advantages in terms of its isomorphous compounds Bi12SiO20 and Bi12GeO20, including lower optical activity, larger electro-optic coefficient and higher sensitivity to red light [7].

The electrical and dielectrical properties of Bi12TiO20 single crystals have been studied previously by impedance spectroscopy [8], the obtained values of electrical resistivity are ranged from 1.933 × 105 Ω to 1.073 × 103 Ω cm in the temperature range of 400–700 °C [8].

In this study, we report the synthesis of materials in the Bi2O3–TiO2–MgO with varying \(\frac{{Mg^{2 + } }}{{Ti^{4 + } + Bi^{3 + } }}\) ratio in order to establish structural of a sillenite type using XRD and to investigate the effect MgO addition on the electrical conductivity.

The introduction of MgO into Bi2O3–TiO2 system is expected to have an effect on conductivity. Also, by introducing Mg2+, additional oxygen vacancies are created in order to balance the electrostatic charge.

2 Experimental Section

2.1 Sample Preparation

The synthesized compositions type sillenite phase were obtained by solid state reaction. Powder crystalline samples were prepared from mixtures of Bi2O3 (Aldrich, purity 99.9%), TiO2 (Aldrich, purity 99.9%) and MgO (Aldrich, purity 99.9%) in stoichiometric proportion. The raw materials were grounded in an agate mortar and then took at temperatures 700, 750 and 800 °C for 24 h for each thermal treatment with several intermediate grindings and followed by quenching.

2.2 X-ray Diffraction

The final products have been monitored by X-ray powder diffraction (XRD) using a Philips X’Pert PRO diffractometer and Cu-K-alpha (λ = 1.5406 Å) radiation. The structural refinements were undertaken from the powder data. The patterns were scanned through steps of 0.02° (2θ), between 10° and 100° (2θ) with a fixed time counting of 100 s. The study of the structure is conducted by analyzing the profile of X-ray diffraction diagrams of powder with the program Fullprof [9] using the pseudo-Voight function.

2.3 Scanning Electron Microscopy (SEM)

The morphology of the compositions was taken by scanning electron microscopy Zeiss Gemini supra 40 VP with a probe Oxford X-max 20 mm2.

2.4 Elecrtical Impedance Spectrometer (EIS)

The electrical study was performed using an electrical impedance spectrometer SOLARTRON SI 1260 coupled with an electrical cell operating under air and in the temperature range from 200 to 700 °C. The samples were cylindrical pellets initially compacted at 5 kbar under ambient conditions.

3 Results and Discussion

3.1 X-ray Diffraction Analysis

The doping of Bi2O3 by 10% of TiO2 then simultaneously by 10% of TiO2 and 10% of MgO allowed us to stabilize sillenite phase. The synthesized compositions inside the ternary system Bi2O3–TiO2–MgO are represented in Fig. 1.
Fig. 1

Different phases localized in ternary system Bi2O3–TiO2–MgO

The XRD patterns can be fully indexed in a cubic system with the space group I23. Several compositions of part and others of A, B and C have been synthesized. Their Analysis by XRD have shown the existence of a mixture of phases Sillenite, α-Bi2O3 [10], γ-Bi2O3 [11] and Bi4Ti3O12 [12]. The lattice parameters derived from the XRD patterns are listed in Table 1. In order to have more information about these materials, the structural refinement was carried out by means of the Rietveld method using the Fullprof program [9]. All the structural parameters and the registration requirements of Bi0,9Ti0,05Mg0,05O1,5 are summarized in Table 2.
Table 1

The characterization by DRX of different compositions

Composition

Obtained phases

Parameter

a (Å)

Cell volume

V (Å3)

99% BiO1,5–1% TiO2

αBi2O3; γBi2O3

98% BiO1,5–2% TiO2

αBi2O3; γBi2O3; Sillenite

97% BiO1,5–3% TiO2

αBi2O3; γBi2O3; Sillenite

–̶

95% BiO1,5–5% TiO2

γBi2O3; Sillenite

90% BiO1,5–10% TiO2

Sillenite

10.1723 ± 0.0002

1052.59

85% BiO1,5–15% TiO2

Sillenite; Bi4Ti3O12

–̶

90% BiO1,5–5% TiO2–5% MgO

Sillenite

10.1633 ± 0.0007

1049.81

80% BiO1,5–10% TiO2–10% MgO

Sillenite

10.1725 ± 0.0002

1052.66

Table 2

Structural parameters and vesting conditions of the composition Bi0,9Ti0,05Mg0,05O1,5 type sillenite

