Electron density distribution and crystal structure of 27<i>R</i>-SiAlON, Si₃₋<i>_x</i>Al₆₊<i>_x</i>O<i>_x</i>N₁₀₋<i>_x</i> (<i>x</i> ~ 1.9)

Abstract

The 27R-SiAlON crystal with the general formula Si3−xAl6+xOxN10−x (Z = 3) was characterized using laboratory X-ray powder diffraction (Cu Kα1) and energy dispersive X-ray spectroscopy. The [Si:Al] molar ratios were determined to be [0.12(1):0.88(1)], corresponding to x = 1.9(1). The Si1.1(1)Al7.9(1)O1.9(1)N8.1(1) compound is trigonal with space group R3m (centrosymmetric). The hexagonal unit-cell dimensions are a = 0.305991(4) nm, c = 7.1454(1) nm and V = 0.57940(1) nm3. The structural parameters of the initial model were taken from those of 27R-AlON (Al9O3N7), which were subsequently refined by the Rietveld method. The final structural model showed the positional disordering of two of the five types of (Si,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 R3m. The atom arrangements in the four other domains are noncentrosymmetric with the space group R3m. 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 was also reported for 27R-AlON.