Distribution of Rings and Intermediate Range Correlations in Silica Glass Under Pressure - A Molecular Dynamics Study
- PDF / 392,937 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 83 Downloads / 191 Views
Concurrent Computer Laboratory for Materials Simulations Department of Physics & Astronomy and Department of Computer Science Louisiana State University, LA 70803-4001 E-Mail: [email protected] Web: http://www.cclms.lsu.edu/ ABSTRACT Using the molecular dynamics (MD) method, we have studied the effect of pressure on the distribution of rings and their relationship to intermediate range correlations manifested as the first analysis of the modifications observed sharp diffraction peak (FSDP) for Si0 2 glass. A systematic in the FSDP for densities ranging from 2.0 to 3.2 g/cm 3 and temperatures from 0 to 1500 K is reported. The decrease in the height of the FSDP with increasing density is found to be proportional to the decrease in the number of 6-fold rings. For the density and temperature ranges studied in Si0 2 glass, the full width at half maximum (FWHM) of the FSDP remains unchanged. INTRODUCTION
Structural order in silica glass can be divided into short-range order (SRO) and intermediate range order (IRO) [1-5]. X-ray [2], neutron [3] and nuclear magnetic resonance (NMR) [4] experiments reveal the presence of corner-sharing Si(O1/ 2 )4 tetrahedra in silica glass at normal density (2.2 g/cm 3). The IRO is manifested as the first sharp diffraction peak (FSDP) in the static structure factor, S(q) in neutron and x-ray diffraction experiments [1-3]. One of the anomalous features of the FSDP in many network glasses, especially chalcogenide glasses such as GeSe 2 and SiSe2, is that the height of the FSDP increases with increasing temperature [1,5,6]. This is contrary to the normal behavior where the height peaks in S(q) decrease with increasing temperature. The FSDP in GeSe2 and SiSe2 have been shown to arise mainly from Ge-Ge and Ge-Se (Si-Si and Si-Se) correlations between 4 and 10 A. The temperature dependence of the FSDP is found to be a balance between two competing effects: the disorder due to thermal vibrations and thermal expansion [5]. Thermal vibrations broaden the peaks in the pair distribution functions and hence reduce the amplitude of peaks in its Fourier transform, i.e., the static structure factor. On the other hand, the thermal expansion decreases the frustration associated with the disordered arrangement of GeSe 2 tetrahedral units, thereby increasing the height of the FSDP. Neutron diffraction study by Susman et al. [7] has revealed that in Si0 2 glass the height of the FSDP decreases with increased temperature contrary to other glasses. Normal behavior of the FSDP in Si0 2 glass is a consequence of an essentially zero thermal-expansion coefficient so that the effect due to thermal vibration dominates. Using neutron diffraction, Susman et al. [8] have studied the changes in the intermediate range order up to 20%
333 Mat. Res. Soc. Symp. Proc. Vol. 408 01996 Materials Research Society
densification in SiO 2 glass. Recently, Meade, Hemley and Mao [9] have carried out in situ x-ray measurements of the static structure factor S(q) of Si0 2 glass subjected to pressures of 8-42 GPa. With increasing
Data Loading...
