HIGH TEMPERATURE X-RAY DIFFRACTION STUDY OF REACTION RATES IN CERAMICS

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1 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume ABSTRACT HIGH TEMPERATURE X-RAY DIFFRACTION STUDY OF REACTION RATES IN CERAMICS M.S. Peterson, C.A. Say, S.A. Speakman,* and S.T. Misture New York State College of Ceramics, Alfred, NY *Presently at Oak Ridge National Labs, Oak Ridge, TN The reaction rates and formation mechanisms of various n=3 Aurivillius phases were studied. Current research has shown that in order to make phase pure materials via solid-state synthesis, over 100 hours of heat treatment are necessary. A custom high temperature X-ray diffractometer (HTXRD) was utilized to observe in-situ formation reactions. The Bi 2 Sr 2 Nb 2 TiO 12 formation reaction could not be followed to completion in-situ, but the initial reactions were clarified. Starting with precursor powders, at least two Aurivillius phases formed by 650 C, and at 1250 C there was a mixture of SrTiO 3, Bi 2 O 3, SrBi 2 Nb 2 O 9, Bi 2 Sr 2 Nb 2 TiO 12, and SrBi 4 Ti 4 O 15. A phase purity study was done of Aurivillius phases containing aluminum and gallium (Bi 2 Sr 2 Nb 2 GaO 11.5 and Bi 2 Sr 2 Nb 2 AlO 11.5 ). Phase purity is in question for PDF cards and , identifying Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Ta 2 AlO Three distinct phases were found present in polished samples investigated using back-scattered electron imaging and analytical transmission electron microscopy. INTRODUCTION Aurivillius phases have great potential as electrically active materials for a variety of electrochemical and ferroelectric applications. High quality phase pure precursor powders are required for the development of internal circuits, ferroelectric thin films, and electrochemical membranes. The formation mechanisms of Bi 2 Sr 2 Nb 2 TiO 12 were studied in-situ, using a custom high-temperature X-ray diffractometer (HTXRD). Additionally, the phase composition and purity of Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Ta 2 AlO 11.5 were investigated. In 1949, Bengt Aurivillius published a series of papers [1-3] exploring the discovery of mixedmetal oxides having bismuth oxide layers alternating with perovskite structure layers with the general composition Bi 2 A n-1 B n O 3n+3 (where A=Ca, Sr, Ba, Pb, Bi, Na, K, etc., and B=Ti, Nb, Ta, Mo, W, Fe, etc.). The A cations can be mono-, di-, or trivalent ions; or a mixture. The Bi 2 O 2 2+ sheets are separated by perovskite-type blocks (A n-1 B n O 3n+1 ) 2- of variable thickness according to the value of n. Because of their ionic structural framework, Aurivillius phases exhibit great flexibility with respect to metal cation substitution; therefore, these phases have high potential for systematic control of their properties [4]. EXPERIMENTAL In order to study the in-situ formation of Bi 2 Sr 2 Nb 2 TiO 12, a custom high temperature diffractometer (Figure 1a) was used. Stoichiometric mixtures of precursor powders (Bi 2 O 3, Alfa

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Aesar, % pure; SrCO 3, Alfa Aesar, 97.5% pure; Nb 2 O 5, Alfa Aesar, 99.9% pure; TiO 2, Alfa Aesar, 99.9% pure) were mixed in a mortar and pestle. The mixture then was placed on the sample stage shown in Figure 1b, and heated from 25 C to 1300 C at a ramp rate of 30 C/min. Data (Figure 2) was collected at room temperature, and then from 650 C to 1300 C in 50 increments to monitor phase transformations. (a) (b) Figure 1. (a) Custom high-temperature diffractometer (patents pending) and (b) HTXRD sample holder used in this study. Furthermore, phases in the composition region Bi 2 Sr 2 (Nb (2-x) Ta x )(Al (1-y) Ga y )O 11.5 (where 0<x<2 and 0<y<1) were synthesized via solid-state processing to study phase purity. Solid-state synthesis was followed according to temperatures and hold times similar to literature [5,6]. Samples underwent the following heating steps: 800 C for 24 hours, 950 C for 35 hours, 1025 C for 40 hours, and 1120 C for 60 hours. Samples were ground in a mortar and pestle between heating steps to ensure maximum reaction. RESULTS AND DISCUSSION Bi 2 Sr 2 Nb 2 TiO 12 has been produced in pure form using solid-state synthesis techniques. However, obtaining phase pure Bi 2 Sr 2 Nb 2 TiO 12 was not possible on the HTXRD; instead a mixture of n=2, n=3, n=4 Aurivillius phases formed. The in-situ evolution of Bi 2 Sr 2 Nb 2 TiO 12 is shown in Figure 2. All phase identification was done with Jade [7]. The XRD data shows that an n=2 and n=4 Aurivillius phase forms by 650 C from the precursor powders. Between 800 C and 850 C the formation of SrTiO 3 occurs. The final composition of the sample at 1250 C is SrTiO 3, SrBi 2 Nb 2 O 9, Bi 2 Sr 2 Nb 2 TiO 12, and SrBi 4 Ti 4 O 15, but the major phase is Bi 2 Sr 2 Nb 2 TiO 12. Partial melting of the sample occurs between 1250 C and 1300 C. Upon cooling, the sample was found to contain SrTiO 3, Bi 2 O 3, SrBi 2 Nb 2 O 9, and Bi 2 Sr 2 Nb 2 TiO 12. The SrBi 4 Ti 4 O 15 had disappeared upon cooling to room temperature. The phase evolution results are diagrammed pictorially in Figure 3.

