Bart Olinger

Structures and phase diagrams of N2 and CO to 13 GPa by x‐ray diffraction

The structures and phase transitions of N2 and CO were studied by powder x‐ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases, β, δ, and e, were observed in each material. The known β and δ solids were confirmed to have hexagonal space group P63/mmc and cubic space group Pm3n, respectively. From refinements using photographic x‐ray intensities, the new e‐N2 and e‐CO structures were determined to be rhombohedral R3c. There are eight ordered molecules in the e‐N2 unit cell with a=5.928 A and α=85.14° at 110 K and 7.8 GPa, and eight ordered molecules in the e‐CO unit cell with a=6.059 A and α=85.73° at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The δ–e transition takes place through an ordering and small displacement of the N2 and CO molecules, accompanied by a slight extension of the lattice along a cube diagonal. Molar volumes are presented over an expanded P‐T region. Recent theoretical calculations using lattice energies, molecular dynamics, and symmetry c...

The isothermal linear and volume compression of pentaerythritol tetranitrate (PETN) to 10 GPa (100 kbar) and the calculated shock compression

The isothermal linear and volume compressions of the explosive pentaerythritol tetranitrate (PETN) were measured to 10 GPa using a high pressure x‐ray diffraction technique. The fits to the data are a/a0 = 1−2.052×10−2P + 2.230×10−3P2 − 1.041×10−4P3, and c/c0 = 1−2.832×10−2P + 3.295×10−3P2 − 1.458×10−4P3, for P<10 GPa, and ust = 2.16 + 3.007 upt − 0.682 upt2, for upt < 0.95 km/s, and ust = 2.76 + 1.71 upt, for upt ≳ 0.95 km/s, where ust = [PV0/ (1 − V/V0)]1/2, upt = [PV0(1 − V/V0)]1/2, and ρ0 = 1/V0 = 1.774 g/cm3. The shock compression Hugoniot of PETN calculated from the isothermal compression fit is us = 2.24 + 2.95 up − 0.605 up2, for up < 1.00 km/s, and us = 2.81 + 1.75 up, for up ≳ 1.00 km/s, where us is the shock velocity and up is the particle of mass velocity behind the shock wave. The isothermal compression fit and the Hugoniot are initially quadratic because of the rapid change in the strength of repulsive forces between the PETN molecules during initial compression.

Compression and bonding of ice VII and an empirical linear expression for the isothermal compression of solids

The volume of ice VII was measured between 3.0 and 8.0 GPa at 298 K using a high−pressure x−ray diffraction technique. The specific volume (0.708, +0.023, −0.012 cm3/g), the isothermal bulk modulus (12.54±0.27 GPa), and the modulus’ pressure derivative (5.56±0.14) for ice VII at 298 K and zero pressure were determined using an empirical equation for isothermal compression. That equation is [P V02/(V0 − V)]1/2 = CT + ST [P (V0 − V)]1/2, where CT and ST are constants, V0 is the ambient volume, and V is the volume at pressure P. This linear relation, which describes the state of nonporous materials along their Hugoniots, is shown to characterize the isothermal compression of solids as well as does the Murnaghan equation. The zero pressure, 298 K oxygen−oxygen distance in ice VII extrapolated from the present data and a simple bonding model for the hydrogen−bonded oxygen atoms strongly support Kamb’s description of the ice VII structure as two interpenetrating ice Ic frameworks.

The compression of solid CO2 at 296 K to 10 GPa

The volume of solid CO2 was measured at 296 K from 1 to 10 GPa using a high pressure, x‐ray diffraction technique. The compression of NaF was used as the pressure gauge. The data are converted to the velocity plane and linearly fit. The volumes and bulk moduli are calculated from the fit as a function of pressure. Previous compression studies are discussed and a recent a priori calculation is shown to agree well with the present data.

Lithium, compression and high-pressure structure

Lithium is found to transform from a body-centered cubic (bcc) to a face-centered cubic (fcc) structure at 6.9 gigapascals (69 kilobars) and 296 kelvin. The relative volume of the bcc structured lithium at 6.9 gigapascals is 0.718, and the fcc structure is 0.25 percent denser. The bulk modulus and its pressure derivative for the bcc structure are 11.57 gigapascals and 3.4, and for the fcc structure are 13.1 gigapascals and 2.8. Extrapolation of the bcc-fcc phase boundary and the melting curve indicate a triple point around 15 gigapascals and 500 kelvin.

Bulk A15, high TC Nb3Si synthesized by shock compression

Abstract A shock wave compression technique has been used to convert bulk quantities of Nb3Si into the A15 phase. The recovered material has a lattice parameter of 5.091 ± 0.006 A. It also has an inductive TC of 18.6 K and shows a large specific heat transition at 18.0 K.

A method for the accurate measurement of lattice compressions of low‐Z materials at pressures up to 12 GPa by x‐ray diffraction

The technique developed by Jamieson and Lawson for x‐ray diffraction at high pressure has been modified so that low‐Z materials can be studied under high pressure conditions to 12 GPa. A beryllium annulus replaces the commonly used boron annulus, allowing the use of Cu Kα or Fe Kα radiation, thus improving the accuracy of d‐space determinations. The inclusion of a 4:1 methanol‐chanol mixture developed by Piermarini, et al. relieves pressure gradients and anisotropy, improving both the quality and range of compression data. Excellent compression results are obtained in the 0–1 GPa region where x‐ray techniques have formerly been inaccurate.

Compression of solid nitromethane to 15 GPa at 298 K

The measurement of the isothermal compression of solid nitromethane to 15 GPa at 298 K using high pressure x‐ray diffraction techniques is described. The compression data are fit to a model from which bulk moduli are calculated. Most interesting are the linear compression data. The a axis direction, along which the C–N bonds are aligned, shows little increase in repulsion with increasing pressure above 5 GPa. This indicates that the nitro and methyl groups of neighboring molecules may be interacting.

Structures and transitions in solid O2 to 13 GPa at 298 K by x‐ray diffraction

The structures and phase transitions in solid O2 were studied at 298 K from 6 to 13 GPa using powder x‐ray diffraction in an anvil cell. Pressures were determined from the compression of in situ NaF. X‐ray photographs at various pressures showed patterns from five different phases of O2, two of which were previously unreported. Values of the bulk modulus and volume were derived up to 11 GPa where transition to e‐O2 occurs. A compromise fit to all available data indicates that e‐O2 may be an orthorhombic distortion of δ‐O2.

The compression of solid N2 at 296 K from 5 to 10 GPa

The volume of solid N2 having Pm3n space group was measured at 296 K from 5 to 10 GPa using a high pressure, x‐ray diffraction technique. The compression of NaF was used as the pressure gauge. From a fit of the compression of the Pm3n structure data converted to the velocity plane, the volumes and bulk moduli are calculated as a function of pressure. From this fit and simultaneous volume measurements of the β‐N2 phase and the Pm3n structure, the pressure of transformation from the β‐N2 phase to the Pm3n structure is found to be 4.8 GPa. Recently published a priori calculations are shown to agree well with the present data.