REAL-SPACE MANIFESTATION OF THE 1ST SHARP DIFFRACTION PEAK IN THE STRUCTURE FACTOR OF LIQUID AND GLASSY MATERIALS

The problem of those discernible features of the intermediate range order (IRO) which can be attributed to the first sharp diffraction peak (FSDP) observed in the structure factor of many liquid and glassy materials is approached by treating this peak as a distinct feature. It is found, by considering the measured partial structure factors, S(alphabeta)(k), for molten ZnCl2, GeSe2, MgCl2, NiBr2 and NiI2 and the measured total structure factors, F(k), for glasSY SiO2, PS4 and liquid CCl4, that the propensity of the FSDP to have a prominent effect on the underlying features of the IRO depends noticeably on the system type. Specifically, the FSDP confers a marked oscillatory character of periodicity 2pi/k1 (where k1 is the FSDP position) on the IRO when the local structural units, which give rise to the density fluctuations on the IRO Scale, exist as stable entities for a timescale tau much greater than 5 x 10(-12) s. The FSDP therefore accounts for the discernible features of the underlying IRO for the viscous glass forming liquids ZnCI2 and GeSe2, for the glasses SiO2 and PS4, and for the molecular liquid CCl4. The influence of the FSDP on the IRO is less pronounced for molten MgCl2 and is negligible for molten NiBr2 and NiI2, both of which have a high cation mobility which leads to a relative instability of the Ni2+ centred structural units. The effect on the FSDP of temperature and pressure a.re briefly considered as are the development of the FSDP in molten ZnX2 (when X is changed from Cl to I to Br) and the minimum size of r-space model which is required if the FSDP is to be accurately predicted.