SG

a (Å)

Angular domain

Step

Number of refined parameters

Rp

Rwp

Rexp

RB (%)

RF (%)

I23

10,1633

10° < 2θ < 70°

0.01°/min

67

32.0

33.3

6.64

3.53

2.51

The parameter of unit cell of the composition Bi0,9Ti0,1O1,55 is 10,172 Å and the composition Bi0,9 Ti0,05Mg0,05O1,5 is 10,163 Å. The parameter decreases because of the substitution of the cation 0.05 Ti4+ by 0.05 Mg2+ (r Ti4+ = 0.74 Å, r Mg2+ = 0.72 Å [13]). The parameter of unit cell of the composition Bi0.8 Ti0.1 Mg0.1O1.5 knew a light increase in comparison with the composition Bi0,9 Ti0,05Mg0,05O1,5. It had substitution 0.1 Bi3+ by (0.05 Ti4+ + 0.05 Mg2+) the effect of size of the cations does not have signification on increase, but it would seem that the replacement of a cation (Bi3+) by two cations (Mg2+ and Ti4+) has created one light distortion of the cell.

Rietveld refinements of X-ray powder diffraction data indicate that the atomics positions for Bi0.9Ti0.1O1.55 are: Bi, Ti occupy the sites 24f and 2a respectively, the anions O1, O2, and O3 are localized in the sites 24f, 8c and 8c respectively. For the compositions Bi0,9Ti0,05Mg0,05O1,5 and Bi0.8Ti0.1Mg0.1O1.5, the only difference is that the cations Ti and Mg occupy the same site 2a.

After refinement, the reliability factors stabilize in the following values: Rb = 3.53%, Rf = 2.51%, Rp = 32.0%, Rwp = 33.3%. Table 3 includes the atomic coordinates and thermal factors refined agitation. Observed, calculate and difference powder XRD patterns are given in Fig. 2.
Table 3

Atomic coordinates and thermal factors refined agitation of the composition Bi0,9Ti0,05Mg0,05O1,5 type sillenite

Atom

Site

x

y

z

Biso

Occ

Bi

24f

0.1761 (7)

0.3182 (5)

0.0155 (7)

0.9859 (3)

0.8835 (4)

Ti

2a

0.0000

0.0000

0.0000

0.9859 (3)

0.0735 (4)

Mg

2a

0.0000

0.0000

0.0000

0.9859 (3)

0.025

O1

24f

0.1387 (6)

0.2555 (6)

0.4854 (2)

0.2000

0.98 (4)

O2

8c

0.1995 (6)

0.1995 (6)

0.1995 (6)

0.2000

0.2835 (3)

O3

8c

0.9016 (3)

0.9016 (3)

0.9016 (3)

0.2000

0.2075 (9)

Fig. 2

Observed, calculated and difference powder XRD patterns of Bi0,9Ti0,05Mg0,05O1,5

The bond-valence analyses are 2.94 and 3.668 for bismuth and titanium/magnesium respectively, which corresponds to the theoretical values for different atoms. The main interatomic distances and bond angles are summarized in Table 4.
Table 4

Main interatomic distances (Å), angles (°) and bond valences in the C: Bi0,9Mg0,05Ti0,05O1,5

Ti/Mg

O1(3)

O2(3)

O3(3)

O4(3)

Valence

O1(3)

1.7251

2.82 (17)

2.82 (17)

2.82 (17)

0.917

O2(3)

109 (11)

1.7251

2.82 (17)

2.82 (17)

0.917

O3(3)

109 (11)

109 (11)

1.7251

2.82 (17)

0.917

O4(3)

109 (11)

109 (11)

109 (11)

1.7251

0.917

 

3.668

Bi

O1(1)

O2(1)

O3(1)

O4(2)

O5(3)

Valence

O1(1)

2.26

2.84 (16)

3.09 (11)

3.01 (9)

4.88 (14)

0.685

O2(1)

70 (5)

2.62

3.21 (10)

4.74 (11)

4.55 (14)

0.079

O3(1)

92 (5)

85 (4)

2.05

2.77 (11)

3.18 (14)

1.208

O4(2)

84 (4)

150 (6)

80 (5)

2.24

3.36 (14)

0.723

O5(3)

171 (8)

118 (6)

84 (6)

87 (5)

2.64

0.245

 

2.94

3.2 Structural Study

The sillenite structure composes of two structural units which are TiO4 tetrahedra and BiO5 polyhedra and allows the charge trapping sites. Ti-atoms are located at the corners and at the center of the elementary cell surrounded by four equidistant oxygen atoms and BiO5 polyhedra are arranged by each of 24 bismuth atoms surrounded with five oxygen atoms to form an octahedral arrangement together with stereochemically active 6 s2 lone electron pair of Bi+3 [14].

