THE PHASE-TRANSITION OF HBN-CBN IN THE B-N-H-O SYSTEM

The physical-chemical processes are responsible for hBN-cBN conversion in the B-N-H-O system were studied in region of pressure 3.5-7.2 GPa and temperature up to 1400 degrees C by means of in situ differential thermal analysis (DTA) and the quenched method. The absence of interme diate solid phases in products of the interaction of hBN with melts of anhydrous ammonium berates has confirmed the supposition about the ac tivating effect of these melts on the kinetic of the conversion. A sch eme of the part of T,c phase diagram on the (NH4)(2)O-B2O3 line was bu ilt at 6 GPa. Three peritectics corresponding to the dissociation of p roposed NH4BO2, (NH4)(2)B4O7 and (NH4)(4)B10O17 were found. The connec tion of the lower-temperature limits of cBN synthesis regions with the found peritectics in the range of 5.5-7.2 GPa was established. The fo rmal critical composition of the melt was 3(NH4)(2)O:7B(2)O(3), becaus e the appearance of cBN was fixed in the product beginning with just t his composition. However, the question about critical melt composition activating hBN-cBN conversion has no correct solution without informa tion about short-range order structure and relaxation kinetics of the melts. Two unknown anhydrous ammonium berates were found in HPHT produ cts. One of these compounds was determined to be (NH4)(4)B10O17. It cr ystallizes in a orthorhombic cell with a=12.82 Angstrom, b=11.30 Angst rom, c=9.52 Angstrom, a measured density of 2.10 g cm(-3), a calculate d density of 2.21 g cm(-3) at Z=4. At normal conditions (NH4)(4)B10O17 is metastable but it can be preserved a long time in a ''dry'' atmosp here.