Gem quality diamond crystals are employed as anvils in high-pressure diamond cell research. Homoepitaxial growth experiments by microwave plasma-assisted chemical vapor deposition (MPCVD) have produced 1.76 mm (diameter) by 0.65 mm (thickness) sized diamonds. We report fundamental studies on diamond growth rate and quality as a function of reactor pressure and methane concentration, in a hydrogen plasma. By varying the growth conditions, large, single crystal diamond can be produced, which is ideal for manufacturing high pressure anvils.

Traditional high pressure, high temperature (HPHT) techniques for production of synthetic diamond anvils are extremely expensive and chemical vapor deposition (CVD) provides an economically viable alternative. We report diamond growth rates up to 0.32 mg/hr, which are comparable to HPHT growth rates, and crystal quality approaching that of gem diamond. When perfected, diamond anvils produced from chemical vapor deposition methods could replace those manufactured by high pressure, high temperature synthesis.

The physical behavior of silicate liquids at high pressure is important in modeling the dynamic processes of the deep earth. The density of the silicate liquids determines the transport properties of silicate melts and volumetric properties of silicate melts are therefore of prime importance. However, the partial molar volumes of melt components are difficult to evaluate since it is difficult to observe the molten state at high pressure. A variety of approaches are used to investigate silicate melt properties. Individual melt components can be used and complex mixing terms used to describe mixing behavior. Alternatively the melt species themselves can be investigated, although there is little information as to what these actually are. In both cases the accurate determination of the dP/dT slope of the melting curves is critical.

Speciation reactions can be studied in situ at one atmosphere through high temperature calorimetry. An approach has been developed where the dissolution of melt components which are readily soluble in silicate solvents and which have a large heat effect can be used to determine the dependence of the activities of these components on melt structure. In addition the high temperature calorimetrie data can be used in combination with solution calorimetrie data at lower temperatures to evaluate the temperature-dependent stability of these melt species. These studies are independent of and complementary to spectroscopie determinations and provide a description of the temperature-dependence of melt structure at one atmosphere. Recent studies have focused on the solution of La2O3 and TiO2 in model silicate melts to assess the role of melt structure in the complexing of these components. The next obvious step is to extrapolate to high pressure. One possible approach is to combine one atmosphere data with existing melting relationships to determine the optimized melt compositions for a model natural systems. The next step is to then assess the stability of melt species from the partial molar properties and the partitioning of the elements between coexisting melts and mineral phases at pressures and temperatures of interest.

We report on a high pressure study of emission from a series of GaInP(ordered)/GaAs heterostructures. A so-called “deep emission” band at 1.46 eV is observed in all our samples. At high excitation power, quantum well emission emerges in only one structure where thin GaP layers are inserted on both sides of the GaAs well. From the pressure dependent emission in this sample we have determined its band alignments. The role of the GaP layers in suppressing the deep emission is elucidated.

We have studied the pressure dependence of the type-I and type-II transitions in (GaAs)m/(AlAs)m superlattices by photoluminescence (PL) spectroscopy. From the study of PL linewidths of the type-I exciton as a function of pressure and temperature, we determine the intervalley deformation potential. Beyond the type-I and type-II crossover, the PL linewidth increases both as a function of pressure and temperature. We find that the electron-phonon deformation potential for Γ-X intervalley scattering varies with temperature.

In this paper the galvanomagnetic properties (4.2≤T≤300 K) of Pb1-xGexTe(x≤0.038) doped with Yb were investigated both under atmospheric pressure and under hydrostatic compression up to 12 kbar. The variation of the energy diagram of Pb1-xGexTe<Yb> under variation of Ge concentration and under pressure were proposed. It was shown that change of the energy spectrum under pressure leads to the insulator-metal transition due to redistribution of electrons between the valence band and the deep Yb level. By comparing theoretical and experimental dependences of hole concentration on pressure the parameters of the Yb-induced deep level were determined.

The temperature dependence of the resistance and thermoelectric power under hydrostatic pressure between 10 and 300 K are reported for two compositions, x = 0.044 and x = 0.05, of the narrowband system La1-xSrxTiO3. The application of hydrostatic pressure at room temperature induces a transition from a positive p-type to a negative n-type thermoelectric power for x = 0.044. This behavior is interpreted within a model of itinerant-electron antiferromagnetism in LaTiO3.

