R. L. Mills

Raman spectroscopy of solid nitrogen up to 374 kbar

Abstract We measured the Raman spectrum of room-temperature solid nitrogen in a diamond cell at pressures up to 374 kbar. Only the molecular stretching mode was observed. The frequency of this mode appears to increase smoothly from 2340 cm-1 at 39 kbar to 2394 cm-1 at 374 kbar. Between about 50 and 120 kbar, the Raman line is split into a second, higher-frequency peak, which may be associated with molecular ordering in the solid. Measurements of broadening and splitting in fluorescence lines from the ruby pressure gauge indicate that solid nitrogen is reasonably plastic up to 130 kbar. We suggest that nitrogen might serve as a useful pressure medium.

Experimental evidence for a second-order transformation prior to melting in ammonia, organic compounds and ice I

Abstract The isothermal compressibility of ammonia and expansivity of different classes of molecular organic compounds are observed to have a power-law behavior with an exponent around 1 2 in the so-called premelting region, showing that these solids undergo a second-order phase transformation prior to melting. This viewpoint is consistent with dynamic results in this region indicating, for molecular organic compounds, the onset of progressive orientational disorder. In ammonia the second-order transition proceeds up to first-order melting, whereas in molecular organic compounds the transition is interrupted by another regime closely connected to melting. The stability limit bounding the second-order process is found close to the melting line.

Crystal structures of N2O to 12 GPa by x‐ray diffraction

The structures and phase transitions of N2O were studied by powder x‐ray diffraction in a tungsten–carbide anvil device from about 100 to 300 K and 2 to 12 GPa. Two solid phases, α‐N2O and β‐N2O, were observed. The α pattern is consistent with the known low‐pressure low‐temperature ordered cubic form, space group Pa3, up to 4.8 GPa where transition to a new β solid occurs. From refinements using photographic x‐ray intensities, the β‐N2O structure was determined to be orthorhombic Cmca. There are four molecules in a unit cell with a=4.954 A, b=4.497 A, and c=6.201 A at 5.81 GPa and 298 K. The molecular axes lie parallel to the bc plane and are tipped at an angle of 37.2° to the b axis. Random head‐to‐tail orientation is probable in both solids. The phase diagram and values of the molar volume for N2O are compared with published data for the isoelectronic molecule CO2. Recent theoretical calculations correctly predict the β‐N2O structure.

Longitudinal nuclear relaxation measurements in hcp H2

Abstract Measurements of T 1 in the hep phase of H 2 , over the temperature range 2°–12°K and the ortho concentration range between 0.5 and 0.97 are presented. At temperatures below 10°K, the thermally activated self-diffusion is negligible and the mechanism for nuclear relaxation is that attributed by Moryia and Motizuki and by Harris to intramolecular dipolar interaction, modulated by intennolecular electric quadrupole-quadrupole (EQQ) interaction. The gaussian approximation for the correlation function was used by these authors to predict T 1 . From the comparison between experiment and theory, we determine the EQQ parameter Γ k B to be 0.67°K. Above 10°K the effect of diffusion influences T 1 , and the experimental results for an 88 per cent ortho H 2 sample up to the melting point suggest that the relaxation mechanisms resulting from EQQ interaction and diffusion are not independent of one another.

NMR in solid D2 near the orientational ordering transition

A study of the NMR lineshapes in solid D2 for various concentrations C of (J = 1) molecules is presented. Below the rotational ordering temperature Tλ (C), the lineshape has two doublets. The outer one, attributed to the J = 1 molecules, has a splitting of about 75 kHz for T ⪡ Tλ. This splitting, which is proportional to the order parameter, was studied as a function of temperature. For the region 0.02 < 1 - T/Tλ < 0.08, the data could be fitted to a power law (1 - T/Tλ)β with β ≅ 0.33. The inner doublet was attributed to J = 0 molecules. Its splitting is proportional to Tλ (C) and is extrapolated to 8.5 kHz for C = 1 in good agreement with the prediction of Harris.

Raman study of SO2 at high pressure: aggregation, phase transformations, and photochemistry

Abstract We report Raman measurements made on SO2 in a diamond cell up to 75 kbar showing two new phases, solid II and solid III, which differ from known, zero-pressure solid I. Spectra indicate that SO2 molecules aggregate in solid III to possibly form a cyclical trimer; solid III is shown to be photochemically active to blue radiation.

High-pressure gases in diamond cells

Abstract We developed an apparatus and technique for loading a diamond-anvil cell with high-pressure gas. Experiments were carried out at various temperatures on CO2 and D2 in which final pressures of 34.0 and 44.7 kbar, respectively, were achieved.

X-ray study of xenon to 23 GPa

Abstract X-ray diffraction photographs of polycrystalline xenon in a diamond- anvil cell show that the crystal structure of Xe remains fcc up to at least 23 GPa at 298 K. The atomic volumes are in good agreement with earlier x-ray data at lower pressures and with calculated volumes along the O-K isotherm.

Pressure changes accompanying the phase transitions in solid H2 and D2

Abstract Pressure changes at constant volume accompanying hexagonal-to-cubic and cubic-to-hexagonal phase transitions have been measured in solid H 2 and D 2 as a function of ortho concentration.

Equation of State of Fluid NH3 from P-V-T and Ultrasound Measurements to 12 Kbar

A piston-cylinder apparatus was used to measure the pressure, volume, temperature, and ultrasonic velocity (P,V,T,u) of fluid NH/sub 3/ from 195 to 320 K at pressures up to 12 kbar. Over 1200 sets of P-V-T-u and P-V-T data were fitted to a Tait-type equation of state (EOS) by non-linear least-squares minimization. With quadratic terms in T for the two fitted parameters, the rms derivation is +-0.2% in V and +-0.9% in u, which is comparable to the experimental error. The simple Tait EOS is useful over the entire fluid range between the vaporization and melting curves up to T exceeding room temperature. Measurements of u and calculations of the constant-pressure heat capacity Cp show regular behavior, differing noticeably from values given by a 44-parameter EOS recently published by NBS. It is concluded that incorporating u or Cp in the formulation of an EOS improves its self consistency markedly.