B.J. Kennedy, J. Neutron Res. 13, 183 (2005). 11. Ismunandar, T. Kamiyama, A. Hoshikawa, Q. Zhou, B.J.. Kennedy, Y. Kubota and K. Kato, J. Solid State Chem.
Ferroelectric Properties of Ba2Bi4Ti5O18 Doped with Pb2+, Al3+, Ga3+, In3+, Ta5+ Aurivillius Phases A. Rosyidah1, D. Onggo1, Khairurrijal2 and Ismunandar1 1
Inorganic & Physical Chemistry Research Division, Institut Teknologi Bandung, Indonesia Physics of Electronic Materials Research Division, Institut Teknologi Bandung, Indonesia
2
Abstract. In recent years, bismuth layer structured ferroelectrics (BLSFs) have been given much attention because some materials, such as Ba2Bi4Ti5O18, are excellent candidate materials for nonvolatile ferroelectric random access memory (FRAM) applications. BLSFs are also better candidates because of their higher Curie points. Recently, we have carried out computer simulation in atomic scale in order to predict the energies associated with the accommodation of aliovalent and isovalent dopants (Pb2+, Al3+, Ga3+, In3+, Ta5+) in the Aurivillius structure of Ba2Bi4Ti5O18. In this work, the predicted stable phases were synthesized using solid state reactions and their products then were characterized using powder X-ray diffraction method. The cell parameters were determined using Rietveld refinement in orthorhombic system with space group of B2cb. The cell parameters for Ba2Bi4Ti5O18 doped with Pb2+, Al3+, Ga3+,In3+, Ta5+ were a = 5.5006(6) b = 5.4990(5) c = 50.5440(7) Å; a = 5.5012(4) b = 5.4986(8) c = 50.5449(7) Å; a = 5.5006(3) b = 5.4999(3) c = 50.5437(9) Å; a = 5.5007(4) b = 5.4989(7) c = 50.5446(6) Å; and a = 5.5000(5) b = 5.4995(8) c = 50.5436(6) Å. Results from the ferroelectric properties measurement for Ba2Bi4Ti5O18 doped with Pb2+, Al3+, Ga3+, In3+, Ta5+ were Pr = 16.7 µC/cm2, Ec = 35.1 kV/cm; Pr = 15.9 µC/cm2, Ec = 33.8 kV/cm; Pr = 15.6 µC/cm2, Ec = 34.2 kV/cm; Pr = 15.3 µC/cm2, Ec = 34.0 kV/cm; Pr = 16.9 µC/cm2, Ec = 35.6 kV/cm. Keywords: Aurivillius phase, Rietveld refinement, ferroelectric properties, Ba2Bi4Ti5O18 PACS: 77.80.-e, 61.10.Nz
electromechanical coupling factors and low temperature coefficients of resonant frequency[2,3]. Ba2Bi4Ti5O18 is one such BLSFs material. The Bismuth layer-structured ferroelectrics (BLSFs) is crystal structure of Ba2Bi4Ti5O18 is composed of a the common acronym for the Aurivillius phase pseudo-perovskite (Ba2Bi2Ti5O16)2- block interleaved materials that are ferroelectric. The Aurivillius with (Bi2O2)2+ layers along the c-axis. Ba2Bi4Ti5O18 family[1], with general formula of (Bi2O2) with an odd number of BO6 octahedra is expected to (Am-1BmO3m+1) can be described as the combination of exhibit spontaneous polarization along the c-axis. The regular stacking between the (Bi2O2)2+ slabs and Ba2Bi4Ti5O18 polycrystalline ceramic has been perovskite-like (Am-1BmO3m+3)2- blocks. The integer, m, reported[4] to have a dielectric permittivity of 850 at describes the number of sheets of corner-sharing BO6 325 oC (the Curie temperature) and 360 at room octahedra forming the ABO3-type perovskite blocks. temperature; and the electrical properties in the single The 12 coordinate perovskite-like A-site could be a crystal also has been reported[5]. mono-, di- or trivalent element (or combination) with One interesting feature of the Aurivillius phases large cation such as Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, resides in the compositional flexibility of the Bi3+ or Ln3+ and the 6-coordinate perovskite-like B-site perovskite blocks which allows incorporating various could be filled by smaller cations such as Fe3+, Cr3+, cations. It is thus possible to modify the ferroelectric Ti4+, Nb5+ or W6+. Whereas the perovskite blocks properties[6,7] according to the chemical composition. offer large possibilities in terms of compositional Although this phenomenon was observed since many flexibility due to numerous possible combinations of A 2+ years, its structural origin not yet clearly elucidated. and B cations, the cation sites in the (Bi2O2) layers 3+ ATTACHMENT I Dopants are added into a wide variety of Aurivillius in are almost exclusively occupied by Bi . BLFs CREDIT LINE (BELOW) TO BE INSERTED ON THE to FIRST PAGE OF EACH PAPER order modify their properties. The goal in some ceramics are characterized by high Curie points, low EXCEPT THE PAPER ON P. 28 (FOR THIS cases PAPERisUSE THE CREDIT LINE to create or enhanceINdesirable properties, dielectric constants, low dielectric losses,ATTACHMENT low aging, IIwhile BELOW.) in other this is to eliminate or reduce undesirable high dielectric breakdown strengths, strong anisotropic
