NEUTRON-DIFFRACTION STUDY OF LIQUID LIXSI1-X ALLOYS WHERE X=0.80, 0.65 AND 0.57

We report a neutron diffraction experiment on liquid 7LixSil-x alloys (x = 0.80, 0.65 and 0.57) just above the liquidus and on 7Li57Si43 at about 200 K above the liquidus. We obtained absolutely normalized and fully corrected structure factors and radial distribution functions, which provide indications for the occurrence of chemical ordering in all three alloys. We have analysed the data by fitting models to the structure factor and to the radial distribution function, and by using the reverse Monte Carlo (RMC) method. We find strong indications for the presence of covalently bonded Si in liquid Li65Si35 and Li57Si43. The Si-Si coordination number Z(SiSi) varies from about 1.5 in Li65Si35 to about 2 in Li57Si43. For c(Si) > 0.2, Si forms poly-anions ranging from dumbbells to large three-dimensional structures. Z(SiSi) in Li57Si43 is not strongly temperature dependent and consequently the poly-anions are fairly stable. Covalently bonded Si is absent in liquid Li80Si20. The RMC modelling indicates that individual Si atoms are well separated. This suggests that the covalent character of Si in the Si-rich alloys (Li65Si35 and Li57Si43) changes to a more ionic character in Li80Si20.