4 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume ~ Partial melt Bi 2 Sr 2 Nb 2 TiO C 900 C 650 C 25 C Figure 2. In-situ evolution of Bi 2 Sr 2 Nb 2 TiO 12 using custom high-temperature X-ray diffractometer. Bi 2 O 3 * 25 o C 650 o C 750 o C 850 o C 950 o C 1050 o C 1150 o C 1250 o C SrCO 3 Nb 2 O 3 TiO 2 SrTiO 3 SrBi 2 Nb 2 O 9 (n=2) Bi 2 Sr 2 Nb 2 TiO 12 (n=3) SrBi 4 Ti 4 O 15 (n=4) *Goes through muliple phase changes as temperature is increased Figure 3. The phase evolution of Bi 2 Sr 2 Nb 2 TiO 12 as a function of temperature. The phase purity of cation substituted Aurivillius phases (Bi 2 Sr 2 Nb 2 GaO 11.5, Bi 2 Sr 2 Nb 2 AlO 11.5, Bi 2 Sr 2 Ta 2 GaO 11.5, and Bi 2 Sr 2 Ta 2 AlO 11.5 ) was also studied. Comparison of XRD patterns to literature [5] and PDF cards and , identifying Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Ta 2 AlO 11.5 respectively, appeared to indicate phase purity. However, a combination of Bi 2 Sr 2 Nb 2 TiO 12 and Bi 2 O 3 PDF cards also matched the observed diffraction pattern (Figure 4).

5 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume SEM analysis (Figure 5) further indicated that the sample, identified as phase pure according to PDF cards and , actually consisted of a mixture of phases. Backscattered electron imaging using the SEM revealed second and third phases present in the sample. Secondary electron imaging ensured that porosity was not misinterpreted as a compositional secondary phase in backscattered imaging. Bi 2 Sr 2 Ta 2 GaO 11.5 Bi 2 Sr 2 Ta 2 AlO 11.5 Bi 2 Sr 2 Nb 2 GaO 11.5 Bi 2 Sr 2 Nb 2 AlO 11.5 Figure 4. XRD patterns of pertinent phases and relevant PDF cards.

6 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume Figure 5. a) Secondary e - image of Bi 2 Sr 2 Nb 2 AlO 11.5 specimen, b) Backscattered e - image of Bi 2 Sr 2 Nb 2 AlO 11.5 showing presence of three phases (dark, gray, and bright), c) Backscattered e - image of Bi 2 Sr 2 Nb 2 GaO 11.5 specimen, and d) Backscattered e - image with composition Bi 2 Sr 2 Nb 2 GaO CONCLUSION A custom high temperature X-ray diffractometer was used to investigate the formation of Bi 2 Sr 2 Nb 2 TiO 12. Although the formation reaction of Bi 2 Sr 2 Nb 2 TiO 12 could not be followed to completion, the initial reactions were clarified. Upon heating to 1250 C, the precursor powders form into a variety of different phases, including SrTiO 3, SrBi 2 Nb 2 O 9, Bi 2 Sr 2 Nb 2 TiO 12, SrBi 4 Ti 4 O 15. Cation substituted Aurivillius phases containing Nb, Ta, Al, and Ga were determined to contain three distinct phases instead of the single assumed composition. These unexpected phases were found present in samples using backscattered electron imaging. PDF cards, and , identifying Bi 2 Sr 2 Nb 2 AlO 11.5 and Bi 2 Sr 2 Ta 2 AlO 11.5 describe multiphase specimens and need correction. ACKNOWLEDGMENTS Support was provided in part by the NSF, grant DMR , and by the NYS Center for Advanced Ceramic Technology at Alfred University.

7 Copyright JCPDS - International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Volume REFERENCES [1] B. Aurivillius, "Mixed Bismuth Oxides with Layer Lattices: II. Structure of Bi 4 Ti 3 O 12," Arkiv For Kemi, 1 [58] (1949). [2] B. Aurivillius, "Mixed Bismuth Oxides with Layer Lattices: I. The Structure Type of CaNb 2 Bi 2 O 9," Arkiv For Kemi, 1 [54] (1949). [3] B. Aurivillius, "Mixed Oxides with Layer Lattices: III. Structure of BaBi 4 Ti 4 O 15," Arkiv For Kemi, 2 [37] 519 (1950). [4] T. Rentschler, "Substitution of Lead into the Bismuth Oxide Layers of the n=2 and n=3 Aurivillius Phases," Materials Research Bulletin, 32 [3] (1997). [5] K. R. Kendall, J. K. Thomas, and H.C. z. Loye, "Synthesis and Ionic Conductivity of a New Series of Modified Aurivillius Phases," Chemistry of Materials, 7 [1] 50-7 (1995). [6] W. J. Yu, Y. I. Kim, D. H. Ha, J. H. Lee, Y. K. Park, S. Seong, and N. H. Hur, "A New Manganese Oxide with the Aurivillius Structure: Bi 2 Sr 2 Nb 2 MnO 12-δ," Solid State Communications, 111 [12,14] (1999). [7] Jade, 6, Materials Data Inc., Livermore, CA, 2002.