Origin of complex crystal structures of elements at pressure

We present a unifying theory for the observed complex structures of the sp-bonded elements under pressure based on nearly free electron picture (NFE). In the intermediate pressure regime the dominant contribution to crystal structure arises from Fermi-surface Brillouin zone (FSBZ) interactions - structures which allow this are favoured. This simple theory explains the observed crystal structures, transport properties, the evolution of internal and unit cell parameters with pressure. We illustrate it with experimental data for these elements and ab initio calculation for Li.Comment: 4 pages 5 figure

Theoretical and computational study of high pressure structures in barium

Recent high pressure work has suggested that elemental barium forms a high pressure self-hosting structure (Ba IV) involving two `types' of barium atom. Uniquely among reported elemental structures it cannot be described by a single crystalline lattice, instead involving two interpenetrating incommensurate lattices. In this letter we report pseudopotential calculations demonstrating the stability and the potentially disordered nature of the `guest' structure. Using band structures and nearly-free electron theory we relate the appearance of Ba IV to an instability in the close-packed structure, demonstrate that it has a zero energy vibrational mode, and speculate about the structure's stability in other divalent elements.Comment: 4 pages and 5 figures. To appear in PR

Structural transformations in a simple-hexagonal Hg-Sn alloy under pressure

The structure of Sn alloyed with Hg (10 at. %) was studied under pressure up to 66 GPa with energy-dispersive x-ray diffraction using synchrotron radiation. The ambient pressure simple hexagonal (hP1)phase transformed at 15 GPa to a body-centered tetragonal (tI2) phase similar to the high-pressure forms of Sn and InBi. The axial ratio, c/a, of the tI2 phase increased with pressure up to a maximum value of c/a≈0.92 at about 40 GPa and remained nearly constant upon further compression to 55 GPa. At pressures above 55 GPa, a second phase transition, probably to an orthorhombic phase, was observed. The structural trends observed are discussed in terms of a Fermi-sphere–Brillouin-zone interaction

Crystal structure of InBi under pressure up to 75 GPa

InBi is studied by energy-dispersive diffraction with synchrotron radiation in a diamond-anvil cell under pressure up to 75 GPa. Three phase transitions are observed from its initial tetragonal structure tP4with c/a≈0.95 to another tP4 structure with c/a≈0.65 typical for the β-Np structure, than to a disordered tetragonal body-centered structure tI2, and finally some hints are obtained for a transition to a cubic body-centered structure cI2. All these transformations are reversible with large hysteresis. InBi(tI2), phase III, shows initially a steep increase in c/a from 0.91 to 0.94, in the pressure range from 20 to 30 GPa, and a nearly constant value of about 0.96 above 55 GPa. Hints for the possible stability of a new phase IV are noticed in the present experiments on decompression from 75 GPa as admixture of new lines to phase III giving strong indications for a first-order phase transition with large hysteresis. The origin of the tetragonal distortion and the transformation to a cubic phase are discussed from the point of view of nesting the Fermi sphere in the Brillouin zone

Pressure and temperature dependence of the Eu valence in EuNi2Ge2EuNi_2Ge_2 and related systems studied by Mössbauer effect, X-ray absorption and X-ray diffraction

The effect of pressure and temperature on the valence state of Eu in EuNi2Ge2 systems and the related systems EuNi2Si2, EuNi2Si0.5Ge1.5 and EuPl2Ge2 was studied by 151Eu-Mössbauer spectroscopy, EuLIII X-ray absorption and X-ray diffraction. Pressure-induced valence transitions from a divalent towards a trivalent state were observed in EuNi2Ge2 around 5 GPa and in the quasi-ternary EuNi2Si0.5Ge1.5 system around 0.5 GPa. In EuNi2Ge2 we found from Mössbauer spectra measured at various pressures and temperatures a new type of valence transition, where an initially homogeneous valence state near to divalency segregates into two different valent states. From these data we derive for EuNi2Ge2 the pressure and temperature dependence of the Eu valence and magnetism

A First-Order Phase Transformation between Amorphous Phases in a Zn-Sb Alloy under High Pressures

International audienc

High-pressure high-temperature in situ diffraction studies of nanocrystalline ceramic materials at HASYLAB

High-pressure in situ diffraction studies were performed up to 8 GPa in a cubic anvil cell MAX80 (Station F2.1) and up to 45 GPa in a Diamond Anvil Cell (DAC-Station F3 at HASYLAB, Hamburg). A series of nanocrystals of SiC with grain sizes ranging from 2 nm to several μm were examined in non-hydrostatic conditions by pressing pure powders. A new method of evaluation of powder diffraction data measured at high pressures is presented. This method is based on quantitative evaluation of asymmetry of Bragg reflections where each peak is described as a combination of two reflections of two similar crystallographic phases having different compressibilities. The measured changes of the lattice parameters calculated for split Bragg reflections were used for determination of the pressure gradient which occurs across the grain boundaries in the compressed materials. A model of the strain induced in compacts of pure powders under high pressures is proposed. The model accounts for the presence of two phases: a volume phase corresponds to cores of individual grains which are surrounded by a surface phase which is formed of free surfaces in loose powders and of grain boundaries in solids. Due to extreme hardening of the boundaries under non-hydrostatic pressure conditions, the effective pressure in the interior of the grains is much lower than the applied external pressure. It is suggested that additional ‘hardening’ of the grain boundaries results from the presence of dislocations which are generated at the surface of the grains. The actual gradient of the pressure depends on the size of the grains, and also on the method of synthesis of the materials

Phase transformations of the amorphous Zn-Sb alloy under high pressures

International audienc

A First-Order Phase Transformation between Amorphous Phases in a Zn-Sb Alloy under High Pressures

International audienc

Phase transformations of the amorphous Zn-Sb alloy under high pressures

International audienc