Ionic conductivity of rare earth doped phase stabilized Bi2O3: Effect of ionic radius S. Bandyopadhyay, Sk. Anirban, A. Sinha, and A. Dutta
Citation: AIP Conference Proceedings 1832, 110020 (2017); doi: 10.1063/1.4980644 View online: http://dx.doi.org/10.1063/1.4980644 View Table of Contents: http://aip.scitation.org/toc/apc/1832/1 Published by the American Institute of Physics
Ionic Conductivity of Rare Earth Doped Phase Stabilized Bi2O3: Effect of Ionic Radius S. Bandyopadhyaya#, Sk. Anirbana,b, A. Sinhaa and A. Duttaa* a b
Department of Physics, The University of Burdwan, Burdwan-713104, India
Department of Physics, Govt. General Degree College, Salboni, Paschim Medinipur-721516, India *
[email protected]; #
[email protected]
Abstract. Nanostructured Bi2O3 was prepared through citrate auto ignition method and stabilized down to room temperature into rhombohedral phase by 30% doping of rare earth cations (Eu3+, Sm3+ ,Nd3+, La3+), which was experimentally confirmed by the XRD patterns of the doped compositions. The average crystallite size increases with increase of ionic radius. The ionic conductivity of the La-doped compound was found to be highest among other doped compounds. The change in structural and electrical properties were discussed and correlated with the ionic radius of the dopants. Keywords: Rietveld refinement; Impedance Spectroscopy; Relaxation dynamics. PACS: 61.05.cp; 81.20.Ev; 81.07.Bc; 82.47.Ed; 72.15.Eb
INTRODUCTION Among the five crystallographic polymorphs of bismuth oxide (Bi2O3), LQWHUHVWKDVEHHQFHQWHUHGRQįBi2O3 because of its higher oxygen ion conductivity than Yttria Stabilized Zirconia (YSZ) and cubic ceria1. However, pure į SKDVH LV RQO\ VWDEOH LQ D QDUURZ temperature range (730 °C to 820 °C) which is limited by its melting point and a dramatic phase transition to PRQRFOLQLF Į-phase2. This high temperature cubic phase can be stabilized down to room temperature by the additions of isovalent rare earth cations3, 4. But, due to the lower polarizibility of dopant cations, these doped bismuth oxides show lower ionic conductivity. Moreover, the number of available rare earth FDWLRQV WKDW FDQ VWDELOL]H FXELF į-phase of Bi2O3 is limited. To overcome this problem an attempt of phase stabilization of Bi2O3 into other phases without compromising with its conductivity value, has been made in this work. Here, Bi2O3 is stabilized into a rhombohedral phase using chemical route. Objectives of this work are to investigate the effect of ionic radius of different rare earth cations on microsructural and electrical conductivity of the phase stabilized rhombohedral Bi2O3.
respectively] and pure Bi2O3 were prepared by low temperature citrate auto ignition method. Stoichiometric amounts of Bi(NO3)3 , 5H2O (>99 %) and Re2O3 (99.9%) were dissolved in de-ionized water and concentrated HNO3 was added drop wise under constant stirring at temperature 65-700C. Citric acid (99.5%) in a 1:3 molar ratio with the cations, was added to the solution and was stirred at temperature 80-850C. Afterwards, yellowish gel was formed and the auto ignition process was completed within few seconds leaving a grey-yellowish ash. The product was crushed, grinded and sintered at 8000C each for 6 h duration. Powder X-ray diffraction patterns were obtained by X-ray diffractometer (BRUKER, Model D8 AdvanceAXS) using CuKĮ UDGLDWLRQ Ȝ Å LQșUDQJH from 20°–80°, with step size 0.02° and scan rate 0.5 s/step. For electrical measurement, powdered samples were uniaxially pressed into 10 mm diameter cylindrical shaped pellets of average thickness 1.5 mm. The pellets were annealed in air at 800°C for 1 h. Electrical measurements was performed using two probe method in air with an impedance analyzer (HIOKI Model: 3532-50) in the temperature range 200°C to 400°C and in the frequency range of 42 Hz to 5 MHz.
MATERIALS AND METHODS Nano structured Bi0.7Re0.3O1.5-į [Re = Eu, Sm, Nd, La and abbreviated as BEu, BSm, BNd and BLa
DAE Solid State Physics Symposium 2016 AIP Conf. Proc. 1832, 110020-1–110020-3; doi: 10.1063/1.4980644 Published by AIP Publishing. 978-0-7354-1500-3/$30.00
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enhancement of conductivity with the rising temperature, which may be due to the lowering of barrier for which mobility of charge carrier increases5.
RESULTS AND DISCUSSION X-ray Diffraction The XRD patterns of all doped compounds are shown Figs. 1(a). The patterns were compared with standard data JCPDF card no. 71-2274 for pure and card no. 89-4391 for Sm- Bi2O3 sample respectively. In our earlier work3, the phase of pure bismuth oxide ZDV FRQILUPHG DV WKH VLQJOH PRQRFOLQLF Į-phase with space group P21/c and XRD patterns all doped compositions confirmed the presence of rhombohedral phase with space group R3m. All other doped compounds possess similar type of crystal structure. Using Debye-Scherer equation crystallite size of prepared nano-structures have been estimated from the highest intensity peak, which are listed in Table-1. This indicates that particle size increases with increase of ionic radii of the dopant cations.
