The metastable phase equilibria of substances in the disordered, liquid or amorphous, state are considered in T–P coordinates. Application of the thermodynamic model of pseudo-binary solutions is illustrated by means of calculated metastable T–P phase diagrams of water, carbon, and semiconducting compounds, GaSb and Ga38Sb38Ge24. All calculated diagrams include the equilibrium lines between the disordered phases terminating at the critical points and the stability boundaries of these phases, the spinodals. Depending on the mutual disposition of the elements of the stable and metastable T–P diagrams, two additional lines of metastable equilibria between the crystalline high-pressure phase and each of the metastable disordered phases can appear. These lines terminate at the points of intersection with spinodals. These special points are denoted as pseudo-critical points. A combined analysis of the stable and metastable diagrams gives a reasonable interpretation of the experimental data on the phase transformations and on the nature of the anomalous properties of the substances in question as well as allowing prediction of some new effects.

A thermodynamic approach to T P phase diagrams of substances in liquid and amorphous states

This paper presents the results of an investigation of the fine crystal structure of Dalan diamonds synthesized from graphite and carbon black by detonation. The phase composition of the diamond powders was found to vary widely; the structural and structural-morphological states of diamond particles were studied. The main types and subtypes of detonation diamonds are characterized according to transmission electron microscopy data. Assumptions are made concerning diamond formation during detonation synthesis.

Structure and properties of Dalan detonation diamonds

This article is a review on anomalous metastable phases observed in ultrahigh-pressure metamorphic (UHPM) rocks. Quartz incipiently amorphized under pressure and lonsdaleite, a hexagonal polytype of diamond, cannot form at equilibrium in rocks, as they have no P–T stability field. However, they can form metastably in place of coesite and diamond, respectively, if crystallization of the latter phases is hampered by kinetics. Besides impactites and shocked meteorites, these two phases have been reported in a few UHPM rocks. Incipiently amorphized quartz has been evidenced by Raman spectrometry and X-ray microdiffraction in eclogites from Antarctica. This anomalous form of quartz could be more common, but it easily escapes notice, being apparently confined to micro-inclusion and exhibiting an ordinary appearance. Lonsdaleite has been reported from a dozen UHPM rocks, among which the diamond-bearing gneiss from the Kokchetav Massif (Kazakhstan), but many of these occurrences are doubtful and need to be re-evaluated. Indeed, lonsdaleite is difficult to distinguish from diamond, and it can be identified definitively only by X-ray or electron microdiffraction. Incipiently amorphized quartz and lonsdaleite can only form at UHP (typically, P >12.5 GPa at 300 K for amorphized quartz) when unfavorable kinetics prevents the formation of coesite/stishovite and diamond, respectively. This can occur either at medium or high T , during a very short ultrahigh-pressure event (i.e., a shock), or during a longer ultrahigh-pressure event (inclusion-scale overpressure?) at low T when diffusion is hence too slow for enabling any solid–solid transitions. The occurrence of such phases supports the hypothetical role of nonlithostatic pressure in UHP metamorphism, at least either during shocks or on a minute scale.

Origin of High-Pressure Disordered Metastable Phases (Lonsdaleite and Incipiently Amorphized Quartz) in Metamorphic Rocks: Geodynamic Shock or Crystal-Scale Overpressure?

A well-dispersed micron-scale detonation polycrystalline diamond (DPD) was prepared and modified on RDX particles, preparing four DPD-modified RDX (DMR) composites with the modification amount increasing from 1/9 to 1/3. The effect of modification on the thermal decomposition and kinetics of RDX was studied using dynamic pressure measuring thermal analysis (DPTA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) techniques. As the modification amount increased, the gas emission, reaction rate constant, decomposition temperature, and kinetic and thermodynamic parameters of the composites increased firstly and decreased afterwards. The DPD had a catalytic effect on the thermal decomposition of RDX, but this effect was not in linear correlation with the modification amount. The DPD-modification amount of 1/7 had the optimal catalytic effect. For DPD modification of less than 1/7, the thermal decomposition of RDX was accelerated by DPD. As the modification amount exceeded 1/7, the excessive DPD modification conversely hindered the decomposition of RDX. The thermal decomposition kinetics demonstrated that the thermal decomposition of the DMRs conformed to a multi-step reaction mechanism involving a catalytic reaction and a secondary reaction, while they had the same rate-determining step, which is the scission of the N–NO2 bonds of RDX. The DPD modification changes the reaction pathway and reaction rate to affect the decomposition mechanism and kinetics.

The effect of detonation polycrystalline diamond modification on the thermal decomposition of RDX

Detonation is a supersonic motion of a front of exothermic chemical reaction through a medium, i.e. a reactive shock wave.

Detonation Transformation in Materials

Crystalline structure of detonation synthesized diamonds

The diamond-like amorphous carbon phase was obtained by means of detonation compression of mixtures of an explosive with soot or graphite. The pycnometric density of powders after compression and treatment in boiling acids achieved 3.42 g/cm3. The X-ray and TEM study showed that fine-grained crystalline and nanocrystalline diamond was also present in the samples in addition to the diamond-like amorphous phase.

Diamond-like amorphous carbon and nanocrystalline diamond obtained by detonation method

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Crystalline structure of detonation synthesized diamonds

Diamond-like amorphous carbon and nanocrystalline diamond obtained by detonation method