Synthesis of an orthorhombic high pressure boron phase

The densest boron phase (2.52 g cm^-3) was produced as a result of the synthesis under pressures above 9 GPa and temperatures up to ∼1800 °C. The x-ray powder diffraction pattern and the Raman spectra of the new material do not correspond to those of any known boron phases. A new high-pressure high-temperature boron phase was defined to have an orthorhombic symmetry (Pnnm (No. 58)) and 28 atoms per unit cell.     

Synthesis of an orthorhombic high pressure boron phase

Abstract

The densest boron phase (2.52 g cm^-3) was produced as a result of the synthesis under pressures above 9 GPa and temperatures up to ～1800 °C. The x-ray powder diffraction pattern and the Raman spectra of the new material do not correspond to those of any known boron phases. A new high-pressure high-temperature boron phase was defined to have an orthorhombic symmetry (Pnnm (No. 58)) and 28 atoms per unit cell.     

Raman spectroscopic studies of the phase transitions in hexane at high pressure

Raman spectroscopic study of n-hexane was carried out in a cubic zirconia anvil cell up to approximately 2.0 GPa. Under high pressure, the C-H stretching region of the spectrum at 2850-3000 cm(-1) shows measurable changes in frequency, bandwidth, and intensity. These Raman bands shift towards higher frequencies with increasing pressure. At about 1.4 GPa, phase transition from liquid to solid was induced by compression, as was simultaneously observed with the built-in microscope.     

Conductance and phase transition of freestanding ZnO nanocrystals under high pressure

Abstract

The electrical resistances of ZnO powder and nanocrystals produced by a microemulsion method were studied under pressure up to 24 GPa in a diamond anvil cell. The experimental data are different from those previously reported using other techniques. The resistance of ZnO powder rises gradually with increasing pressure until 4.8 GPa and then decreases above 4.8 GPa. The transition point pressure (4.8 GPa) is much smaller than that of 9 GPa for the first order structural transition pressure of bulk ZnO. A similar profile of the electrical resistance versus pressure relationship for ZnO nanocrystals (20 nm) occurs at a higher pressure (8.5 GPa) than that determined for ZnO powder. The cause of these phenomena is discussed.     

On the electronic structure and equation of state in high pressure studies of solids

We discuss the high pressure behaviour of zinc as an interesting example of controversy, and of extensive interplay between theory and experiment. We present its room temperature electronic structure calculations to study the temperature effect on the occurrence of its controversial axial ratio (c/a) anomaly under pressure, and the related electronic topological transition (ETT). We have employed a dense 63 X 63 X 29 k-point sampling of the Brillouin zone and find that the small (c/a) anomaly near 10 GPa pressure persists at room temperature. A weak signature of the anomaly can be seen in the pressure-volume curve, which gets enhanced in the universal equation of state, along with that of K-point ETTs. We attribute the change of slope in the universal equation of state near 10 GPa pressure, mainly to hybridization effects. The temperature effect in fact enhances the possibility of L-point ETT. We find that the L-point ETT is very sensitive to exchange correlation terms, and hence we suggest that further refinements in the theoretical techniques are needed to resolve the controversies on the ETT in Zn.     

Synthesis of a high pressure phase of FePO4

A CrVO₄ type FePO₄ polymorph with a=5.227, b=7.770 and $\text{c}\text{=6.322}\text{A}\text{?}$ has been synthesized at 50 kb and 900° C. The density is 3.90 g/cc, and increase of 22.6% over the quartz modification. The results of magnetic and Mossbauer effect measurements indicate that iron ions are in the high spin trivalent state in both phases. The quadrupole splitting of 1.90 mm/sec for Fe³⁺ of the high pressure phase is much larger than the 0.60 mm/sec for low pressure phase.     

Effect of high pressure on the lattice constants of chromites having the spinel structure

In order to examine the pressure dependence of the compressibility for Ni and Cu chromites with the spinel structure the lattice constants have been measured up to 70kb by the use of a cubic anvil X-ray diffraction press. The high compressibility of NiCr₂O₄ was explained by softening of the lattice for temperatures near the cubictetragonal transition. The change of lattice constant of CuCr₂O₄ under high pressure was anomalous.     

Superconductivity and phase transitions in single-crystal and polycrystal α-U at high pressure

The superconducting transition temperature of single-crystal and polycrystal uranium has been determined as a function of hydrostatic pressure to 24 kbar. Although the general form of theT _c (P) variation is similar for the two types of material, features are present in the detailed behavior for the single-crystal samples which are not found for polycrystal samples. These features are presumed to be associated with the first-order transitions observed at zero pressure. A low-temperature phase diagram has been constructed and the electronic nature of the various phases is discussed.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR contract no. AFOSR-F44620-72-C-0017.Work performed under the auspices of the U.S. Atomic Energy Commission.     

Extraordinarily complex crystal structure with mesoscopic patterning in barium at high pressure

Elemental barium adopts a series of high-pressure phases with such complex crystal structures that some of them have eluded structure determination for many years. Using single-crystal synchrotron X-ray diffraction and new data analysis strategies, we have now solved the most complex of these crystal structures, that of phase Ba-IVc at 19?GPa. It is a commensurate host-guest structure with 768 atoms in the representative unit, where the relative alignment of the guest-atom chains can be represented as a two-dimensional pattern with interlocking S-shaped 12-chain motifs repeating regularly in one direction and repeating with constrained disorder in the other. The existence of such patterning on the nanometre scale points at medium-range interactions that are not fully screened by the itinerant electrons in this metal. On the basis of first-principles electronic structure calculations, pseudopotential theory and an analysis of the lattice periodicities and interatomic distances, we rationalize why the Ba phases with the common densely packed crystal structures become energetically unfavourable in comparison with the complex-structured Ba-IVc phase, and what the role of the well-known pressure-induced s-d electronic transfer is.