Compressibility of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4−xAlxOxN7−x] (x = 2; M = Sr, Ba)

The compressibilities of the nitridosilicate SrYb[Si4N7] and the oxonitridoaluminosilicates MYb[Si4−xAlxOxN7−x] (x = 2; M = Sr, Ba) were investigated by in situ high-pressure X-ray powder diffraction. Pressures up to 42 GPa were generated using the diamond–anvil cell technique. The title compounds are structurally stable to the highest pressure obtained. A fit of a third-order Birch–Murnaghan equation-of-state to the p–V data results in V0 = 302.91 (6) Å3, B0 = 176 (2) GPa and B′ = 4.4 (2) for SrYb[Si4N7]; V0 = 310.4 (1) Å3, B0 = 161 (2) GPa and B′ = 4.6 (2) for SrYb[Si4−xAlxOxN7−x]; and V0 = 317.3 (5) Å3, B0 = 168 (2) GPa and B′ = 4.7 (2) for BaYb[Si4−xAlxOxN7−x]. While the linear compressibilities of the a and c axes of BaYb[Si4−xAlxOxN7−x] are very similar up to 30 GPa, distinct differences were observed for SrYb[Si4N7] and SrYb[Si4−xAlxOxN7−x], with the c axis being the most compressible axis. In all of the investigated compounds the bulk compressibility is dominated by the compression behaviour of the tetrahedral network, while the size of the substituted cation plays a minor role

Coupled Al/Si and O/N order/disorder in BaYb[Si4–xAlxOxN7–x]sialon

The fractions of aluminium, [Al]/[Al + Si], and oxygen, [O]/[O + N], 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 calcu lations 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

Reaction of titanium with carbon in a laser heated diamond anvil cell and reevaluation of a proposed pressure-induced structural phase transition of TiC

The formation of cubic TiCₓfrom the elements was studied in a laser-heated diamond anvil cell at pressures up to 26 GPa. Annealed samples at these pressures show no splitting of the cubic (1 1 1) or (2 2 2) reflection. This is in contrast to an earlier study, in which a splitting of the (1 1 1) reflection was observed above 18 GPa. The recovered sample had a lattice parameter of 4.3238(6) Å, which implies that the synthesis gave fully stoichiometric TiC. Density functional theory-based model calculations were used to study the dependence of the total energy of a rhombohedral distortion. In these model calculations the total energy was minimal for the undistorted (cubic) lattice. Therefore, the results obtained here imply that at least for titanium carbides with a high carbon content no pressure-induced structural phase transition up to at least 26 GPa occurs. The appearance of a trigonal TiCₓ polymorph during the synthesis is discussed in terms of its relative stability with respect to the cubic phase.6 page(s

Stability field of the high-(P, T) Re₂C phase and properties of an analogous osmium carbide phase

The formation of a hexagonal rhenium carbide phase, Re₂C, from the elements has been studied in a laser heated diamond anvil cell in a P, V range of 20-40 GPa and 1000-2000 K. No indication for the existence of cubic rhenium carbide, as suggested in the literature, or any other phase was found and Re₂C is the only phase formed in the Re-C system up to around 70 GPa and 4000 K. A fit of a 3rd-order Birch-Murnaghan equation of state to the Re₂C P, V-data results in a bulk modulus of B₀ = 405 (30) GPa (B' = 4.6). The linear compressibility of Re₂C along [0 0 1] (∼ 500 GPa) is significantly larger than the compressibility in the (0 0 1) plane (∼ 360 GPa ∥[100]). Based on the observations for Re₂C, we predict the structure and elastic properties of an analogous Os₂C phase which is at least in the athermal limit more stable than any other osmium carbide studied previously by density functional theory based calculations.5 page(s

Virtual crystal approximation study of nitrido silicates and oxonitridoaluminosilicates

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In situ observation of the reaction of tantalum with nitrogen in a laser heated diamond anvil cell

8 page(s