Concerning the general structure of sillenite phase, it was drawn using ATOMS program [15]. The Fig. 3 shows the three-dimensional view of Bi0,9Ti0,05Mg0,05O1,5 unit cell indicating the sequence of the Bi–O2 bonds which forms hexagonal cages, trapping thus the site (2a) occupied by titanium and magnesium atoms.
Fig. 3

Three-dimensional view and plan view of Bi0,9Ti0,05Mg0,05O1,5 unit cell indicating the sequence of the Bi–O2 bonds

Each bismuth atom is surrounded by five oxygen atom that formed with the stereochemically active 6 s2 lone electron pair of Bi+3 an octahedral arrangement (Fig. 4). If we take in consideration a longer distance between bismuth and oxygen type 1 (Bi–O1 = 3.14 Å), the bismuth atom will appear as surrounded by six oxygen atom, four oxygen type 1, one type 2 and another one type 3 (Fig. 5), forming a distorted octahedron with distances from 2.05 to 2.64 Å.
Fig. 4

The octahedral arrangement of Bi3+ with five atoms of oxygen and the lone pair of bismuth

Fig. 5

View of the Bi3+ environment

As to titanium and magnesium atoms, they are surrounded by four oxygen atoms (type 3) at the same distance 1.73 Å. Titanium and magnesium are located in the center of the regular tetrahedron formed by the oxygen atoms type 3 (Fig. 6).
Fig. 6

View of the Ti/Mg environment

The structure is formed by a three-dimensional environment of BiO5 and tetrahedron (Ti)O4 which are connected by the oxygen atom (O3) (Fig. 7).
Fig. 7

Three-dimensional environment of octahedron (Bi) O6 and tetrahedron (Ti) O4

3.3 IR Analysis

The Fig. 8 represents the IR spectra of new compositions.
Fig. 8

IR spectra of 10T, 10MT and 5MT compositions

The spectra show a similar profile justifying the isotypy with the sillenite structure and confirm the results obtained by XRD.

Based on the band attribution given in the literature [1, 3], it follows that we can assign the bands to the different modes.

The band that is about 820 cm−1 is due to the Bi–O–Bi angular deformation mode as reported in previous work [16]. We note that there is a shift from this band to the high frequency for the composition Bi0,9Ti0,05Mg0,05O1,5.

The bands that appear at frequencies 465.01, 529.67, 591.70 and 667.18 cm−1 are allocated to BiO5E groups as described in previous work, particularly in the cubic phase Bi12TiO20. The frequencies measured for Bi–O liaisons appear in this structural type at 457, 535, 586 and 663 cm−1 [3, 17].

3.4 SEM Analysis

The SEM image of the compositions Bi0,9Ti0,1O1,55 and Bi0,8 Ti0,1Mg0,1O1,5 are shown in Fig. 9 (a;b). The powder is constituted of large grains in same shapes. However, in magnifying the image, the two samples present the same morphology. Sintered samples showed dense and uniform microstructure. Density of sintered samples obtained is nearly 92% of the theoretical density. The substitution of bismuth by Mg or Ti leads to good grain growth with uniform and regular grains (Fig. 9a′, b′). The global analysis confirms the starting composition of each sample.
Fig. 9

SEM micrographs illustrating the surface morphologies of the compositions Bi0,9Ti0,1O1,55 and Bi0,8Ti0,1 Mg0,1O1,5

3.5 Electrical Conductivity Analysis

Impedance spectroscopy was used to measure the electrical conductivity of the synthesized samples. Selected impedance spectra were recorded in the temperature range of 200–750 °C. The impedance diagrams are shown in Fig. 10 for Bi0.9Ti0.1O1.55 at 400 °C and Bi0.9Ti0.05Mg0.05O1.5 at 550 °C, respectively.
Fig. 10

Nyquist plots obtained for compositions Bi0.9Ti0.1O1.55 (a) at 400 °C and Bi0.9Ti0.05Mg0.05O1.5

For Bi0.9Ti0.1O1.55, the Nyquist representations are formed by a single semicircle (Fig. 10a). However, for the composition Bi0.9Ti0.05Mg0.05O1.5, we noticed the first semicircle in addition of a second one in lower frequencies (Fig. 10b). The bulk resistivity is equal to the grain resistivity.

