Precursor States in the Phase Transformation Accompanying a Solid-Solid Reaction
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PRECURSOR STATES IN THE PHASE TRANSFORMATION ACCOMPANYING A SOLID-SOLID REACTION
0. CHAIX-PLUCHERY,* J. BOUILLOT,* F. FREUND,** J.C. NIEPCE,*** AND M. PEYRARD**** *ILL, BP 156X, 38042 Grenoble Cedex, France; **Mineralogisches Institut, Universitat K3ln, ZUlpicher Strasse 49, 5000 K3ln 1, Fed. Rep. Germany; ***Laboratoire Rdactivit6 des Solides, Facult6 des Sciences Mirande, BP 138, 21004 Dijon Cedex, France; ****Laboratoire Optique du R~seau Cristallin, Facult4 des Sciences Gabriel, BP 138, 21004 Dijon Cedex, France.
ABSTRACT The studied transformation is associated with the decomposition reaction of brucite-type hydroxides into the corresponding oxide. It is shown, by various techniques (X-ray and neutron diffraction, Raman Spectroscopy, proton conductivity) that some modifications take place in the hydroxide structure before its transformation into the oxide structure,i.e., at a temperature lower than the transformation temperature. A mechanism is proposed to account for the experimental data.
INTRODUCTION A lot of research is done for the knowledge of the structural transformation mechanisms, but very little to understand how the structural transformation is prepared in Solid I before it gives rise to Solid 2. The present work is part of the studies related to the thermal decomposition mechanism of brucite-type hydroxides. Three successive steps are involved in the decomposition reaction : (1) formation of H20 molecules from hydroxyl ions, (2) water elimination out of the solid and (3) transformation of the hydroxide structure into the oxide structure. Step (2) and step (3) are well known in Ca(OH)2, Cd(OH) 2 and Mg(OH) 2 [1-5]. Step (1) was firstly evidenced by earlier studies of Mg(OH)2 [6-8] which indicated that some motions of protons, vacancies and H20 molecules occurred in the hydroxide structure long before the elimination of water. In the present work, X-ray and neutron diffraction and Raman spectroscopy were used to study these prereactional changes - step (1) - occurring in Ca(OH)2, Ca(OD)2 and Mg(OD)2 when temperature and pressure conditions lead the hydroxide gradually from its stable form to the resulting oxide. The experimental results are discussed in terms of a simple model.
FIRST EXPERIMENTAL EVIDENCE OF THE PRECURSOR STATES (STEP
1)
The temperature dependence of the conductivity measured by FREUND et al. [6-8] in magnesium, calcium and aluminium hydroxides have shown that a maximum of the conductivity lies 50 degrees below the on-set of the dehydration (Fig. 1). This can be related to very mobile protons in the hydroxide. Moreover, optical data in Mg(OH)2 suggest the existence of a proton conduction band approximately 2.0 eV above the valence band. According to these results, the mobile protons are delocalised protons and thermally activated to this proton conduction band. Thermally stimulated depolarisation experiments and simple calculations indicate a concentration of mobile protons of the order of 1013 mole-].
Mat. Res. soc. Symp. Proc. Vol. 21 (1984) a Elsevier Science Publishing Co
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