AN IN SITU HIGH-TEMPERATURE X-RAY DIFFRACTION STUDY OF PHASE TRANSFORMATIONS IN SILVER BEHENATE

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1 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume AN IN SITU HIGH-TEMPERATURE X-RAY DIFFRACTION STUDY OF PHASE TRANSFORMATIONS IN SILVER BEHENATE Thomas N. Blanton 1, Swavek Zdzieszynski 2, Michael Nicholas 2, and Scott Misture 2 1 Eastman Kodak Company, Research & Development Laboratories, Rochester, NY Alfred University, New York State College of Ceramics, Alfred, New York ABSTRACT In situ high-temperature X-ray diffraction (XRD) data have been collected for silver behenate, CH 3 (CH 2 ) 20 COOAg. In the absence of development chemistry silver behenate exhibits four phase transformations when heated from room temperature to 200 ºC. Combining XRD and differential scanning calorimetry (DSC) results, the phase transformation temperatures and phase types have been determined. Type I, II, and III forms of silver behenate are found to be crystalline phases, whereas Type IV and V forms are liquid crystal phases. INTRODUCTION Silver behenate (Figure 1) is a crystalline long-chain silver carboxylate, CH 3 (CH 2 ) 20 COOAg, utilized in commercially available photothermographic and thermographic imaging elements [1]. Figure 1. Molecular structure of a AgBehenate dimer molecule. The silver metal image formation is based on the heat-induced reduction of silver behenate dispersed in a binder incorporated with development chemistry. In the absence of this development chemistry silver behenate exhibits four phase transformations when heated from room temperature to 200 ºC. Further increases in temperature leads to decomposition. The temperature at which these phase transitions occur was determined by differential scanning calorimetry (DSC), shown in Figure 2.

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 2005, Advances in X-ray Analysis, Volume Figure 2. DSC scan for AgBehenate. Four endotherms are observed in the DSC scan, indicating the polymorph transition temperatures, along with decomposition at 226 ºC. The arrows indicate the temperatures selected for in situ high temperature XRD data collection. The room temperature diffraction pattern (Figure 3) for AgBehenate is characterized by a series of low angle diffraction peaks resulting from a d 001 = Å long period spacing [2], along with a few (hkl) diffraction peaks observed at 2θ > 14º. Figure 3. Room temperature XRD pattern for AgBehenate.

4 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume Upon exposure to elevated temperature the X-ray diffraction pattern for silver behenate is expected to change. This study was carried out to observe the low angle diffraction patterns for AgBehenate as a function of temperature, and to understand the phase type formed after each polymorph transition observed in Figure 2. EXPERIMENTAL Silver behenate powder (Eastman Kodak Company) was used as received. The particles have a plate-like morphology as shown in Figure 4 [3]. Figure 4. SEM micrograph of AgBehenate. In situ high-temperature XRD data were collected using the Alfred University high temperature diffractometer (Figure 5). The base is a Siemens D500 goniometer, equipped with a copper sealed tube X-ray source, Goebel mirrors, custom high temperature stage, anti-scatter soller slits, and a Braun linear position sensitive detector. Specimens were prepared by packing powder in 0.5 mm glass or quartz Debye-Scherrer capillaries (Charles Supper Company). Data collection time was 30 seconds per scan.

5 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume Figure 5. Alfred University high temperature diffractometer (left figure) and sample stage (right figure). RESULTS In situ high-temperature data for AgBehenate are shown in Figure 6. At room temperature and at 120 ºC, there is no observable change in the diffraction pattern. This polymorph is designated as Type I AgBehenate. Figure 6. Low-angle in situ high temperature XRD patterns for AgBehenate. Figure 7 shows an overlay of the diffraction patterns for AgBehenate at 120 ºC and 132 ºC. Note that at 132 ºC there is peak broadening and a significant shift in the diffraction peak positions to higher 2θ (smaller d-spacing). Heating a material should result in a shift in diffraction peak

6 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume positions to lower 2θ due to thermal expansion. However in the case of AgBehenate, the alkyl chain tails in the dimer exhibit folding as the temperature increases. This folding leads to a shortening of the long-period spacing and a different packing of the AgBehenate molecules. The endotherm at 128 ºC in the DSC scan is a result of a phase change and we define this phase as Type II AgBehenate. Figure 7. Overlay of XRD patterns for AgBehenate collected at 120 ºC (black) and 132 ºC (blue). A similar observation is made for AgBehenate analyzed by XRD at 155 ºC and 166 ºC, with the overlay shown below in Figure 8. There is a significant peak shift due to further changes in the overall dimer length and molecule packing. Figure 8. Overlay of XRD patterns for AgBehenate collected at 155 ºC (black) and 166 ºC (blue).

7 Copyright JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume AgBehenate at 155 ºC is defined as Type III, at 166 ºC as Type IV. Further changes in the XRD pattern collected at 200 ºC indicate the existence of an additional polymorph, Type V AgBehenate. DSC data in Figure 2 show large endotherms for the silver behenate polymorph transitions of Type I > Type II and Type II > Type III. In contrast, the Type III > Type IV and Type IV > Type V transitions are small. Based on the endotherm peak area, the Type II > Type III transition required J/g consistent with a crystal-to-crystal phase transition. The Type III > Type IV transition required only J/g, which is consistent with a crystal to liquid crystal or liquid crystal to liquid crystal phase transition [4]. Combining XRD and DSC data, Type I, Type II, and Type III AgBehenate polymorphs are crystalline, and Type IV and Type V AgBehenate polymorphs are liquid crystal phases. We were unable to determine the liquid crystal type based on the diffraction results obtained in this study. SUMMARY Silver behenate is used in imaging elements requiring thermal processing to convert silver behenate to silver metal. Coated photographic product contains chemistry that allows this conversion to occur around 122 ºC. In the absence of the developer chemistry, silver behenate has four phase transitions before decomposing at 226 ºC. Using DSC and XRD the room temperature and high temperature polymorphs of silver behenate have been identified. Types I, II, and III are crystalline, Types IV and V are liquid crystal forms of AgBehenate. ACKNOWLEDGEMENTS The authors would like to thank Patti Scioti of Eastman Kodak Company for collecting DSC data. REFERENCES [1] Cowdery-Corvan, P.J. and Whitcomb, D.R., Photothermographic and Thermographic Imaging Materials, Handbook of Imaging Materials, A. Diamond and D Weiss, Eds., Marcel Dekker Inc., New York, [2] Blanton, T.N., Huang, T.C., Toraya, H., Hubbard, C.R., Robie, S.B., Louer, D., Gobel, H.E., Will, G., Gilles, R., and Raftery, T., Powder Diffraction, 1995, 10(2), 91. [3] Huang, T.C., Toraya, H., Blanton, T.N, Wu, W., J. Appl. Cryst., 1993, 26, 180. [4] Mastrangelo, J.C., Blanton, T.N., Chen, S.H., Appl. Phy. Lett., 1995, 66(17), 2212.