B. K. Godwal

Investigation of the phase stability of LuVO4 at high pressure using powder x-ray diffraction measurements and lattice dynamical calculations

High pressure angle dispersive x-ray diffraction measurements are carried out on LuVO4 in a diamond anvil cell up to 33 GPa at the Elettra synchrotron radiation source. The measurements show that LuVO4 undergoes a zircon to scheelite structure phase transition with a volume change of about 11% at about 8 GPa. A second transition to a monoclinic fergusonite structure occurs above 16 GPa. The data are also recorded while releasing the pressure, and indicate that the scheelite phase is metastable under ambient conditions. The equations of state and changes in internal structural parameters are reported for various phases of LuVO4. Lattice dynamical calculations based on a transferable interatomic potential were also performed and the results support the stability of the scheelite structure at high pressures. The calculated structure, equation of state and bulk modulus for all the phases are in fair agreement with the experimental observations.

Ultrahigh-pressure melting of lead - A multidisciplinary study

Measurements of the melting temperature of lead, carried out to pressures of 1 megabar (1011 pascal) and temperatures near 4000 kelvin by means of a laser-heated diamond cell, are in excellent agreement with the results of previous shock-wave experiments. The data are analyzed by means of first principles quantum mechanical calculations, and the agreement documents the reliability of current experimental and theoretical techniques for studies of melting at ultrahigh pressures. These studies have potentially wide-ranging applications, from planetary science to condensed matter physics.

High-pressure phase transitions in adamantane

Abstract We report angle dispersive X-ray diffraction (ADXRD) measurements on adamantane carried out at SPRING-8 to the pressures of 25 GPa. The tetragonal phase observed at 0.5 GPa remains stable up to 12.5 GPa. In this pressure range the intermolecular hydrogen separation reduces from 2.37 to1.87 A with relative angle of rotation of the two molecules increasing from 8.5° to 10.5° in agreement with values from energy minimization. At 16 GPa, the diffraction pattern could be indexed either by a tetragonal or a monoclinic cell. Beyond 22 GPa only monoclinic cell indexes the patterns. The present findings corroborate the earlier Raman results.

Seismic anisotropy in the Earth's innermost inner core: Testing structural models against mineral physics predictions

Using an updated data set of ballistic PKIKP travel time data at antipodal distances, we test different models of anisotropy in the Earths innermost inner core (IMIC) and obtain significantly better fits for a fast axis aligned with Earths rotation axis, rather than a quasi-equatorial direction, as proposed recently. Reviewing recent results on the single crystal structure and elasticity of iron at core conditions, we find that an hcp structure with the fast c axis parallel to Earths rotation is more likely but a body-centered cubic structure with the [111] axis aligned in that direction results in very similar predictions for seismic anisotropy. These models are therefore not distinguishable based on current seismological data. In addition, to match the seismological observations, the inferred strength of anisotropy in the IMIC (6–7%) implies almost perfect alignment of iron crystals, an intriguing, albeit unlikely situation, especially in the presence of heterogeneity, which calls for further studies.

On the relative stability of orthorhombic and hcp phases of beryllium at high pressures

High-pressure electronic properties of Be have been investigated theoretically by means of ab initio electronic structure calculations. The calculations have been carried out by the semi-relativistic full-potential, linear muffin-tin orbital (FPLMTO) method, within the local density approximation. The crystal structure stability among the hcp, bcc and orthorhombic (distorted hcp) phases has been studied as a function of compression. The bcc structure is found to be energetically stable at pressures above 180 GPa. From the results of our calculations, the orthorhombic phase cannot occur as an intermediate phase between the ambient pressure hcp phase and the high-pressure bcc structure. Our work thus suggests the need for more accurate high-pressure x-ray data.

