Chiu Tang High Resolution Powder Diffraction

Chiu Tang is the Principal Beamline Scientist for Beamline I11, the High Resolution Powder Diffraction Beamline. Before joining Diamond, he was a senior beamline scientist at the SRS with a wide range of scientific interests and experience in the development of sample cells. He continues his main research work in the structural and mechanical properties of materials using synchrotron powder diffraction techniques, including the studies of structural changes in applied materials and minerals at non-ambient conditions.

Email: Chiu Tang
Tel: +44 (0) 1235 778407
Beamline I11: High Resolution Powder Diffraction

Key Research Areas

Structural properties of materials Structure determination Phase identification Microstructures of materials Structural transitions/transformations

Current Research Interests

One of my research interests is in the calcium carbonate system, which is fundamental to understanding cycling of CO₂ in the Earth over geologic time. Formation of monoclinic CaCO₃.6H₂O (ikaite) at low temperatures (£ 273K) has prompted the use of this phase as a palaeo-indicator of global climate change. Upon warming of ikaite (280-340 K), calcium carbonate pseudo-morphs (glendonites, vaterite) are formed. These mineral formations are widespread in the geologic record, and indicate either periods of glaciation or polar latitude. Establishing the physical chemistry of CaCO₃.6H₂O decomposition is important for understanding the CaCO₃ - H₂O system, and for predicting the effects of climate change on these phases. Therefore powder diffraction patterns were obtained at SPring-8 and the SRS during transformation to determine the nature of the structural changes. Results from initial data analysis have clearly established rapid ikaite-vaterite transformation with the co-existence of the two phases at room temperature. The vaterite phase appears to have adopted a hexagonal lattice of a=4.1259 and c=8.4880 Å. The monoclinic morphology of the particles was essentially retained through the transformation, but the starting ikaite crystallites (~ 1-3 m m) were much bigger than the vaterite crystalline domains (~ 50 nm).

Another area of my research is in the interesting structural behaviour exhibited by the microwave ceramics of Ba_6-3xNd_8+2xTi₁₈O_54, which have desirable dielectric properties for high frequency communication applications. Using synchrotron powder diffraction to study their structural properties, we found thermal expansion of the Pnma orthorhombic cell (x=0.5) from 10 to 295 K to be anisotropic, with the largest expansion occurs along the b-cell edge and the least along the a-cell edge. The anisotropy is due to enhanced bending of the TiO₆ polyhedra chain along the b-axis.

When metals are strained, the built up of dislocations (lattice defects) can seriously affect their mechanical performance. As diffraction peaks from polycrystalline materials are broadened by structural defects, I used x-ray line profile analysis to study dislocation density in Cu foils under strain of up to 5%. A tensonometer for stretching metal foils has been developed specifically for the study.

Selected Publications

1. Synchrotron x-ray diffraction study of Ba_4.5Nd₉Ti₁₈O₅₄ microwave dielectric ceramics at 10-295 K. C.C. Tang, M.A. Roberts, F. Azough, C. Leach & R. Freer (2002), J. Materials Research 17, 675-682
2. The structure and thermal expansion behaviour of ikaite, CaCO₃.6H₂O, from T=100 to 295 K. A.R. Lennie, C.C. Tang and S.P. Thompson (2004), Mineralogical Magazine, 68, 135-146.
3. The study of Attic black gloss sherds using synchrotron x-ray diffraction. C.C. Tang, E.J. MacLean, M.A. Roberts, D.T. Clark, E. Pantos and A.J.N.W. Prag (2001), J. Archaeological Science, 28, 1015-1024.