New investigations on shock-wave synthesized high-pressure phases in the system Si-Al-O-N
Abstract
The shock-wave synthesis of nanostructured high-pressure phases at a gram-scale permits the analysis of spinel type nitrides with different chemical composition using methods not suitable for microgram amounts of material. Methods with a significant mass loss through the analytical process like TG-MS or FT-IR or bulk methods at the g-scale like 29Si-MAS-NMR or neutron diffraction were used. The synthesis of pure high-pressure modifications (gamma-phases) of different SiAlON-compounds using amorphous H-bearing precursors at pressures of 30-40 GPa is a necessary prerequisite for precise determinations of crystal chemical features. Etching with HF is a well-known method to purify the high-pressure nitrides (Sekine 2002). The etched parts were analyzed by neutron diffraction, TG-MS, and carrier gas hot extraction (CGHE). Volatile elements like H2 and Cl2, as well as non-stoichiometric oxygen and nitrogen, and NOx, H2O are enriched in the disordered rims. This degassing process ends at temperatures of approximately 600°C, while the spinel structure remains well preserved up to 1300°C. Under these conditions the gamma-phases stay unchanged under air, argon and vacuum. Furthermore chlorine, an important impurity of the H-bearing precursors neither influences the synthesized products nor the synthesis process itself. IR-spectroscopy of gamma-Si3(O,N)4 shows that peak shifts of octahedral lattice vibrations (≈ 680 cm-1) and both tetrahedral vibrations (ny3 and ny4) (Jeanloz 1980, Preudhomme & Tarte 1971) to higher frequencies with decreasing oxygen content occur. This effect is also visible in samples contaminated with impurities of low pressure modifications. The more complex structure of gamma-SiAlON and the simultaneously exchange of the cation- and the anion-positions prevents the appearance of this important feature. Yet to be synthesized pure gamma-SiAlON using similar H-bearing precursors is necessary to resolve its structure. Sekine, T., H. He, T. Kobayashi, K. Kimoto, and M. Mitomo (2002), Spinel Type Sialon, Spinel Type Silicon Oxinitride and Methods for producing their Powders, US 2002/0045529 A1. Jeanloz, R. (1980), Infrared spectra of olivine polymorphs: α, β phase and spinel, Phys. Chem. Mineral, 5(4), 327-341. Preudhomme, J., and P. Tarte (1971), Infrared studies of spinels—II: The experimental bases for solving the assignment problem, Spectrochimica Acta Part A: Molecular Spectroscopy, 27(6), 845-851.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI41A2285S
- Keywords:
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- 3944 MINERAL PHYSICS / Shock wave experiments;
- 3947 MINERAL PHYSICS / Surfaces and interfaces;
- 3994 MINERAL PHYSICS / Instruments and techniques