Raja Chellappa

Pressure-induced complexation of NH3BH3–H2

High pressure Raman spectroscopy of NH(3)BH(3)-H(2) mixtures up to 60 GPa reveals unusual pressure-induced complexation and intermolecular interactions. Stretching modes of H(2) in the complex arise at 6.7 and 10 GPa, increasing in frequency with pressure of up to 60 GPa with different pressure coefficients, and at approximately 40 GPa, the lower frequency mode approaches vibron frequency of bulk H(2). Pressure-induced transformations in pure NH(3)BH(3) studied up to 60 GPa reveal a disorder-order transition at 1 GPa (phase II) and further transitions at 5 (phase III) and 10 GPa (phase IV). The spectra of both pure NH(3)BH(3) and the NH(3)BH(3)-H(2) complex provide evidence for strengthened of the N-H(delta+)...H(delta-)-B dihydrogen bonding linkages up to 50 GPa, beyond which they weaken. The dihydrogen bonding breaks down due to interactions with H(2) between 15 and 20 GPa in the NH(3)BH(3)-H(2) complex. The behavior of the nu(NH(3)) modes in the NH(3)BH(3)-H(2) complex indicates a dominant role of the NH(3) functional group in the observed interactions.

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Phase diagram calculations of organic “plastic crystal” binaries: (NH2)(CH3)C(CH2OH)2–(CH3)2C(CH2OH)2 system

Abstract Thermodynamic analysis of a binary, 2-amino-2-methyl-1,3-propanediol (AMPL, (NH 2 )(CH 3 )C(CH 2 OH) 2 )–2,2-dimethyl-1,3-propanediol (NPG, (CH 3 ) 2 C(CH 2 OH) 2 ), “plastic crystal” system has been performed and the AMPL–NPG phase diagram has been calculated. The low temperature phases generally have layered or chain structures ( α or β phases), and when heated transform to the high temperature plastic crystal ( γ ′ or γ ) phase. The AMPL-rich γ ′ phases have a BCC and the NPG-rich γ phases have FCC structure. The low temperature ( α and β ) and liquid phases are assumed to be ideal and the plastic crystal phases ( γ ′ and γ ) are described using subregular solution models. The Gibbs energies of pure components were determined with the inclusion of heat capacity data. The Gibbs energies of metastable modifications were estimated by assuming ideal solutions of the phases. The excess Gibbs energy parameters were optimized using Thermo-Calc (TCC) software to fit the experimental phase diagram data. The magnitudes of the interaction parameters are very small and thus the solution phases are almost ideal.

High Pressure-Temperature Phase Diagram of 1,1-Diamino-2,2-dinitroethylene (FOX-7).

The pressure-temperature (P-T) phase diagram of 1,1-diamino-2,2-dinitroethylene (FOX-7) was determined by in situ synchrotron infrared radiation spectroscopy with the resistively heated diamond anvil cell (DAC) technique. The stability of high-P-T FOX-7 polymorphs is established from ambient pressure up to 10 GPa and temperatures until decomposition. The phase diagram indicates two near isobaric phase boundaries at ∼2 GPa (α → I) and ∼5 GPa (I → II) that persists from 25 °C until the onset of decomposition at ∼300 °C. In addition, the ambient pressure, high-temperature α → β phase transition (∼111 °C) lies along a steep boundary (∼100 °C/GPa) with a α-β-δ triple point at ∼1 GPa and 300 °C. A 0.9 GPa isobaric temperature ramping measurement indicates a limited stability range for the γ-phase between 0.5 and 0.9 GPa and 180 and 260 °C, terminating in a β-γ-δ triple point. With increasing pressure, the δ-phase exhibited a small negative dT/dP slope (up to ∼0.2 GPa) before turning over to a positive 70 °C/GPa slope, at higher pressures. The decomposition boundary (∼55 °C/GPa) was identified through the emergence of spectroscopic signatures of the characteristic decomposition products as well as trapped inclusions within the solid KBr pressure media.

Rhenium reactivity in H2O–O2 supercritical mixtures at high pressures

Rhenium (Re) gaskets are commonly used in diamond anvil cell experiments to contain and pressurize samples. It is found that Re undergoes a series of reactions with supercritical fluid H2O–O2 mixtures at room temperature to form perrhenic acid (HReO4) and its hydrates. Similar reactivity of Re is also observed by destabilization of oxygen clathrate hydrates. The reaction mechanism is consistent with the electrochemistry of metal corrosion in an aqueous media. From a practical perspective for high pressure research, this finding indicates the need to consider the reactivity of Re when it is used as a gasket in experiments on aqueous fluids. More importantly, the documentation of these reactions demonstrates the oxidative potential of H2O–O2 supercritical mixtures for beneficial practical implications.

Chemical stability of molten 2,4,6-trinitrotoluene at high pressure

2,4,6-trinitrotoluene (TNT) is a molecular explosive that exhibits chemical stability in the molten phase at ambient pressure. A combination of visual, spectroscopic, and structural (x-ray diffraction) methods coupled to high pressure, resistively heated diamond anvil cells was used to determine the melt and decomposition boundaries to >15 GPa. The chemical stability of molten TNT was found to be limited, existing in a small domain of pressure-temperature conditions below 2 GPa. Decomposition dominates the phase diagram at high temperatures beyond 6 GPa. From the calculated bulk temperature rise, we conclude that it is unlikely that TNT melts on its principal Hugoniot.

High-Pressure Raman Spectroscopy of Tris(hydroxymethyl)aminomethane

High-pressure Raman spectroscopy has been used to study tris(hydroxymethyl)aminomethane (C(CH(2)OH)(3)NH(2), Tris). Molecules with globular shapes such as Tris have been studied thoroughly as a function of temperature and are of fundamental interest because of the presence of thermal transitions from orientational order to disorder. In contrast, relatively little is known about their high-pressure behavior. Diamond anvil cell techniques were used to generate pressures in Tris samples up to approximately 10 GPa. A phase transition was observed at a pressure of approximately 2 GPa that exhibited relatively slow kinetics and considerable hysteresis, indicative of a first-order transition. The Raman spectrum becomes significantly more complex in the high-pressure phase, indicating increased correlation splitting and significant enhancement in the intensity of some weak, low-pressure phase Raman-active modes.

Pressure-induced Polymerization in Substituted Acetylenes

A fundamental understanding of shock-induced chemical reactions in organics is still lacking and there are limited studies devoted to determining reaction mechanisms, evolution of bonding, and effect of functional group substitutions. The fast timescale of reactions occurring during shock compression create significant experimental challenges (diagnostics) to fully quantify the mechanisms involved. Static compression combined with temperature provides a complementary route to investigate the equilibrium phase space and metastable intermediates under extreme P-T conditions. In this study, we present our results from our ongoing high pressure in situ synchrotron x-ray diffraction experiments on substituted acetylenes: tert-butyl acetylene [TBA: (CH3)3-C=CH] and ethynyl trimethylsilane [ETMS: (CH3)3-SiC=CH]. We observed that the onset pressure of chemical reactions (at room temperature) in these compounds is higher under static compression (TBA: 12 GPa and ETMS: 17.6 GPa) when compared to shock input pressur...