Inelastic x-ray scattering study of supercooled liquid and solid silicon.
Momentum-resolved inelastic x-ray scattering (IXS) technique is one of the powerful methods for the study of dynamical properties of a given system even in extreme conditions like high temperature and high pressure. At the same time, experimental studies of physical and structural properties of liquids have multiplied in recent years with the advent of containerless techniques. These methods reduce the possibility of contamination of specimens and remove external nucleation sites. Therefore, by combining the IXS method with the levitation method, the dynamical properties of stable liquids up to 3000 K and supercooled phase of liquids can be studied. Silicon is a basic material in the semiconductor industry and has been the subject of a large amount of experimental and theoretical studies over a long time. In the crystalline phase at ambient conditions, silicon is a diamond-structured semiconductor, but upon melting it undergoes a semiconductor-to-metal transition accompanied by significant changes in the structure and density. The coordination number increases from 4 in the solid to about 6.5 in the liquid, and liquid density is increased by about 10%. The principal purpose of the present study was to determine silicon's elastic modulus from the measurement of averaged sound speed determined from IXS. The experiments were carried out at the Advanced Photon Source (APS) beamline 3-ID with a high-resolution monochromator consisting of two nested channel-cut crystals and four backscattering analyzer setups in the horizontal scattering plane 6 m from the sample. The requirements for very high energy resolution and the basic principles of such instrumentation are discussed elsewhere as referenced. The levitation apparatus was enclosed in a bell jar specially designed for backscattering geometry with a separation of 10 cm between the sample and the detector. Silicon spheres of 2 to 3 mm in diameter were suspended in an argon gas jet and heated with a 270 W CO{sub 2} laser beam. Temperatures were measured during the experiment with a pyrometer whose operating wavelength was 0.65 {micro}m. The temperature gradient on the sample was estimated to be about +/- 20 K. The energy scans were taken for supercooled-liquid and hot-solid silicon at temperature T=1620 K. Sound velocities were determined from the initial slope of the excitation frequencies. Then, the longitudinal moduli for hotsolid and supercooled-liquid silicon were calculated from L = v{sub L}{sup 2}{rho} using measured velocities. In these calculations, density values were taken from Ohsaka et al. as referenced. Results are presented in Table 1. together with room-temperature, hot-solid single-crystal measurements, and stable-liquid values. Room-temperature longitudinal moduli were calculated from the values of the single-crystal elastic constants. They were measured between 300 K and 870 K. Since there was no phase transition up to temperature 1620 K for hot-solid silicon, it is reasonable to extrapolate these data to 1620 K in order to compare to our results for the hot solid. A significant difference (about 20%) is observed between our measurement and the extrapolated single-crystal value of the longitudinal modulus for solid silicon at temperature 1620K. This reduction of the longitudinal modulus may be an indication of the pre-melting. The factor of more than two change in the elastic modulus between supercooled liquid and hot solid at the same temperature can be attributed to the semiconductor-to-metal transition in silicon associated with melting. Also, the longitudinal modulus of the stable liquid is reported in Table 1. About a 10% difference is observed between the modulus of the supercooled and the stable liquid silicon. This can be interpreted as silicon still maintaining metallic properties with a significant increase in the degree of the directional bonding upon supercooling, as found in the x-ray diffraction and ab initio MD studies. All these results are discussed in reference.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 971930
- Report Number(s):
- ANL/XSD/CP-118874; TRN: US1001421
- Resource Relation:
- Conference: 5th International Conference on Synchrotron Radiation in Materials Science (SRMS 5); Jul. 30, 2006 - Aug. 2, 2006; Chicago, IL
- Country of Publication:
- United States
- Language:
- ENGLISH
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Related Subjects
ADVANCED PHOTON SOURCE
ARGON
BACKSCATTERING
BONDING
CONTAMINATION
COORDINATION NUMBER
ENERGY RESOLUTION
EXCITATION
GEOMETRY
LASERS
LEVITATION
MELTING
MONOCHROMATORS
NUCLEATION
PYROMETERS
SCATTERING
SILICON
SYNCHROTRON RADIATION
TEMPERATURE GRADIENTS
WAVELENGTHS
X-RAY DIFFRACTION