Cation Disorder in Pb-Doped SrBi 2 Nb 2 O 9 Brendan J. Kennedy* Centre for Heavy Metals Research, School of Chemistry, F11, The University of Sydney, Sydney NSW 2006, Australia Brett A. Hunter Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai NSW 2234, Australia Received April 12, 2001. Revised Manuscript Received September 4, 2001 Cation disorder in Pb 1-x Sr x Bi 2 Nb 2 O 9 has been studied using a combination of powder synchrotron X-ray and neutron diffraction methods. The oxides all adopt an orthorhombic structure, space group A2 1 am in which the A-type cations, Sr and Pb, are disordered over the Bi 2 O 2 and perovskite layers. Although the Pb shows a marked preference for the Bi 2 O 2 layers, relative to that of Sr, there are still appreciable amounts of Sr present in the Bi 2 O 2 layers. This is discussed in terms of local bonding effects and bond valence sum analysis. Introduction SrBi 2 Ta 2 O 9 -based ferroelectrics have superior fatigue and thin film conductivity properties to the commonly used lead-zirconium-titanium oxides; however, their fabrication requires higher temperatures and, the avail- able data suggests, their electrical properties are very sensitive to the processing conditions. 1 They also have the significant advantage of not containing any lead. SrBi 2 Ta 2 O 9 (SBT) is an Aurivillius-type layered com- pound. Oxides of this type were first reported in 1949 when Aurivillius described the formation of a series of layeredbismuthoxidesofthegeneralformulaBi 2 A m-1 B m O 3m+3 (m ) 1, 2, 3, 4). These consist of R-PbO-type [Bi 2 O 2 ] 2+ layers interwoven with (m - 1) perovskite-type layers having the composition [A m-1 B m O 3m+1 ] 2- . 2 Shortly there- after, Smolenski and Subbarao 3-5 identified these ma- terials as promising ferroelectrics, prompting numerous studies during the 1960s and early 1970s on the preparation and electronic properties of these types of oxides. A large number of these early studies were the subject of the review by Subbarao 6 in 1973. In general Aurivillius-type oxides exhibit a great variability in the metal cation stoichiometry, thus presenting the poten- tial for systematic control of their physical and electronic properties. The A-site cations include Ca, Sr, Ba, Pb, Bi, Na, rare-earth ions, or mixtures of these, while the octahedral B-site invariably contains small highly charged cations such as Ti 4+ , Nb 5+ , Ta 5+ ,W 6+, or Mo 6+ . The m ) 2 oxides, with the general formula ABi 2 M 2 O 9 (A ) Sr or Pb; M ) Nb or Ta), show small distortions from the archetypal tetragonal structure, resulting in orthorhombic symmetry. 7,8 The orthorhombic structures can described by space group A2 1 am, with a and b ≈ 5 Å and c ≈ 25 Å. In comparison with the ease of substitution into the perovskite layers, it was long believed that it was not possible to substitute other cations into the Bi 2 O 2 layers without destroying the structure. 3,9 The Bi 2 O 2 layers are comprised of a square planar net of oxygen anions with the Bi 3+ cations alternatively above and below the plane and can be described as forming caps of BiO 4 square pyramids. The asymmetric coordination environment of the Bi cations is due to the stereochemical activity of the 6s 2 lone pair electrons. It is this distorted environ- ment that is thought to limit cation substitution into the Bi 2 O 2 layers. 9 Recently, it has been established that other cations with sterochemically active lone pair electrons such as Sn 2+ , Sb 3+ , Pb 2+ , or Te 4+ can be introduced, at least in part, into the Bi 2 O 2 layers. 10-15 The first direct experimental evidence for thermally induced disorder in the distribution of the Pb 2+ and Bi 3+ ions over the two different sites in PbBi 2 Nb 2 O 9 was provided by Srikanth, Subbarao, and co-workers using powder neutron diffraction methods. 16 As noted by these authors, the small difference in the neutron scattering lengths of Pb and Bi limits the precision of their * To whom correspondence should be addressed. Phone: 61-2-9351- 2742. Fax: 61-2-9351 3329. E-mail: Kennedyb@chem.usyd.edu.au. (1) Scott, J. F.; Paz de Araujo, C. A.; Scott, M. C.; Huffman, M. Mater. Res. Soc. Bull. 1996, 21, 33. (2) Aurivilllius, B. Ark. Kemi 1949, 1, 463. (3) Subbarao, E. C. J. Phys. Chem. Solids 1962, 23, 665. (4) Subbarao, E. C. J. Am. Ceram. Soc. 1962, 45, 166. (5) Smolenski, G. A.; Isupov, V. A.; Agranovskaya, A. I. Sov. Phys. Solid State 1959, 3, 651. (6) Subbarao, E. C. Ferroelectrics 1973, 5, 267. (7) Rae, A. D.; Thompson, J. G.; Whithers, R. L. Acta Crystallogr., Sect B 1992, 48, 418. (8) Shimakawa, Y.; Kubo, Y.; Nakagawa, Y.; Kamiyama, T.; Asano, H.; Izumi, F. Appl. Phys. Lett. 1999, 74, 1904. (9) Newnham, R. E.; Wolfe, R. W.; Dorrian, J. F. Mater. Res. Bull. 1971, 6, 1029. (10) Rentschler, T. Mater. Res. Bull. 1997, 32, 351. (11) Rentschler, T.; Karus, M.; Wellm, A.; Reller, A. Solid State Ionics 1996, 90, 49. (12) Millan, P.; Castro, A.; Torrance, J. B. Mater. Res. Bull. 1993, 28, 117. (13) Millan, P.; Ramirez, A.; Castro, A. J. Mater. Sci. Lett. 1995, 14, 1657. (14) Castro, A.; Millan, P.; Martinez-Lope, M. J.; Torrance, J. B. Solid State Ionics 1993, 63-65, 897. (15) Castro, A.; Millan, P.; Enjalbert, R. Mater. Res. Bull. 1995, 30, 871. (16) Srikanth, V.; Idink, H.; White, W. B.; Subbarao, E. C.; Rajagopal, H.; Sequeira, A. Acta Crystallogr., Sect. B 1996, 52, 432. 4612 Chem. Mater. 2001, 13, 4612-4617 10.1021/cm010327e CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2001