Synchrotron radiation study of MgB2 formation under high pressure

X-ray diffraction with synchrotron radiation was used to study the MgB2 formation in the Mg-B-N system at pressures up to 5.5 GPa and temperatures up to 1900 K. The sequence of chemical reactions and phase transformations in the Mg-B, Mg-B-BN and Mg-BN mixtures was established from energy-dispersive diffraction patterns collected in situ at a constant pressure in the course of a linear heating (or cooling). Over the whole pressure and temperature ranges under study the appearance of the 101 line of MgB2 in diffraction patterns was observed prior to the melting of magnesium in all the cases except the Mg-BN mixture. No other new phases apart from MgB2 were detected in the experiments. Cooling of reaction mixtures leads to the crystallization of MgB2 as the main phase along with the occurrence of MgB4 as a minor impurity.