Full elastic constant tensors (cij) of three minerals namely, MgSiO3 perovskite, MgO and SiO2, are obtained as a function of pressure up to 140 GPa using first-principles computer simulations based on the local density and pseudopotential approximations. The zero pressure values and initial pressure dependence of athermal elastic constants derived from stress-strain relations are in excellent agreement with available experimental data. We find that elastic moduli, wave velocities and anisotropy of the minerals are strongly pressure dependent, particularly, in the vicinity of the structural transformations. In the view of the present experimental limitations at realistic conditions of the inner Earth, our results for high pressure elasticity are expected to be of substantial geophysical significance. Comparisons based on compressional and shear wave velocities support the prevailing hypothesis of Mg-rich silicate perovskite dominated composition for the lower mantle.

High-pressure phase transformations occurring in materials are important for a wide range of problems in materials science and engineering. Most of the results in this field have been obtained using high-pressure cells.

In this work, we demonstrate that high-pressure phase transformations also take place during mechanical contact of hard solid materials. These solid-state transformations can be studied by simply combining hardness indentation tests with micro-Raman spectroscopy. In contrast to diamond anvil cell (DAC) experiments, the stress conditions during mechanical contacts are highly non-hydrostatic, i.e. very high shear stresses affect the transformation behavior. Nevertheless, the obtained results can be discussed in the light of the phase transformations as known from DAC experiments. The examples of our research presented here include Si, Ge, SiO2 and ZrO2, and show that phase transformations and amorphization at mechanical contacts are a quite common phenomenon, which has important implications.

This paper is devoted to some recent applications of density-functional perturbation theory to the prediction of lattice instabilities occurring in ionic solids at high pressure. We first briefly review some work aimed at understanding the interplay between shear instabilities and phonon softening in the pressure-induced phase transformations of Cesium halides and hydride. We then report on preliminary results of an attempt of ours to apply similar techniques to the long-standing problem of the pressure-induced amorphization of quartz.

We predict a new pressure induced phase for silica via first principles variable-cell-shape molecular dynamics. The structure results from annealing a-quartz at pressures near a phonon instability. The mechanism which produces this new phase is similar to that proposed for structural transitions in silicate melts. The diffraction pattern of the new phase compares favorably with that of the unidentified intermediate crystal phase found in silica near the pressure induced amorphization transformation.

A new method [1] of calculating the free energy difference between two crystalline structures is presented. The method involves a single simulation which repeatedly transforms the system between the two crystal phases. Since the configurations of both structures are sampled within a single Monte Carlo process, the difference between their free energies can be evaluated directly from the ratio of the measured probabilities of each. Compared with traditional techniques, the method is most advantageous when applied to highly anharmonic systems. To illustrate the method, an application to the free energy difference between the fee and hep structures of hard spheres is described.

Diamond anvil cells revolutionized high pressure research because they provided easy to generate extremely high pressures with an optical window on the sample and an easy-to-use pressure gauge based on the shift with pressure of the ruby fluorescence spectrum. The ruby scale provides an accurate method of determining pressure distributions within samples. However, at pressures above 100 GPa measurement of the ruby fluorescence begins to become difficult due to weakening signals, interference from intense diamond fluorescence, and ultimately, strong absorption of the exciting laser by diamond itself. We shall discuss a number of methods which have been devised to extend the ruby pressure optical gauge to 500 GPa, including quasi-direct pumping of the ruby R-lines.

Here we summarize our investigation of the effect of a recently observed phase transformation on ruby's optical transitions. This study involved a first principles calculation of the electronic and structural properties of a chromium impurity in alumina host lattices and a subsequent calculation of the multiple structure using eigenvalues and eigenvectors derived from the first principles calculation. This investigation is relevant to clarify the behavior of the fluorescent optical transitions which are used as pressure sensor in diamond anvil experiments across the structural transformation.

The Advanced Photon Source (APS) is a third-generation synchrotron now in operation at Ar-gonne National Laboratory. A national user facility for research in the Earth sciences is being constructed by the GeoSoilEnviro group of the Consortium for Advanced Radiation Sources (GSECARS). One component of this effort is the program for diamond anvil cell (DAC) research, to study materials at ultrahigh pressure and temperature conditions using a variety of techniques.

In this paper, we describe the DAC facility that is being developed at the GSECARS. Recent progress in developing a laser heating system is presented.

We describe the multi-anvil, large-volume, high-pressure facility that is being constructed at the GeoSoilEnviroCARS (Sector 13) at the Advanced Photon Source, Argonne National Laboratory. Various multi-anvil, high-pressure apparatus will be used to cover pressure and temperature conditions up to 40 GPa and 3000 °C, respectively, with milimeter to centimeter sized samples. This national facility is open to all users, providing excellent opportunities for high pressure, high temperature experiments.

