Refined Phase Diagram of Boron Nitride

Abstract

The equilibrium phase diagram of boron nitride thermodynamically calculated by Solozhenko in 1988 has been now refined on the basis of new experimental data on BN melting and extrapolation of heat capacities of BN polymorphs into high-temperature region using the adapted pseudo-Debye model. As compared with the above diagram, the hBN ⇆ cBN equilibrium line is displaced by 60 K toward higher temperatures. The hBN−cBN−L triple point has been calculated to be at 3480 ± 10 K and 5.9 ± 0.1 GPa, while the hBN−L−V triple point is at T = 3400 ± 20 K and p = 400 ± 20 Pa, which indicates that the region of thermodynamic stability of vapor in the BN phase diagram is extremely small. It has been found that the slope of the cBN melting curve is positive whereas the slope of hBN melting curve varies from positive between ambient pressure and 3.4 GPa to negative at higher pressures.

RETRACTED ARTICLE: Fundamental Discovery of New Phases and Direct Conversion of Carbon into Diamond and hBN into cBN and Properties

“…10-BN phase diagram showing P-T phase space for stability of hBN, cBN, and L (liquid BN), dotted lines due to Bundy [11] and recent modifications, [21,22] and dash-dot line extension for super undercooling. [8] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 47A, APRIL 2016-1491…”

“…[20] According to the P-T phase diagram, the transformation from hBN into cBN under equilibrium processing can occur only at high temperatures and pressures, as the hBN-cBN-Liquid triple point is at 3500 K/9.5 GPa or 3700 K/7.0 GPa with a recent theoretical refinement. [11,21,22] Using nonequilibrium nanosecond laser melting, we have created super undercooled state and shifted this triple point to as low as 2800 K and atmospheric pressure. The rapid quenching from super undercooled state leads to formation of a new phase, named Q-BN.…”

“…[11,21,22] Thus, phase-pure cBN can be synthesized only near the triple point at high temperatures and pressures. Recently, phase-pure cBN has been synthesized at slightly lower pressures and temperatures (1700 K/5.5 GPa) by using a lattice matching diamond substrate for cBN growth from melt.…”

“…[26][27][28] These observations tend to favor Corrigan and Bundy phase diagram (curve 1 in Figure 10) showing hBN as the stable phase and cBN metastable in the entire temperature range at ambient pressures. [11] Thermodynamic calculations based upon BN melting and scaling of specific heats C p 0 (T) to high temperatures by Solozhenko et al [21,22] have suggested curve 2, where triple point is shifted to lower pressures but higher temperatures and made cBN to be the stable phase 0 to 1600 K and hBN beyond that. In view of the multiple fitting parameters in C p 0 (T) extrapolation to higher temperatures for calculating enthalpies and entropies (Table I), the accuracy of free energy values to better than 5 pct is questionable.…”

Comment on “On the mechanism of the cubic phase formation in the boron nitride thin-film systems” [Appl. Phys. Lett. 79, 353 (2001)]

“…The P -T phase boundary line between c-BN and h-BN in the BundyWentorf phase diagram was derived from high-pressure high-temperature experiments and extrapolated to low temperatures in analogy to the carbon system. However, the BN phase diagram has been revised in the mid 90s 4 and it is today well accepted that the cubic phase is the thermodynamic stable modification of BN at normal conditions. 5 Additional evidence for c-BN to be the thermodynamical stable phase at normal conditions was provided by ab inito calculations 6 and experimental calorimetry measurements.…”