The multiferroic BiFeO3 crystals were studied under high pressures up to 70 GPa created in diamond anvil cells. Electronic, magnetic and structural transformations were found in the region 40-55 GPa. Electronic transition with high-to-low spin crossover of Fe3+ ion, suppression of magnetism and metallization were carefully investigated and documented. All transitions are completely reversible. The proposed theoretical consideration explains some details of the high-pressure magnetic, optical and electronic properties.

We investigate the lattice dynamics and thermodynamics of nonmagnetic bcc vanadium as a function of temperature and pressure, using the first principles linear response linear-muffin-tin-orbital method. The calculated phonon density of states (DOS) show strong temperature dependence, different from inelastic neutron scattering measurements where the phonon DOS show little change from room temperature up to 1273 K. We obtain the Helmholtz free energy including both electronic and phonon contributions and calculate various equation of state properties such as the bulk modulus and the thermal expansion coefficient. A detailed comparison has been made with available experimental measurements.

In this work we investigate the effect of processing at high pressure-high temperature (HPHT) on the adhesion of CVD diamond coatings on WC-Co substrates. The samples consisted of WC-Co substrates coated with thin diamond films (10 – 40 μm thick) grown by microwave plasma (MWCVD) CVD. The substrates were previously etched in order to remove the Co from the surface region. The adhesion of the film and its wear resistance improved after the HPHT treatment. SEM images of the cross section of the coated substrate revealed that Co infiltrated back to the region where it was previously removed. The results indicate that it is possible to take advantage of the HPHT plants already available around the world to produce, besides PCD's and diamond powder, high-performance CVD diamond cutting tools with the advantage of requiring less demanding processing conditions.

Fluid phosphorus exhibits a macroscopic phase separation during a pressure-induced first-order phase transition between low-density fluid and high-density fluid. Solidification process of the phase-separated fluid sample was monitored by x-ray radiography and the obtained solid sample was investigated by optical and scanning electron microscopy. A solid mixture of black parts and red parts was obtained from the phase-separated sample. Three different morphologies were observed in the red part of the sample.

We have designed new materials, CaN and SrN in a rock-salt structure (RS). They are ferromagnetic nitrides. Their ferromagnetic states are quite stable. Our calculations for the formation energy and first-principles MD simulations suggested that RS-CaN is at least metastable and stable in normal condition. Based on our results we have proposed the synthesis process of RS-CaN. Our proposed process is (1) heat up α-Ca3N2 until it is transformed into β-Ca3N2, (2) compress it above 50 GPa until 2Ca3N2 →2CaN+Ca reaction occurs, (3) cool down them to room temperature and (4) decompress them into an ambient pressure. We consider that this high pressure and high temperature synthesis is one of the hopeful method to synthesize new materials.

We present results of studies of the xenon-chlorine system to 60 GPa and temperature range 300 to 2000 K. Using a combination of Raman spectroscopy, infrared spectroscopy and synchrotron x-ray diffraction; we characterize the products of diamond cell experiments. In the 2-20 GPa pressure interval, we observe the formation of a deep red crystalline phase. Above 15 GPa, and at high temperature, a solid solution of Xe and Cl2 appears to form. Comparison of the ultraviolet-visible and infrared absorption measurements on the proposed alloy of Xe and Cl2 show that band edge moves into the visible range at high pressure

A natural chromite single crystal was compressed in a helium pressure medium to just above 35 GPa at ambient temperature and studied with energy-dispersive x-ray diffraction. The volume compression of the cubic, spinel-type structure was fit to a third-order Birch-Murnaghan equation of state with parameters a0= 8.338(4) Å, V0 = 579.6(9) Å3, K0 =179(10) GPa, and K' = 3.9(9) up to 29 GPa. A distortive phase transition was discovered at higher pressures to a CaAl2O4-type orthorhombic structure, with two cubic unit-cell axes increasing and the third decreasing with increasing pressure. The transition can be fit to a Landau-type strain-order formulism with approximately 5% volume decrease from the cubic phase at 35 GPa. The transition may be triggered by electronic or magnetic transitions in the 3d elements Fe and Cr cations.

