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.

The structural phase stability of 1,1-diamino-2,2-dinitroethylene (FOX-7) has been studied up to 10 GPa through isothermal compression at 100 °C and 200 °C using synchrotron mid- and far-infrared spectroscopy. During isothermal compression at 100 °C changes are observed in vibrational spectra with increase in pressure that are indicative of significant distortion to monoclinic α phase or a possible structural transformation to a high pressure α(') phase at 2.2 GPa and α(") phase at 6.1 GPa. At 200 °C, for the far- and mid-IR regimes, the similar changes were observed at 2.1 (2.0) GPa and 5.3 (5.5) GPa, respectively. The observed change is nearly isobaric, consistent with previously reported high pressure and room temperature values, up to the highest temperature of 200 °C reached in our experiments. Over the total P-T range investigated, up to ∼10 GPa and 200 °C, we observed no evidence of sample decomposition. The observed changes are partially reversible with only slight evidence of the high pressure distortion remaining upon complete decompression. Additional isobaric heating at 1.07 GPa was performed in the mid-IR regime, which clearly revealed an onset of decomposition at 360 °C. Further x-ray or neutron diffraction, which are needed to fully resolve the cause of observed changes above 2 and 5 GPa, are ongoing.

Feature Metal Hydrides Recent developments in light metal complex hydrides show that there is a potential for hydrogen storage using these hydrides in fuel cells for on-board vehicular and other applications. The search for new alloys promises to have practical signifi cance with the realization that hydrogen as a fuel holds the key to fi lling energy needs and solving environmental problems. This review presents the U.S. Department of Energy FreedomCAR goals for hydrogen storage, storage capacities of important hydrides, and current developments in light-metal complex hydrides.

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 ϳ40 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 II͒ and further transitions at 5 ͑phase III͒ and 10 GPa ͑phase IV͒. 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 ␦+¯H␦− -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 ͑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 pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.

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.

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.

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