Boron nitride phase diagram. State of the art

Abstract

From recent experimental data on BN thermodynamic propeties, the equilibrium phase diagram for boron nitride has been plotted, which differs from the generally accepted Bundy-Wentorfs one. At atmospheric pressure cubic boron niride has been shown to be a thermodynamically stable modification up to temperatures of 1600 K, which drastically changes the established notions of BN polymorphism, based on assumed analogy of phase diagrams for carbon and boron nitride. These studies have shown that according to the proposed equilibrium phase diagram the threshold pressure of cBN crystallization can be reduced from 4 down to 2 GPa with the supercritical fluids present, which opens new fields for developing methods for cBN low-pressure synthesis.     

Compact multianvil device for in situ studies at high pressures and temperatures

We describe a recently developed device for in situ studies at pressures up to 25 GPa and temperatures up to 2300 K. The system consists of a 450 ton V7 Paris-Edinburgh press combined with a Stony Brook ‘T-cup’ multianvil stage. Such a compact large-volume set-up has a total mass of 100 kg only and can be readily used on most synchrotron radiation facilities. The optimization of the set-up by off-line tests is detailed, and we present some X-ray diffraction results which demonstrate the potential of the technique.     

Identification of the diamond-like B-C phase by confocal Raman spectroscopy

The new diamond-like B-C phase was obtained from the graphite-like BC phase in a laser-heated diamond anvil cell at high temperature 2230+/-140 K and high pressure 45 GPa. Raman spectra of the new phase measured at ambient conditions revealed a peak at 1315 cm(-1), which was attributed to longitudinal-optical (LO) mode. The X-Y Raman mapping was used to investigate spatial distribution of the diamond-like phases and was shown to be a powerful tool in studying the sp(2)-to-sp(3) phase transformations occurring in the diamond cell under high temperature and high pressure.     

Rhombohedral boron subnitride,B13N2, by X‐ray powder diffraction

The structure of the title compound consists of distorted B₁₂ icosahedra linked by N—B—N chains. The compound crystallizes in the rhombohedral space group Rm (No. 166). The unit cell contains four symmetry‐independent atom sites, three of which are occupied by boron [in the 18h, 18h (site symmetry m) and 3b (site symmetry m) Wyckoff positions] and one by nitrogen (in the 6c Wyckoff position, site symmetry 3m). Two of the B atoms form the icosahedra, while N atoms link the icosahedra together. The main feature of the structure is that the 3b position is occupied by the B atom, which makes the structure different from those of B₆O, for which these atom sites are vacant, and B_4+xC_1−x, for which this position is randomly occupied by both B and C atoms.     

Melting of B12P2 boron subphosphide under pressure

Melting of boron subphosphide (B₁₂P₂) to 26?GPa has been studied by in situ synchrotron X-ray powder diffraction in a laser-heated diamond anvil cell, and by quenching and electrical resistance measurements in a toroid-type high pressure apparatus. B₁₂P₂ melts congruently, and the melting curve has a positive slope of 23(6)?K/GPa. No solid-state phase transition was observed up to the melting in the whole pressure range under study.     

First and second‐order Raman scattering of B6O

First and second‐order Raman spectra of B₆O and their dependence on the wavelength of the excitation line from IR (infrared) to deep UV (ultraviolet) has been studied. The first‐order Raman spectra contain 11 well‐resolved lines of the 12 expected modes 5 A_1g + 7 E_g (space group R‐3m, point group D_3d). The second‐order Raman spectra contains eight lines that are resolved only in the case of the 244‐nm excitation line. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparing Nonsmooth Nonconvex Bundle Methods in Solving Hemivariational Inequalities

Hemivariational inequalities can be considered as a generalization of variational inequalities. Their origin is in nonsmooth mechanics of solid, especially in nonmonotone contact problems. The solution of a hemivariational inequality proves to be a substationary point of some functional, and thus can be found by the nonsmooth and nonconvex optimization methods. We consider two type of bundle methods in order to solve hemivariational inequalities numerically: proximal bundle and bundle-Newton methods. Proximal bundle method is based on first order polyhedral approximation of the locally Lipschitz continuous objective function. To obtain better convergence rate bundle-Newton method contains also some second order information of the objective function in the form of approximate Hessian. Since the optimization problem arising in the hemivariational inequalities has a dominated quadratic part the second order method should be a good choice. The main question in the functioning of the methods is how remarkable is the advantage of the possible better convergence rate of bundle-Newton method when compared to the increased calculation demand.     

Interaction between explosive and analyte layers in explosive matrix-assisted plasma desorption mass spectrometry

An HMX/insulin two-layer system was chosen as a model for further investigation of the matrix properties of explosive materials for protein analytes in plasma desorption mass spectrometry. The dependencies of the molecular ion yield and average charge state as a function of the analyte thickness were studied. An increase in the charge state of multiply protonated molecular species was confirmed as the major matrix effect, with the average charge state z at the smallest thickness studied being higher than in matrix-assisted laser desorption/ionization and closer to the value obtained in electrospray ionization under standard acidic conditions. Observed charge state distributions are significantly narrower than the corresponding Poisson distributions, which suggests that the protonation of insulin is limited in plasma desorption by the number of basic sites in the molecule, similar to electrospray ionization. Both the curve displaying total molecular ion yield and the one showing the total charge (proton) yield as a function of the insulin thickness have maxima at a thickness different from an insulin monolayer. These observations diminish the significance of a matrix/analyte interface mechanism for the explosive matrix assistance. Instead, a mechanism related to the chemical energy release during conversion of the explosive after the ion impact is proposed. As additional mechanisms, enhanced protonation of the analyte through collisions with products of the explosive decay is considered, as well as electron scavenging by other products, which leads to a higher survival probability of positively charged protein molecular ions. Copyright 1999 John Wiley & Sons, Ltd.     

Portable multi‐anvil device for in situ angle‐dispersive synchrotron diffraction measurements at high pressure and temperature

A recently developed portable multi‐anvil device for in situ angle‐dispersive synchrotron diffraction studies at pressures up to 25 GPa and temperatures up to 2000 K is described. The system consists of a 450 ton V7 Paris–Edinburgh press combined with a Stony Brook `T‐cup' multi‐anvil stage. Technical developments of the various modifications that were made to the initial device in order to adapt the latter to angular‐dispersive X‐ray diffraction experiments are fully described, followed by a presentation of some results obtained for various systems, which demonstrate the power of this technique and its potential for crystallographic studies. Such a compact large‐volume set‐up has a total mass of only 100 kg and can be readily used on most synchrotron radiation facilities. In particular, several advantages of this new set‐up compared with conventional multi‐anvil cells are discussed. Possibilities of extension of the (P,T) accessible domain and adaptation of this device to other in situ measurements are given.