We report that C60 nanorods of three polymeric phases have been synthesized under high pressure and high temperature and that the photoluminescence properties of these polymeric nanorods also have been studied. C60 nanorods were fabricated by a facile solution evaporation method. Raman spectra indicate that this material transformed into orthorhombic, tetragonal and rhombohedral polymeric phases under various high pressure and high temperature conditions. SEM measurements show that the shape of the nanorods can be kept under quasi-hydrostatic pressure condition. The photoluminescence intensity of the polymeric C60 nanorods has been enhanced compared with that of the pristine C60 nanorods. The main fluorescence band shifted from 1.70 eV in the monomeric phase to near infrared in the polymeric phase when pressure and temperature were increased. The enhanced fluorescence with adjustable frequency for different polymeric C60 nanorods makes possible potential application in luminescent nanomaterials.

The strong electron correlations play a crucial role in the formation of a variety of electronic and magnetic properties of the transition metal oxides. In strongly correlated electronic materials many theoretical predictions exist on pressure-induced insulator-metal transitions, which are followed by a collapse of localized magnetic moments and by structural phase transitions [1]. The high-pressure studies provide additional degree of freedom to control the structural, electronic, optical, and magnetic properties of transition metal oxides. With the development of the high-pressure diamond-anvil-cell technique the experimental studies of such transitions are now possible with the advanced synchrotron techniques. In our studies, the iron monooxide Fe0.94O was studied under high pressures up to 200 GPa in diamond anvil cells. The single crystals enriched with Fe57 isotopes have been prepared for nuclear resonance measurements. The results of synchrotron Mössbauer spectroscopy (nuclear forward scattering -NFS), and electro-resistivity measurements suggest a complicated scenario of magnetic interactions governed by band-broadening effects.

An overview of the pressure-induced transformations and order-disorder phenomena in LiAlH4 (up to 7 GPa) and LiNH2 (up to 25 GPa) are presented. The analysis of pressure-induced changes in Raman spectra suggest a phase transition at ∼3 GPa for LiAlH4 and ∼14 GPa for LiNH2. New results on the metastable recovery of the high pressure β-LiAlH4 phase are also presented. An examination of the lattice translational and librational modes reveals that the high pressure β-LiAlH4 phase is disordered while there is evidence of orientational ordering in the high pressure beta-LiNH2 phase.

We have developed a novel method of reactive sintering under high pressure and high temperature conditions and have successfully synthesized bulk and nanostructured BC2N, BC4N, and diamond-SiC composites. The keys to this success are the careful preparation of precursor materials through high energy ball-milling and sensible tuning of the sintering conditions (pressure, temperature, and sintering time), leading to the formation of nano-scale features in the final synthesis products. The indentation measurements indicate that BC2N and BC4N have the Vickers hardness of 62 and 68 GPa, respectively, making them the second hardest materials known, second only to diamond. The high P-T reactive sintering results in a nanostructured diamond-SiC composite of high uniformity, low porosity, minimum residual silicon, and minimal quantities of graphite. It tunes subtle interplays of P-T-t to have fine control of thermodynamics and kinetics in the formation of diamond-SiC nanocomposites with nanocrystalline SiC matrix. The surface defects of micron/nano-diamonds are consumed by amorphous/molten silicon in the formation of nanocrystalline SiC matrix. It in turn greatly reduces the forming/mobility of microcracks thus leads to significant enhancement of fracture toughness (by as much as 50%) of the nano-composites without much compromising of hardness. The present study shows how to design/proceed a well controlled nano-synthesis so as to achieve great improvement in nano-mechanics for advance technological applications.

The galvanomagnetic effects in the n-Pb1-xGexTe:Cr (x=0.02-0.13) alloys at the temperatures 4.2≤T≤300 K and under hydrostatic compression up to 17 kbar have been investigated. The metal-insulator transition under the increase of germanium content in the alloys and insulator-metal transition, induced by pressure in Pb1-xGexTe:Cr (x=0.10) alloy, were revealed. Using the experimental data in the frame of two-band Kane dispersion relation the dependences of the free electron concentration and the Fermi level position on the matrix composition and on the pressure were calculated. The composition and pressure coefficients of chromium deep level movement were obtained and the models of the electronic structure reconstruction under varying the alloy composition and under pressure were proposed.

