Atomic and electronic structures of binary silicate glasses Materials Science : Structure 58 Glass is abundant in nature and has been made by man for over 3000 years. Glass has evolved from a basic structural material to an enabling material for advanced electronic, biological and photonic products as well as high-volume window and fiber glass devices. Despite its rich history, many aspects of glass remain mysterious: The theory of glass transition is one of the most challenging problems in physics and chemistry in the 21st century. The PbO-SiO 2 system is interesting even though lead compounds are unfavorable materials nowadays. It is well known that PbO acts both as a glass former and a glass modifier and hence the PbO-SiO 2 system shows a wide glass formation composition range. Therefore, it is interesting to know the underlying reason through the atomic structure of glass. Atomic structures of magnesium silicate melts are the key to understanding the processes related to the evolution of the Earth’s mantle and represent precursors of the formation of most igneous rocks. Magnesium silicate compositions also represent a major component of many glass ceramics, and, depending on their composition, can span the entire fragility range of glass formation. It is worth mentioning that glass of Mg 2 SiO 4 composition shows an extremely low glass forming ability (GFA), while MgSiO 3 composition shows a higher GFA. Therefore, we have tried to understand the relationship between glass structure and GFA at atomic and electronic levels by a combination of high-energy X-ray diffraction ( BL04B2 ) and neutron diffraction measurements, reverse Monte Carlo (RMC) simulations, and density functional theory (DFT) calculations [1,2]. Figure 1 shows the cavity distribution in SiO 2 and PbO-SiO 2 glasses obtained by RMC simulation. It is well known that SiO 2 glass shows a large fraction of cavities (~30%), but we can see a significant amount of cavities in PbO-SiO 2 . This is very unusual, since cavity sites are usually occupied by cations in typical binary silicate glasses (e.g., Na 2 O-SiO 2 and CaO-SiO 2 glasses). Lead is known to act as a network former and a network modifier in binary oxide glasses. Thus, we have succeeded in visualizing the role of lead in PbO-SiO 2 glass, suggesting that the large fraction of cavities is the reason for the high GFA in a wide composition range. On the other hand, both MgSiO 3 and Mg 2 SiO 4 glasses do not have any cavities, because magnesium occupies cavity sites in binary silicate glasses. To understand the relationship between GFA and atomic structure, we compared the distribution of “-Si(Mg)- O-Si(Mg)-O-Si(Mg)-” rings in MgSiO 3 and Mg 2 SiO 4 glasses with the distribution of “-Si-O-Si-O-Si-” rings in SiO 2 glass in Fig. 2. The ring distribution of SiO 2 glass shows the maximum fraction of 6-fold rings (comprising 6 SiO 4 tetrahedra) and is broad up to SiO 2 glass 34 mol% PbO - 64 mol% SiO 2 glass 50 mol% PbO - 50 mol% SiO 2 glass 65 mol% PbO - 35 mol% SiO 2 glass 35.7 Å 43.1 Å 44.0 Å 42.6 Å Fig. 1. Atomic configurations and voids of SiO 2 and PbO-SiO 2 glasses. Color key: light gray, silicon; red, oxygen; gray, lead; and cyan; voids [1]. 59 10-fold rings. According to Gupta and Cooper, this distribution is the signature of “topological disorder”, since the crystalline SiO 2 (cristobalite) has only 6-fold rings [3]. On the other hand, Mg 2 SiO 4 glass shows the narrowest ring distribution, suggesting that it is “topologically ordered”, which is related to the low GFA of the glass. The coordination numbers of oxygen around magnesium derived from the RMC model are 4 and 5 for MgSiO 3 and Mg 2 SiO 4 , respectively, which do not support the formation of MgO 6 octahedra confirmed by recent NMR measurements for both glasses [4,5]. To obtain insight regarding this inconsistency, we optimized the RMC structures by DFT calculations. Furthermore, we calculated chemical strength (bond orders) for Mg-O bonds in both MgSiO 3 and Mg 2 SiO 4 crystals and glasses. The Mg-O bond orders of glasses shown in Fig. 3 are larger than those of the corresponding crystalline phases owing to the fact that the Mg-O coordination is smaller in the glassy phase, where the cations compensate the smaller number of oxygen contacts by increasing the ionic bond strength correspondingly. The DFT calculations explain the discrepancy between the NMR and diffraction results, because NMR probes chemical shifts that are very sensitive to the electronic environment of the nuclei, while diffraction is a direct probe of the average coordination number through known neutron scattering lengths or the number of electrons surrounding an atom (provided that the partial functions are known). Previous studies [4,5] indicated that for the MgSiO 3 and Mg 2 SiO 4 glasses, the NMR shifts are in line with the octahedral crystalline environment, although in this study we find that the Mg-O coordination is actually smaller in the glasses. The structure of disordered materials is very ambiguous owing to the lack of long-range periodicity manifested by a broad diffraction pattern. However, a combination of synchrotron X-ray diffraction measurements and theoretical simulations is a powerful technique for studying the relationship between atomic/electronic structure and physico- chemical properties, which is crucial for revealing and understanding the origin of unique functional properties in disordered materials. 50 40 30 20 10 0 40 30 20 10 0 40 30 20 10 0 12 11 10 9 8 7 6 5 4 3 2 1 n –fold ring Fraction (%) GFA Topological order SiO 2 Mg 2 SiO 4 MgSiO 3 Chemical Strength (bond order) Mg-O Distance (Å) Mg-O Distance (Å) Chemical Strength (bond order) MgSiO 3 glass MgSiO 3 crystal Mg 2 SiO 4 glass Mg 2 SiO 4 crystal 0.3 0.2 0.1 0.0 4.0 3.5 3.0 2.5 2.0 0.3 0.2 0.1 0.0 4.0 3.5 3.0 2.5 2.0 Fig. 3. Scatter plot of the chemical bond order as a function of distance for Mg-O pairs in MgO-SiO 2 glass. The crystalline reference values are included in red. [2] Fig. 2 The distribution of “-Si(Mg)-O-Si(Mg)-O-Si(Mg)-” rings in MgSiO 3 and Mg 2 SiO 4 glasses and the distribution of “-Si-O-Si-O-Si-” rings in SiO 2 glass. Shinji Kohara a, * and Jaakko Akola b a SPring-8/JASRI b Dept. of Physics, Tampere University of Technology, Finland *E-mail: kohara@spring8.or.jp References [1] S. Kohara, H. Ohno, M. Takata, T. Usuki, H. Morita, K. Suzuya, J. Akola and L. Pusztai: Phys. Rev. B 82 (2010) 134209 . [2] S. Kohara, J. Akola, H. Morita, K. Suzuya, J.K.R. Weber, M.C. Wilding and C.J. Benmore: Proc. Nat. Acad. Sci. USA 108 (2011) 14780 . [3] P.K. Gupta and A.R. Cooper: J. Non-Cryst. Solids 123 (1990) 14. [4] K. Shimoda et al. : Am. Mineral. 92 (2007) 695. [5] S. Sen et al. : J. Phys. Chem. B 113 (2009) 15243.
home
- JP
- EN