Structural changes on supercooling liquid silicon

Abstract

X-ray diffraction experiments and ab initio molecular dynamics (AIMD) simulations have been performed to investigate the atomic structure of liquid silicon into the deeply supercooled region. The levitation technique used has made it possible to extend the measurements down to a temperature of 1458 K, 230 K below the equilibrium melting point. The x-ray and AIMD results, which are in reasonable agreement, show conclusively that the tetrahedral order is reinforced and that the coordination number decreases as the liquid is supercooled, with implications for the liquid–liquid phase transition.

In situHigh-Energy X-Ray Diffraction Study of the Local Structure of Supercooled Liquid Si

“…For direct comparison, the results of previous x-ray measurements are reproduced in Fig. 3(b) [7][8][9]. For liquids, the first shell coordination number is poorly defined, and several methods for determining N can be found in the literature [16].…”

“…3, the data clearly show a constant coordination number of approximately 6:0 0:5 over the entire temperature range from above T M down to a supercooling of 316 K. Our results for the temperature independence of the coordination number are in conflict with the results of several previously published experimental studies and with expectations from most simulation studies based upon the SW potential. Using the conical nozzle technique for sample levitation and diffraction measurements for x rays in the energy range of 7-25 keV, Ansell et al [7] and Jakse et al [8] have reported that the coordination number decreases with decreasing temperature. Working in a reflection geometry using relatively low energy x rays requires significant data corrections for the scattering geometry as well as absorption and multiple scattering.…”

“…For liquids, the first shell coordination number is poorly defined, and several methods for determining N can be found in the literature [16]. The method chosen in previous experiments [7][8][9][10] on liquid Si was to integrate the radial distribution function, R r , up to the first minimum after the main peak. Here, R r 4 r 2 0 1 G r =4 r 0 and 0 is the average number density.…”

“…Nevertheless, containerless x-ray studies of supercooled liquid Si have largely produced contradictory results. For example, x-ray measurements made using conical nozzle levitation [7,8] have provided structural information for a supercooling of 230 K below the melting point (T M 1685 K) and identified a decrease in the coordination number from 6.3 at 1767 K to about 5.6 at 1458 K. This result is consistent with the predicted decrease in coordination number obtained from simulations using the SW potential as the temperature is reduced below T M , and was argued to provide strong evidence for the existence of an underlying first-order L-L transition. However, other x-ray measurements by Kimura et al [9] employing electromagnetic levitation (EML) for a supercooling of 290 K below the melting point found an increase in the average coordination number from 5.5 at 1793 K to 6.1 at 1403 K. Recent measurements by Higuchi et al [10], using EML to a supercooling of 150 K, found no change in coordination number with temperature, but obtained a rather low value for the coordination number (N 5) relative to previous investigations.…”

Structures at Glassy, Supercooled Liquid, and Liquid States in La-Based Bulk Metallic Glasses

“…On the other hand, the temperature dependences of the coordination number for some nonmetallic amorphous materials have been studied by several groups. [12][13][14] Ansell et al [12] and Jakse et al [13] reported the coordination number decreases with decreasing temperature for supercooled liquid silicon, while Kim et al [14] presented evidence to emphasize the temperature independence of the coordination number for the same material. In this work, local atomic structures at glassy, supercooled liquid, and liquid states for La 62 Al 14 Cu 11.7 Ag 2.3 Ni 5 Co 5 BMG are investigated by in-situ high-temperature X-ray diffraction (XRD).…”