Ji-an Xu

Raman spectroscopic characteristics of Mg-Fe-Ca pyroxenes

Abstract Raman spectra of several compositions of (Mg, Fe, Ca)SiO3 pyroxenes were collected at ambient conditions. More than 10 Raman vibrational modes were observed for these pyroxenes in the wavenumber range between 200 and 1200 cm-1. In general, these pyroxenes are characterized by (1) the Si-O stretching modes above 800 cm-1; (2) the Si-O bending modes between 500 and 760 cm-1; (3) SiO4 rotation and metal-oxygen translation modes below 500 cm-1. For a constant Ca content, frequencies of the Raman modes in the enstatite-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decrease with an increase in Fe content. This phenomenon is attributed to an increase in both the bonding lengths and the reduced mass as Fe2+ is substituted for Mg. However, two modes at ~900 cm-1 in the enstatite-ferrosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds when the M2 sites are preferentially occupied by the iron cation. The effect of Fe substituting for Mg on the frequency shift in cpx is less profound than opx because the larger M2 was occupied by calcium and the substitution of iron and magnesium in the M1 site results in a less significant change in the bond length. The major-element composition of the (Mg, Fe, Ca)-pyroxenes, especially the orthopyroxene series, can be semi-quantitatively determined on the basis of the peak positions of their characteristic Raman modes.

Raman study at high pressure and the thermodynamic properties of corundum; application of Kieffer's model

Abstract A Raman spectroscopic study was performed in a diamond-anvil cell to measure the optical vibrational modes of corundum as a function of pressure up to 20 GPa. The integrals concerning the optic continuum in Kieffer’s model can be calculated as the weighted sum of each mode in the continuum. Thus, on the basis of Kieffer’s model and the experimental results of this study, thermodynamic parameters such as heat capacity, entropy, thermal expansion coefrcient, and Gruneisen parameter for corundum at various pressures and temperatures were determined. The contribution to the heat capacity of corundum approaches the classic (Dulong-Petit) limit in the interior of the Earth beneath a depth of 50 km.

The compression of diaspore, AlO(OH) at room temperature up to 27 GPa

Diaspore AlO(OH) was compressed in a diamond anvil cell up to 27 GPa at room temperature. X-ray diffraction experimental data on this sample show that the bulk modulus KT is 230 GPa assuming its pressure derivative Ko′ is 4.

High-pressure phase transition in Al(OH)3: Raman and X-ray observations

High-pressure Raman spectra and X-ray diffraction patterns of gibbsite have been investigated up to 23.3 GPa and 8.7 GPa, respectively in diamond anvil cells at room temperature. More than 20 Raman-active modes were observed at the ambient conditions and these modes were predominately distributed in two regions : the low-wavenumber (LW, 200 to 1200 cm -1 ) and high-wavenumber regions (HW, 3000 to 3700 cm -1 ). The wavenumber of Raman modes in LW region increases linearly with pressure while the wavenumber of those in the HW region decreases with pressure. The later observation indicates the enhancement of hydrogen bonds between octahedral layers in the lattice upon compression. A significant change in the diffraction patterns and Raman spectra was observed for Al(OH) 3 above 3 GPa where a phase transition takes place. The high-pressure phase of Al(OH) 3 is quenchable and is tentatively determined as nordstrandite, a triclinic polymorph of gibbsite, on the basis of its diffraction pattern and Raman spectrum.

Ultrahigh pressures in gem anvil cells

Abstract In analogy with the bevelled diamonds in diamond anvil cells, some transparent gems are used as anvils for generating static ultrahigh pressure. The maximum pressure within which these gem anvil cells could generale was 16.7 GPa in cubic-zirconia anvil cell and 25.8 GPa in sapphire anvil cells, respectively.

Pressure-induced phase transitions in gypsum

Abstract We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO4 · 2H2O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO4 · 2H2O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure. Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.

Phase transitions of Zn0.84Fe0.16Se under high-pressure

Abstract The energy-dispersive X-ray-diffraction (EDXD) and Raman spectroscopy were employed to study the pressure induced phase transitions of Zn 0.84 Fe 0.16 Se crystal up to 21.0 and 32.0 GPa, respectively. A semiconductor-metal transition was identified by EDXD and Raman spectroscopic methods at 11.4 ± 0.5 GPa. Other two phase transitions were found only in the Raman scattering study below 11.4 GPa. In addition, three unidentified Raman peaks were still observable above the metallization pressure. The existence of Fe impurity in the ZnSe up to a concentration of 0.16 reduced prominently the semiconductor-metal phase transition pressure. For Raman works, the three unidentified Raman peaks of Zn 0.84 Fe 0.16 Se above 10.9 GPa are possibly the TO modes in the thin surface of the high pressure metallic phase.