By performing constant pressure ab initio molecular dynamics simulations we analyse the high pressure phases of molecular solid hydrogen. We use a gradient corrected LDA, and a freshly implemented efficient technique for Brillouin zone sampling. An extremely good k-point sampling turns out to be crucial for obtaining the correct ground state. Our constant pressure approach allows us to optimize simultaneously the ori-entational degrees of freedom, the lattice constants, and the space group. This can be done either by a local optimization tehcnique, or by running molecular dynamics (MD) trajectories. The MD allows for the system to undergo structural transformations spontaneously. In the lower pressure, namely for the broken symmetry phase (BSP or phase II), we find a quadrupolar orthorhombic structure, of Pca21 symmetry. By means of an MD investigation, we find, at higher pressures, a slightly distorted orthorhombic structure of Cmc21 symmetry. This structure cannot be straightforwardly identified with the H-A phase (or phase III) because: 1) it is metallic, and 2) the Raman vibron discontinuity would be far too large compared to experiment. In fact, we argue that this phase is the first metallic molecular phase of hydrogen. Metallization would happen then, not via a band-overlap mechanism, but due to a structural transformation. By comparing total enthalpies, we also give suggestions for the structure of phase III.

The experimental search for metallic hydrogen has been active for over two decades with the first claims being published in 1975, followed by several others all of which failed to pass the test of time. The present paper describes experiments in which several important changes were made in the experiment, compared to those performed previously, which enabled us to reach a pressure of 342 GPa. The path was strewn with 15 pairs of carefully chosen diamonds. At 342 GPa visible light was readily transmitted through the sample proving that it was not yet an alkali metal. The reasons why it has been so difficult to achieve the required pressure for metallization will be discussed.

GaN is recently considered as the most important material for blue and ultraviolet optoelectronics. The device structures are usually grown on foreign substrates which results in high density of dislocations above 108cm−2. The application of high N2 pressure gives a unique possibility of growing of GaN single crystalline substrates which allows to lower dislocation density in epitaxial layers by 3–4 orders of magnitude.

In this paper, the results of high nitrogen pressure study of properties of Al-N, Ga-N and In-N systems are presented. The results include the phase diagrams in large range of pressures and temperatures (up to 2 GPa and 2000K) and also growth of GaN single crystals from atomic nitrogen solution in liquid gallium. The kinetic limitations of dissolution of N2 in liquid Al, Ga and In will be discussed. It follows, that the best conditions for crystal growth at available pressures and temperatures can be achieved for GaN.

The high nitrogen pressure experimental system equipped with multi-zone internal furnace was used for growth of high quality GaN crystals. At present both n-type and semi-insulating substrate quality GaN crystals with surface area up to 1cm2, with dislocation density below 105 cm-2 are routinely obtained and successfully used for homoepitaxy.

Some results concerning homoepitaxial growth by the MOCVD and MBE methods are shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

We present results of photoluminescence (PL) studies of GaN, InxGa1-xN and AlxGa1-xN alloys, as well as related thin film heterostructures under hydrostatic pressure using the diamond-anvil-cell technique. The GaN PL spectra are dominated by strong and sharp near-band-edge luminescence associated with annihilations of bound excitons and intrinsic free excitons in the crystals. The spectrally well-resolved emission lines allow us to accurately determine their pressure. The PL spectra of InxGa1-xN and AlxGa1-xN epitaxial films were found to exhibit strong near-band-edge luminescence emissions. By examining the pressure dependence of these spectral features, the pressure coefficients for the PL emissions associated with the direct Γ band gap of InxGa1-xN and AlxGa1-xN were determined for the first time.

Among the various external disturbances used in the study of semiconductors, including electric and magnetic fields as well as uniaxial pressure, large hydrostatic pressures can be employed to induce dramatic changes in host lattice, dopant and defect properties. Diamond anvil cells with an appropriate pressure medium (e.g. liquid N2 or alcohol mixtures) allow the application of pressures up to hundreds of kbar. In this pressure range the global conduction band minimum of a semiconductor can become a local minimum. GaAs for example changes near 45 kbar from a direct (Γ-band) to an indirect (X-band) semiconductor. Donors in GaAs and InP transform from their shallow, hydrogenic state to the DX configuration at hydrostatic pressures near 23 and 82 kbar, respectively. This donor configuration change has been studied using local vibrational mode (LVM) spectroscopy in the far infrared region of the electromagnetic spectrum. Recently we have investigated several LVM's of H-containing complexes in GaAs as a function of hydrostatic pressure at liquid He temperatures. Depending on the specific complex we find the LVM frequencies to vary either linearly, sub- or superlinearly with hydrostatic pressure. In the case of O in Si the vibrational mode changes its character from that of a harmonic oscillator to a rotor as pressure is applied. The implications of the pressure dependences of LVM's are discussed.

In this paper we review the use of high pressure techniques to investigate the optical properties of partially ordered GalnP epitaxial layers grown on GaAs substrates. In particular, we demonstrate the ability of high pressure to modify the band alignment at the GalnP/GaAs interface and hence to alter the optical properties of heterostructures fabricated from partially ordered GalnP on GaAs.

