High-pressure x-ray scattering and computer simulation studies of density-induced polyamorphism in silicon

Abstract

Wilson, M., Wilding, M. C., Daisenberger, D., Machon, D., Cabrera, R. Q. (2007). High-pressure x-ray scattering and computer simulation studies of density-induced polyamorphism in silicon. Physical Review B: Condensed Matter and Materials Physics, 75 (22). Sponsorship: ESRFA low- to high-density pressure-driven phase transition in amorphous silicon is investigated by synchrotron x-ray diffraction in the diamond anvil cell. Complementary atomistic molecular dynamics computer simulations provide insight into the underlying structural transformations and allow us to interpret the structure factors obtained from experiment. During compression the form of the scattering function S(Q) changes abruptly at 13.5 GPa, indicating significant structural rearrangement in the amorphous solid. In particular, the first peak in S(Q) shifts to larger Q values. The changes are correlated with the occurrence of a low- to high-density (LDA-HDA) polyamorphic transition observed previously using Raman scattering and electrical conductivity measurements. The data are analyzed to provide real space (pair distribution function) information. The experimental data are compared with results from molecular dynamics (MD) simulations using a modified Stillinger-Weber many-body potential energy function in order to extract structural information on the densified amorphous material. We deduce that the polyamorphic transition involves an abrupt increase in the proportion of 5- and 6-coordinate Si atoms. The overall structure of the HDA polyamorph can be related to that of the LDA form by creation of highly-coordinated "defects" within the tetrahedrally-bonded LDA network. However classical and quantum MD simulations indicate that an even higher density amorphous state might exist, based on structures that resemble the densely-packed metallic polymorphs of crystalline Si.publishersversionPeer reviewe

Simulations of nanocrystals under pressure: Combining electronic enthalpy and linear-scaling density-functional theory

“…Such reversible transformations between LDA, HDA, and ph phases have also been observed for amorphous bulk Si (a-Si). [72][73][74] Theoretical investigations have attempted to characterize these amorphous phases in bulk Si [73][74][75][76][77][78] and hydrogenated Si nanocrystals. [20][21][22][23][24] The LDA phase has been described as a disordered tetrahedrally coordinated network, the HDA as a deformed tetrahedral network 75 with the presence of interstitials increasing the coordination to 5-6, and finally a veryhigh-density amorphous phase (VHDA) has been postulated 78 with coordinations 8-9 as found also in ice.…”

High-pressure studies with x-rays using diamond anvil cells

“…multiple structures of amorphous materials with the same composition separated by a first order phase transition) remains a fundamentally important and unresolved phenomenon [408]. A wealth of indirect evidence for such transitions has already been established in H2O [407], SiO2 [200,409,410], GeO2 [61,224,411], Si [198,412,413], I [414], Se [414,415], S [415,416], P [197,417], Ce [406]. However, unambiguous evidence for liquid-liquid transition has yet to be presented in the liquid state for any system.…”

Polyamorphism in network structures

“…Thus this discontinuity also supports occurrence of a transition. It is well known that the densities of amorphous solids and liquids scale like (Q 1 ) 3 in analogy with the diffraction in crystalline solids [22,23]. The Q 1 of the samples recovered from 17 GPa is found to be smaller than that for samples recovered from 33 GPa, suggesting that the two recovered amorphous phases have different densities.…”