Accepted Manuscript

Investigation of the phase formation and dielectric properties of Bi7Ta3O18

M.P. Chon, K.B. Tan, C.C. Khaw, Z. Zainal, Y.H. Taufiq Yap, S.K. Chen, P.Y.Tan

PII: S0925-8388(13)02758-8DOI: http://dx.doi.org/10.1016/j.jallcom.2013.11.028Reference: JALCOM 29857

To appear in:

Received Date: 12 June 2013Revised Date: 6 November 2013Accepted Date: 6 November 2013

Please cite this article as: M.P. Chon, K.B. Tan, C.C. Khaw, Z. Zainal, Y.H. Taufiq Yap, S.K. Chen, P.Y. Tan,Investigation of the phase formation and dielectric properties of Bi7Ta3O18, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.11.028

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Investigation of the phase formation and dielectric properties of Bi7Ta3O18

M.P. Chona, K.B. Tana, C.C. Khawb, Z. Zainala, Y.H. Taufiq Yapa, S.K. Chenc, P.Y. Tana

aDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.

bDepartment of Mechanical and Material Engineering, Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, 53300 Setapak, Kuala Lumpur.

cDepartment of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia.

*Corresponding author: Tel: + 603 – 89467491; fax: + 603 – 89435380.

E–mail: tankb@science.upm.my (K.B. Tan)

Abstract

Polycrystalline Bi7Ta3O18 was synthesised at the firing temperature of 950°C over 18

hours via conventional solid state method. It crystallised in a monoclinic system with space

group C2/m, Z = 4 similar to that reported diffraction pattern in the Inorganic Crystal

Structure Database (ICSD), 1–89–6647. The refined lattice parameters were a = 34.060 (3)

Å, b = 7.618 (9) Å, c = 6.647 (6) Å with α = γ = 90° and β = 109.210 (7), respectively. The

intermediate phase was predominantly in high–symmetry cubic structure below 800°C and

finally evolved into a low–symmetry monoclinic structured, Bi7Ta3O18 at 950°C. The sample

contained grains of various shapes with different orientations in the size ranging from 0.33 –

22.70 µm. The elemental analysis showed the sample had correct stoichiometry with

negligible Bi2O3 loss. Bi7Ta3O18 was thermally stable and it exhibited a relatively high

relative permittivity, 241 and low dielectric loss, 0.004 at room temperature, ~ 30°C and

frequency of 1 MHz.

Keywords: Ceramics, Sintering, Powder diffraction, Dielectric properties.

1. Introduction

High temperature cubic δ–Bi2O3 polymorph is known to display a disordered cubic

fluorite structure and exhibit high ionic conductivity of 2.3 S cm-1 at 800°C. The electronic

structure of Bi3+ is characterised by the inert pair effect of its 6s2 lone pair electrons, leading

to non–equivalent and highly polarisable cation networks. This unique feature enables Bi3+ to

accommodate highly disordered structure and possible large number of oxygen vacancies in

the fluorite–type lattice which allow high anion mobility. In attempts to stabilise cubic δ–

Bi2O3 polymorph from high temperature, a large number of metal ions including Fe, Ti, Ta

and Nb had been doped chemically through different methods yielding new phases with a

wide range of properties which found potential applications as semiconductors, dielectrics,

multiferroics and photocatalysts [1-6].

Ta– or Nb–doped bismuth derivatives have triggered tremendous research interests

due to their promising properties in a broad spectrum of applications. Studies on the

structural and electrical properties on different phases in Bi2O3–M2O5 (M = Ta or Nb) binary

systems have been accounted in literature [4–9]. Bi3TaO7 was claimed to possess an

incommensurately modulated fluorite structure with space group Fm–3m similar to that of

reported Bi3NbO7 analogue [6]. Substitution of Nb dopant in Bi3Ta1-xNbxO7 subsolidus

system was also investigated and a full pseudo–cubic solid solution was confirmed with the

ratio of Bi: M = 3: 1 (M = Ta or Nb) in an equivalent flourite structure [7]. On the other hand,

two structural transitions were observed in samples containing 25 mol% Nb2O5. Bi3NbO7

demonstrated an unusual polymorphism with a tetragonally ordered phase existing between a

very narrow temperature ranges from 850°C–880°C, but such characteristic was not found in

