Formation of cubic boron nitride in the system Mg3N2\mathrm{Mg_3N_2}-BN: a new contribution to the phase diagram

New experimental results for cubic boron nitride (cBN) formation in the system Mg3N2-BN are presented and discussed. Generally, two different regions may be distinguished. At high pressures and temperatures precipitation processes from a melt via nucleation and growth mechanisms dominate. Owing to the occurrence of two different intermediate compounds, Mg3BN3 at the beginning of the cBN-formation region and a newly discovered compound more BN-rich at higher temperatures, the kinetics changes remarkably in that region. A generally different formation mechanism is observed at low pressures (0.5 GPa) and only at enhanced temperatures (480 K) after a quench. Under these conditions the new BN-rich compound becomes unstable and decomposes into cBN and another product by fast solid-solid transformation processes. Consequences for the resulting phase diagram are discussed

Rapid formation of cubic boron nitride in the system Mg3N2−\mathrm{Mg_3N_2-}hBN

The catalytic transformation from hBN to cBN in the system Mg3N2-BN was investigated in more detail under high pressures and temperature using in-situ sensing methods. At the threshold of the cBN formation region (approximately 1550 K at 5.5 GPa), very fast formation processes leading to submicron cBN grains are observed in a small temperature interval (140 K at 5.5 GPa). The transformation proceeds in a eutectic melt. Model assumptions of dissolution and precipitation could be confirmed. The only intermediate phase formed under these thermodynamic conditions is Mg3BN3 thus forming the solvent for this process

High pressure X-ray diffraction study of CdAl<SUB>2</SUB>Se<SUB>4</SUB> and raman study of AAl<SUB>2</SUB>Se<SUB>4</SUB> (A=Hg, Zn) and CdA<SUB>l2</SUB>X<SUB>4</SUB> (X=Se, S)

We report the results of an X-ray diffraction study of CdAl2Se4 and of Raman studies of HgAl2Se4 and ZnAl2Se4 at room temperature, and of CdAl2S4 and CdAl2Se4 at 80 K at high pressure. The ambient pressure phase of CdAl2Se4 is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at 9 GPa is about 10%. The analysis of the Raman data of HgAl2Se4 and ZnAl2Se4 reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl2S4 and CdAl2Se4, which shows a very weak temperature dependence of the Raman-active phonon modes