Masao Wakatsuki

Proper understanding of down-shifted Raman spectra of natural graphite: Direct estimation of laser-induced rise in sample temperature

Laser-induced heating during micro-Raman spectroscopy, in some cases, gives rise to considerable down-shifts in Raman spectra of powder graphite samples. Employing the most typical experimental condition for graphitic matter of geochemical and/or cosmochemical interest, 10 mW laser power at the sample surface and ~2 μm laser beam, powdered artificial graphite samples revealed the E2g mode in low Raman peak position ranging from 1567 to 1576 cm−1. The energy range agrees very well with the previously reported frequency range for anomalously down-shifted Raman spectra of natural graphites. In order to extract spectroscopic information inherent in samples from the E2g frequency, it is essential to remove the laser-induced down-shift and to know the temperature of sample surface. We showed that the intensity ratio of the Stokes line to the anti-Stokes line for the E2g Raman band of graphite is potential to monitor the temperature of graphite surface. Furthermore, our measurement employing enough low laser power without laser-heating showed that natural graphite sample containing the rhombohedral polymorph exhibited the same G-band frequency as the E2g (1582 cm−1) frequency of hexagonal graphite. Thus, stacking sequences of graphite did not affect the intralayer vibrational energy.

Analysis of trace Co in synthetic diamonds using synchrotron radiation excited X-ray fluorescence analysis

Abstract Synchrotron radiation excited X-ray fluorescence analysis was utilized for characterization of trace impurities in synthetic diamonds. Advantage of the energy tunability was fully utilized to evaluate the attenuation of X-rays through the sample, and the absorption corrected X-ray fluorescence yield was utilized for quantitative analysis. Diamonds grown with several types of metallic solvents were investigated, and quantitative analysis of trace Co was carried out. It was found that Co prefers to be dissolved into {1 1 1} growth sector and that Co concentration in the {1 1 1} growth sector decreases one order of magnitude with the existence of Ni in the solvent. XANES spectra of dissolved Co shows characteristic pre-edge feature similar to that reported with the dissolved Ni. Experimental results suggest that both Ni and Co occupy in the similar site in the diamond lattice and that Ni is easier to be dissolved into diamonds.

NOTES ON COMPRESSIBLE GASKET AND BRIDGMAN-ANVIL TYPE HIGH PRESSURE APPARATUS.

The squeezing process of a disc-shaped gasket, which is compressed between Bridgman anvils, is analyzed by a simple model. It is made clear that plastic flow and pressure distribution in the disc is governed by its thickness. A normalized characteristic thickness, which can be determined for each disc material by thickness measurement on squeezed discs, is defined and correlated experimentally with shear strength of the material under pressure. According to the correlation, a simple method is developed to evaluate substances for solid pressure materials. A Bridgman anvil apparatus and sample assemblies for it are so designed as to provide the best performance in a range of from 100 kbar to 200 kbar. Noncatalytic syntheses of diamond, stishovite and cubic boron nitride are described as examples of utilizing the apparatus. Some features of anvil fracture are also described.

NEW HIGH-PRESSURE PHASES OF CARBON

Reversible phase transformations with volume contractions were found at 18 GPa and 22 GPa when graphite was compressed at room temperature. Volume contraction was detected in a diamond anvil cell by observing a plateau in the curve of generated pressure versus the cell load. The phase realized at 18 GPa is thought to work as an intermediate phase in the transformation from graphite to hexagonal diamond. A structure of this phase, though not yet identified, is also discussed.

