Simulational analysis of the local structure in liquid germanium under pressure

Abstract

Recently, a high-pressure x-ray diffraction study was performed on liquid germanium (l-Ge͒, in order to clarify the change in the local structure of a liquid under pressure. Inspired by this work, we perform tightbinding ͑TB͒ molecular dynamics simulations for l-Ge at seven different pressures. To this end, we construct a new TB scheme by modifying a previously reported nonorthogonal TB scheme. By explicitly taking into account the nonorthogonality of the atomic wave functions, we obtain a TB scheme which is accurate at both low and high pressures. The pair distribution function g(r), static structure factor S(Q), and bond-angle distribution function g (3) (r c ,) are calculated, along with other quantities. The calculated g(r) and S(Q) are in excellent agreement with the above-mentioned experiments, which confirms the validity of our TB scheme. From a detailed analysis of atomic configurations, we elucidate the change in the local structure of l-Ge under pressure. We find that bonding in l-Ge is interpreted as a mixture of covalent and metallic bonding, and that as pressure increases, the ratio of covalent bonds decreases, although they still exist at pressures as high as 24.0 GPa. We further find that, by dividing g (3) (r c ,) into covalent and isotropic parts, the covalent contribution to the local structure of l-Ge changes from a ''disordered'' ␤-Sn structure in the low-pressure region to a ''pure'' ␤-Sn structure in the high-pressure region.

Intermediate range chemical ordering in amorphous and liquid water, Si, and Ge

“…Recently, higher density forms have been measured using high pressure diffraction techniques for both Si ͑Ref. 8͒ and Ge, 11 which show strong structural similarities to that of very high density amorphous ice 10,12 and with liquid water at a high pressure. 13 The very high density forms all show increased intermediate range ordering compared to the high density form, but differences occur in the way additional atoms or molecules are pushed into the nearest neighbor shell in water.…”

“…11 However, in liquid Si there is an abrupt coordination change and density increase between 8 and 14 GPa, as the coordination number rises abruptly from 7 to ϳ9. 8 Polyamorphic, low to high density amorphousamorphous transitions under pressure have also been reported for densified porous silicon at ϳ10 GPa ͑Ref.…”

“…The structure of tetrahedral low density, 1-4 high density, [5][6][7] or very high density, [8][9][10][11][12][13] glassy and liquid forms has been investigated in several materials over the past two decades, with much attention focused on amorphous ice. 1, 5,9,10,12,14,15 Although the structural changes in densified amorphous or liquid water are usually associated with that of glassy SiO 2 under pressure, 16,17 in this paper we show that the structures of low, high, and very high density amorphous ice ͑denoted LDA, HDA, and VHDA, respectively͒ exhibit remarkable similarities with amorphous and liquid silicon and germanium.…”

Tight-Binding Molecular Dynamics Study on the Structural Change of Amorphous Germanium with the Increase of Density

“…15) The temperature T is set at T ¼ 300 K. The thermostat parameter is chosen as in ref. 9. We use a system with 512 atoms.…”

Structure change of liquid GaSb under pressure: An ab initio molecular-dynamics simulation

“…The recent experimental results indicated that the local structure of liquid Si exhibits abruptly changes under pressure within the relatively small interval, 2 while the local structure of liquid Ge exhibits gradually changes with increasing pressure in a wide region. 3 Recently, Hattori et al [4][5][6] systemically studied the structure changes of the liquid III-V compounds ͑liquid GaSb, liquid InSb, and liquid InAs͒ under pressure, and suggested that they show different pressure dependences from those in the crystalline phase.…”