No abstract

We have developed synchrotron x-ray absorption and diffraction techniques for measuring the density and structure of noncrystalline materials at high pressures and have applied them to studying the behavior of SiO2 glass. The density, coordination number, and Si-O bond length at a pressure of 50 GPa were measured to be 4.63 g/cm;{3}, 6.3, and 1.71 A, respectively. Based on the density data measured in this study and the sound velocity data available in the literature, the bulk modulus at 50 GPa was estimated to be 390 GPa, which is consistent with the pressure dependence of the density in the vicinity of 50 GPa. These results, together with the knowledge from our exploratory study, suggest that SiO2 glass behaves as a single amorphous polymorph having a sixfold-coordinated structure at pressures above 40-45 GPa up to at least 100 GPa.

In situ X ray experiments on MgSiO 3 perovskite at pressures of 21 -29 GPa and temperatures of 300-2000 K were carried out using an MA8-type high-pressure apparatus, employing sintered diamond anvils, combined with synchrotron radiation. The thermal expansion at 25 GPa up to 2000 K was determined from interpolation of the P-V-T data obtained in the present study. The 95% confidence level was estimated by taking all possible errors into account, including statistical error and systematic error caused by uncertainty of pressure scales, etc. The thermal expansivity at 25 GPa is expressed as 0[T,25 '-a25 + b25T -c25T 2 with the bestfit parameters ofa25=2.11x10-SK -• b25= 1 80x10-OK-2, andc25 1.93 K. The equation of state of MgSiO 3 perovskite has been determined using our new data combined with the lower-pressure data of Wang et al. [ 1994] and Utsumi et al. [ 1995]. The optimal set of parameters of the third-order Birch-murnaghan equation of state, which is expressed as P-(3/2)Kr,0[(VT,0/V) 7/3-(VT.0/V)S/3]{ 1 -(3/4)(4-KT,O')[(VT,o/V) 2/3-1]}, where KT, o -K3oo, o + (•KT,o/•T)p(T -300), Kr,o' -K3oo,o', VT,(, = T • __ Voexp .[3{•)ø•T,odT, and CZr, o % + boT -Co T-2, is K3oo, o = 261 GPa, K3(•),o '= 4, a o 1.982x10 -5 K -•, b o = 0.818x10-8 }(2, Co = 0.474 K, and (3Kr,o/3T)p = -0.0280 GPa/K. The reliability of the result is discussed in detail.

SiO 2 glass has a network structure with a significant amount of interstitial voids. Gas solubilities in silicates are expected to become small under high pressure due to compaction of voids. Here we show anomalous behaviour of SiO 2 glass in helium. Volume measurements clarify that SiO 2 glass is much less compressible than normal when compressed in helium, and the volume in helium at 10 GPa is close to the normal volume at 2 GPa. X-ray diffraction and Raman scattering measurements suggest that voids are prevented from contracting when compressed in helium because helium penetrates into them. The estimated helium solubility is very high and is between 1.0 and 2.3 mol per mole of SiO 2 glass at 10 GPa, which shows marked contrast with previous models. These results may have implications for discussions of the Earth's evolution as well as interpretations of various high-pressure experiments, and also lead to the creation of new materials.

High-pressure and high-temperature x-ray diffraction measurements indicate that liquid silicon contracts with increasing pressure without significant changes in the local structure up to 8 GPa and then transforms to a denser structure between 8 and 14 GPa. In spite of volume contraction, the nearest-neighbor interatomic distance expands by about 1.6% within this pressure interval, accompanied by an anomalous increase in the coordination number. These findings reveal that the drastic pressure-induced structural change can take place in three-dimensional-network liquids with rather isotropic bonding.