INTRODUCTION
CP989, Neutron and X-ray Scattering in Materials Science and Biology, International Conference on Neutron and X-ray Scattering 2007, edited by A. Ikram, A. Purwanto, Sutiarso, A. Zulfia, S. Hendrana, and Z. Nurachman © 2008 American Institute of Physics 978-0-7354-0508-0/08/$23.00
117
different as prepared materials are listed in Table 1. Refining the site mixing between the Bi3+ an A-sites proved to be impossible without introducing a refinement constraint. Because it was necessary to allow for atoms to fractionally occupy one site, a constraint was set up to ensure that the total site occupancy added to unity. In order to accomplish the atoms mixing, a new fictive atom was created by splitting one atom into two atoms. Oxygen anion parameters were not refined. From the development of the cell parameters, several distinct effects can be deduced depending on the fraction of iso- and aliovalent substitutions. For all groups of materials, the cell volume remains increase because of the doping.
effects. Subbarao[7] and Newnham[8] showed that it to be a ferroelectric with the highest known Curie point in the bismuth layer-structured ferroelectrics family at that time of 940 oC. However, the information about the effect of doping in this material is still limited. This paper will present results of systematics doping in Ba2Bi4Ti5O18 with the aim that a better understanding of doping effect will be gained. The discussion will be restricted to the substitutional isovalent and aliovalent cation in Ba2Bi4Ti5O18.
EXPERIMENTAL METHOD The polycrystalline sample of Ba2Bi4Ti5O18 with aliovalent and isovalent dopants (Pb2+, Al3+, Ga3+, In3+, Ta5+) were prepared by the standard solid-state reaction method. Stoichiometric quantitaties of Bi2O3, TiO2, BaCO3, PbO, Al2O3, Ga2O3, In2O3 and Ta2O5 (Aldrich Chem. Co.), all with a purity of 99.99%, were thoroughly mixed and ground, and heated in an alumina crucibles at elevated temperature until phase purity was establised. Typical reaction conditions were heated for 24 h at 700 oC, 24 h at 850 oC, 24 h at 1000 o C, and a further 24 h at 1100 oC, with intermediate regrindings between each stage. The sample was slowly cooled to room temperature in air. The purity of the product was monitored by powder X-ray diffraction using monochromatized Cu Kα1 radiation λ = 0.1541 nm. Unit cell parameters were least squares refined by the RIETICA program[9]. The ferroelectric properties of Ba2Bi4Ti5O18 with aliovalent and isovalent dopants were evaluated from the P-E hysteresis curves, using a high-voltage test system (Model RT-66A, Radiant Technologies, Albuquerque, NM)
(a)
RESULTS AND DISCUSSION The X-ray diffraction pattern of the as-prepared powders showed that structures of Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In), Ba2Bi4Ti4.95Ta0.05O18 are orthorombic with space group B2cb. Preliminary examination of the raw X-ray powder diffraction data for the doped composition suggested that these powders are single phase compounds and also can be indexed with the orthorombic symmetry. The Rietveld refinement with orthorombic and B2cb space group were then carried out and proceed without incident. Typical Rietveld plot are shown in Figure 1. The powder X-ray diffraction patterns of all composition Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18 suggest that all doped samples are isostructure with the parent Ba2Bi4Ti5O18[10,11]. The cell parameters of the
(b) FIGURE 1. Rietveld refinement plot showing the observed (+), calculated (solid line) and difference for Ba2Bi3.95Pb0.05Ti5O18 (a) and Ba2Bi3.95Al0.05Ti5O18 (b). The tick marks show the positions of the allowed Bragg reflections in space group B2cb.