High-pressure behavior of osmium : an analog for iron in Earth's core

High-resolution x-ray diffraction with diamond-anvil cells, using argon as a quasi-hydrostatic pressure medium, documents the crystal structure and equation of state of osmium to over 60u2009GPa at room temperature. We find the zero-pressure bulk modulus in fair agreement with other experiments as well as with relativistic electronic band-structure calculations: Osmium is the densest but not the most incompressible element at ambient conditions. We also find no evidence for anomalies in the ratio of unit-cell parameters, c/a, or in the compressibility of osmium as a function of pressure. This is in agreement with other experiments and quantum mechanical calculations but disagrees with recent claims that the electronic structure and equation of state of osmium exhibit anomalies at pressures of ∼15-25u2009GPa; the discrepancies are may be due to the effects of texturing.

Clapeyron slope reversal in the melting curve of AuGa2 at 5.5 GPa

We use x-ray diffraction in a resistively heated diamond anvil cell to extend the melting curve of AuGa2 beyond its minimum at 5.5 GPa and 720 K, and to constrain the high-temperature phase boundaries between cubic (fluorite structure), orthorhombic (cottunite structure) and monoclinic phases. We document a large change in Clapeyron slope that coincides with the transitions from cubic to lower symmetry phases, showing that a structural transition is the direct cause of the change in slope. In addition, moderate (~30 K) to large (90 K) hysteresis is detected between melting and freezing, from which we infer that at high pressures, AuGa2 crystals can remain in a metastable state at more than 5% above the thermodynamic melting temperature.

Deriving equations of state from non-hydrostatic data

Modeling of non-hydrostatic strains allows extraction of a reliable (quasi-hydrostatic) equation of state from diamond-cell x-ray measurements at high pressures, as illustrated by new data on Os collected to 60 GPa at room temperature: axial- and radial-diffraction measurements are in good agreement with data collected using Ar and He pressure media, as well as with first-principles calculations, in confirming that osmium is the densest but not the most incompressible element. Dynamic-loading methods can generate much higher pressures than static compression, however, shock compression leads to high temperatures there is much interest in compression using ramp waves. This can be accomplished with graded-density mechanical impacts, or with laser-driven pressure waves; other means of maintaining low temperatures include pre-compression and cooling of the sample before it is dynamically compressed by ramp- or multiple-shock waves. Reduced temperatures lead to enhanced strength, which makes it necessary to model both temperature and strength effects in order to extract the equation of state. A unified approach combining analysis of static and dynamic compression measurements offers a means of determining pressure?density equations of state to high compressions.

Investigation of phase transition of mercury decomposed from mercury oxide up to 20 GPa

The high pressure behavior of mercury decomposed from mercury oxide up to 20.4 GPa was investigated using angular-dispersive X-ray diffraction. The results showed that liquid mercury solidified at 2.0 GPa and was resolved as α hexagonal, R-3m, a=3.3743±0.0007 Å and c=6.8199±0.0013 Å. When compressed up to 5.7 GPa, α mercury transformed into orthorhombic γ phase directly, which is not the case of transforming from an α structure to a body-centered tetragonal structure (β). The space group of orthorhombic γ phase was interpreted successfully as Pmmn, with a=2.7722±0.0010 Å, b=4.0792±0.0028 Å and c=6.8285±0.0029 Å at 8.9 GPa.

High pressure and temperature structure of liquid and solid Cd: implications for the melting curve of Cd

The structure of cadmium was characterized in both the solid and liquid forms at pressures to 10 GPa using in situ x-ray diffraction measurements in a resistively heated diamond anvil cell. The distorted hexagonal structure of solid cadmium persists at high pressures and temperatures, with anomalously large c/a ratio of Cd becoming larger as the melting curve is approached. The measured structure factor S(Q) for the melt reveals that the cadmium atoms are spaced about 0.6 Angstroms apart. The melt structure remains notably constant with increasing pressure, with the first peak in the structure factor remaining mildly asymmetric, in accord with the persistence of an anisotropic bonding environment within the liquid. Evolution of powder diffraction patterns up to the temperature of melting revealed the stability of the ambient-pressure hcp structure up to a pressure of 10 GPa. The melting curve has a positive Clausius–Clapeyron slope, and its slope is in good agreement with data from other techniques. We find deviations in the melting curve from Lindemann law type behavior for pressures above 1 GPa.