X-ray powder diffraction with synchrotron radiation was used to study the f
ormation of cubic boron nitride (cBN) in the MgB2-BN system at pressures to
6.8 GPa and temperatures to 2000 K. For the formation of cBN, when it crys
tallizes from a BN solution in the melt of the system under study, the thre
shold pressure (4.5 +/- 0.1 GPa) and low-temperature boundary (T (K) = 1633
- 9.2p (GPa)) have been established. We have calculated pressure and tempe
rature dependences of the rate of cBN nucleation and have found that the po
sition of the high-temperature boundary of the p,T region of the cBN crysta
llization is specified by the nucleation of the cubic phase. The existence
of the threshold pressure of cBN crystallization is dictated by the strong
pressure dependence of the nucleation rate. It is just this dependence that
is responsible for the fact that the region of the cBN spontaneous crystal
lization is well off the hBN reversible arrow cBN equilibrium line by sever
al hundred degrees.