ELECTRON-DENSITY DISTRIBUTION IN ALN FROM POWDER X-RAY-DIFFRACTION DATA BY THE MAXIMUM-ENTROPY METHOD

The electron density distribution in A1N, which is a hexagonal wurtzite type with noncentrosymmetric structure, has been obtained using powder X-ray diffraction intensity data. Two kinds of A1N samples were used in the present study. One was ultrafine particles, in which 69 volume % aluminum was contained, prepared by a ''nitrogen plasma-metal'' reaction method and the other was pure A1N powder with an average particle size of 1.2 mum. The profile of the X-ray powder data was decomposed into integrated intensities, which were the combined ones owing to violation of Friedel's law. The individual structure factor was determined by an ordinary least-squares analysis. Subsequently the maximum-entropy method (MEM) has been used to obtain an electron-density distribution map. In spite of the fact that the former powder contains only 31 volume % A1N, the electron-density distribution maps for the two kinds of the powder agreed well. As a consequence, the effective charge on the nitrogen ion was -0.84(1)e. The maps agree well with the results from single crystal X-ray diffraction which indicate the lack of a distinct bond charge.