SPring-8, the large synchrotron radiation facility

Date&Time: April 10 14:00 - 15:00

Speaker: FProf. E. Bychkov
Affiliation: LPCA, UMR 8101 CNRS, Université du Littoral, 59140 Dunkerque, France

Abstract
　　A prominent feature of the diffraction pattern in glasses is the First Sharp Diffraction Peak (FSDP) which appears in the low scattering vector range, Q₁ =< 2 Å^-1, and exhibits a number of unusual phenomena such as its temperature and pressure dependences. The FSDP was extensively studied for many different glass families but underlying theory is far to be complete and still needs new experimental results. In a particular case of binary chalcogenide systems, neutron diffraction measurements with isotopic substitution and/or anomalous x-ray scattering results clearly show that the main contribution to the FSDP comes from network-forming cation correlations (As-As, Ge-Ge, etc.) at a typical distance L = 2π/ Q₁ ~ 5 - 7 Å. This estimation of the order of two to three interatomic distances as well as high intensity of the FSDP strongly suggests the existence of intermediate-range order in the disordered network, well documented by many researchers. However, in contrast to pressure and temperature dependences, little is known about systematic FSDP evolution with the glass composition except a few examples (P-Se, Si-Se). In the present talk, a summary of our recent neutron and high-energy x-ray diffraction results will be discussed with a special emphasis to relate the FSDP parameters (position, width and amplitude) to the short- and intermediate-range ordering in glasses. I will show that in case of binary A-X glassy systems (A = Si, P, Ge, As; X = S, Se, Te) the FSDP reflects structural rearrangements on different length scales, also detected by spectroscopic techniques (NMR, NQR, Raman) and consistent with macroscopic changes evidenced by viscosity, specific heat and DSC measurements. In contrast, the rigidity percolation seems do not have any clear signature in the observed phenomena.