118
TABLE 1. Cell parameters for Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and for Ba2Bi4Ti4.95Ta0.05O18 were determined using Rietveld refinement applying orthorhombic system, in space group B2cb. Parameters a b c V
(Å) (Å) (Å) (Å3)
2(a−b) (a+b) r(A) +r(O) t= 2[r(B) +r(O)]
Ba2Bi4Ti5O18*)
Pb2+
Dopant Ga3+
Al3+
In3+
Ta5+
5.4985(3) 5.4980(4) 50.3524(8) 1522.3(1)
5.5006(6) 5.4990(5) 50.5440(7) 1528.88(1)
5.5012(4) 5.4986(8) 50.5449(7) 1528.94(5)
5.5005(3) 5.4999(3) 50.5437(9) 1528.86(4)
5.5007(4) 5.4989(7) 50.5446(6) 1528.89(8)
5.5000(5) 5.4995(8) 50.5436(6) 1528.83(6)
1.82 x 10-4
2.91 x 10-4
4.73 x 10-4
1.09 x 10-4
3.27 x 10-4
0.91 x 10-4
1.006
1.002
0.991
0.992
0.995
1.006
Rp 4.37 7.94 9.73 6.46 7.28 8.25 Rwp 5.51 3.85 4.66 3.05 3.23 5.76 Rexp 1.22 1.25 1.23 1.26 1.30 1.21 RBragg 2.94 8.62 10.80 8.98 8.38 7.54 GOF 1.97 2.36 4.72 3.91 4.67 4.85 Rp = 100∑|yobs-ycal|/|yobs|; Rwp = 100{∑wi(yoi-yci)2/∑wi(yoi)2}1/2; R Bragg = 100∑|Io-Ic|/|Io|; GOF = Rwp/Rexp *) Ismunandar et al. (2004)
Bi(2) sites in (Bi2O2)2+ block and unlikely to occur at any of the perovskite sites (Bi sites and Ti sites). This seems in contrary to the expected, since in term of coordination preference and ionic size these three cations are suitable to substitute Ti4+. However, this reconfirms that vacancy creation in perovskite block needs very high energy. On the other hand, the calculation on Ta5+ dopant shows that substitution is likely to occur at any of the perovskite sites on energetic grounds. The formation single phase compounds and the similarity of the diffraction patterns of Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18 gave further evidence of these site preferences. The observed cell parameters are larger than Ba2Bi4Ti5O18 in agreement with the larger ionic radii of Ba2+ and Bi3+, as could be seen on the VIII coordinated Bi3+ and Ba2+ (XII coordinated Bi3+ ionic radii is unvailable). Compared the degree of the orthorombic splitting with Ba2Bi4Ti5O18, given by 2(a − b) obtained for Ba2Bi3.95Ga0.05Ti5O18 and (a + b)
Ba2Bi4Ti4.95Ta0.05O18 compounds are smaller than those obtained Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, In). These can be explained using tolerance factor, t, defined as r(A) + r(O) where rA is the radius of the 12t= 2[r(B) + r(O)] coordinate A2+ cation, rO, the radius of the 4 coordinate oxygen anion and rB the radius of the six-coordinate Ti4+ cation. As the size of the A-type cation increases through the series Al (0.53) < Ga (0.61) < In (0.76) < Ta (0.78) < Pb (1.49) then t increases, from less then
FIGURE 2. Structure of Ba2Bi4Ti5O18 consist of [Bi2O2]2+ layer interleaved with perovskite-like [Ba2Bi2Ti5O16]2blocks.
