Transformation of the corundum structure upon high-temperature reduction
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TRUCTURE OF INORGANIC COMPOUNDS
Transformation of the Corundum Structure upon HighTemperature Reduction A. Ja. Dan’ko, M. A. Rom, N. S. Sidelnikova, S. V. Nizhankovskiy, A. T. Budnikov, L. A. Grin’, and Kh. Sh-o. Kaltaev Scientific and Technological Corporation “Institute for Single Crystals,” National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkov, 61001 Ukraine e-mail: [email protected] Received November 1, 2006
Abstract—This paper reports on the results of investigations into the transformation of the corundum structure upon reducing annealing at high temperatures in the range from 1700 to 2050°C. It is established that the reduction results in the transformation of corundum into new phases with a lower oxygen content, including the phase with a spinel structure. These structures are assumed to be stabilized by anion vacancies. A model of the crystal structure of the spinel phase is proposed. This model provides an adequate description of the compound obtained in the experiment. PACS numbers: 61.66.-f DOI: 10.1134/S1063774508070031
INTRODUCTION At present, the only well-studied oxide that exists in the condensed state in the aluminum–oxygen system is aluminum oxide Al2O3. The data available in the literature on the existence of oxide compounds of aluminum with the lowest valence in the condensed state are very scarce. Filonenko et al. [1] were the first to report on the formation of the AlAl2O4 phase (the alumina spinel) upon the carbothermal reduction of corundum. There are only few publications containing information on the preparation of some compounds (including the aforementioned phase) in the aluminum–oxygen system; however, up to now, special investigations in this direction have not been performed. The present study continues a series of investigations [2–4] concerning the transformation of corundum into phases with a different symmetry upon reducing annealing at high temperatures.
and reducing additives (CO, H2, and CO + H2). In the case when the chamber was filled with argon, the formation of the reducing medium in the chamber occurred spontaneously as a result of the interaction of adsorbed oxygen and water vapors with carbon–graphite materials. The concentration of the reducing component in this case did not exceed 5%. The concentration of the reducing component (CO + H2) in the gaseous medium was determined with the use of a Kristall 2000M gas chromatograph. The duration of annealing was varied from 2 to 10 h. The temperature of the samples was controlled using a Marathon MRISCSF integrated infrared pyrometer. The X-ray powder diffraction analysis was carried out on a DRON-1.5 diffractometer (Cu K α1, 2 radiation, pyrolytic graphite(002) monochromator, θ–2θ scan mode). The thickness of the polycrystalline zone formed on a sapphire substrate was determined according to standard techniques [2].
OBJECTS AND METHODS OF INVESTIGATION
RESULTS AND DISCUSSION
For our investigations, we used plane-parallel sapphire samples and layers of finely dispersed α-Al2O3 powders (with particle sizes of
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