The phase transition of hBN-cBN in the B-N-H-O system

Abstract

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°C by means of in situ differential thermal analysis (DTA) and the quenched method. The absence of intermediate solid phases in products of the interaction of hBN with melts of anhydrous ammonium borates has confirmed the supposition about the activating effect of these melts on the kinetic of the conversion. A scheme of the part of T,c phase diagram on the (NH 4) 2O-B 2O 3 line was built at 6 GPa. Three peritectics corresponding to the dissociation of proposed NH 4BO 2, (NH 4) 2B 4O 7 and (NH 4) 4B 10O 17 were found. The connection 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 formal critical composition of the melt was 3(NH 4) 2O:7B 2O 3, because the appearance of cBN was fixed in the product beginning with just this composition. However, the question about critical melt composition activating hBN-cBN conversion has no correct solution without information about short-range order structure and relaxation kinetics of the melts. Two unknown anhydrous ammonium borates were found in HPHT products. One of these compounds was determined to be (NH 4) 4B 10O 17. It crystallizes in a orthorhombic cell with a=12.82 Å, b=11.30 Å, c=9.52 Å, a measured density of 2.10 g cm −3, a calculated density of 2.21 g cm −3 at Z=4. At normal conditions (NH 4) 4B 10O 17 is metastable but it can be preserved a long time in a “dry” atmosphere.