Z. Kristallogr. Proc. 1 (2011) 235-240 / DOI 10.1524/zkpr.2011.0035 235 © by Oldenbourg Wissenschaftsverlag, München Core-Shell structure of nanocrystalline AlN in real and reciprocal spaces E. Grzanka 1,* , S. Stelmakh 1 , B. Palosz 1 , S. Gierlotka 1 , Th. Proffen 2 , M. Drygas 3 , J. F. Janik 3 1 Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw, Poland 2 Los Alamos National Laboratory, LANSCE, Lujan Center, Los Alamos, NM, USA 3 AGH University of Science and Technology, Faculty of Fuels and Energy, Krakow, Poland * Contact author; e-mail: elesk@unipress.waw.pl Keywords: nanocrystals, PDF, surface structure Abstract. Investigation of the atomic structure of nanocrystalline AlN approximated by a core-shell model was performed based on neutron diffraction data obtained at NPDF/LANL diffractometer and collected up to about Q=40 Å -1 . The measurements were done at room temperature and at 823 0 C. Both PDF function (real space) and apparent lattice parameters, alp, based on analysis of Bragg scattering (reciprocal space) were examined. PDF analysis shows that all inter-atomic distances r for nanocrystalline powder are smaller than those measured for microcrystalline material, what conforms with alp-Q analysis. The alp-s calcu- lated for c and a lattice parameters change differently with Q and, therefore, also (c/a) ratio is Q-dependent. This is interpreted as that there is an anisotropic strain in the surface shell. At elevated temperatures, due to difference in thermal expansion between surface shell and grain core there is an increasing strain in the surface shell. 1. Introduction Aluminum Nitride is of a paramount importance in ceramics and electronics (as an insulator with advantageous heat conductivity) and is known to be sintered with an addition of sinter- ing additives which are causing property deterioration [1]. Other studies have indicated that under extreme temperatures (in the 1700-1900ºC range) certain AlN nanopowders can be sintered without additives [2]. Using anaerobic synthesis method (conversion of aluminum amide-imide by nitridation) low oxygen content powders of hexagonal AlN with crystallite sizes ranging from several to several tens of nanometers were prepared [3]. The ultimate goal of our studies is to investigate the process of sintering of pure AlN nanopowders without additives. The purpose of present paper is application of a core-shell model to structural analysis of AlN samples and determination of their thermal expansion coefficients, discern- ing between the core and shell components. Data analysis in terms of core-shell models al- lows to derive information about interatomic distances of the interior and surface of nano- grains [4]. A difference between bond length in the grain core and that in a relaxed lattice of a single crystal may be interpreted as a result of a presence of internal pressure (hydrostatic type pressure) which corresponds to either compression or expansion of the lattice.