Electron density distribution and disordered crystal structure of 15R-SiAlON, SiAl4O2N4

Abstract

The crystal structure of SiAl4O2N4 was characterized by laboratory X-ray powder diffraction (CuKα1). The title compound is trigonal with space group R3̄m. The hexagonal unit-cell dimensions (Z=3) are a=0.301332(3)nm, c=4.18616(4)nm and V=0.3291825(5)nm3. The initial structural model was successfully derived by the charge-flipping method and further refined by the Rietveld method. The final structural model showed the positional disordering of one of the three (Si,Al) sites. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from the MPF were Rwp=5.05%, S (=Rwp/Re)=1.21, Rp=3.77%, RB=1.29% and RF=1.01%. The disordered crystal structure was successfully described by overlapping three types of domains with ordered atom arrangements. The distribution of atomic positions in one of the three types of domains can be achieved in the space group R3̄m. The atom arrangements in the other two types of domains are noncentrosymmetrical with the space group R3m. These two structural configurations are related by the pseudo-symmetry inversion.