Bi3TaO7 analogue. Compositions prepared at below or above this temperature range would

result in the formation of the δ–Bi3NbO7 solid solution (δ–BNss) end members in a

homogeneity range. Over a limited temperature range, δ–BNss transformed into the tetragonal

Bi3NbO7 which exhibited much lower dielectric losses (Q x f = 730 GHz), a positive

temperature coefficient of resonant frequency (100 ppm / K) and a relative permittivity of 91

[8].

A structurally related phase with higher Ta content, Bi7Ta3O18 had been studied

carefully for its structural characteristics; whilst, the information on electrical properties of

this phase was unavailable. Bi7Ta3O18 had triclinic crystal symmetry with space group, C1 as

determined by a combined single crystal X–ray and electron diffraction analysis; however,

the structure could be of a closely similar monoclinic phase, C2/m if neutron powder

diffraction characterisation was applied. The metal–atom array of Bi7Ta3O18 was

approximately face–centred cubic, but the projection of atoms and the coordination

environments showed no strong relationship to any fluorite–like structure type. In this case,

the fully ordered metal atom populations and O atom positions were found to be Ta5+ cation

in a six-fold coordinated TaO6 octahedra forming columns together with the remaining O

atoms in distorted fluorite positions [9]. Meanwhile, the eight-coordinated Bi3+ cation had

been distorted by the presence of TaO6 octahedra that could be reduced into octahedral,

square pyramidal or trigonal prismatic sites within the O–atom array. The strain of the latter

coordination environment appeared to be responsible for the lowering of symmetry from

monoclinic to triclinic [9].

To date, limited information is available on the synthesis and electrical properties of

Bi7Ta3O18 phase. Note that electrical properties of functional materials are relying strongly on

the control of composition and quality of oxides; therefore, highlight has been placed on the

characterisations on their phase compatibility, sintering and micro–structure. In this work, we

discuss the reaction pathways, structural and dielectric properties of the prepared Bi7Ta3O18

and this study may serve to provide further insight into the understanding of its correlation

between composition–structure–property.

2. Experimental

Bi7Ta3O18 was prepared at the temperature of 950°C over 18 hours in a platinum boat

by conventional solid state technique. The high purity oxides, Bi2O3 (99.9%, Acros

Organics), Ta2O5 (99.85%, Alfa Aesar) with mole ratio of 7:3 were preheated for a duration

of 2 hours at temperatures, 300°C and 600°C, respectively and then mixed with acetone in an

agate mortar. The phase purity was analysed by X–ray powder diffraction (XRD) analysis

using a Stoe STADI P diffractometer (Darmstadt, Germany) with a Ge monochromator,

CuKa1 (1.5406 Å) radiation. XRD data were collected at 0.01° step size, 2 theta range of 10°

– 70° and the lattice refinement was performed with the aid of Chekcell software [10]. Single

phase Bi7Ta3O18 was pelletised and fired at the sintering temperature of 950°C to improve its

density. The densified pellet, ~ 8 mm in diameter and thickness of ~ 2 mm was coated with

platinum electrodes for dielectric measurement by an AC impedance analyser (HP4192A)

while gold sputtering was performed on the pellet for surface morphology study using

scanning electron microscopy (LEO–VPSEM). The electrical data were normalised by

geometric factor and also corrected for stray capacitance of open jig. Electrical measurement

was performed in the frequency range of 5 Hz – 13 MHz on both heating and cooling cycles,

at the temperature range of ~30°C – 800°C with 50°C intervals and 15 minutes equilibration

time. Elemental analyses of sample triplicates were carried out by inductively coupled

plasma–atomic emission spectroscopy (ICP–AES, Perkin Elmer Optima 2000DV). Metal–

oxygen bonds were determined by Fourier transform infrared spectroscopy (Perkin Elmer,

100 series FTIR). Thermal properties were characterised by differential thermal analysis

(Perkin Elmer DTA7) and thermogravimetric analysis (Perkin Elmer TGA).