Atomistic simulation techniques have been employed to investigate the Aurivillius oxides phases: Bi3TiNbO9, Bi4Ti3O12, BaBi4Ti4O15 and Ba2Bi4Ti5O18[12,13]. The simulation suggested that M3+ dopants (Al, Ga, In) and Pb2+ are favorable in
119
unity in Ba2Bi3.95Al0.05Ti5O18 to greater than 1 in Ba2Bi3.95Pb0.05Ti5O18 and Ba2Bi4Ti4.95Ta0.05O18.
forming O-Ba/Ti-O octahedra. Thus Ba2Bi2Ti5O16 units pose a remarkable similarity to the perovskitetype structure. The height of the perovskite-type layer sandwiched between Bi2O2 layers in Ba2Bi4Ti5O18 is equal to 10 for O-Ba/Ti-O distances or approximately to m = 5 ABO3 perovskites. Aurivillius ceramics are interesting ferroelectrics. The layered structure makes this kind of ferroelectrics have good fatigue endurance[13]. We would like to investigate, what are the effects of doping on the ferroelectric properties. Figure 3 shows the P-E hysteresis loops of the Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) a-d and e for Ba2Bi4Ti4.95Ta0.05O18 ceramics, which indicates the ferroelectricity in this compound. The remanent polarization (Pr) of the different as prepared materials are listed in Table 2. The disadvantage of the layer-structure perovskite materials for high-temperature piezoelectric applications is their relatively high ferroelectricity. This ferroelectricity is electronic-type and therefore, can be suppressed by doping[15-16]. The contribution of each constituent ion to the total spontaneous ferroelectric Polarization is calculated
(a)
(b)
(m ×∆xi ×Qie) Ps = ∑ i V i (c)
(d)
(e) FIGURE 3. The P-E hysteresis loops of the Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) a-d and e for Ba2Bi4Ti4.95Ta0.05O18 ceramics.
The bismuth barium-titanat, Ba2Bi4Ti5O18 have the orthorhombic structure as shown in Figure 2. The structure of Ba2Bi4Ti5O18 is thus built up of [Bi2O2]2+ layer betwen which [Ba2Bi2Ti5O16]2- layers are inserted. In the Ba2Bi2Ti5O16 units, Ba/Ti ions are enclosed by oxygen octahedra which are linked through corners
120
(3)
where mi is the site multiplicity, ∆xi is the atomic displacement along the a-axis from the corresponding position in the tetragonal structure, Qie is the ionic charge of the ith constitute ion, and V is the volume of the unit cell. Figure 4 compares the Polarization calculation results for Ba2Bi3.95Al0.05Ti5 and Ba2Bi4Ti5O18. This clearly shows that the substitution of Al for Bi in the perovskite enhances the Polarization. This is in contrast with the result of similar substitution in two layer Aurivillius, where the substitution results in reducing the total Polarizations[17]. Although it should be noted that considering the estimated standart deviation and the contribution of other ions, the total Polarizations could be reduced. However, the precise oxygen atoms positions would be needed, i.e. neutron powder diffraction experiment is need. Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18 samples for examination of the ferroelectric properties show the saturated hysteresis curves at room temperature along the ab-axis. The ferroelectric properties based on the saturated hysteresis curves, in which the remanent polarization (Pr) and the coercive field (Ec) are in the saturated states. Generally, ferroelectrics depend on Curie temperature have a atomic displacement, which leads to a spontaneous polarization and a switching field[18,19]. In addition, the increase in the strain
energy of the BO6 octahedra from the bismuth layer also is assumed to be increased in Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18, which allows easy movement of the octahedral cations in the electric-field direction. These findings are consistent with those expected for BLSF materials with odd- and evennumbered m, respectively. Polarization reversal occurs by the spontaneous polarization vector
“rocking” up to ~10° [5,20]. Therefore, the Pr and Ec values along the c-axis are smaller than those along the ab-axis. This theory can be applied to Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18. Therefore, in Ba2Bi3.95A0.05 Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18, the Pr and Ec values along the c-axis were smaller than those along the ab-axis.
