USE OF IN-SITU XRD TO DEVELOP CONDUCTING CERAMICS WITH THE AURIVILLIUS CRYSTAL STRUCTURE

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1 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume USE OF IN-SITU XRD TO DEVELOP CONDUCTING CERAMICS WITH THE AURIVILLIUS CRYSTAL STRUCTURE Scott A. Speakman, Viral B. Modi, Mike S. Haluska, and Scott T. Misture New York State College of Ceramics Alfred University, Alfred NY ABSTRACT In-situ X-ray diffraction was used to study structural transformations in several n=3 Aurivilliustype phases: Bi 4 Ti 3 O 12, Bi 2 La 2 Ti 3 O 12, Bi 2 Sr 2 Nb 2 TiO 12, Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO Order-disorder transformations have been linked to transitions to fast ion conduction in certain oxygen-deficient phases, such as Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO Bi 2 La 2 Ti 3 O 12 and Bi 2 Sr 2 Nb 2 TiO 12 were synthesized and indexed as body-centered tetragonal with 4/mmm Laue symmetry. At room temperature, Bi 4 Ti 3 O 12 has an orthorhombic unit cell with space group symmetry B2ab. As previously documented, it transforms to tetragonal above its Curie temperature of 675 C. Indexing of XRD data collected for this study indicates the high temperature phase is body-centered tetragonal with 4/mmm Laue symmetry, similar to the structures of Bi 2 La 2 Ti 3 O 12 and Bi 2 Sr 2 Nb 2 TiO 12. None of these phases undergo additional phase transformations up to 1100 C. Both Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 have oxygendeficient perovskite-layers, with either primitive tetragonal or orthorhombic unit cells at room temperature. Both materials undergo multiple phase transformations. Above 875 C, the unit cells are similar to those of the non-oxygen deficient n=3 Aurivillius-type phases, being bodycentered tetragonal with 4/mmm Laue symmetry. This behavior agrees with the expectations for oxygen-vacancy order-disorder transformations. The intermediate phases are not yet fully understood: they may be associated with partial disordering of oxygen vacancies or with a ferroelectric transition. INTRODUCTION Materials that are fast oxide-ion conductors are necessary for electrochemical devices such as solid oxide fuel cells. Ionic conductivity in these materials is often intimately linked to crystal structure. Materials with fluorite, pyrochlore, perovskite, and brownmillerite crystal structures have been commonly investigated as conducting ceramic oxides.[1] This study uses in-situ X-ray diffraction to investigate materials with the Aurivillius crystal structure. These are bismuth oxide/perovskite layered-intergrowth structures, often described with the composition Bi 2 O 2 A n-1 B n O 3n+1. Bismuth-oxygen layers, Bi 2 O 2 2+, sandwich blocks of perovskite or oxygen-deficient perovskite layers, A n-1 B n O 3n+1 2-.[2] Aurivillius-type phases are categorized according to n, the number of perovskite layers between each bismuth-oxygen layer. Several n=3 Aurivillius-type materials were studied: Bi 4 Ti 3 O 12, Bi 2 La 2 Ti 3 O 12, Bi 2 Sr 2 Nb 2 TiO 12, Bi 2 Sr 2 Nb 2 AlO 11.5, and Bi 2 Sr 2 Nb 2 GaO The latter two are oxygen-deficient and have been shown to become fast ion conductors at elevated temperatures.[3] They possess a high concentration of intrinsic oxygen vacancies because of cation substitutions. These vacancies are ordered at lower temperatures, and are not highly mobile. At elevated temperatures, the vacancies disorder and become mobile. In-situ X-ray diffraction was used to study these order-

