High pressure xray diffraction studies on solid N2 up to 43.9 GPa J. Chem. Phys. 93, 8968 (1990); 10.1063/1.459236 Structures and transitions in solid O2 to 13 GPa at 298 K by xray diffractionThe structures and phase transitions ofN2 and CO were studied by powder x-ray diffraction from 100 to 300 K and 4 to 13 GPa. Three solid phases,,8, 8, and E, were observed in each material. The known,8 and 8 solids were confirmed to have hexagonal space group P 6~mme 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 rhombohedralR 3e. There are eight ordered molecules in the E-N2 unit cell with a = 5.928 A. and a = 85.140 at 110 K and 7.8 GPa, and eight ordered molecules in the E-CO unit cell with a = 6.059 A. and a = 85.73 0 at 100 K and 5.5 GPa. The CO molecules are randomly oriented head to tail. The k 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 correlations correctly predict features in the N2 and CO phase diagrams.

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.

PBX 9502 is a plastic‐bonded explosive that contains 95 wt.‐% TATB, a graphitic‐structured high explosive known to undergo “ratchet growth,” i.e., irreversible volume change that accompanies temperature excursions. Earlier studies have reported changes in TATB‐based composites as a function of thermal cycling and density change, however, a clear distinction between density and ratchet‐growth effects has not been made. In the work reported here, an “as‐pressed density” baseline for the mechanical response of recycled PBX 9502 is established over a density range of interest, then high‐density specimens are thermally cycled between −55 and 80 °C to achieve “ratchet‐grown” parts in the same low‐density region. As‐pressed and ratchet‐grown specimens with identical densities are then analyzed using microX‐ray computed tomography and USANS techniques to obtain information about pore‐size distributions. Data show that after ratchet‐growth, PBX 9502 specimens contain, in general, more numerous and smaller voids than specimens that were pressed with lower compaction pressures to match the same density. The mechanical response of the ratchet‐grown material is consistent with damage, showing lower tensile stress and modulus, lower compressive modulus, and higher tensile and compressive strain, than as‐pressed specimens of the same density.

X-ray diffraction studies made in situ under conditions of high pressure and high temperature revealed the direct transition of rutile to the alpha lead dioxide form in titanium dioxide. Compressibility studies of this alpha lead dioxide form at room temperature showed anomalous behavior in that its molar volume converges close to, but not equal to, that of the rutile form. Under this circumstance an unexpectedly large error appears in the calculations of the equilibrium pressure for the two forms at 298 degrees K.

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.