3. Results and discussion

3.1. Phase formation and reaction pathways

The phase formation of Bi7Ta3O18 is studied at different annealing temperatures,

700°C – 950°C using powder X–ray diffraction data that measured at room temperature (Fig.

1). The major phase at below 800°C is the high–symmetry cubic Bi3TaO7 subsolidus

solution. The diffraction planes gradually diminish above 800°C with the emergence of lower

symmetry, orthorhombic, BiTaO4 and monoclinic, Bi7Ta3O18 phases, respectively. A careful

analysis of the diffraction profiles reveals that the presence of Bi7Ta3O18 initiates at 800°C

and this could be attributed to the structural rearrangement of metastable Bi3TaO7 giving rise

to a mixture of BiTaO4, Bi7Ta3O18 and Bi3TaO7 phases. At this stage, the sample is not fully

reacted or yet to reach the thermal equilibrium state. Further heat treatment at higher

temperatures is therefore required for the formation of Bi7Ta3O18 as both BiTaO4 and Bi3TaO7

phases would be eliminated eventually. Single phase Bi7Ta3O18 is finally formed at 950°C.

The overall phase formation mechanism of Bi7Ta3O18 is proposed as below:

3/2 Bi2O3 + 1/2 Ta2O5 Bi3TaO7 T < 800°C

Bi3TaO7 3/2 Bi2O3 + 1/2 Ta2O5

11/2 Bi2O3 + 5/2 Ta2O5 Bi7Ta3O18 + BiTaO4 + Bi3TaO7 800°C ≤ T ≤ 900°C

BiTaO4 + 2 Bi3TaO7 Bi7Ta3O18 900°C < T ≤ 950°C

Monophasic Bi7Ta3O18 crystallised in a monoclinic system (Fig. 2) with space group

C2/m, Z = 4, similar to that reported diffraction pattern in the Inorganic Crystal Structure

Database (ICSD), 1–89–6647. The refined lattice parameters are: a = 34.060 (3) Å, b = 7.618

(9) Å, c = 6.647 (6) Å with α = γ = 90° and β = 109.210 (7). All diffraction planes are fully

indexed (Table 1) and no characteristic peak of other secondary phase is discernible.

Weight loss study indicates that Bi2O3 volatilisation is not a problem and this is

confirmed by the results of elemental analysis as summarised in Table 2; good agreement

between expected and analysed composition is obtained with no evidence of any systematic

deviation from stoichiometry and in particular, no significant Bi2O3 loss is detected. The

percentage of oxygen is determined by the difference of cations. This serves to confirm

further doubts on the possibility of Bi7Ta3O18 phase changes into lower bismuth content due

to volatilisation of Bi. On the other hand, this sample is thermally stable as neither phase

transitions nor losses of weight are noticed throughout the studied temperature (Fig. 3).

3.2. Microstructure and surface morphology analyses

The crystallite sizes are evaluated by using Williamson–Hall (W–H) and Scherrer

methods and these values are highly inter–correlated, with the values of 45 nm and 39 nm,

respectively. Based on Scherrer method, crystallite size (T) is expressed as below:

T = 0.9 x λ / β cos θ (1)

and for W–H method, the strain–induced broadening and crystallite size are deduced as:

β cos θ = kλ / D + 4 ε sin θ (2)

where β is the full width at half maximum (FWHM, in rad); θ is the scattering angle; λ =

1.5406 Å; k is the shape factor, 0.9; D is the coherent scattering length (crystallite size) and ε

is the internal strain. For quantitative comparison, the five most intense diffraction planes are

chosen in the gradient determination of β cos θ as a function of sin θ. The ε of pure Bi7Ta3O18

is relatively low (0.00045) and therefore, it can be inferred that the expansion of unit cell in

the structure is negligible [11]. On the other hand, low strain value also indicates low degree

of distortion and / or less disordered Bi7Ta3O18 phase in which the metal cations are bonded

comfortably with oxide anions in a preferred coordination environment.

