We have successfully developed a method to manufacture spinel-type Si_3N_4 and also a chemical treatment method to separate the spinel-type phase from the low-pressure phases. Similar methods could be applied for the SiAlON systems. In order to explore high-pressure nitrides and oxynitrides, we extended toward the system Si_3N_4-AlN-Al_2O_3 SiO_2. According to the results of in situ measurements of the high-pressure behavior up to pressures of 200 GPa, there appears to be post-spinel phase in the system. This is consistent with the results from the first principles calculations. However we could not obtain the post-spinel phase by the shock recovery experiments at present. We also carried out shock recovery experiments on carbon nitrides and related materials. Experimental results showed formation of a new carbon nitride, high stability of melamine up to a shock pressure of 37 GPa, and production of amorphous C-N materials with the highest N/C ration of 1.26 from the reaction between carbon tetrahalide and sodium dicyanoamide. We tried further to extend toward the systems C_3N_4-Si_3N_4 and Mg_2SiO_(4-) Si_3N_4, after taking into account the results on shock wave synthesis of spinel-type nitrides.
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