A. Barry Kunz

An excitonic mechanism of detonation initiation in explosives

A novel mechanism for detonation initiation in solid explosives is proposed. This is based on electronic excitations induced by an impact wave propagating through the crystal. We illustrate the model by using the RDX (C3H6N6O6) crystal as an example. In our model, a key role belongs to lattice defects, in particular edge dislocations, which promote dramatic changes in the electronic structure, primarily a reduction of the optical gap due to the splitting off of local electronic states from both valence and conduction bands. The pressure inside the impact wavefront further reduces the band gap, making it close to zero. This promotes highest occupied molecular orbital–lowest unoccupied molecular orbital HOMO–LUMO transitions resulting in N–NO2 bond breaking and the creation of favorable conditions for the initiation of a chain reaction. Experimental facts supporting the suggested mechanism are discussed.

Molecular orbital calculations on (MgO)n and (MgO)n+ clusters (n=1-13)

We report ab initio molecular orbital calculations on neutral and single‐ionized stoichiometric clusters of MgO containing up to 26 atoms. Geometrical parameters of the neutral clusters are optimized at the Hartree–Fock level, whereas for the ionized clusters we have applied the vertical approximation. Correlation corrections in the clusters with 2–12 atoms are included at the equilibrium geometries by means of second order Moller–Plesset calculations. We have found that the structures based on the (MgO)3 subunit are preferred in comparison to cubelike configurations, although the energy difference decreases with the increase in cluster size. The relative stability of neutral and single‐ionized clusters has been studied by means of the fragmentation path involving the loss of a neutral MgO molecule. The calculated ‘‘magic numbers’’ for the charged clusters, (MgO)n+, are in complete agreement with the abundance maxima observed in the mass spectra. Finally, we explore the size dependence of structural, ener...

Ab initio simulation of defects in energetic materials: Hydrostatic compression of cyclotrimethylene trinitramine

An isotropic compression of both the perfect solid cyclotrimethylene trinitramine (C3H6N6O6), also known as RDX, and of the solid containing vacancies is simulated using the ab initio Hartree–Fock method combined with two different crystal models: a periodic (band structure) and a molecular cluster. We show that an external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with vacancies. The solid RDX is found to be highly compressible; a pure crystal could be compressed to 57% of its equilibrium volume, whereas crystals containing vacancies are even more compressible. The critical pressure necessary for the insulator–metal transition is also predicted. It is shown that the voids present in a real RDX solid lower the metallization pressure by about 30%. Theoretical results are in close agreement with the experimental data on solid and porous RDX. The influence of defects present in the crystal and the relation to control of the sensitivity to detonati...

Electronic structure of molecular crystals containing edge dislocations

An attempt to model the electronic structure of molecular crystals containing an edge dislocation at the ab initio Hartree–Fock level is performed. The experimentally determined configurations for edge-type dislocations with the Burgers vector [001] in crystalline cyclotrimethylene trinitramine (RDX) and pentaetythritol tetranitrate (PETN) are theoretically simulated. It is shown that a shear stress, induced by the dislocations, produces local electronic states in the fundamental band gap of the crystal. These states are mainly formed by molecular orbitals of critical bonds (which are the N–NO2 group in RDX and the O–NO2 group in PETN) responsible for the stability of the materials. Optical absorption attributed to these electronic states is predicted and compared to the available experimental data. Properties of the defective solids are compared with those of the perfect crystals. Correlation of the electronic structure and sensitivity of the materials to initiation of a chemical reaction as well as some...

Compression-induced effect on the electronic structure of cyclotrimethylene trinitramine containing an edge dislocation

An effect of a hydrostatic compression on the electronic structure of cyclotrimethylene trinitramine (C3H6N6O6), also known as RDX, with an edge dislocation has been studied by means of the ab initio Hartree–Fock method for a periodic system combined with the many-body perturbation theory. An external pressure causes a significant decrease of the optical gap for both the perfect material and the crystal with dislocations. The edge dislocations produce local electronic states in the optical gap whereas the external pressure moves these states deep within the band gap. This contributes strongly to properties of the RDX crystals creating favorable conditions for the N–NO2 chemical bond rupture due to exciton formation. The relationship between the edge dislocations, hot spot formation, and the sensitivity of RDX to detonation are discussed in detail.

