Electron density distribution and crystal structure of 27R-AlON, Al9O3N7

Abstract

The crystal structure of Al 9 O 3 N 7 was characterized by laboratory X-ray powder diffraction (Cu K α 1 ). The title compound is trigonal with space group R 3̄ m (centrosymmetric). The hexagonal unit-cell dimensions ( Z =3) are a =0.30656(2) nm, c =7.2008(3) nm and V =0.58605(5) nm 3 . The initial structural model was derived by the powder charge-flipping method and subsequently refined by the Rietveld method. The final structural model showed the positional disordering of two of the five types of Al sites. The maximum-entropy method-based pattern fitting 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 disordered crystal structure was successfully described by overlapping five types of domains with ordered atom arrangements. The distribution of atomic positions in one of the five types of domains can be achieved in the space group R 3 ¯ m . The atom arrangements in the four other domains are noncentrosymmetric with the space group R 3 m . Two of the four types of domains are related by a pseudo-symmetry inversion, and the two remaining domains also have each other the inversion pseudo-symmetry. The very similar domain structure has been also reported for 21 R -AlON (Al 7 O 3 N 5 ) in our previous study. A bird’s eye view of electron densities up to 50% (0.074 nm −3 ) of the maximum on the plane parallel to (110) with the corresponding atomic arrangements of Al 9 O 3 N 7 . • Crystal structure of Al 9 O 3 N 7 is determined by laboratory X-ray powder diffraction. • The atom arrangements are represented by the split-atom model. • The maximum-entropy method-based pattern fitting method is used to confirm the validity of the model. • The disordered structure is described by overlapping five types of domains with ordered atom arrangements.