TABLE 2. The remanent polarization (Pr) of the Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18. Parameters
Ba2Bi4Ti5O18[5]
2
Pr (µC/cm ) Ec (kV/cm)
12.0 30.0
Pb2+ 16.7 35.1
Al3+ 15.9 33.8
Dopant Ga3+ 15.6 34.2
In3+ 15.3 34.0
Ta5+ 16.9 35.6
REFERENCES 1. 2. 3. 4. 5.
B. Aurivillius, Arkiv foer Kemi 1, 463 (1949) T. Takenaka, K. Sakata, J. Appl. Phys. 55, 1092 (1984) B. Frit, J.P. Mercurio, J. Alloys Compd. 188, 27 (1992) B. Aurivillius, P.H. Fang, Phys. Rev. 126, 893 (1962) H. Irie, M. Miyayama, T. Kodo, J. Am. Ceram. Soc. 83, 2699 (2000) 6. Y. Wu, M.J. Forbess, S. Seraji, S.J Limmer, T.P. Chou, C. Nguyen, G. Cao, J. App. Phys. 90, 5296 (2001) 7. E.C. Subbarao, J. Am. Ceram. Soc. 45, 564 (1962) 8. R.W. Wolfe, R.E. Newnham, D.K. Jr. Smith, M.I. Kay, Ferroelectrics 3, 1 (1971) 9. C.J. Howard, B.A. Hunter, Lucas Heights Research Laboratories, NSW, Australia, 1998, pp 1-27. 10. Ismunandar, T. Kamiyama, A. Hoshikawa, Q. Zhou and B.J. Kennedy, J. Neutron Res. 13, 183 (2005) 11. Ismunandar, T. Kamiyama, A. Hoshikawa, Q. Zhou, B.J. Kennedy, Y. Kubota and K. Kato, J. Solid State Chem. 177, 4188 (2004) 12. A. Rosyidah, D. Onggo, Khairurrijal, Ismunandar, Prosiding Seminar Nasional Kimia dan Kongres Nasional Himpunan Kimia Indonesia, (2006), 321-328. 13. A. Rosyidah, D. Onggo, Khairurrijal, Ismunandar, J. Chin. Chem. Soc. (2007), Accepted. 14. J.G. Thompson, A.D. Rae, R.L. Withers, D.C. Craig, Acta Cryst. B 47, 174 (1991) 15. H.S. Shulman, M. Testorf, D. Damjanovic, N.J. Setter, J. Am. Ceram. Soc. 79, 3124 (1996) 16. A. Voisard, D. Damjanovic, N. Setter, J. Eur. Ceram. Soc. 19, 1251 (1999) 17. Y. Shimakawa, Y. Kubo, Y. Nakagawa, S. Goto, T. Kamiyama, H. Asano, Phys. Rev. B. 61, 6559 (2000) 18. S.K. Kim, M. Miyayama, H. Yanagida, J. Ceram. Soc. Jpn. 102, 722 (1994) 19. H. Irie, M. Miyayama, T. Kudo, Jpn. J. Appl. Phys. 38, 5958 (1999) 20. S.E. Cummins, L.E. Cross, J. Appl. Phys. 39, 2268 (1968)
FIGURE 4. The Polarization contribution in Ba2Bi3.95Al0.05 Ti5O18 compared with those in Ba2Bi4Ti5O18.
In summary this experiment has shown that structure of new five layers Aurivillius compound Ba2Bi4Ti5O18 has determined. The non Bi cations which exclusively occupy the inner of perovskite layers result in enhancement of ferroelectricity.
CONCLUSIONS The combined X-ray diffraction and Rietveld refinement confirm that series of Ba2Bi3.95A0.05Ti5O18 (A = Pb, Al, Ga, In) and Ba2Bi4Ti4.95Ta0.05O18 adopt orthorhombic system, in space group B2cb. The the PE hysteresis loops of the ceramics indicates the ferroelectricity in this compound.
ACKNOWLEDGMENT AR thank for financial support from Direktorat Jenderal Pendidikan Tinggi (Dikti), as well as Chemistry Department, Institut Teknologi Bandung.
121