Generation of an X-ray microbeam for spectromicroscopy at SPring-8 BL39XU

A pair of elliptical mirrors (KB mirror) was designed and fabricated to realize an energy tunable x-ray microbeam for spectromicroscopy at SPring-8 BL39XU. As is commonly recognized, the obtainable beam size with the aspherical total reflection mirrors is strongly affected with the slope error of the mirror. Considering that the extremely high brilliance of the undulator radiation from the SPring-8, the small mirror size and the small mirror-to-focus distance were employed to minimize effects of the slope error. Preliminary evaluation of the KB mirror was carried out using 10 keV monochromatized undulator radiation. Alignment of the mirror was assisted by the beam monitor system composed of a scintillator and a CCD, and the beam size less than 5 microm can be easily achieved even when the source was fully used. The beam size obtained with this experiment was 2 x 4 microm2 with the photon flux of 1 x 10(10) photons/s. Smaller beam size may be expected with the use of intermediate slits. Characterization of trace elements with the spatial resolution will be carried out by using x-ray fluorescence (XRF) analysis and x-ray absorption fine structure (XAFS) measurements with XRF yield method.

An optical high pressure cell with spherical sapphire anvils

An optical high pressure cell has been constructed of spherical sapphire anvils (9.53 mm in diameter). The anvil has a small flat compression face (1.5 mm or 2.5 mm in diameter), which is perpendicular to the crystal c axis. Pressure is generated using a gasket, typically made of phosphorus bronze. A maximum pressure of 12.6 GPa was generated at room temperature in a liquid pressure medium. Pressures up to 10 GPa have been generated stably without appreciable damage of the anvils. The capability of optical observations for a photon energy region up to 6.4 eV was confirmed with this anvil, at high pressure. An advantage of the spherical anvil is ease in production.

High temperature X-ray study of single crystal stishovite synthesized with Li2WO4 as flux

Single crystal stishovite with a square prismatic habit and maximum length 0.8 mm was grown from α-quartz at 120 kbar and ∼1,300° C. Li2WO4, chosen as a result of a previous experiment in growing coesite, was also successful as flux for stishovite. Single crystal X-ray structure analysis of the crystals thus obtained has been carried out at high temperatures under ambient pressure. Lattice constant measurements give a larger thermal expansion coefficient along the a-axis than along the c-axis. The bond distances and bond angles show a decreasing distortion of the SiO6 octahedron with increasing temperature. The increasing amplitude of thermal vibrations of oxygen atoms with increasing temperature results in increasing O-O repulsion in the basal plane, which explains the observed crystallographic changes.

Characteristics of Link-Type Cubic Anvil, High Pressure-High Temperature Apparatus

Operational characteristics are investigated for evaluation and improvement of a high pressure apparatus which has a new anvil-driving mechanism. Analysis and measurement of anvil motion reveales that a very accurate anvil-alignment is achieved and maintained throughout hundreds of usages. Pressures are generated up to 100 kbar with avils having square faces 10 mm on an edge, and 120 kbar with anvils having square faces 6.4 mm on an edge. A method is devised to correct the effect of elastic deformation in structural parts, and completely isotropic compression is realized. Temperatures higher than 2000°C are attained in samples, 4 mm in diameter and 8 mm in length, while anvil-temperatures do not exceed 300°C. Operational ease and long anvil-life in the apparatus are attributed to the new anvil driving mechanism.

Diamond formation from a system of SiC and a metal

Abstract This paper deals with the behaviour of SiC in contact with cobalt (or any metal normally used as the solvent in diamond synthesis from graphite) for 1 h under a pressure of 4.5 or 5.5 GPa at temperatures of 1400–1550°C. At a pressure of 5.5 GPa and temperatures of 1430–1550°C SiC decomposed and diamond and graphite appeared. However, only graphite appeared at 4.5 GPa when the temperature remained at 1430–1500°C, and SiC did not decompose at 5.5 GPa when the temperature was lowered to 1400°C.

Development of an optical liquid high pressure cell and its application to visual observation of pressure-driven crystal growth

Abstract An optical liquid high pressure cell generating pressures up to 1 GPa was developed for visual observation of crystal growth, which can settle a supersaturation very rapidly by a pressure operation. By using a soft transparent sample bag, the cell provides high reliability of pressure control (less than ± 1 MPa) and easy handling. The method of constructing a phase diagram was developed for deducing the supersaturation. The pressure and supersaturation dependences of the nucleation and the growth form of p-xylene crystals were observed, which can be explained by the pressure and concentration dependence of the surface and the edge free energy, respectively.