Fig. 4 shows the SEM micrograph of Bi7Ta3O18. The sample has a relative density of

higher than 80% after firing at 950°C for 18 hours and contains grains of variable sizes with

irregular shapes in which the average size falls in the range of 0.33 – 22.70 µm. The grain

sizes of sample are much larger than the crystallite sizes and this implies that large grains are

the agglomerates of smaller crystallites. Meanwhile, inter–granular connectivity is somewhat

reasonably good especially dense dielectric materials are much desired for better electrical

performance.

3.3. FTIR spectroscopy

FTIR analysis is carried out in the absorption range of 1000 – 250 cm-1 which

attributed to the vibrations of ions in the crystal lattice [12] and the spectrum is shown in Fig.

5. The absorption bands at 308 cm-1, 502 cm-1 and 589 cm-1 are featured to be stretching

modes of Bi–O’ under various coordination environments, e.g. octahedral coordination basal

planes of square (or pentagonal pyramid) and trigonal prisms [9, 13]. The bond length

differences for the Bi–O’ within the structure would correspond to the vibration of the longer

bond at a phonon mode around ~ 300 cm-1; whilst, the vibration of the shorter Bi–O’ bond

may assign to the phonon mode at ~ 600 cm-1. On the other hand, those peaks at 687 cm-1,

874 cm-1 could be attributed to Ta–O stretching mode, respectively [14, 15]. The Ta–O–Ta

stretching located at 874 cm-1 exhibits a sharper band which is also commonly observed in the

Bi–rich materials [16].

3.4. Dielectric properties

Fig. 6 shows the complex Cole–Cole plots of Bi7Ta3O18 at various temperatures. The

sample behaves as typical dielectric material with high resistivity, in the order of 106 Ω cm at

~ 350°C. Perfect semicircles are seen at temperatures ≥ 350°C but turning into incomplete

arcs in the high frequency region at temperatures higher than 500°C. A typical equivalent

circuit consisting of a parallel RC element is used to model the electrical response of the

sample (Fig. 6). The conducting species is unlikely to be an ionic conduction as no evidence

of additional arc or spike is observed. The peaks of M’’ and Z’’ are almost coincident (Fig. 7)

and the full width at half maximum (FWHM) of the M’’ peak is approximately 1.53 decades,

which deviates from the perfect Debye response (1.14 decades) indicating the sample is

electrically non–homogeneous [17].

The conductivity values are in the order of 10-7 – 10-6 S cm-1 at temperatures below

350°C (Fig. 8) and increase with higher temperatures, probably due to the increase of

thermally activated charge carrier in the sample [17]. Such phenomenon is common for most

of the dielectric materials as energy or electrical field supplied beyond the critical magnitude

may cause local destruction which provides alternative low–conductivity pathway.

Nevertheless, Bi7Ta3O18 is highly insulating at low temperature and only data above 350°C

are extracted for Arrhenius conductivity plot (Fig. 9), based on the Arrhenius’s law as shown

in the equation below:

σ = σo exp (–Ea / kT) (3)

where σo represents pre–exponential factor and Ea, k and T are, respectively, activation

energy for conduction, Boltzmann’s constant and the absolute temperature. High activation

energy of 0.97 eV generally associated with hopping type of electronic transport mechanism

if it is not linked to ionic conduction [18].

