Search

Fast compression experiments performed using dynamic diamond anvil cells (dDACs) employing piezoactuators offer the opportunity to study compression-rate dependent phenomena. In this paper, we describe an experimental setup which allows us to perform time-resolved x-ray diffraction during the fast compression of materials using improved dDACs. The combination of the high flux available using a 25.6 keV x-ray beam focused with a linear array of compound refractive lenses and the two fast GaAs LAMBDA detectors available at the Extreme Conditions Beamline (P02.2) at PETRA III enables the collection of x-ray diffraction patterns at an effective repetition rate of up to 4 kHz. Compression rates of up to 160 TPa/s have been achieved during the compression of gold in a 2.5 ms fast compression using improved dDAC configurations with more powerful piezoactuators. The application of this setup to low-Z compounds at lower compression rates is described, and the high temporal resolution of the setup is demonstrated. The possibility of applying finely tuned pressure profiles opens opportunities for future research, such as using oscillations of the piezoactuator to mimic propagation of seismic waves in the Earth. [ABSTRACT FROM AUTHOR]

Two binary phases in the system Hf–O have been synthesized at pressures between 12 and 34 GPa and at temperatures up to 3000 K by reacting Hf with HfOusing a laser-heated diamond anvil cell.In situX-ray diffraction in conjunction with density functional theory calculations has been employed to characterize a previously unreported tetragonal HfOphase. This phase has a structure which is based on an fcc Hf packing with oxygen atoms occupying octahedral interstitial positions. Its predicted bulk modulus is 223(1) GPa. The second phase has a composition close to HfO, where oxygen atoms occupy octahedral interstitial sites in an hcp Hf packing. Its experimentally determined bulk modulus is 128(30) GPa. The phase diagram of Hf metal was further constrained at high pressures and temperatures, where we show that α-Hf transforms to β-Hf around 2160(150) K and 18.2 GPa and β-Hf remains stable up to at least 2800 K at this pressure. [ABSTRACT FROM PUBLISHER]

Structural and vibrational changes due to the incorporation of Cu in the CuxZn2-xTiO4 inversespinel solid solution have been investigated by X-ray diffraction, Raman spectroscopy, and abinitio calculations. Both X-ray diffraction and Raman spectroscopy show that the structure remainscubic while the unit-cell volume decreases on Cu2+ incorporation. The compositional dependenciesof the Raman frequencies and linewidths indicate the incorporation of Cu2+ into tetrahedral sites. The A1g tetrahedral mode frequency becomes independent on composition for x > 0.6. This isattributed to the limited incorporation of Cu2+ in the tetrahedral sites at Cu-rich compositions. Abinitio calculations with quasi-random structures reveal only a slight energetic preference of Cu2+ for octahedral over tetrahedral sites. [ABSTRACT FROM AUTHOR]

The high pressure high-temperature stability of tantalum mononitrides was investigated at 8–23.5 GPa and 1400–3500 K byin situpowder synchrotron X-ray diffraction using the laser-heated diamond anvil cell and ϵ-TaN as starting material. The transformation from ϵ-TaN to ϑ-TaN was observed and ϑ-TaN was confirmed as the stable high-(p, T) tantalum mononitride. At temperatures above 3000 K at 16–23.5 GPa, the formation of a novel quenchable high pressure high-temperature phase was observed. Under nitrogen excess ϵ-TaN reacted with nitrogen to the high-(p, T) phase η-Ta2N3as the main stable phase at 8–13 GPa and about 2000 K. This behaviour is similar to the reaction of pure Ta with nitrogen at high-(p, T) conditions. In addition, we have determined the bulk moduli of ϑ-TaN (B=360(15) GPa), which is less compressible than ϵ-TaN and η-Ta2N3, and of TaON (B=267(8) GPa). [ABSTRACT FROM PUBLISHER]

Abstract: In order to directly observe the effect of daughter product formation on the stability of a crystalline compound, 55Fe was produced at the Isotope Production Facility at Los Alamos National Laboratory. After purification, the metal was oxided to 55Fe2O3. Samples were loaded in diamond anvil cells, with neon as a pressure transmitting medium, and pressurized to pressures between 5 and 15 GPa. The samples were then characterized by synchrotron powder diffraction and micro-Raman spectroscopy. The present study serves as a reference for future studies, in which the formation of 55Mn2O3 from 55Fe2O3 by radioparagenesis will be explored. [Copyright &y& Elsevier]

We revise the high pressure, high temperature phase diagram of CaCO 3 using Raman spectroscopy in conjunction with laser heated diamond anvil cell experiments. We confirm numerous aspects of earlier studies, including a recent X-ray diffraction study about the stability field of CaCO 3 -VII. Our Raman results show that CaCO 3 -VII is stable in the lower mantle at a depth of 690–1010 km. Our DFT calculations show that the phase transition from aragonite to CaCO 3 -VII at ≈ 25 GPa and from CaCO 3 -VII to post-aragonite at ≈ 40 GPa are accompanied by density changes of 2% and 3.5%, respectively. Shear sound velocities change by 9% and −12% across the transitions, respectively. Hence, a sufficient amount, at least locally, of CaCO 3 in the Earth’s mantle can be detectable by an increase of the shear velocity at 690 km and a decrease of the shear velocity at 1010 km depths. [ABSTRACT FROM AUTHOR]

