A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of Ϸ100 GPa. A hexagonal carbon phase was formed at Ϸ75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp 3 -rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a 0 ‫؍‬ 2.496(4) Å, c0 ‫؍‬ 4.123(8) Å, and V0 ‫؍‬ 22.24(7) Å 3 . There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 ؎ 0.2 g͞cm 3 , which is at least compatible to that of diamond (3.52 g͞cm 3 ). The bulk modulus was determined to be 447 GPa at fixed Kϵ4, slightly greater than the reported value for diamond of Ϸ440 -442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.

Recent work by Gupta and Shen [Appl. Phys Lett. 58, 583 (1991)] has shown that in a nonhydrostatic environment, the frequency of the ruby R, line provides a reliable measure of the mean stress or pressure. When using the frequency of either the R, or R, line to measure pressure at nonambient temperature, it is necessary to know the temperature dependence of the line shift. Unfortunately, the shift of the R2 line with temperature has not been reported. The ruby R i and R2 fluorescence shifts have been determined as a function of temperature from 15 to 600 K. Both can be fitted very well to the simple cubic forms R,(T) =14423+4.49~10-~T-4.81~10-~T~+3.71~10-'T~ cm-' and R2(T) = 14 452 $3.00~ 10m2T-3.88x 10e4T2+2.55x lo-'T3 cm-'. From 300 to 600 K the shifts fit well to linear functions of temperature. In addition, it is found that the RI-R2 splitting changes by about 3 cm-' over the 600 K temperature range. Linewidths were found to vary both with temperature and from sample to sample.

The lattice constants and the atomic position parameter, z, of a high purity, low strain, single‐crystal of arsenic have been determined. Low extinction reflections of filtered Mo Kα and Ag Kα radiation were used for the determination of z. The Bond precision technique with Mo Kα radiation was used for the determination of the unit‐cell dimensions a and c. Average values for a and c in Å, and for z are: 4.2°K z = 0.22764 a = 3.7597 c = 10.4412; 78°K z = 0.22754 a = 3.7595 c = 10.4573; 299±3°K z = 0.22707 a = 3.7598 c = 10.5475. The estimated standard deviation of z is ±0.00004 at 4.2°K, ±0.00002 at 78°K and ±0.00005 at 299°K. The precision of a is estimated to be ±0–0001 Å and that for c to be ±0.0002 Å. The results for As are compared with those reported earlier for Sb and Bi.

2315proton transfer, desorption of the neutrals, recombination in the vapor phase. The C-N bond must cleave for this to happen with PTTN. Some of the nitrogen oxides that appear could result from oxidation-reduction reactions associated with the C-N bond fission.(3). Within 1 s of the initial product formation, the hydrocarbon residue is attacked, probably by HN03 and the nitrogen oxides, resulting in the formation of H 2 0 , C02, CO, HNCO, and HCN by what is, no doubt, a complex series of reactions. The fact that HCN(g) becomes increasingly prevalent while NH4N03(g) becomes unimportant when external gas pressure is applied suggests that C-N bond rupture leading to N H 3 evolution is strongly suppressed by pressure.In summary, the decomposition of PTTN begins at the extremities of the cation with N-H bond fission. It progresses inward, first cleaving the C-N bonds and finally involves the C-C bonds. This pattern is consistent with degradation of the cation by reaction with an external agent. H N 0 3 appears to be that agent (23) Inami, S. H.; Rosser, W. A.; Wise, H. Supplementary Material Available:Tables SI-SI11 containing listings of observed and calculated structure factors, the anisotropic temperature factors, hydrogen atom coordinates (9 pages). Ordering information is given on any current masthead page.Single crystals of nitromethane in diamond-anvil cells at pressures of 0.3 to 6.0 GPa have been studied by X-rays at room temperature. The crystal structure is similar to the low-temperature, ambient-pressure structure. Below 3.5 GPa hydrogen atoms could not be located. At 3.5 GPa the methyl group is rotated about 4 5 O from the position in the low-temperature form. Raman data taken at pressures up to 11.7 GPa showed all molecular vibrational frequencies to increase with pressure. IntroductionThe pressureinduced chemistry of small molecules has recently been shown to be a fruitful area of investigation. In particular, studies of the decomposition of N O a t moderate pressures have shown a facile transformation to N 2 0 , NO2, and N203.1 An especially interesting result in the N O system is that two different structural modifications of NzO4 exist but only one of the forms exhibits a further pressure-induced reaction to nitrosonium nitrate, N O N 0 3 . This result suggests the importance of topochemical pathways for pressure-induced chemical changes.Nitromethane is one of the simplest organic explosive compounds. As such, it is a good candidate for study under temperature and pressure extremes. Brasch, has reported that at 150 O C and 5.0 GPa nitromethane undergoes an irreversible decomposition to C02 and a solid material that is probably ammonium oxalate. More recently3 the formation of methazonate ion has been observed in the thermal base-catalyzed decomposition of nitromethane at ambient pressure.In an attempt to learn more about the chemical and physical properties of nitromethane, we have made single-crystal X-ray studies in a diamond-anvil cell at nine pressures from 0.3 to 6.0 GPa at room temperature. I...