According to detailed complex impedance formalisms, both plots of imaginary part of

electric modulus, M” or impedance, Z” as a function of frequency will have same functional

form, showing Debye peaks which is scaled according to C-1 and R, respectively [19]. The

change of capacitance, C and resistance, R in the proposed parallel RC model of Bi7Ta3O18

could be correlated in a formula of dielectric relaxation time, τ = RC. In other words, the

appearance of Z’’ and M’’ peaks at a characteristic frequency (ωmax = 2πfmax) is related to the

type and strength of the electrical relaxation phenomenon in the material [20]. Fig. 10

illustrates the variations of electric modulus plots, M’’ versus frequency on a logarithmic

scale over a wide range of temperatures. The existence of a peak in the spectrum that changes

its position with increased temperature confirms the presence of temperature–dependent

dielectric relaxation and this phenomenon is noticed in typical ferroelectric or relaxor–type

materials [20–23]. The peak maxima undergo a systematic shift towards higher frequency

with increase of temperature, indicating a drop in capacitance values within Bi7Ta3O18. M’’

peaks are broadened at low temperature region and such Debye peak disappears at higher

temperatures due to the restriction of measured frequency.

Debye response peaks are also evidently observed in plots of imaginary part of

impedance, Z” as a function of frequency on a logarithmic scale (Fig. 11). This indicates the

existence of polarisation process [17, 18, 24] as a displacement of curve maxima towards

higher frequencies is seen with increasing temperatures. At elevated temperatures, R values

of Bi7Ta3O18 are expected to decrease gradually due to the influx of highly thermal activated

charge carriers. In general, Z’’ peak broadened with the increase of temperatures showing an

asymmetric pattern and this arises due to the distribution of relaxation times in the sample.

The relaxation process in this case might be attributed to the relaxation of hopping charged

species between the adjacent neighbouring lattice sites via dipole formation [20]. Meanwhile,

the frequency at the maximum value of each peak in Z’’ is then plotted as an Arrhenius-type

plot to represent its variation of the critical frequency with temperature (Fig. 12) based on the

equation below:

υ = υo exp (–Ea / kT) (4)

where υ and υo are measuring frequency and attempt jumping frequency, respectively. It

follows the Arrhenius’s law with apparent activation energy, Ea = 1.07 eV, which is close to

the Ea obtained in Fig. 9. The close agreement between these two values provides additional

evidence that the conduction mechanism could be of hopping type [18, 24].

Different postulations and models, e.g. micro–macro domain transition, dipolar glass,

inhomogeneous micro region and local field model have been proposed for the strange origin

of polarisation mechanisms that take place in dielectrics with high relative permittivity, ε’,

low dielectric loss, tan δ and high electrostriction coefficient [25]. There is yet any single

satisfactorily theory which could fully explain the overall mechanism but, it is generally

believed to be associated with the polar region within the materials. Temperature dependence

of relative permittivity of Bi7Ta3O18 at different frequencies is examined in a temperature

range of ~30 – 600oC as shown in the figures. The relative permittivity (dielectric constant)

can be deduced as a complex number: ε* = ε’ – jε’’ where ε’ and ε’’ are the real and

imaginary parts of the complex permittivity. Theoretically, the alteration of ε’ can be featured

by different types of polarisation mechanisms, including electronic, ionic, orientation

(dipolar) and space charge (diffusion) polarisations, which come into play at different stages

of its response to varying temperature and frequency in an applied alternating field. Below

~300°C, the frequency–independent ε’ gradually decreases with temperature (Fig. 13). An

increase in ε’ is observed beyond that temperature showing a dielectric anomaly where the

transition peaks shift towards the right with both increasing temperatures and frequencies;

whilst, the peaks height are also found to decrease with a rise in frequency. This suggests that

the Bi7Ta3O18 shows a relaxor character, e.g. it exhibits higher dispersion behaviour at 10

kHz in high temperature region. A similar behaviour was also observed in tungsten bronze

ferroelectrics [26]. It is worthwhile to mention relaxor ferroelectrics have been considered for

applications in phase conjugate mirrors, piezoelectric sensors and actuators [27]. On the other

hand, lower ε’ values at higher frequencies are probably attributed to the limited time of

switching AC frequency for the occurrence of polarisation process. The response of

temperature–dependent relative permittivity could probably best explained by invoking two

proposed models in relaxor ferroelectrics, (a) resonance polarisation at low temperature and

