Atomic and Electronic Structure in MgO-SiO2

Abstract

Understanding disordered structure is difficult due to insufficient information in experimental data. Here, we overcome this issue by using a combination of diffraction and simulation to investigate oxygen packing and network topology in glassy (g-) and liquid (l-) MgO-SiO₂ based on a comparison with the crystalline topology. We find that packing of oxygen atoms in Mg₂SiO₄ is larger than that in MgSiO₃, and that of the glasses is larger than that of the liquids. Moreover, topological analysis suggests that topological similarity between crystalline (c)- and g-(l-) Mg₂SiO₄ is the signature of low glass-forming ability (GFA), and high GFA g-(l-) MgSiO₃ shows a unique glass topology, which is different from c-MgSiO₃. We also find that the lowest unoccupied molecular orbital (LUMO) is a free electron-like state at a void site of magnesium atom arising from decreased oxygen coordination, which is far away from crystalline oxides in which LUMO is occupied by oxygen's 3s orbital state in g- and l-MgO-SiO₂, suggesting that electronic structure does not play an important role to determine GFA. We finally concluded the GFA of MgO-SiO₂ binary is dominated by the atomic structure in terms of network topology.

Figure 1

Methodology of the persistent diagram.

Figure 2

Measured density of l-Mg₂SiO₄ as a function of temperature. Error bar was estimated to be 3%.

Figure 3

Diffraction data for MgO–SiO₂ glasses and liquids. (a) Neutron (upper) and X-ray (lower) structure factors, S(Q) for g-^, and l-MgSiO₃. (b) Neutron (upper) and X-ray (lower) structure factors, S(Q) for g-^, and l-Mg₂SiO₄. (c) Neutron (upper) and X-ray (lower) total correlation functions, T(r) for g-^, MgSiO₃ (bule line) and Mg₂SiO₄ (red line). (d) Neutron (upper) and X-ray (lower) total correlation functions, T(r) for l-MgSiO₃ (blue line) and Mg₂SiO₄ (red line). Dashed lines are guides for the eyes.

Figure 4

Neutron and X-ray total structure factors, S(Q), for g,l-MgO–SiO₂ derived from DF–MD simulations (blue line) and experimental (red line) data.(a) Neutron (upper) and X-ray (lower) structure factors, S(Q) for g-^, MgSiO₃. (b) Neutron (upper) and X-ray (lower) structure factors, S(Q) for g-^, Mg₂SiO₄. (c) Neutron (upper) and X-ray (lower) structure factors, S(Q) for l-MgSiO₃. (d) Neutron (upper) and X-ray (lower) structure factors, S(Q) for l-Mg₂SiO₄.

Figure 5

Partial structure for MgO–SiO₂ glasses and liquids.(a) Partial structure factors, S_ij(Q). (b) Partial pair distribution functions, g_ij(r). Black, l-MgSiO₃; red, g-MgSiO₃; blue, l-Mg₂SiO₄; cyan, g-Mg₂SiO₄. Dashed lines are a guide to the eyes.

Figure 6

Analysis of intermediate-range structure in MgO–SiO₂. (a) BADs. (b) King ring size distributions for −O–Si(Mg)–O– rings. Black, l-MgSiO₃; red, g-MgSiO₃; blue, l-Mg₂SiO₄; cyan, g-Mg₂SiO₄.

Figure 7

Topological analysis for MgO–SiO₂. (a) Si-centric PD₁, (b) O-centric PD₁, and (c) Mg-centric PD₁.

Figure 8

Electronic structure of MgO–SiO₂ glasses and liquids. Electron DOSs for (a) MgSiO₃ and (b) Mg₂SiO₄ glasses and liquids calculated by DF–MD simulations employing PBE0 (0.3).

Figure 9

Behaviors of HOMO and LUMO in MgO–SiO₂.(a) Isosurface plots of the partial charge density around the HOMO and the LUMO levels for g-MgSiO₃. (b) Schematic illustration for HOMOs and LUMOs in crystals, glasses, and liquids. (c) Schematic illustration of LUMO in glasses and liquids. (d) Schematic illustration for electron repulsions in liquids.