Electrical Transport Properties DC Conductivity The dc conductivity for the prepared samples has been calculated from complex impedance spectra. The Cole-Cole plot along with theoretical fit for the composition La- Bi2O3 is shown in Fig. 2(a). The fit of the impedance plot is achieved by equivalent circuit, (shown in the inset of Fig. 2(a)), using EC-Lab software. In the pictorial representation of the equivalent circuit, shown in the inset of Fig. 2(a), Rb, Re, and Qb, Qe are the resistances and phase elements of bulk and electrode contributions respectively. ‘Q’ represents the modified capacitive phase element, deviated from the ideal behavior. The true value of capacitance (C) is calculated from the expression – (1 a ) a
1 a
(1) C R Q Where, a lies between 0 and 1; Depressed and broad semicircular arcs indicate the presence of non-Debye type relaxation process. All the other samples show similar behavior. The radius of the semicircular arc decreases with increase in temperature indicating TABLE 1. Ionic radii (r) of dopant cations, particle size (D), bulk capacitance (Cb), activation energy for d.c conductivity ( Eaı) and for most probable relaxation time (EaIJ) : Sample
r (Å)
D (Å)
Cb (pF)
BEu30
1.066
1791.6368
BSm30
1.079
BNd30 BLa30
32.6578
Eaı (eV) 1.03
EaIJ (eV) 1.12
1547.2301
48.5612
0.92
0.87
1.109
1801.6157
56.1235
0.87
0.78
1.16
1891.8368
172.145
0.76
0.74
FIGURE 1. XRD patterns of all doped Bi2O3 compositions.
It can be observed that, the dc conductivity increases with increase in measuring temperature, indicating the semiconducting nature of the samples. The reciprocal temperature dependence of the dc conductivity for bulk contribution of all the compositions, shown in Fig. 2(b), obeys the Arrhenius relation. The activation energy EDı has been calculated from the least-squares straight-line fits. The variation of dc conductivity at 4000 C and corresponding activation energy for conduction with ionic radius is shown in the inset of Fig. 2(b). The conductivity value increases to a high extent for the La3+ doped nano structured Bi2O3 in comparison with that of other doped Bi2O3 systems DQG ıdc value of BLa30 is even comparable with that of Dy3+ VWDELOL]HGį-Bi2O3, which is the highest conducting room temperature stable phase of Bi2O3 nano-structures known so far6. Since polarizibility of dopant cations is proportional to the cube of ionic radius7, La3+ is more preferable cation with larger and comparable ionic radius (1.16 Å) with that of Bi3+ (1.17 Å).
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The frequency variation of the imaginary part of the complex impedance (Z'') at different temperatures for Nd- Bi2O3 (BNd30) sample is shown in Fig. 2(c), which can be characterized by some important features such as, (i) Z'' value shows a maxima peak (Z''max) at a particular frequency (UHOD[DWLRQIUHTXHQF\Ȧmax or fmax) that shifts towards higher frequency side with increasing temperature. (ii) The peak value Z''max decreases with increasing temperatures, (iii) with the increase in temperature a typical asymmetrical peak broadening can be observed. The peak frequency is characterized by composition and structural arrangements of atoms in the sample. This peak frequency (fmax) satisfies the theoretical Arrhenius fit –
f max
§ E · f 0 exp¨¨ aW ¸¸ © k BT ¹
(2)
Where, kB Boltzmann constant, T is absolute temperature and EaIJ is the activation energy for relaxation, estimated from the linear fit of log10(fmax) vs 1000/T curve as shown in the inset of Fig. 2(c). Two types of activation energies for all doped compositions are listed in Table-1. Comparable values of Eaı and EaIJ indicate a common mechanism for conduction as well as for relaxation.
Z'ƍ(Ȧ) is scaled by Zƍƍmax. The perfect overlap of all the curves in the relaxation region at different temperatures on a single master curve indicates that the relaxation mechanism is temperature independent. This also infers that, the change in temperature only changes the number of charge carriers without changing the conduction mechanism8.
CONCLUSION The single phase (monoclinic) Bi2O3 has been stabilized into nano structured Bi2O3 rhombohedral phase by doping of rear earth cations Eu3+, Sm3+, Nd3+, La3+. XRD study confirmed the stabilized phase formations in the compositions. Effect of ionic radii of dopant cations on ionic conductivity is investigated and it has been seen that among these four compositions, La stabilized sample shows the highest conductivity due to it closeness in radius with Bi3+. The comparable values of two types of activation energies calculated from complex impedance and modulus spectra indicate a common mechanism for conduction as well as for relaxation. The electrical relaxation mechanism, occurring in the materials, has been found to be temperature independent.
ACKNOWLEDGMENTS The financial assistance and the instrumental support from Department of Science and Technology (Govt. of India) (Grant no: SR/FTP/PS-141-2010) and under departmental FIST programme (Grant no: SR/FST/PS-II-001/2011) and University Grants Commission (UGC) for departmental CAS scheme (Grant no. F.530/5/CAS/2011 (SAP-I)) is thankfully acknowledged.
REFERENCES
FIGURE 2. (a)The complex impedance spectra of BLa30 (the equivalent model is shown in the inset), (b) Arrhenius plots of bulk conductivity ıb) and 9DULDWLRQ RI ıb and Eaı with ionic radius is in the inset (c) Frequency dependence of Z'' and 1000/T variation of log10(fmax) (inset) for BNd30. (d) Master curve of Z'' for BNd30.
The frequency-dependent complex impedance (Z'') isotherms of BNd30 composition are scaled in a master plot and shown in Fig. 2(d), in which each IUHTXHQF\ LV VFDOHG E\ WKH SHDN IUHTXHQF\ Ȧmax and
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