Structure determination of magnesium boron nitride, Mg₃BN₃, from X-ray powder diffraction data

The crystal structure of magnesium boron nitride in the low-pressure phase, Mg₃BN3(L), has been solved ab initio from X-ray powder data. The cell is hexagonal (space group P6₃/mmc, Z = 2) with a = 3.54453 (4), c = 16.03536 (30) Å. Initial positional parameters for the Mg atoms were obtained from Patterson functions generated by 50 integrated intensities derived from a whole-powder pattern decomposition. The remaining atoms were located by trial-and-error model building, followed by Rietveld refinements (R_wp = 8.5%). The structure can be described as consisting of ABB'BACC'CA... layers perpendicular to the c axis with linear N=B=N molecular anions at position A, Mg²⁺ at positions B and C and Mg²⁺ with three coordinating N atoms at positions B' and C', although Mg₃BN₃(L) is not a layer compound. A very similar structure has also been obtained by applying standard direct methods to the same intensity data. A high-quality electron-density map has been calculated from the structure factor data using the maximum-entropy method.