High-pressure Diamond Anvil Cell Experiments Research Articles

Pressure dependence of melting temperature and shear modulus of hcp-iron

ABSTRACTThe pressure effects on melting temperature and shear modulus of hcp-iron have been studied based on the semi-empirical approach in the Debye model. The recent well-established pressure-dependent Grüneisen parameter has been applied to derive the analytical expressions of the Debye frequency, the Debye temperature, melting temperature and shear modulus which are of importance to geophysical implications. Numerical calculations have been performed for hcp-iron as functions of pressure up to the pressure of Earth's inner core. Our calculations are compared with those of previous experimental and theoretical data showing the good and reasonable agreements. The present results contribute to the database of high pressure melting, especially Earth's inner core boundary temperature, and could also be used to verify as well as analyze the future high pressure diamond-anvil cell experiments.

How “Hollow” Are Hollow Nanoparticles?

Diamond anvil cell (DAC), synchrotron X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) techniques are used to probe the composition inside hollow γ-Fe(3)O(4) nanoparticles (NPs). SAXS experiments on 5.2, 13.3, and 13.8 nm hollow-shell γ-Fe(3)O(4) NPs, and 6 nm core/14.8 nm hollow-shell Au/Fe(3)O(4) NPs, reveal the significantly high (higher than solvent) electron density of the void inside the hollow shell. In high-pressure DAC experiments using Ne as pressure-transmitting medium, formation of nanocrystalline Ne inside hollow NPs is not detected by XRD, indicating that the oxide shell is impenetrable. Also, FTIR analysis on solutions of hollow-shell γ-Fe(3)O(4) NPs fragmented upon refluxing shows no evidence of organic molecules from the void inside, excluding the possibility that organic molecules get through the iron oxide shell during synthesis. High-pressure DAC experiments on Au/Fe(3)O(4) core/hollow-shell NPs show good transmittance of the external pressure to the gold core, indicating the presence of the pressure-transmitting medium in the gap between the core and the hollow shell. Overall, our data reveal the presence of most likely small fragments of iron and/or iron oxide in the void of the hollow NPs. The iron oxide shell seems to be non-porous and impenetrable by gases and liquids.

In situx-ray diffraction measurements of thec/aratio in the high-pressureεphase of shock-compressed polycrystalline iron

The structure of laser-shock-compressed polycrystalline iron was probed using in situ x-ray diffraction over a pressure range spanning the $\ensuremath{\alpha}$-$\ensuremath{\epsilon}$ phase transition. Measurements were also made of the $c/a$ ratio in the $\ensuremath{\epsilon}$ phase, which, in contrast with previous in situ x-ray diffraction experiments performed on single crystals and large-scale molecular dynamics (MD) simulations are close to those found in high-pressure diamond anvil cell experiments. This is consistent with the observation that significant plastic flow occurs within the nanosecond time scale of the experiment. Furthermore, within the sensitivity of the measurement technique, the fcc phase that had been predicted by MD simulations was not observed.

Doping Effect on High-Pressure Structural Stability of ZnO Nanowires

We report reversible phase transitions of ZnO nanowires subjected to high-pressure diamond anvil cell experiments. It was found that Mn doping obviously changes the elastic stability of ZnO nanowires and leads to low critical pressures of the reversible transitions between wurtzite to rocksalt phases during compression and decompression. First-principles calculations demonstrate that the lower critical pressures caused by Mn doping are associated with the decrease of internal energy difference and the increase of relative volume change between wurtzite to rocksalt phases.

On compressibility of osmium metal

On the basis of the high-pressure diamond anvil cell experiments on Os metal, Cynn et al. [Phys. Rev. Lett. 88, 135701-1 (2002)] have reported that this metal has lower compressibility than diamond. In the present work we have reanalysed the experimental data of Cynn et al. We find that the bulk moduli of Os and diamond are close to each other, implying that Os metal is as incompressible as diamond, but not more so. Our first principles total energy calculations using the full potential linearised augmented plane wave method on Os and diamond also suggest the same results.

Amorphous boron gasket in diamond anvil cell research

Recent advances in high-pressure diamond anvil cell experiments include high-energy synchrotron x-ray techniques as well as new cell designs and gasketing procedures. The success of high-pressure experiments usually depends on a well-prepared sample, in which the gasket plays an important role. Various gasket materials such as diamond, beryllium, rhenium, and stainless steel have been used. Here we introduce amorphous boron as another gasket material in high-pressure diamond anvil cell experiments. We have applied the boron gasket for laser-heating x-ray diffraction, radial x-ray diffraction, nuclear resonant inelastic x-ray scattering, and inelastic x-ray scattering. The high shear strength of the amorphous boron maximizes the thickness of the sample chamber and increases the pressure homogeneity, improving the quality of high-pressure data. Use of amorphous boron avoids unwanted x-ray diffraction peaks and reduces the absorption of incident and x rays exiting the gasket material. The high quality of the diffraction patterns makes it possible to refine the cell parameters with powder x-ray diffraction data under high pressure and high temperature. The reactivity of boron prevents its use at high temperatures, however. When heated, boron may also react with the specimen to produce unwanted phases. The relatively porous boron starting material at ambient conditions also poses some challenges for sample preparation.

Theoretical studies of iridium under pressure

Recent experiments on Ir under pressure (Cerenius and Dubrovinsky 2000 J.Alloys Compounds 306 26) show a transition to a superlattice structurecomprising 14 atomic layers. This observation has implications for high-pressureapplications since Ir, with its high bulk modulus and high thermal stability,is ideally suited for use as a gasket for high-temperature, high-pressurediamond anvil cell experiments. We perform first-principles total energycalculations to study the crystal phases and defect structures of Irunder pressure. We have extended the bond-orientation model (Chettyand Weinert 1997 Phys. Rev. B 56 10844) to compute all of the ∼ 2Ndefect structures as a function of atomic volume. We find Ir in the FCC structureto be extremely stable for pressures up to about 60 GPa. We also calculate thestacking fault energies of Ir.

Constraints from molecular dynamics on the liquidus and solidus of the lower mantle

A method is proposed for calculating freezing point depressions in silicate systems of geologic interest using molecular dynamics simulations. The method directly determines the slope of the liquidus at a given composition and temperature. The construction of a phase diagram is then possible using a fitting procedure, or by inference of a set of liquid phase components whose mixing behavior approximates thermodynamic ideality. Application of this method to the system Mg 2SiO 4MgSiO 3 at low pressures gives results in qualitative agreement with experimentally determined phase diagrams. At 100 GPa this method gives depression curves compatible with curves calculated assuming ideal mixing of the liquid phase species MgOSiO 2 or MgO Si 1 2 O . This result, together with the melting temperatures of periclase and of MgSiO 3 perovskite extrapolated from high-pressure diamond anvil cell experiments, allows the construction of a eutectic phase diagram in the system MgOMgSiO 3. Throughout the lower mantle the eutectic temperature is 280–560 K below the melting temperature of MgSiO 3 perovskite. Taking account of other oxide components in the mantle drops the eutectic by another 530–730 K and gives tighter upper bounds for the Earth's solidus and geotherm.

Elimination of pressure-induced fluorescence in diamond anvils

At pressures above one megabar (100 GPa) in high-pressure diamond anvil cell experiments the ruby fluorescence signal needed for pressure calibration is increasingly difficult to measure. A primary cause of this difficulty is the presence of an intense pressure-induced diamond fluorescence. We give a tentative identification of this pressure-induced fluorescence and report on a technique for the elimination of this fluorescence. We demonstrate that weak ruby signals, completely hidden by diamond fluorescence, are now easily measured with our technique.