Neutron diffraction ofγ-aluminium oxynitride

Abstract

Netherlands Energy Research Foundation, POB 1, 1755 ZG Petten, The Netherlands 7-Aluminium oxynitride (Alon) is a ceramic mater- ial which can be sintered to transparency [1]. It has a spinel-type crystal structure (space group Fd3m) and its defect structure can be described with the constant anion model proposed by McCauley [2]. In this model all anion sites are completely occupied whereas the cation sites are only partly occupied. In the spinel-type crystal structure the position of the cation sites can vary according to a certain spinel parameter (or u-parameter; see, for instance, Galasso [3]). Neutron diffraction was used to estab- lish the exact positions and occupation of sites in the Alon lattice for three compositionally different Alons. Reports of neutron diffraction studies on Alon of a single composition have appeared previ- ously in the literature [4, 5], but the results are inconsistent with respect to the occupation of the cation sites. Neutron diffraction experiments were performed on Alon powders with compositions corresponding to 67.5, 73 and 77.5 tool% A1203. The 73 mol% powder was produced by reacting A1203 and A1N powders for 3 h at 1750 °C. After reaction the resultant powder was ground with a mortar and pestle to eliminate large particles. X-ray diffraction (XRD) of the powder revealed that residual A1N was present. Because the other two powders are not stable at 1750 °C [6], these powders were made at the higher temperature of 1850 °C. When reacting the powders at this temperature a large amount of sintering occurs and side reactions contaminate the powder (mostly with A1N). For these reasons the 67.5 and 77.5 mol % A1203 powders were made in a different way. Tablets of about 10 g were reaction sintered; after sintering, the outer layer was removed and the tablets were crushed in an iron mortar and milled for 8-24 h in isopropanol with iron balls in an iron container. The iron contamination was then re- moved with HNO3 and