Nenad Velisavljevic

A high-pressure far- and mid-infrared study of 1,1-diamino-2,2-dinitroethylene

High pressure infrared (IR) measurements of the insensitive explosive, 1,1-diamino-2,2-dinitroethylene (DADNE or FOX-7), have been performed in the far- and mid-IR spectral regions up to ∼28u2009GPa using a synchrotron source. Petroleum jelly and KBr were employed as the pressurizing media for the far- and mid-IR experiments, respectively. In both experiments, IR spectra were collected at various pressures both in compression and decompression to determine reversibility under pressure cycling. There is evidence for at least two and possibly three phase transitions in the samples at pressures near 2, 5, and10 GPa, respectively. In the high frequency region (∼3000–3300 cm−1), the NH2 symmetric and antisymmetric vibrational modes soften with increasing pressure, suggesting strengthening intermolecular hydrogen bonding, and then stiffen with further pressure increase. At higher pressures (above 5u2009GPa), we suspect progressive flattening of the zig zag structure characteristic of the α phase occurs, which was obser...

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.

Distortion of alpha-uranium structure in praseodymium metal to 311 GPA

It is generally observed that the rare earth metals adapt an orthorhombic alpha-uranium (α-U) structure at high pressures following the delocalization of 4f shell under compression. We examine the stability of the α-U structure in praseodymium metal at ultrahigh pressures of 313 GPa (volume compression V/V 0u2009=u20090.343) in a diamond anvil cell at room temperature. X-ray diffraction data show a transformation from the α-U structure to a primitive orthorhombic P212121 phase at 147±5 GPa, which is characterized by the anisotropic compressibility of various crystallographic axes. This anisotropic compressibility leads to an interesting situation when the b-axis and the c-axis of the orthorhombic phase become nearly equal above 260 GPa and the structure can be regarded as a pseudo-tetragonal phase. Our present study shows that the 4f band metal Pr does not adapt a body centred tetragonal phase as predicted by theory, but instead novel crystallographic phases are observed at extreme compressions. The present results have a broader impact on the stability of the α-U phase in a variety of f-band systems at high pressures.

1,1-Diamino-2,2-dinitroethylene under high pressure-temperature.

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.

ELECTRICAL MEASUREMENTS ON PRASEODYMIUM METAL TO 179 GPA USING DESIGNER DIAMOND ANVILS

The electrical and magnetic properties of light rare-earth metals and trans-plutonium actinide metals are of interest to study the f-shell delocalization phenomenon under high compressions. Using designer diamond anvil technology, sensitive electrical four-probe measurements were performed on light rare-earth metal praseodymium to pressures of 179 GPa at room temperature. We document an average drop in resistivity of 53% at a pressure of 20 GPa in a series of high-pressure experiments. This large drop in resistivity provides the strongest experimental evidence yet for the view that the 20 GPa phase transition is indeed associated with f-electron delocalization. Our results show that the precise electrical measurements are ideally suited for f-delocalization studies, especially where structural data do not provide clear evidence of this transition.

Bioceramic hydroxyapatite at high pressures

A bioceramic hydroxyapatite, Ca10(PO4)6(OH)2 polycrystalline sample was studied under high pressures in a diamond anvil cell to investigate its structural, electrical, and mechanical properties under compression. Anisotropic compression effects were observed in x-ray diffraction studies below 8 GPa. Nanoindentation hardness measurements on pressure-treated hydroxyapatite samples show hardness value of 4.0±0.5u2009GPa which is comparable to the plasma-sprayed samples. Electrical studies show that the bioceramic sample retained its insulating properties to 65 GPa. The present studies demonstrate that a fully dense and translucent hydroxyapatite sample is attained above 10 GPa at 300 K.

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.

Calibration of an isotopically enriched carbon-13 layer pressure sensor to 156GPa in a diamond anvil cell

An isotopically enriched C13 homoepitaxial diamond layer of 6±1μm thickness was grown on top of a brilliant cut diamond anvil by a microwave plasma chemical vapor deposition process for application as a pressure sensor. This isotopically enriched diamond tip was then used in conjunction with a natural isotopic abundance diamond anvil to generate high pressure on the sample. We provide a calibration for the C13 Raman mode of this extremely thin epitaxial layer to 156GPa using ruby fluorescence and the equation of state of copper as secondary pressure standards. The nonlinear calibration of the C13 Raman mode pressure sensor is compared with similar calibrations of C12 Raman edge and a good agreement is obtained. The Raman signal from the C13 epitaxial layer remained a distinct singlet to 156GPa, and pressure calibration is independent of sample mechanical strength or the diamond anvil geometry. The use of even thinner layer would allow calibration further into ultrahigh pressure regime where the use of oth...

Isotopically pure C13 layer as a stress sensor in a diamond anvil cell

Isotopically pure C13 homoepitaxial diamond layer of 20±5microns thickness was grown on top of a brilliant cut diamond anvil by a microwave plasma chemical vapor deposition process for application as a stress sensor. This isotopically pure diamond tip was then used in conjunction with a natural abundance C12 diamond anvil to generate high pressure on a soft sample containing carbon nanotubes to 100GPa. The C13 diamond Raman signal remained a distinct single peak showing that the mean normal stress experienced by this thin layer is uniform to the highest pressure. The stress-induced shift of the C13 layer is related to the quasihydrostatic pressure in the sample chamber using a ruby pressure sensor and shows a remarkably linear behavior to extreme pressures.

Physical and mechanical properties of C60 under high pressures and high temperatures

The physical and mechanical properties of a C60 fullerene sample have been investigated under high pressure–high temperature conditions using a designer Diamond Anvil Cell. Electrical resistance measurements show evidence of C60 cage collapse at 20 GPa, which leads to the formation of an insulating phase at higher pressure. Energy dispersive X-ray diffraction (EDXD) data indicated that the characteristic fcc reflections gradually decrease in intensity and eventually disappear above 28 GPa. A C60 sample was laser-heated at a pressure of 35 GPa to a temperature of 1910±100 K and, subsequently, decompressed to ambient conditions. The photoluminescence spectra and the Raman spectrum of the pressure–temperature-treated sample were measured at a low temperature of 80 K. Raman peak at 1322.3 cm−1 with full-width half-maximum of 2.9 cm−1 was observed from the sample, which is attributed to the hexagonal diamond phase in the sample. The room temperature photoluminescence spectra showed a symmetric emission band centered in the red spectral range with a peak at 690 nm. The structural analysis of the pressure–temperature-processed C60 sample using EDXD method showed strong internal structure orientation and a phase close to hexagonal diamond. Mechanical properties such as hardness and Young’s modulus were measured by nanoindentation technique and the values were found to be 90±7 and 1215±50 GPa, respectively and these values are characteristic of sp3-bonded carbon materials.