Chang-Sheng Zha

Internal resistive heating in diamond anvil cell for in situ x-ray diffraction and Raman scattering

An internal resistive heater contained completely between the anvil faces of a diamond anvil cell has been used to conduct experiments up to 10 GPa and temperatures up to 3000 K. There is evidence that pressures up to 50 GPa can be achieved with smaller anvil faces and smaller heater assembly. The technique offers a very homogeneous temperature profile and excellent time stability for studying both metallic and nonmetallic materials. Temperature is measured by spectroradiometry as an image of the incandescent sample is projected onto the end of a fiber optic cable leading to a spectrometer. A very linear relationship between temperature and power provides accurate temperature measurements even when the temperature is below incandescence. The melting of gold determined by loss of diffraction peaks occurred at temperatures in good agreement with published values. In situ Raman spectra of SiO2 revealed the conversion of quartz to coesite. This method offers a larger heating volume with more stable and unifor...

X-ray diffraction study on pressure-induced phase transformations in nanocrystalline anatase/rutile (TiO2)

An in situ x-ray diffraction study was conducted to study the pressure-induced phase transformation in nanocrystalline anatase/rutile (TiO2) to 35.1 GPa. The nano-anatase phase remains stable to approximately 16.4 GPa, and then transforms to an amorphous phase, which is returned upon release of pressure. The nano-rutile phase starts to transform to the baddeleyite (ZrO2) structure at ~8.7 GPa, and the transformation is complete at approximately 16.4 GPa. On release of pressure the ZrO2 structure transforms to the α-PbO2 structure. The results are compared to previous work on phase changes in TiO2 with different particle sizes.

High pressure synchrotron x-ray powder diffraction study of Sc2Mo3O12 and Al2W3O12

Synchrotron x-ray powder diffraction was used to study Sc2Mo3O12 and Al2W3O12 at high pressure in a DAC. Both compounds adopt the orthorhombic Sc2W3O12 structure under ambient conditions and exhibit anisotropic negative thermal expansion. A phase transition from orthorhombic (Pnca) to monoclinic (P 21/a) symmetry was observed at ~0.25 GPa for Sc2Mo3O12 and at ~0.1 GPa for Al2W3O12 associated with a volume reduction of ~1.5–2%. A second crystalline to crystalline phase transition was clearly seen only for Sc2Mo3O12 (2.5–3.0 GPa). Peak broadening and almost complete amorphization were observed for Sc2Mo3O12 at ~8 GPa, and this was not fully reversible on decompression. At 7 GPa, the amorphization of Al2W3O12 was not as advanced as for the molybdate and on decompression crystalline material was recovered. The compressibility of orthorhombic Sc2Mo3O12 is highly anisotropic, but it is almost isotropic for both monoclinic Sc2Mo3O12 and Al2W3O12. Both compounds show a reduction in their bulk moduli (K0) at the orthorhombic to monoclinic transition: 32(2) GPa for orthorhombic and 16(1) GPa for monoclinic Sc2Mo3O12, and 48 GPa for orthorhombic and 28(1) GPa for monoclinic Al2W3O12. Sc2Mo3O12 displays very similar high pressure behaviour to the previously studied Sc2W3O12.

Technique for x-ray markers at high pressure in the diamond anvil cell

X-ray markers as powder or foil can interfere with optical studies on a sample. Use of the gasket itself as an x-ray marker requires careful collimation of the x-ray beam so that only the gasket material adjacent to the sample is studied. (The pressure drops rapidly as the radius increases in the gasket.) By depositing a thin half-micron thick marker on the wall of the sample hole, these problems are eliminated and a large beam can be used, but for pressure measurements only the submicron layer will be involved.