The conductivity data for all compositions are summarized in Table 5. With the increase in temperature, the values of conductivity have been increased. We noticed the contribution of Warburg element for the composition Bi0.9Ti0.05Mg0.05O1.5, which is due to the ionic diffusion.
Table 5

Conductivity parameter for composition

 

Bi0.9Ti0.05Mg0.05O1.5

Bi0.9Ti0.1O1.55

σ (S/cm)

ρ

σ (S/cm)

ρ

400 °C

1.79E−10

5.59E+09

1.83E−09

5.47E+08

450 °C

4.60E−10

2.17E+09

6.70E−09

1.49E+08

500 °C

1.10E−09

9.08E+08

1.96E−08

5.09E+07

550 °C

2.02E−09

4.96E+08

7.61E−08

1.31E+07

600 °C

4.04E−09

2.48E+08

1.39E−07

7.21E+06

Activation Energy (eV)

0.78(2)

1.12(4)

The highest conductivity is observed for Bi0.9Ti0.1O1.55 (σ = 1.39E−07 at 600 °C). This value is close to that obtained by Singla et al. [18] for Bi12TiO20 at x = 0.20 (σ = 2.7E−07 at 650 °C).

The results obtained from the conductivity of the compositions Bi0,9Ti0,05Mg0,05O1,5 (σ = 4.04E−09 at 600 °C) and Bi0,9Ti0,01O1,55 (σ = 1.39E−07 at 600 °C) are relatively weak in comparison with αBi2O3 (σ ~ 10E−04), βBi2O3 (σ ~ 10E−04) and γBi2O3 (σ ~ 3E−03) in 600 °C [19].

The low conductivity of the compositions, Bi0,9Ti0,05Mg0,05O1,5 and Bi0,9Ti0,01O1,55 is due on one hand to the decrease of the number of defects of oxygen (for Bi2O3 25% of defect, for Bi0,9Ti0,01O1,55 22.5% of defect and for Bi0,9Ti0,05Mg0,05O1,5 21.25% of defect) and on the other hand in the presence probably single phases of Ti4+ and Mg2+ stop the circulation of the phases of oxygen (see Fig. 11a′, b′).
Fig. 11

Arrhenius plots for Bi0.9Ti0.1O1.55 (a) and Bi0.9Ti0.05Mg0.05O1.5 (b)

The Arrhenius plots of total conductivity are shown in Fig. 11. Each conductivity plot can be presented by a single linear region. The activity energy determinate by Arrhenius equation [20] is 0.78 eV and 1.12 eV for Bi0.9Ti0.1O1.55 and Bi0.9Ti0.05Mg0.05O1.5 respectively. These results are in good agreement with the literature [21].

4 Conclusion

The sillenite phase has been successfully synthesized inside the ternary system Bi2O3–TiO2–MgO with formulas Bi0.9Ti0.1O1.55, Bi0.9Ti0.05Mg0.05O1.5, and Bi0.8Ti0.1Mg0.1O1.5. A structural refinement was realized. The ion Mg2+ is placed on the same site as Ti4+. The analysis by SEM showed that grains are densified well. Thus, the electrical properties of the synthesized compositions can be described with an impedance model including constant phase element and Warburg impedances. The electrical conductivity follows the Arrhenius law with the activation energy of 0.78 eV and 1.12 eV for Bi0.9Ti0.1O1.55 and Bi0.9Ti0.05Mg0.05O1.5 respectively. The highest conductivity is observed for Bi0.9Ti0.1O1.55 (σ = 1.39E−07 at 600 °C).

The results obtained for electrical measurements show that these materials are good insulators. They can be used for industrial applications [22].

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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2018

Authors and Affiliations

  • Hajar AitOulahyane
    • 1
    Email author
  • Leila Loubbidi
    • 1
  • Abdeslam Chagraoui
    • 1
  • Lamia Bourja
    • 1
  • Sylvie Villain
    • 2
  • Omar Ait Sidi Ahmed
    • 1
  • Abdennajib Moussaoui
    • 1
  • Aziz Menichi
    • 1
  1. 1.Laboratory of Analytical Chemistry and Physico-Chemistry of Materials (LCAPM), Faculty of Sciences Ben M’SikUniversity Hassan IICasablancaMorocco
  2. 2.Laboratory IM2NP, Building R 83957 The CEDEX GuardUniversity of ToulonToulonFrance

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