X-ray diffraction with synchrotron radiation was used to study MgB2 formation in the Mg–B–N system at pressures up to 5.5GPa and temperatures up to 1900K. 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 studied, the appearance of the 101 line of MgB2 in diffraction patterns was observed prior to the melting of Mg in all cases except the Mg–BN mixture. No other new phases apart from MgB2 were detected. Cooling of reaction mixtures led to the crystallization of MgB2 as the main phase along with the occurrence of MgB4 as a minor impurity.

Synchrotron Radiation Study of MgB2 Formation under High Pressure. A.N.Baranov, V.L.Solozhenko, C.Lathe, V.Z.Turkevich, Y.W.Park: Superconductor Science and Technology, 2003, 16, 1147-51