Recently, the use of graded density impactors for dynamic compression experiments has received increasing interest. These gas gun experiments have demonstrated complex loading paths which can last microseconds, and may be capable of bridging the timescales of existing static and dynamic compression experiments. A tape casting technique has been developed for fabrication of the impactors. In the technique, a series of tapes were prepared in the Mg-Cu system with compositions ranging from 100% Mg to 100% Cu. The tapes were characterized for their layer thickness, density, and sound wave velocity. Impactors were fabricated by punching individual layers from the tapes, stacking and laminating them, removing the organic matrix, and hot-pressing the laminated structure. The density profile is determined by the order in which the tapes are stacked in the laminate and is therefore highly flexible. The resultant thickness and average density of the impactors is consistent with the data for the individual layers. Impactors were characterized for uniformity by ultrasonic C-scan and white light interferometry. Dynamic compression experiments were performed on a two-stage helium gas gun using the graded density impactors. Results will be presented and shown to agree well with hydrocode modeling.

High-purity nano-polycrystalline diamonds have been synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures. The polycrystalline diamond synthesized from graphite at ≥15 GPa and 2300-2600 °C has a mixed texture comprising a homogeneous fine structure (particle size: 10-30 nm, formed in a diffusion process) and a lamellar structure (formed in a martensitic process), and has a very high Knoop hardness of 120-145 GPa. In contrast, the polycrystalline diamonds made from the non-graphitic carbons at ≥15 GPa and 1600-2000 °C have a single texture consisting of a very fine homogeneous structure (5-10 nm, formed in a diffusion process) without a lamellar structure. The hardness values of the nano-polycrystalline diamonds made from non-graphitic carbons (70-90 GPa) are significantly lower than that of polycrystalline diamond made from graphite. The investigation of the microstructure beneath the indentation of these nano-polycrystalline diamonds revealed that the existence of the lamellar structure and the bonding strength of the grain boundary have a decisive effect on the hardness.

Transition metals of the platinum group (Os, Ir, Pt, Ru, Re, Rh) do not form carbides and nitrides at ambient pressure. Osmium carbide seems to have been synthesized at zero pressure by Kempter and Nadler forty six years ago. According to the authors, OsC crystallizes in WC-type structure and has a hardness equal to 2000 kg mm-2. Up to date, no other experimental confirmation is available. We studied the electronic and mechanical properties of this hypothetical carbide using an approach based on the density-functional theory. We found that the work of the above mentioned authors is sound. The calculated lattice parameters are in good agreement with that given by those authors and a rough estimate also showed that the hardness given by them is reasonable. However, we found that the hexagonal structure of osmium carbide is electronically and mechanically unstable.

Pressure-induced structural aspects of NaCl-type (B1) to CsCl-type (B2) structure in MgX [X = S, Se, Te] semiconductors are presented. An effective interionic interaction potential (EIOP) with long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach is developed. Particular attention is devoted to evaluate the vdW coefficients following the variational method, as both the cation and the anion are polarizable. Our result on vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from B1 to B2 structure. The estimated value of the phase transition pressure (Pt) is consistent with results previously published. The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

In the systems with strong electron correlations, many theories predict the high-pressure-induced dielectric-metal transition, which is followed by collapse of localized magnetic moment and structural phase transition. In this report, summary results of many last experiments on the influence of high pressure on the magnetic and crystal structure as well on the electronic and transport properties of 3d metal oxides is presented. Along with X-ray diffraction, optical absorption, Raman scattering and electroresistivity measurements, several synchrotron radiation techniques have also been applied to perform the high-pressure experiments with compound iron oxides having different crystal structures.

We describe the synthesis of nitrides of iridium and palladium using the laser-heated diamond anvil cell. We have used the in-situ techniques of x-ray powder diffraction and Raman scattering to characterize these compounds and have compared our experimental findings where possible to the results of first-principles theoretical calculations. We suggest that palladium nitride is isostructural with pyrite while iridium nitride has a monoclinic symmetry and may be isostructural with baddeleyite.

The x-ray structure factor of water has been measured along the melting line to 57 GPa and 1500 K using focused monochromatic synchrotron radiation and laser heated diamond anvil cell. The oxygen radial distribution function, g(r) is determined from these data. We have also calculated g(r) using ab initio methods and find a good agreement with the experiment. Based of the similarity of the measured and calculated structure factors determined density of water under extreme conditions unattainable previously.

We prepared a carbon nitride compound by treating the molecular precursor C2N4H4 in a laser-heated diamond-anvil-cell (LH-DAC) at pressures exceeding 27 GPa and temperatures above 1700°C. After quenching we recovered single crystals of the new material with diameters of about 1 μm. Using several experimental characterization techniques in conjunction with computational methods we determined composition, hybridization of constituting atoms, and the crystal structure. The new carbon nitride imide, C2N2(NH), adopts a defect-wurtzite structure and is isotypic to Si2N2(NH).