(b) high temperature polarisation which is associated with thermally activated flips of polar

regions [22, 25]. In our study, the actual polarisation process remains unclear but it is

believed that high ε’ of Bi7Ta3O18 at lower temperatures may have resulted from distribution

of highly polarisable, Bi3+ over the reduced coordination environment and / or internal strain

which mentioned earlier [9]. Meanwhile, variations of the dielectric loss, tan δ as a function

of temperature at different frequencies are illustrated in Fig. 14. The dielectric loss can be

described as tan δ = ε’’ / ε’. Low tan δ values of Bi7Ta3O18 at temperatures below 350oC

indicate this ceramic sample is highly resistive and the substantial increment at higher

temperatures is featured to be the loss of increased conduction which mentioned earlier. In

general, Bi7Ta3O18 shows a relatively high dielectric constant, 241 and low dielectric loss,

0.004 at room temperature, 1 MHz. Dielectric response of this material is found to be

temperature and frequency dependent; whilst, different type of polarisations may possible

take place as an influence of its crystal structure and temperature.

4. Conclusions

The synthesis condition for polycrystalline Bi7Ta3O18 had been determined at the

firing temperature of 950°C for 18 hours and good compositional stoichiometry was obtained

as confirmed by elemental analysis. Phase formation mechanism of Bi7Ta3O18 was associated

with a recombination of BiTaO4 and Bi3TaO7 intermediate phases at elevated temperatures.

Thermally stable Bi7Ta3O18 showed bulk dielectric relaxation with high dielectric constant, ε’

= 241 and low dielectric loss, tan δ = 0.004 at room temperature and frequency of 1 MHz.

Acknowledgements

This work is supported by Research University Grant Scheme (RUGS) from Ministry

of Higher Education, Malaysia (MOHE). The authors are grateful for high resolution Stoe

STADI P X-ray measurement at Department of Materials Science and Engineering, The

University of Sheffield, UK.

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Figure captions

Fig. 1: X–ray diffraction patterns for phase evolution of Bi7Ta3O18 at different

annealing temperatures.

Fig. 2: Refined and indexed X–ray diffraction pattern of prepared single phase

Bi7Ta3O18.

Fig. 3: DTA and TGA thermograms of single phase Bi7Ta3O18.

Fig. 4: SEM image of Bi7Ta3O18 captured at 3k magnification.

Fig. 5: FTIR spectrum of Bi7Ta3O18 recorded at wave number of 1000 – 250 cm-1.

Fig. 6: Complex Cole–Cole plots of Bi7Ta3O18 at different temperatures and

incomplete arcs at temperatures above 500°C are shown in the inset of figure.

Fig. 7: Combined spectroscopic plots of M” and Z” as a function of frequency at the

temperature of 352°C.

Fig. 8: Real admittance plots, Y’ as a function of frequency at various temperatures for

Bi7Ta3O18.

Fig. 9: Arrhenius conductivity plot of Bi7Ta3O18 with activation energy, Ea ~ 0.97 ev.

Fig. 10: Imaginary part of electric modulus plots, M” as a function of frequency at

various temperatures for Bi7Ta3O18.

Fig. 11: Imaginary part of impedance plots, Z’’ as a function of frequency at different

temperatures for Bi7Ta3O18.

Fig. 12: Arrhenius plot of the peak frequency extracted from the plots of imaginary

impedance, Z’’ as a function of temperature.

Fig. 13: Real part of permittivity plots, ε’ as a function of temperature at various

frequencies.

Fig. 14: Dielectric loss, tan δ measured over a wide range of temperatures at different

frequencies.

