Satoshi Fukura

Change in compressibility of δ-AlOOH and δ-AlOOD at high pressure: A study of isotope effect and hydrogen-bond symmetrization

Abstract The compression behaviors of δ-AlOOH and δ-AlOOD were investigated under quasi-hydrostatic conditions at pressures up to 63.5 and 34.9 GPa, respectively, using results from synchrotron X-ray diffraction experiments conducted at ambient temperature. Because of the geometric isotope effect, at ambient pressure, the a and b axes of δ-AlOOD, which define the plane in which the hydrogen bond lies, are longer than those of δ-AOOH. Under increasing pressure, the a and b axes of δ-AlOOH stiffen at 10 GPa, although the c axis shows no marked change. Identical behavior was found in δ-AlOOD, but the change in compressibility was observed at a slightly higher pressure of 12 GPa. Axial ratios a/c and b/c first decrease rapidly with increasing pressure, then begin to increase at pressures >10 GPa in δ-AlOOH and >12 GPa in δ-AlOOD. At these pressures, the pressure dependence of a/b also changes from increasing to decreasing. The unit-cell volumes of δ-AlOOH and δ-AlOOD become slightly less compressible at high pressures. Assuming K0′ = 4, the calculated bulk moduli of δ-AlOOH below and above 10 GPa are 152(2) and 219(3) GPa, respectively. Those of δ-AlOOD below and above 12 GPa are 151(1) and 207(2) GPa, respectively.

Factors Determining the Stability, Resolution, and Precision of a Conventional Raman Spectrometer

We verified the performance of a conventional Raman spectrometer, which is composed of a 30 cm single polychromator, a Si based charge-coupled device (CCD) camera, and a holographic supernotch filter. For that purpose, the time change of the peak positions of Raman spectra of naphthalene and fluorescence spectra of ruby (Cr-doped Al2O3) were monitored continually. A time-dependent deviation composed of two components was observed: a monotonous drift up to 0.4 cm−1 and a periodic oscillation with a range of 0.15 cm−1. The former component was stabilized at approximately 2000 s after the CCD detector was cooled, indicating that incomplete refrigeration of the CCD detector induced the drift. The latter component synchronized with the periodic oscillation of the room temperature, indicating that thermal expansion or contraction of the whole apparatus induced this oscillation. The implemental deviation is reduced when measurements are conducted using a sufficiently cooled CCD detector at a constant room temperature. Moreover, the effect of the room temperature oscillation is lowered in a spectrum acquired over a duration that is longer than one cycle of this oscillation. Applying the least squares fitting method to carefully measured spectra enhanced the precision of the determination of the peak position to 0.05 cm−1 using the spectrometer with pixel resolution of 1.5 cm−1.

Infrared and Raman spectroscopic observations of Central African carbonado and implications for its origin

Carbonado diamonds from the Central African Republic (CAR) were investigated using spectroscopic observations. Raman spectra for a polished section of carbonado showed the average Raman frequency as 1333.0 cm −1 ; measurements 10 μm below the sample surface showed a bimodal distribution of the Raman frequency with the average at 1333.5 cm −1 . These contrasting results indicate that the carbonado interior retains considerable residual pressure. The maximum residual pressure detected in this study from the 10 μm subsurface was 0.49 GPa. From infrared (IR) absorption spectra for crushed carbonado samples, no absorption attributable to diamond was observed because absorption bands of mineral and fluid inclusions were too strongly observed. Acid leaching treatment of the crushed grains elicited IR bands assignable to intrinsic diamond vibration and nitrogen impurity in the diamonds. Nitrogen atoms in the CAR carbonado were not much aggregated and the degree of aggregation was intermediate between type Ib and type IaA. In the chemically treated samples, IR absorption bands from liquid water and carbonate were ubiquitous suggesting that the CAR carbonado samples contain fluid inclusions and are similar to diamonds containing mantle-derived fluids. The experimental results of this study suggest that the CAR carbonado originated from rapid heating event with a presence of fluid in the mantle and subsequent rapid cooling before aggregation of nitrogen impurity in the diamond lattice.

High Precision in Raman Frequency Achieved Using Real-Time Calibration with a Neon Emission Line: Application to Three-Dimensional Stress Mapping Observations

A three-dimensional (3D) Raman mapping system with a real-time calibration function was developed for detecting stress distributions in solid materials from subtle frequency shifts in Raman spectra. An atomic emission line of neon at 918.3 cm−1 when excited at 514.5 nm was used as a wavenumber standard. An emission spectrum of neon and a Raman spectrum from a sample were introduced into a single polychromator using a bifurcated optical fiber. These two spectra were recorded simultaneously on a charge-coupled device (CCD) detector using double-track mode. Energy deviation induced by the fluctuation of laboratory temperature, etc., was removed effectively using the neon emission line. High stability during long measurements was achieved. By applying curve fitting, positions of the Raman line were determined with precision of about 0.05 cm−1. The present system was applied to measurements of residual pressure around mineral inclusions in a natural diamond: 3D stress mapping was achieved.

Pressure-induced change in the compressional behavior of the O-H bond in chrysotile: A Raman high-pressure study up to 4.5 GPa

Abstract A change in the linear pressure behavior of the chrysotile Raman O-H band is revealed by an insitu high-pressure Raman study using a diamond anvil cell (DAC) at 0.1-0.4 GPa pressure intervals. The peak of 3701 cm-1 can be accurately determined in the pressure range of 0.2-4.5 GPa regardless of peak fitting models. The pressure (P)-wavenumber (ν) relationship for the peak is closely approximated by two linear functions with the slopes (dν/dP) of 4.3 cm-1/GPa and 1.7 cm-1/GPa at pressures above and below 1.7 GPa, respectively. The spectral resolution given by a peak fitting method (0.5 cm-1) implies that these functions for changes in the positions of the 3701 cm-1 peak provide pressure estimates with resolutions of 0.1-0.2 GPa. This method can be used to estimate remnant pressure in natural samples in the form of thin sections. The pressure shift characterized by the change in slope and the associated decrease of the peak width can be explained by a model where the change in compressibility of the tetrahedral layer affects the interaction between the inner O-H and Si atoms forming a six-membered ring. The high-pressure dependence at pressures lower than 1.7 GPa may be a contribution of a dominant layer-parallel (in-plane) compression that compensates a distortion in the tetrahedral layer of chrysotile. The conclusion that chrysotile changes its compressibility at around 1.7 GPa is significant for understanding of the properties of chrysotile nano-fibers and possibly for thermodynamic consideration on serpentine phase relations