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3 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume disorder transitions. This should provide insight into the nature of these materials, so that they might better be exploited. PROCEDURE The Aurivillius phases were prepared by mixing stoichiometric amounts of Bi 2 O 3, SrCO 3, TiO 2, Nb 2 O 5, Al 2 O 3, and Ga 2 O 3. The batches were ball-milled in a water or isopropanol slurry. After the powders were dried and recovered, they were pressed into pellets at approximately 34.5 MPa. These pellets were calcined in covered MgO crucibles at 800 C for 24 hours, 850 C for 8 hours, then 900 C for 10 hours. The dwell time at 800C was required to react Bi 2O 3, as at higher temperatures it tended to melt and react with the crucible or to vaporize. The specimens were cooled, ground, repressed into pellets, and then calcined in covered MgO crucibles at 1000 C for 24 hours. Powder X-ray diffraction was used to evaluate phase purity. The batches were then repeatedly ground, pressed into pellets, and fired at 1100 C in ~24 hour cycles until phase purity was achieved. Bi 4 Ti 3 O 12 required 48 hours of total calcining time at 1100 C to achieve phase purity; Bi 2 Sr 2 Nb 2 AlO 11.5 required 72 hours; and Bi 2 La 2 Ti 3 O 12, Bi 2 Sr 2 Nb 2 TiO 12 and Bi 2 Sr 2 Nb 2 GaO 11.5 required 100 hours of total calcining time at 1100 C. For indexing room temperature patterns, a Siemens d500 diffractometer with Cu radiation was used. Samples were prepared on a zero-background holder. NIST 640C silicon was used as an external standard. Patterns were calibrated and peaks profile fit using Jade 5.[4] Chekcell was used to refine the unit cell and determine the most likely space group.[5] LSQWIN, an in-house version of NBSLSQ modified for the MS Windows environment, was then used for final cell refinement and indexing.[6] In-situ XRD was performed using a Siemens d500 equipped with a custom built furnace. This furnace consists of a high-vacuum capable stainless steel canister, a Be window, and two hemispherical heating elements. To minimize sample displacement during heating, the sample holder rests on alumina pins located outside the heating zone. The sample holder was a plate of alumina. A linear position sensitive detector (PSD) was used to collect the diffraction patterns. The 10 º2è collection window of the PSD allows for quick measurements, and therefore time resolved studies. Ká-Cr radiation was used. X-ray data was used for phase analysis and timetemperature resolved studies of phase transitions. A temperature calibration curve was produced for the in-situ X-ray diffractometer using the phase transformations of NIST DTA standards, SRM 759 and SRM 760. Phase purity of Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 was also analyzed using an EVEX system energy dispersive spectrometer attached to a Philips 515 SEM. DISCUSSION Table I summarizes the room temperature indexing results for Bi 4 Ti 3 O 12, Bi 2 La 2 Ti 3 O 12, Bi 2 Sr 2 Nb 2 TiO 12, Bi 2 Sr 2 Nb 2 AlO 11.5, and Bi 2 Sr 2 Nb 2 GaO Bi 4 Ti 3 O 12, Bi 2 La 2 Ti 3 O 12, and Bi 2 Sr 2 Nb 2 TiO 12 are related in that they are n=3 Aurivillius phases with perovskite blocks that are not oxygen deficient. Thus, they are not expected to undergo oxygen vacancy order-disorder transformations.

4 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Table I. Indexing results for n=3 Aurivillius phases at room temperature. Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 may actually be orthorhombic, with an orthorhombic distortion near 1. Space Group or (Laue Group) Lattice Parameters in Å (esd) Bi 4 Ti 3 O 12 B2cb 5.449(1) x (2) x (1) Bi 2 La 2 Ti 3 O 12 (4/mmm) (2) x (4) Bi 2 Sr 2 Nb 2 TiO 12 I4/mmm (1) x (1) Bi 2 Sr 2 Nb 2 AlO 11.5 (4/m) (4) x (4) Bi 2 Sr 2 Nb 2 GaO 11.5 (4/m) (2) x (2) Figure 1. XRD patterns of Bi 4 Ti 3 O 12 above and below the Curie temperature, using Cr-Ká radiation. Bi 2 La 2 Ti 3 O 12 and Bi 2 Sr 2 Nb 2 TiO 12 are indexable as body-centered tetragonal with 4/mmm Laue symmetry. Bi 2 Sr 2 Nb 2 TiO 12 has been described with the I/4mmm space group.[3] Bi 2 La 2 Ti 3 O 12 and Bi 2 Sr 2 Nb 2 TiO 12 do not undergo observable phase transformations up to 1100 C. Bi 4 Ti 3 O 12 is dissimilar, in that at room temperature it has an orthorhombic unit cell with space group symmetry B2ab.[7] Literature notes that above its Curie temperature of 675 C, its structure transforms to tetragonal. This is not an order-disorder transition, however. It is more akin to the transformations of other perovskite-type ferroelectrics.[8] The lattice parameters differ between the two phases, but can be related as ao bo at 2, where subscripts o and t denote the orthorhombic and tetragonal phases. This transformation is shown in Fig. 1. Indexing of this data indicates that above the Curie temperature the lattice is body-centered tetragonal, as listed in Table II, with the space group symmetry likely belonging to the 4/mmm Laue group. Bi 4 Ti 3 O 12 is therefore similar to Bi 2 La 2 Ti 3 O 12 and Bi 2 Sr 2 Nb 2 TiO 12, but only at temperatures above 675 C. None of these phases are observed to undergo additional phase transformations up to 1100 C. Both Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 have crystal structures in which oxygen vacancies are introduced through cation substitution for quadvalent ions. These oxygen vacancies can be treated as a second anion species, where 1 of every 20 oxygen ions in the perovskite block are replaced with an oxygen vacancy. At room temperature, these vacancies are ordered, which lowers the symmetry. Indexing of the room temperature patterns remains ambiguous at this time. As listed in Table I, both patterns are well indexed as primitive tetragonal unit cells, with