AN AB INITIO INVESTIGATION OF THE ELECTRONIC STRUCTURE OF LITHIUM AZIDE (LIN3), SODIUM AZIDE (NAN3), AND LEAD AZIDE PB(N3)2

Solid energetic substances have long played an important technological role as explosives, as well as for fuels. In this article, the authors concentrate on a type of explosive considered a primary explosive, lead azide, and its related compounds, lithium azide and sodium azide. Recent interest in more fundamental questions relating to the basic properties of these systems as materials, coupled with a desire to probe fundamental questions relating to the initiation and sustaining of the chemical reactions leading to combustion/detonation, is generating significant interest in the basic solid-state properties of such energetic systems. In particular, recent analysis of detonation by Gilman emphasizes the need to include excitation of the electronic system in obtaining an understanding. In this article, the band structures of the three solid metal azides are studied. This is done for both the normal lattice geometry and also in isotropically compressed geometries. These studies found that the alkali azide band gaps are far wider than is the lead azide gap and the lead azide gap is far more sensitive to narrowing with lattice compression than are the gaps for the alkali azides. In fact, the gap for sodium azide is found to widen with compression rather than narrow. The authors found that there is much seen in the band structures of these azides to lend some support to the Gilman model and also to demonstrate the importance of solid-state effects on the electronic structure and possible behavior of such energetic systems.

An Ab Initio Investigation of Crystalline PETN

Solid energetic substances have long played an important technological role as explosives and also as fuels. Much of the research on the solid phases has been concentrated on ground state properties, and also on the chemistry of the molecules comprising the solid. In addition, significant understanding of the detonation properties has been obtained by semi-empirical continuum mechanical modeling. Traditional solid state studies of this important class of materials have mostly been ignored. This may be due to the apparent success of the semi-empirical models in describing the detonation properties, as well as the practical difficulties in performing band theoretical studies. Recent interest in more fundamental questions relating to the basic properties of these systems as materials, coupled with a desire to probe fundamental questions relating to the initiation and sustaining of the chemical reactions leading to combustion/detonation is generating significant interest in the basic solid state properties of such energetic systems. In particular, recent analysis of detonation by J. J. Gilman emphasizes the need to include excitation of the electronic system in obtaining an understanding. In this manuscript, the basic solid state properties of PETN are considered.

Study of point defects in alkaline-earth sulfides

The results of a computer simulation study of point defects including vacancy, interstitial, and F/sup +/ center in alkaline-earth sulfides are presented. The study is based on ICECAP/HADES simulation procedures and uses empirical interionic potentials obtained from the analysis of macroscopic data for these materials. The results predict the dominance of Schottky disorder and suggest that vacancy migration predominates in alkaline-earth sulfides. Furthermore, the calculated F/sup +/ center absorption energy is in good agreement with the experimental data deduced from the optical stimulated studies in these materials.

An Effect of Hydrostatic Compression on Defects in Energetic Materials: AB Initio Modeling Maija

First-principle theoretical investigation of the basic defects such as a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack in organic explosive molecular crystals is presented. As an example the authors considered solid RDX (C{sub 3}H{sub 6}N{sub 6}O{sub 6}) which is a well studied unstable solid. It was established that external hydrostatic pressure changes optical properties of defect-free RDX as well as of the crystal with defects narrowing the band gap. The lattice defects (especially dislocations) are identified with the so-called hot spots. The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbital of N-NO{sub 2} group is analyzed. Favorable conditions of molecular dissociation due to electronic excitation are shown.

Modeling of shock compression of RDX with defects

A first-principles theoretical modeling of basic defects (a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack) in organic explosive molecular crystals, particularly solid RDX (C3H6N6O6), is presented. It is established that shock compression changes the optical properties of both defect-free RDX and RDX with defects by narrowing the optical band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots” in triggering the explosive detonation. The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for RDX explosion as a result of shock compression are discussed.A first-principles theoretical modeling of basic defects (a molecular vacancy, a vacancy dimer, an edge dislocation, and a micro-crack) in organic explosive molecular crystals, particularly solid RDX (C3H6N6O6), is presented. It is established that shock compression changes the optical properties of both defect-free RDX and RDX with defects by narrowing the optical band gap. The lattice defects (especially dislocations) are identified with the so-called “hot spots” in triggering the explosive detonation. The nature of local electronic states introduced in the band gap by the edge dislocation and formed mainly by molecular orbitals of N-NO2 group is analyzed. Favorable conditions for RDX explosion as a result of shock compression are discussed.