X-ray diffraction and Raman spectroscopy were employed to study the phase diagram of SF 6 at pressures up to 32 GPa. The known phase transformation of SF 6 phase I to phase II at around 2 GPa was confirmed. Discontinuities in the pressure dependence of the Raman frequencies and positions of X-ray reflections indicate further phase transitions at 10(1) and 19(1) GPa. The crystal structures of phases I and II were determined using single crystal X-ray diffraction. The structure of phase I corresponds to the “plastic” body-centred cubic low-temperature structure and the structure of phase II to the ordered monoclinic low-temperature structure. The bulk moduli of phase I and II were determined to be B 0 , I = 6.3 ( 2 ) GPa with B I ′ = 4 (fixed) and B 0 , II = 8.5 ( 8 ) GPa with B II ′ = 7.4 ( 9 ) , respectively. The linear compressibilities of phase II showed a slight anisotropy, which can be rationalised by consideration of the packing of the SF 6 molecules. [ABSTRACT FROM AUTHOR]

The decomposition of SF 6 in the presence of glassy carbon was induced in laser heated diamond anvil cells at 10–11 GPa and 2000–2500 K. The reaction products were characterised by synchrotron X-ray diffraction, including high pressure pair distribution function analysis, and micro-Raman spectroscopy combined with atomistic model calculations. The decomposition leads to elemental amorphous helical sulfur and crystalline CF 4 -III. Two different sulfur phases, namely helical S μ and crystalline α-S 8 , were observed after recovering the laser heated samples of different experiments at ambient conditions. [ABSTRACT FROM AUTHOR]

Abstract: The high-pressure, room temperature behavior of otavite (CdCO3) was investigated by angle-dispersive synchrotron radiation powder diffraction up to 40GPa, Raman spectroscopy up to 23GPa and quantum mechanical calculations based on density functional theory. The calcite-type structure of CdCO3 is stable up to at least ∼19GPa as shown by Raman spectroscopy. The compression mechanism was obtained from structure refinements against the diffraction data. The quantum mechanical calculations propose a calcite–aragonite phase transition to occur at about 30GPa. The existence of a pressure-induced phase transition is supported by the Raman and diffraction experiments. Evidence for the transformation is given by broadening of X-ray reflections and external Raman bands starting from about 19GPa in both experiments. [ABSTRACT FROM AUTHOR]

The high-pressure behaviour of α-PbF2 was investigated by angular-dispersive synchrotron X-ray powder diffraction up to 6.55(8) GPa. The fit of a third-order Birch–Murnaghan equation-of-state gave the volume at zero pressure V0=194.14(4) A˚3 and the bulk modulus b0=47.0(6) GPa with the pressure derivative b′=7.9(4). The continuous-symmetry-measure approach has been used for the quantification of the distortion of the coordination polyhedron in α-PbF2 revealing an increasing distortion with pressure. [Copyright &y& Elsevier]

The low-temperature behavior of tetragonal copper sulfide, Cu2S, was investigated by powder and single-crystal x-ray diffraction, calorimetry, electrical resistance measurements, and ambient temperature optical absorption spectroscopy. The experiments were complemented by density-functional-theory-based calculations. High-quality, polycrystalline samples and single crystals of tetragonal copper sulfide were synthesized at 5 GPa and 700 K in a large volume multianvil press. Tetragonal Cu2S undergoes a temperature-induced phase transition to an orthorhombic structure at around 202 K with a hysteresis of ±21 K, an enthalpy of reaction of 1.3(2) kJmol-1, and an entropy of reaction of 6.5(2) Jmol-1K-1. The temperature dependence of the heat capacity at the transition temperature indicates that the transition from the tetragonal to the low-temperature polymorph is not a single process. The structure of the low-temperature polymorph at 100 K was solved in space group Pna21. The structure is based on a slightly distorted cubic close packing of sulfur with copper in threefold coordination similar to the structure of tetragonal copper sulfide. The electrical resistance changes several orders of magnitude at the transition following the temperature hysteresis. The activation energy of the conductivity for the tetragonal phase and the low-temperature polymorph are 0.15(2) and 0.22(1) eV, respectively. The direct band gap of the tetragonal polymorph is found to be 1.04(2) eV with the absorption spectrum following Urbach's law. The activation energies and the band gaps of both phases are discussed with respect to the results of the calculated electronic band structures. [ABSTRACT FROM AUTHOR]

The pressure-induced phase transition of monazite-type LaPO4 at ≈26 GPa is studied by single-crystal x-ray diffraction and second harmonic generation (SHG) up to 31 GPa. The structure of the postmonazite phase of LaPO4 has been obtained and it is shown that it corresponds to a post-barite-type structure with an acentric space group P212121. A strong increase of the SHG signal at the transition confirms that the high-pressure polymorph is noncentrosymmetric. The phase transition involves a significant discontinuous decrease of the unit-cell volume by 6%, which is mainly due to a strong contraction of the a lattice parameter. Enthalpy differences between polymorphs with monazite-, barite-, and post-barite-type structures have been obtained from density-functional-theory-based calculations. They also yield a transition pressure of 21 GPa for the monazite to postbarite phase transition, but indicate that the barite-type phase is significantly more stable, implying that the transition into this phase may be kinetically hindered. As the transition pressure for the monazite-type to post-barite-type structure is reproduced with reasonable accuracy, we predict the corresponding phase-transition pressures of NdPO4 and GdPO4 to be 35 and 45 GPa, respectively. [ABSTRACT FROM AUTHOR]