5 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Table II. Indexing of Aurivillius phases after their final observed transformation. Patterns are not calibrated. All unit cells index as body-centered tetragonal. T c (C) Indexing Laue Lattice Parameters in Å (esd) Temperature (C) Group Bi 4 Ti 3 O /mmm (6) x 33.23(1) Bi 2 Sr 2 Nb 2 AlO /mmm 3.946(1) x (8) Bi 2 Sr 2 Nb 2 GaO /mmm 3.959(1) x 33.66(1) Figure 2. In-situ XRD patterns for Bi 2 Sr 2 Nb 2 AlO 11.5, using Cr-Ká radiation. space group symmetry most likely belonging to the 4/m Laue group. However, initial structural refinements suggest that the unit cell may actually be orthorhombic, with the cell slightly distorted from the tetragonal version. Both Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 undergo multiple phase transformations on heating. Indexing results, as best understood presently, are listed in Table II. As shown in Fig. 2, Bi 2 Sr 2 Nb 2 AlO 11.5 undergoes transformations between 610 and 660 C, between 730 and 775 C, and between 825 and 875 C. Above 875 C, the unit cell is similar to that of the non-oxygen deficient n=3 Aurivillius-type phases, being body-centered tetragonal with 4/mmm Laue symmetry. Bi 2 Sr 2 Nb 2 GaO 11.5 undergoes phase transformations between 635 and 680 C, and between 825 and 875 C, as shown in Fig 3. Again, above 875 C the unit cell is similar to that of the nonoxygen deficient n=3 Aurivillius-type phases, being body-centered tetragonal with 4/mmm Laue symmetry. This behavior agrees with the expectations for oxygen-vacancy order-disorder transformations. The room temperature structure has a lower symmetry due to the ordering of oxygen vacancies. Once the oxygen vacancies are fully disordered, the structure is similar to that of defect-free phases.

6 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume Figure 3. In-situ XRD patterns of Bi 2 Sr 2 Nb 2 GaO 11.5, using Cr-Ká radiation. The intermediate phases are not yet fully understood. These transformations may be associated with partial disordering of oxygen vacancies, or may related to a ferroelectric transition such as observed in Bi 4 Ti 3 O 12. Investigation with EDS was performed on Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO 11.5 before these conclusions were made. The room temperature XRD patterns seemed appropriate for phase pure materials. However, the XRD patterns could also conceivably fit a binary mixture of a bodycentered tetragonal phase and a doped-bi 2 O 3 secondary phase. The behavior of peaks during phase transformations coincides with the behavior of doped- Bi 2 O 3, with a phase transition at ~675 C and the peaks disappearing between 800 and 900 C (the temperature at which Bi 2O 3 melts). Therefore, an EDS system was used in conjunction with an SEM to confirm phase purity. A powder sample was examined for particles that appeared different, and EDS spectra were collected for several different particles. In prior tests, the procedure was able to identify a Bi-Sr-Nb-Ga-O phase with Ga-doped Bi 2 O 3 impurities. For the samples in this study, however, no inhomogeneities were found. All particles and sample regions tested had similar elemental compositions consistent with Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Nb 2 GaO Thus, it is probable that the samples were indeed phase pure. However, the EDS capabilities of a TEM will be used in the near future to further study phase purity. The qualitative observation and indexing of phase transformations in these n=3 Aurivillius-type phases indicates trends which fit the current understanding of oxygen-vacancy order-disorder transitions. Additional refinement of the crystal structures will allow additional details of these transformations and behavior to be understood. REFERENCES 1. Mazanec, T.J., Get Your O 2 's. Electrochem. Soc. Interface, (4): pp K.R. Kendall, C.N., J.K. Thomas, and H.C. zur Loye, Recent Developments in Oxide Ion Conductors: Aurivillius Phases. Chem. Mater., : pp

7 Copyright (c)jcpds-international Centre for Diffraction Data 2002, Advances in X-ray Analysis, Volume K.R. Kendall, J.K.T., and H.C. zur Loye, Synthesis and Ionic Conductivity of a New Series of Modified Aurivillius Phases. Chem. Mater., : pp MDI Jade 5.0,. 2000, Materials Data, Inc.: Livermore, CA. 5. Bochu, J.L.e.B., CelRef,, Laboratoire des Matériaux et du Génie Physique Ecole Nationale Supérieure de Physique de Grenoble: Saint Martin d'hères. 6. Hubbard, C.R., Lederman, S.M., and Pyrros, N.P. JCPDS-NBS*LSQ82, U.S. National Bureau of Standards, (1982). Updated to Windows by S.I. Zdzieszynski and S.T. Misture, NYS College of Ceramics, Alfred, NY (1998). 7. J.F. Dorrian, R.E.N., D.K. Smith, Crystal Structure of Bi 4 Ti 3 O 12. Ferroelectrics, : pp Subbarao, E.C., Ferroelectricity in Bi 4 Ti 3 O 12 and Its Solid Solutions. Physical Review, (3): pp