We develop and apply high pressure techniques to study the modification of electronic structure, magnetic properties, and local crystal structure, in magnetic materials under high pressure. The variation in inter-atomic distances and atomic volumes in the high pressure regime provides new information for elucidating electronic processes in solids, and for understanding the connection between electronic structure and magnetic properties of matter.

For our samples we used rare-earth orthoferrites, which are of the perovskite-type structure, and Heusler alloys, with the cubic structure. Those materials are known to be magnetic insulators and magnetic metals, respectively.

We studied the following properties induced by high pressure: electronic structure, spin crossover effects, magnetic to non-magnetic transitions, and insulator-to-metal transitions. The problems of delocalization, distribution of spin, and charge density distribution in dielectric materials, semiconductors, and metals, as well as chemical bonding can be investigated in this way.

We have recently devised a new method for the ab initio simulation of solid-solid phase transformations under pressure. The method is here applied to the study of the solid-state polymerization of acetylene under pressure and the generation of hydrogenated amorphous carbon from compression of polyacetylene.

An integrated technique of diamond-anvil cell, laser-heating and synchrotron x-ray diffraction technologies is capable of structural investigation of condensed matter in an extended region of high pressures and temperatures above 100 GPa and 3000 K. The feasibility of this technique to obtain reliable data, however, strongly depends on several experimental issues, including optical and x-ray setups, thermal gradients, pressure homogeneity, preferred orientation, and chemical reaction. In this paper, we discuss about these experimental issues together with future perspectives of this technique for obtaining accurate data.

The structural transformations in the II-VI compound MgSe have been studied under pressure using energy dispersive x-ray diffraction. MgSe transforms ‘continuously’ from the rocksalt (Bl) structure to a FeSi (B28) or Au-Be structure beginning at 99 ± 8 GPa. At 202 GPa, MgSe is approaching 7-fold coordination with u = 0.0828 and w = 0.4173. The method for intensity calculation of the diffraction peaks is presented. Using the Birch equation, the equation of state to 146 GPa was determined with Bo = 62.8 + 1.6 GPa and Bo′ = 4.1.

A high-pressure phase of gadolinium is completely described for the first time. This phase, HR24, is found to exist at 46 GPa. The structure is examined for evidence of delocalization of the f shell, and the evolution of the HR24 phase with increasing pressure is discussed.

When ice Ih in an emulsion is compressed below 250 K, it melts to supercooled liquid water, avoiding the formation of other crystal phases. Here, we create emulsified high-pressure ices under high pressure and low temperature, and measure their temperature while these ices are decompressed at a constant rate at different temperatures. We detect metastable melting points of high-pressure ices, and identify their melting lines. We find what could be possibly two new ice phases, and discuss the relationship between decompression-induced melting and decompression-induced amorphization. Finally, we discuss briefly the analysis of experimental data and simulation results that are consistent with the hypothesized “second critical point” with temperature and pressure coordinates of approximately 200 K and 100 Mpa.

The stoichiometry of boron suboxide (B6O1-x) synthesized at high pressure lies closer to the nominal composition (x = 0) than materials obtained at atmospheric pressure. The crystallinity of materials obtained in the presence of molten B2O3 is also higher than for sintered powders. Further, for syntheses at 1700–1800 °C between 4 and approximately 5–6 GPa, the well-crystallized particles are dominated by large (up to ∼40 μm in diameter) icosahedral multiply-twinned particles. This unusual morphology is obtained by Mackay packing; i.e., by assembly of successive shells of icosahedral B12 units around a central icosahedral nucleus. The result is a multiply-twinned particle in which each of the 20 elements has the R 3 m space group of the α-B structure.

Results on lanthanum-modified lead zirconate-titanates (PLZTs) have revealed a pressure-induced crossover from a normal ferroelectric to a relaxor state and the continuous evolution of the dynamics and energetics of the relaxation process. This crossover appears to be a general feature of soft mode ferroelectrics with random site dipolar impurities or polar nanodomains and results from a large decrease in the correlation radius among polar nanodomains - a unique property of soft mode ferroelectrics.

We report some unusual phase behaviour, of general implication for condensed matter, on the polymer poly-4-methyl pentene-1 (P4MP1) induced by changes in pressure (P) and temperature (T), as observed by in-situ X-ray diffraction and high pressure DSC. Upon increasing pressure beyond a threshold value, the polymer, crystalline at ambient conditions, looses its crystalline order isothermally. The process is reversible. This behaviour is observed in two widely separated temperature regions, one below the glass transition temperature (< 50°C) and one close to the melting temperature (250°C), thus showing solid state amorphization and inversion in the melting temperature with increasing pressure. This further suggests inverse melting, i.e. re-entrant of the two widely separated liquid and amorphous phases along the T-axis at fixed P. This is confirmed experimentally as disordering in the crystalline structure on cooling. The inverse melting in P4MP1 raises the possibility of exothermic melting and endothermic crystallization as anticipated by Tammann (1903). The anticipated exothermic melting and endothermic crystallization is confirmed experimentally in the one component system P4MP1. We are observing similar features in a range of polymers.