IN-SITU PHASE TRANSITION CHARACTERISATION PROBED BY SYNCHROTRON RADIATION

Claude Landron

Centre de Recherches sur les Materiaux a Haute Temperature, 45071 Orléans, France.
E-mail: landron@admin.cnrs-orleans.fr

Keywords: synchrotron Radiation, refractory oxides, structure, X-ray Absorption, X-ray Diffraction, high-temperature, coordination shell, phase transition.

Structural characterization of condensed matter at elevated temperature by X-ray constitutes a powerful structural tool in material science. This technique is playing an increasingly important role in determining the atomic distribution in high temperature materials. The third generation synchrotron radiation facilities, now available, produce very intense focused X-ray beams used to efficiently detect transformations at the angstrom scale during phase transition. The brightness of these very intense X-ray sources permits to increase the performance of this probe at the atomic level in extreme conditions of temperature. [1]

We have developed a new high temperature analysis chamber for performing combined Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) measurements on solids and liquids by using a laser heating system and aerodynamic levitation [2]. EXAFS is based on the determination of the attenuation of X-ray beam propagation in a condensed medium. The oscillations of an absorption spectrum depend essentially on the number, the distance, the disorder and the type of the nearest neighbor of the excited atom. Due to the geometry of the system, X-ray absorption spectra have been recorded in the fluorescence mode. The position of the photodiodes detectors has been adjusted close to the sample in order to achieve an optimum fluorescence signal emitted by the sample. Beryllium windows have been used to transmit X-rays and to absorb visible light. The signal/noise ratio was sufficiently high to quantitatively describe the coordination sphere of the cations. The aim of characterization by XRD includes: phase identification, lattice characterization and texture analysis, that is critically important for improving the production of metals, ceramics and other crystalline materials.

In order to perform time resolved diffraction measurements on sample during heating and cooling, we have used a Position Sensitive Detector (PSD). The real time characterization by diffraction permits to follow phase transition phenomena as rapid as 50 ms. we have also recorded diffusion spectra on various liquid oxides with a scintilator coupled to a photomultiplyer. The spectra were recorded on an angular range of 120° giving a complementary information to the pair function data calculated by EXAFS analysis. The interest of measurement of X-ray diffusion oscillation results from the reliability of the information obtained at low k values.

The heating by infrared radiation associated to levitation offers many advantages resulting from the withdraw of the contamination of the sample by the wall of the container and suppression of the heterogeneous nucleation during cooling [3]. Aerodynamic levitation proposes a unique opportunity of studying continuously the solid and the liquid state by covering a wide range of temperature [4]. Our structural results show that gas levitation associated to laser heating is a potential device for contactless characterization of materials allowing the control of particular properties by in-situ synchrotron analysis at elevated temperature. In our experiments, a levitated sample is heated by a sealed 100 W CO₂ laser from room temperature up to 2500°C. This analysis chamber can operate under various oxidizing or reducing atmosphere conditions. The in-situ characterization during the fabrication process benefits both industrial and academic users of synchrotron facilities in materials science providing information on the crystallization route during the solidification. The production of materials as ceramics and glasses becomes more explicit and more reliable.

The performance of the analysis cell developed for combined XAS and XRD experiments allows us to show the complementary properties of these techniques. Experiments have been achieved at LURE (Orsay, France) and at ESRF (Grenoble, France). We have studied the phase transitions in various oxides including lanthanoid sesquioxides. We have shown that combined X-ray absorption and X-ray diffraction data contain valuable information on the evolution of disorder in refractory oxides at very high temperature.

[1] G. Jacob, I. Egry, K. Maier, D. Platzek, J. Reske and R. Frahm, Rev. Sci. Instrum. 67, (1996) 3683-3690.
[2] C. Landron in Mass Charge Transport in Ceramics, Ceramic Transaction, Volume 71. Edited by K. Koumoto, L.M. Sheppard and H. Matsubara. Pub. The American Ceramic Society, Westerville, USA, (1996) 201-212.
[3] C. Landron, X. Launay, J.C. Rifflet, Y. Auger, D. Ruffier, J.P. Coutures, M. Lemonier, M. Gailhanou, M. Bessiere and D. Bazin and H. Dexpert. - Nuclear Instruments and Methods B 124, (1997) 627-632.
[4] S. Ansell, S. Krishman, J.K. Weber, J.J. Felten, P.C. Nordine, M.A. Beno, D.L. Price and M.L. Saboungi, Phys. Rev. Lett., 78 (1997) 464-468.