od-Sialon ceramics have the general composition Mx(Si,A1)12(O,N)16, where x < 2 and M is a cation such as Li ÷, Ca 2÷, y3+ or most of the lanthanides [1,2]. The structure of the od-sialons is derived from o~-silicon nitride, which has a trigonal space group P31c [3], by partial replacement of Si by A1 and N by O. The M-cation maintains electrical neutrality by occupying interstitial sites within the lattice. Crystallographic studies [4,5] suggest that these M-cations occupy large closed interstices in the (Si,A1)-(N,O) lattice and are positioned at (0.333, 0.667, z) and (0.667, 0.500 + z) [4-8] but do not state whether there is any preferred siting. Initial extended X-ray absolution fine structure (EXAFS) studies of an erbium od-sialon suggest that the Er cation adopts preferred orientation along the c-axis and a sevenfold co-ordination shell of five nitrogens and two oxygens [9].This letter describes further EXAFS spectroscopy of hot-pressed I,n-o/-sialons, where Ln = Yb, studied to establish the "local" environment surrounding it. EXAFS exploits an aspect of X-ray absorption. As X-rays pass through a material, the material absorbs some of the radiation, the degree of which varies smoothly as the energy of the targeted X-rays is increased. However, at the absorption edge there is a sharp increase due to the X-rays having just the right energy to remove a core electron from a particular atom type within the sample. EXAFS is manifested by the oscillations observed on the high-energy side of absorption due to the interference effects between the "outgoing" photoelectron wave and the "b, ackscattering" wave (caused by the reflection of "outgoing wave" by neighbouring atoms). The intensity and frequency of the EXAFS can be interpreted in terms of the number, type and distance of neighbouring atoms. A more detailed description and mathematical representation of EX-AFS is outlined in the initial EXAFS investigation of E r -od-sialon [9].The starting composition of Yb0.ssSia.3sA13.65-O0.8N15.2 was prepared from a mixture of Yb203 (supplied by Rare Earth Products Ltd, UK), Si3N 4 and A1N (both supplied by H. C. Starck, Berlin, Germany). These were wet-mixed in isopropanol for 30 min, followed by dry mixing to overcome the 0261-8028 © 1993 Chapman & Hall sedimentary effects caused by evaporating off the alcohol. The powder mixture was then first preformed into a flat cylinder, with dimensions 17.5 mm (diameter) and 4 m m (height), before being embedded in boron nitride under a pressure of 10 MPa for 30 min at 1750 °C in a graphite die. The sintered compact was then pulverized and submitted to X-ray powder diffraction analysis for phase identification. The sample (together with the Yb203 model compound which was used to establish phase-shift parameters) were then ready for the EXAFS analysis.The EXAFS measurements were performed at the SERC Synchrotron Radiation Source (SRS) at Daresbury (UK), where the YbLIII edge of the Yb-o/-sialon ceramic material and Yb20 3 model compound were recorded. The datasets were collected usi...
