| Summary: | The fractions of aluminium, [Al]/[Al+Si], and oxygen, [O]/[O] in crystallographically distinct sites of BaYb[Si4-xAlxOxN7-x] oxonitridoaluminosilicate (space group P63mc, No. 186) were refined based on the results of neutron powder diffraction for a synthetic sample with the composition of x 2.2(2) and simulated as functions of temperature for the compositions x 2 and x 2.3 using a combination of static lattice energy calculations (SLEC) and Monte Carlo simulations. The SLEC calculations have been performed on a set of 800 structures differing in the distribution of Al/Si and O/N within the 2 2 2 supercell containing 36 formula units of BaYb[Si4-xAlxOxN7-x]. The SLEC were based on a transferable set of empirical interatomic potentials developed within the present study. The static lattice energies of these structures have been expanded in the basis set of pair-wise ordering energies and on-site chemical potentials. The ordering energies and the chemical potentials have been used to calculate the configuration energies of the oxonitridoaluminosilicates (so-called sialons) using a Monte Carlo algorithm. The simulations suggest that Al and O are distributed unevenly over two non-equivalent T(Si/Al) and three L(N/O) sites, respectively, and the distribution shows strong dependence both on the temperature and the composition. Both simulated samples exhibit order/disorder transitions in the temperature range 500-1000 K to phases with partial long-range order below these temperatures. Above the transition temperatures the Si/Al and N/O distributions are affected by short-range ordering. The predicted site occupancies are in a qualitative agreement with the neutron diffraction results.
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