[en] Diamond anvil cells (DAC) as opposed anvil devices produce non-hydrostatic stress state, especially in experiments at very high pressures. We develop an algorithm for analysis of the effect of non-hydrostatic stress on the shape and width of the diffraction lines acquired with an area detector in angle-dispersive powder diffraction experiments. Numerical simulations show that deviatoric stress changes d-spacings and produces broadening of all diffraction lines. Both these phenomena depend on the elastic properties of the material, value of uniaxial stress component, and orientation of the cell (loading direction) with respect to incident beam

[en] Iron is the main component of the Earth's core and its structure and properties are important for interpretation of geophysical observations and modeling dynamics of the core. We argue that the diffraction lines in the high temperature high pressure X-ray diffraction pattern, presented by Tateno et al., 2010 and interpreted as those of solely hot hcp-Fe, correspond indeed to the insufficiently heated part of the sample. We show that observed diffraction spots are either due to bcc-Fe or carbides. (Author) 11 refs.

[en] A simple method for detection of melting event in laser-heated diamond anvil cells (DACs) is introduced. The melting is registered optically by the formation of spherical drops of the investigated material as heated in an inert pressure transmitting medium. Feasibility of the method is demonstrated on the examples of metal (iron and gold) and iron oxide (Fe₂O₃), materials molten at pressures over 40 GPa employing a portable laser heating system. (Author) 25 refs.

[en] The first in situ Raman study of C₆₀ isothermal compression at 800 K and up to 32 GPa was performed using rhombohedral and tetragonal phases as starting materials. The rhombohedral phase shows a phase transition to 3D polymer above 10 GPa, similar to that in experiments where isobaric heating was used at pressures of 9-13 GPa. It is shown that the T-P diagram of C₆₀ polymeric phases (temperature increase followed by pressurizing) is significantly different from the known P-T diagram (pressurizing followed by heating). Tetragonal polymer exhibited significantly stronger stability and can be followed at least up to ∼15 GPa. Heating up to 800 K of tetragonal polymer at pressures of 6-8 GPa confirms that, due to geometrical frustrations, the tetragonal phase remains stable even at pressure and temperature conditions at which rhombohedral polymer is usually formed

[en] The high-pressure, room temperature behavior of otavite (CdCO₃) was investigated by angle-dispersive synchrotron radiation powder diffraction up to 40 GPa, Raman spectroscopy up to 23 GPa and quantum mechanical calculations based on density functional theory. The calcite-type structure of CdCO₃ is stable up to at least ∼19 GPa 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 30 GPa. 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 19 GPa in both experiments.

[en] A tetragonal σ phase of the equiatomic FeCr alloy was investigated by x-ray diffraction under pressure up to 77 GPa. The phase was found to be stable in the whole pressure range studied. The equation of state for the tetragonal phase was found to have a value of bulk modulus K₀ = 217(5) GPa and its pressure derivative was K₀' = 5.8(2), which makes it less compressible than the constituent elements. Electronic factors governing the σ phase stability are discussed in relation to the Hume-Rothery mechanism.

[en] Despite great technological importance and many investigations, a material with a measured hardness comparable to that of diamond or cubic boron nitride has yet to be identified. Our combined theoretical and experimental investigations led to the discovery of a new polymorph of titanium dioxide, where titanium is ninefold coordinated to oxygen in the cotunnite (PbCl₂) structure. Hardness measurements on this phase, synthesized at pressures above 60 GPa and temperatures above 1000 K, reveal that this material is the hardest oxide yet discovered. Furthermore, it is one of the least compressible (with a measured bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa) polycrystalline materials studied so far

[en] High pressure ⁵⁷Fe Moessbauer spectroscopy measurements (up to ∼41 GPa at room temperature) have been carried out for investigating the magnetic properties of γ (fcc) ⁵⁷Fe₅₃Ni₄₇ alloy using diamond anvil cell (DAC) technique. The Moessbauer spectrum at 0 GPa shows a six line magnetic pattern with broad outer peaks and an average hyperfine field of ∼31 T characteristic of a disordered alloy. In the pressure range (2< P<20 GPa) we observe Moessbauer spectra with additional low hyperfine field component resembling spectra of γ (fcc) Fe-Ni Invar alloys (30-40 at % Ni). Our data indicate a pressure induced Invar effect for ⁵⁷Fe₅₃Ni₄₇ alloy at ∼7-12 GPa. Above 20 GPa the hyperfine field breaks down and the alloy becomes non-magnetic showing only a single line Moessbauer spectrum.

[en] The diamond anvil cell (DAC) technique coupled with laser heating has become the most successful method for studying materials in the multimegabar pressure range at high temperatures. However, so far all DAC laser-heating systems have been stationary: they are linked either to certain equipment or to a beamline. Here, a portable laser-heating system for DACs has been developed which can be moved between various analytical facilities, including transfer from in-house to a synchrotron or between synchrotron beamlines. Application of the system is demonstrated in an example of nuclear inelastic scattering measurements of ferropericlase (Mg_0.88Fe_0.12)O and h.c.p.-Fe_0.9Ni_0.1 alloy, and X-ray absorption near-edge spectroscopy of (Mg_0.85Fe_0.15)SiO₃ majorite at high pressures and temperatures. Our results indicate that sound velocities of h.c.p.-Fe_0.9Ni_0.1 at pressures up to 50 GPa and high temperatures do not follow a linear relation with density.

[en] An investigation into the high-pressure behavior of AX₂-type iron pnictides was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Our results demonstrate that a phase transition from the marcasite to the CuAl₂ occurs at 108 GPa for FeP₂, at 92 GPa for FeAs₂, and at 38 GPa for FeSb₂, accompanying a semiconductor-to-metal crossover. A linear relationship between bulk moduli and the inverse specific volume is proposed to be B₀ = 17 498/V₀-45.9 GPa for the marcasite-type phase and B₀ = 31 798/V₀-67.5 GPa for the CuAl₂-type phase. According to the observed structural evolutions, we claim that the regular marcasite transforms to the CuAl₂-type phase and the anomalous marcasite transforms to the pyrite-type phase at high pressures.