Table 1: Refined and indexed X–ray diffraction data of Bi7Ta3O18.

h k l 2θ

(Observed)

2θ

(Calculated)

Δ(2θ)

4 0 0 10.898 10.993 -0.095

6 0 0 16.610 16.522 0.088

6 0 1 25.083 25.123 -0.040

-2 2 1 26.973 26.972 0.001

10 0 0 27.717 27.710 0.007

0 0 2 28.425 28.413 0.012

-6 2 1 29.541 29.542 -0.001

4 2 1 31.323 31.321 0.002

-10 0 2 32.618 32.628 -0.010

6 2 1 34.567 34.554 0.013

8 2 1 38.390 38.395 -0.005

-12 2 1 39.708 39.706 0.002

4 2 2 41.466 41.544 -0.078

5 3 1 42.475 42.454 0.021

-14 2 1 44.153 44.157 -0.004

16 0 0 45.107 45.057 0.050

-11 1 3 45.860 45.785 0.075

10 0 2 46.597 46.567 0.030

-4 2 3 47.550 47.529 0.021

-14 0 3 48.246 48.196 0.050

-16 2 1 48.983 48.978 0.005

-10 2 3 49.626 49.573 0.053

15 1 1 50.693 50.700 -0.007

13 3 0 51.572 51.529 0.043

14 2 1 52.470 52.471 -0.001

4 2 3 54.390 54.380 0.010

-20 0 2 54.864 54.868 -0.004

15 3 0 56.084 56.089 -0.005

17 1 1 56.492 56.479 0.013

-16 2 3 57.360 57.373 -0.013

9 3 2 57.849 57.793 0.056

-10 4 2 58.996 59.004 -0.008

1 1 4 61.301 61.243 0.058

-9 3 4 67.703 67.641 0.062

1 5 2 68.916 68.937 -0.021

Table 2: ICP–AES elemental analysis of single phase Bi7Ta3O18.

Concentration, mg/L Atomic %

Elemental Calculated Experimental Calculated Experimental

Bi 51.02 51.03 25.00 23.78 ± 0.012

Ta 18.93 18.04 10.71 9.71 ± 0.014

O 10.04 10.93 64.29 66.52 ± 0.026

Fig. 1: X–ray diffraction patterns for phase evolution of Bi7Ta3O18 at different annealing

temperatures.

Fig. 2: Refined and indexed X–ray diffraction pattern of prepared single phase Bi7Ta3O18.

.

Fig. 3: DTA and TGA thermograms of single phase Bi7Ta3O18.

Fig. 4: SEM image of Bi7Ta3O18 captured at 3k magnification.

Fig. 5: FTIR spectrum of Bi7Ta3O18 recorded at wave number of 1000 – 250 cm-1

.

Fig. 6: Complex Cole–Cole plots of Bi7Ta3O18 at different temperatures and incomplete arcs

at temperatures above 500°C are shown in the inset of figure.

Fig. 7: Combined spectroscopic plots of M” and Z” as a function of frequency at

temperature of 352°C.

Fig. 8: Real admittance plots, Y’ as a function of frequency at various temperatures for

Bi7Ta3O18.

Fig. 9: Arrhenius conductivity plot of Bi7Ta3O18 with activation energy, Ea ~ 0.97 ev.

Fig. 10: Imaginary part of electric modulus plots, M” as a function of frequency at various

temperatures for Bi7Ta3O18.

Fig. 11: Imaginary part of impedance plots, Z’’ as a function of frequency at different

temperatures for Bi7Ta3O18.

Fig. 12: Arrhenius plot of the peak frequency extracted from the plots of imaginary

impedance, Z’’ as a function of temperature.

Fig. 13: Real part of permittivity plots, ε’ as a function of temperature at various frequencies.

Fig. 14: Dielectric loss, tan δ measured over a wide range of temperatures at different

frequencies.

Highlights

• Synthesis condition of Bi7TaO3O18 had been determined • Recombination of intermediate BiTaO4 and Bi3TaO7 phases are required for the

Bi7TaO3O18 phase formation • Stable material as confirmed by thermal and structural analyses • Typical ferroelectric showing high dielectric constants and low losses • Resonance and thermal activated polarisation processes are responsible for the

excellent dielectric characteristic