Shear induced phase transition in PbO under high pressure

We have studied the structural behavior of lead monoxide (PbO) as a function of pressure via angular dispersive X-ray diffraction employing two different pressure transmitting media that were quasi-hydrostatic (N₂) and non-hydrostatic (MgO), respectively. Besides litharge (-PbO) and massicot (β-PbO), which are both stable at ambient pressure, there is an orthorhombic γ-PbO phase which appears upon application of pressure to -PbO. We have found that the orthorhombic γ-PbO phase is favored by shear stress under non-hydrostatic conditions. -PbO shows strong anisotropy in compressibility. The a-axis is rather incompressible with a linear stiffness coefficient of K_a0=540(30) GPa whereas the c-axis stiffness is K_c0=25(1) GPa. The bulk modulus of -PbO is K₀=23.1(3) GPa and its derivative .     

161Dy Mössbauer study of the endohedral metallofullerenes Dy@C n (n = 80, 82, 84)

Mössbauer studies of Dy@C _n (n = 80, 82, 84) metallofullerenes were performed at 4.2, 9.6 and 78 K with the ¹⁶¹Dy (25.6 keV) resonance. The observed spectra consist of two subspectra splitted by magnetic hyperfine fields near to the full moment value of trivalent Dy. Paramagnetic relaxation is observed even at 4.2 K. The observed isomer shift is consistent with the Dy³⁺ state and indicates a full charge transfer to the fullerene cage.     

Phonon Spectroscopy of Oriented hcp Iron

High-pressure phonon spectroscopy was performed on iron in the bcc and hcp phase up to 40 GPa using the nuclear inelastic scattering (NIS) of synchrotron radiation (SR). In hcp iron we observe differences in the density of phonon states for spectra measured with different orientations of the diamond anvil cell (DAC) with respect to the SR beam. These differences are attributed to a preferred orientation of the hexagonal c -axis along the load axis of the DAC. These texture effects are used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the c -axis of hcp iron.     

High-Pressure/High-Temperature NFS Study of Magnetism in LuFe 2 and ScFe 2

Using the new technique of nuclear forward scattering (NFS) of synchrotron radiation, we studied the magnetic hyperfine fields B hf and ordering temperatures T M of the Laves phases LuFe 2 (cubic C15) and ScFe 2 (hexagonal C14) at pressures up to 90 GPa and temperatures up to 700 K. For LuFe 2 we find for T M first an increase from 562 K at 0 GPa to 603 K at 10 GPa and then a decrease to 295 K around 75 GPa. The hyperfine fields B hf show at 295 K a continuous decrease with pressure, indicating a reduction of the Fe band moment. A similar behaviour of both T M and B hf was observed in ScFe 2.     

High-Pressure Studies of Magnetism and Lattice Dynamics by Nuclear Resonant Scattering of Synchrotron Radiation

A review is given on recent high-pressure experiments using nuclear resonant scattering of synchrotron radiation. We shortly introduce the methodological aspects connected with high-pressure experiments applying nuclear forward scattering and nuclear inelastic scattering of synchrotron radiation. Selected examples for the study of magnetism are given for the Laves phases LuFe₂ and ScFe₂, where we studied the variation of the magnetic ordering temperature and of the Fe band moment as a function of pressure. An actual example for the study of lattice dynamics is a recent investigation of the phonon density-of-states in metallic iron with special emphasis on hcp ε-Fe, where the pressure-induced texture is used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the hexagonal c-axis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Radiation-induced decomposition of explosives under extreme conditions

We present high-pressure and high temperature studies of the synchrotron radiation-induced decomposition of powder secondary high explosives pentaerythritol tetranitrate (PETN) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) using white beam synchrotron radiation at the 16 BM-B and 16 BM-D sectors of the HP-CAT beamline at the Advanced Photon Source. The radiation-induced decomposition rate TATB showed dramatic slowing with pressure up to 26.6 GPa (the highest pressure studied), implying a positive activation volume of the activated complex. The decomposition rate of PETN varied little with pressure up to 15.7 GPa (the highest pressure studied). Diffraction line intensities were measured as a function of time using energy-dispersive methods. By measuring the decomposition rate as a function of pressure and temperature, kinetic and other constants associated with the decomposition reactions were extracted.     

Equivalence of the boson peak in glasses to the transverse acoustic van Hove singularity in crystals

We compare the atomic dynamics of the glass to that of the relevant crystal. In the spectra of inelastic scattering, the boson peak of the glass appears higher than the transverse acoustic (TA) singularity of the crystal. However, the density of states shows that they have the same number of states. Increasing pressure causes the transformation of the boson peak of the glass towards the TA singularity of the crystal. Once corrected for the difference in the elastic medium, the boson peak matches the TA singularity in energy and height. This suggests the identical nature of the two features.     

Radiation-induced decomposition of PETN and TATB under extreme conditions

We conducted a series of experiments investigating decomposition of secondary explosives PETN and TATB at varying static pressures and temperatures using synchrotron radiation. As seen in our earlier work, the decomposition rate of TATB at ambient temperature slows systematically with increasing pressure up to at least 26 GPa but varies little with pressure in PETN at ambient temperature up to 15.7 GPa, yielding important information pertaining to the activation complex volume in both cases. We also investigated the radiation-induced decomposition rate as a function of temperature at ambient pressure and 26 GPa for TATB up to 403 K, observing that the decomposition rate increases with increasing temperature as expected. The activation energy for the TATB reaction at ambient temperature was experimentally determined to be 16 +/- 3 kJ/mol.