Diffusivity D and nuclear spin relaxation times T1 and T2 have been measured by NMR to 4.0 GPa in methanol, using a diamond anvil cell probe. In pure MeOH, D−1 and T2 show essentially identical activation volumes. However, these are ∼18% larger than the activation volume of viscosity. By relating these observations to an average molecular correlation time a pressure-dependent infinite-frequency shear modulus G∞ can be inferred, using two independent approaches. The relation between diffusivity and viscosity shows increasing departure from Stokes-Einstein behavior with increasing pressure, if a constant hydrodynamic radius is assumed. This departure is attributed to the pressure dependence of G∞, and can be described empirically by a simple modification of the Stokes-Einstein relation.

We report the synthesis of a crystalline carbon-nitrogen compound having a zero-pressure bulk modulus of ∼ 249 GPa. Its crystal-structural and compressional properties are in good accord with those predicted theoretically by Teter and Hemley for a cubic C3N4 phase. Experimentally, this phase is synthesized at pressures and temperatures above 18 GPa and 2000 K. It decomposes upon pressure release below 14 GPa, and may offer a route to synthesizing other incompressible carbon nitride phases.

The composition and properties of the sp2-bonded carbon nitride precursor paracyanogen (pCN) has been studied at high pressures and temperatures. Paracyanogen decomposes to carbon and molecular nitrogen with the decomposition temperature increasing with pressure over the range of 3 to 19 GPa. Prior to decomposition, pCN can be transformed to an atmospheric-pressure quenchable phase that is more than 25% higher in density and over an order of magnitude harder than the starting material. Structural analysis of this quenchable phase shows, however, that it consists of a sp2-bonded network. In addition, the decomposition kinetics of paracyanogen have been studied in detail. Rapid, self-propagating decomposition occurs above a threshold temperature. Below this, decomposition rates exhibit an Arrhenius behavior with activation energy and volume of 2.7 eV and 3.9 Å3, respectively The decomposition rates depend on the nitrogen density and decrease significantly with lower nitrogen concentration. Kinetic effects favoring a graphite-like, sp2-bonded structure may preclude the high-pressure synthesis of superhard, sp3-bonded carbon nitride solids below their thermodynamic stability limit, unless an optimally designed precursor is employed.

We prepared α-rhombohedral (α-rh.) B-rich materials in the B-C-N-O system at high-pressures and temperatures. Samples were synthesized using a multianvil device and characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and parallel electron energy-loss spectroscopy (PEELS). The B-C-O compounds were obtained by reducing B2O3 with B, or mixtures of B and C between 1 to 10 GPa and 1200° and 1800° C In the B-O system we synthesized boron suboxide (nominally B6O) of high purity, crystallinity, and close to stoichiometric. Quantitative analyses give B6O0.95 and B6O0.77 for high-pressure and room-pressure samples, respectively. Between 4 to 5.5 GPa, B6O formed as macroscopic near-perfect regular icosahedra (to 30 μm in diameter). In the B-C-O system, intermediate phases were prepared showing evidence of solid solution between B4C and B6O. Boron carbide crystals, to 20 μm, containing a significant amount of O, typically B6C1.1O0.33 and B6C1.28O0.31, were grown for mixtures in which B and C were reacted with excess B2O3 at 7.5 GPa and 1700 °C. We also report the first conclusive bulk synthesis of a new boron nitride, B6N1-x. This subnitride was synthesized from B and hexagonal BN at 7.5 GPa and 1700 °C. XRD and PEELS shows that the boron subnitride has the α-rh. B structure and average composition B6N0.92

Cubic, hexagonal BN, and graphite were heated with the aid of cw CO2 or pulsed YAG laser in a diamond anvil cell at pressures up to 35 GPa. The recovered samples were analyzed by high resolution and scanning electron microscopy, micro-Raman, and X-ray diffraction using synchrotron radiation. Melting of hBN and cBN was achieved. Microcrystals of hBN or cBN were grown from the fluid of BN in nitrogen or argon. Analysis of the hBN-cBN transitions in dependence on pressure showed that the hBN-cBN-liquid triple point is located at 9 GPa. In the transient region between the two phases in the same sample amorphous BN was found revealing the microscopic mechanism of diffusion-like first-order transitions. BN nanotubes growing from an amorphous state at the surface of the crystals were discovered. Under nanosecond pulse laser heating graphite transforms to a diamond-like structure. Spherical droplets and nanocrystals were condensed from the ablated carbon. A compound of nitrogen with carbon was found.