Template for Electronic Submission to ACS Journals - DOC

Document Sample
Template for Electronic Submission to ACS Journals - DOC Powered By Docstoc
					                                       Supporting information

                           Structure and Properties of Dense Silica Glass

                    Min Wu1,2, Yunfeng Liang2,3- Jian-Zhong Jiang1+ and John S. Tse1,2*


      1
          International Center for New-Structured Materials, Zhejiang University, and Laboratory of
          New-Structured Materials, Department of Materials Science and Engineering, Zhejiang
                                 University, 310027 Hangzhou, P.R. China
      2
          Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon,
                                     Saskatchewan, Canada S7N 5E2
      3
          JAPEX Energy and Resources Engineering Laboratory, Kyoto University, Kyoto Daigaku
                                    Katsura, Nishikyo-ku, Kyoto, Japan


Generation of a-SiO2 structural models
First-Principles molecular dynamics calculations were used to generate various structures of silica glass.
The variable cell Car-Parrinello method with the isobaric-isothermal (constant-pressure constant
temperature) (NPT) ensemble was used. The model consists of 24 SiO2 units (72 atoms). Ultrasoft
pseudopotentials [14] were used to describe the valence electron-nuclei interactions. Electron orbitals
were expanded in a plane wave basis set using an electron density cutoff of 240 Ry. The Perdew-Burke-
Enzerhof functional (PBE) [15] and the Generalized Gradient Approximations (GGA) was used as it has
been shown to reproduce the -quartz-coesite and coesite-stishovite transition pressure very well 16].


An initial glass structure was constructed by quenching a liquid configuration obtained by heating
crystalline quartz at a density of 2.2 g/cm3 (a=5.227 Å and c/a=1.1) to 6000 K and then annealed at
3000K for 6.2 ps. The liquid configuration was ensured by monitoring the mean square displacement of
Si and O atoms. The system was then cooled slowly to room temperature within an additional 20 ps. We
then analyzed the behavior of compressed glass by slow, cold compression at 300 K every 2 GPa for 3
ps (Run A: cold compression). In order to study the recovered sample, the compressed sample at 26 GPa
and 46 GPa were decompressed to 0GPa at 300 K at every 4 GPa for 3 ps (Run B and E, respectively).


To investigate whether it is possible to obtain a pure 5-fold silica glass (hypothesized GeO2 glass [1])
and a pure 6-fold silica glass [2, 3, 4], the quenching procedure was repeated from a liquid at selected
densities. The 5- and 6- fold liquid samples were obtained, respectively, by heating a “penta” phase at a
                                                     1
density of 3.93 g/cm3 [5] or stishovite phase at a density of 4.16 g/cm3 [6]. The pressure of “5-fold”
glass, i.e., obtained originally from “penta” phase, is ca. 20 GPa. We have then studied the equation of
state and the structure properties (e.g. coordination number) of the resulting glass structure by cold
compression or decompression at 300 K at every 4 GPa for 3 ps (Run C). The “6-fold” glass, i.e.,
obtained originally from stishovite phase, is around 30 GPa. We have then studied the equation of state
and the structural properties of the glass by cold compression or decompression at 300 K at every 4 GPa
for 3 ps (Run D).


The averaged Si-O coordination of Run C is similar to that of Run A and presents a very similar
compression behavior as Run A above 20 GPa. The averaged Si-O coordination of Run D is similar to
that of Run A and exhibits a very similar compression behavior as Run A and C above 30 GPa. As
shown in Table S1, the glass originally from “penta” (Run C) does not exhibit a particular preference of
5-coordination and the glass obtained originally from stishovite (Run D) also presents a significant
amount of 5-coordination.


It has been suggested that high-pressure amorphous silica could be described as essentially ordered
eutectic arrays of oxygen with disordered silicon atoms [7]. Detailed structural analyses were performed
on the oxygen sublattice along the transformation of silica from the low pressure tetrahedral phase into
the high pressure phase [8]. The results show that silica glass under pressure cannot be described simply
by the arrangement of oxygen in the face-centered cubic (FCC), hexagonal close packed (HCP) or body
centered cubic (BCC) sublattice. Moreover, the oxygen ordering of the cannot be described as a
partially crystalline phase either [1,3,4].


Table S1. Percentage of Si atoms coordinated to oxygen of compressed silica glass at 30 GPa as
obtained by Run A (from “quartz”), Run C (from “penta”) and Run D (from “stishovite”). The cutoff
distance for the calculation of the coordination number was 2.1 Å.




                       4-coodination (%)      5-coodination (%)   6-coordination (%) 7-coodination (%)
Run A (30 GPa)                14.32                 51.41               34.27               0.000
Run C (28 GPa)                9.97                  48.96               39.88                1.19
Run C (32 GPa)                10.94                 38.43               47.09                3.54
Run D (30 GPa)                7.57                  32.70               59.72                0.00


                                                      2
Calculation of core-level x-ray absorption spectra
The O K-edge x-ray absorption spectra (XAS) of a-SiO2 were calculated assuming a supercell with the
full core hole approximation. The core O 1s (K) wavefunction was reconstructed from the projected
augmented wave (PAW) potential. All calculations were performed employing the Xspectra code [9]
distributed with the Quantum-Espresso package [10]. The generalized gradient approximation (GGA)
was used. The energy cutoffs for the plane wave and charge density expansion were 46 Ry. and 500 Ry.
respectively. The  point was used for electronic integration. This is found to be sufficient for the 72-
atom amorphous SiO2 model from the comparison of the calculated density of states with large k-point
meshes. To ascertain the accuracy of the core hole approximation, the O K XAS stishovite (Fig. S1) and
α-quartz (Fig. S2) were computed by solving the Bethe-Salpeter equation (BSE) using the Exciting code
[11]. Convergence with respect to k point and q point sampling were achieved with a 4×4×4 k mesh and
18 irreducible q points. 115 and 60 empty states for the screening and excitation calculations,
respectively. The calculated O K-edge XAS for stishovite and α-quartz by the core-hole approximation
(XPSECTRA) and BSE are in good agreement and also compare well with the respective experimental
spectra [12]. For completeness, the experimental and theoretical BSE O L-edge XAS of quartz are
compared in Fig. S3.




 Fig. S1. Comparison of the observed O K-XAS of stishovite with theoretical spectra calculated by the
                                       supercell and BSE method



                                                   3
Fig. S2. Comparison of the observed O K-XAS of α-quartz with theoretical spectra calculated by the
                                    supercell and BSE method




           Fig. S3. Comparison of observed [13,14] and theoretical BSE O L-edge XAS




                                                4
References
[1] M. Guthrie, C. A. Tulk, C. J. Benmore, J. Xu, J. L. Yarger, D. D. Klug, J. S. Tse, H.-K. Mao and R.
J. Hemley, Formation and structure of a dense octahedral glass. Phys. Rev. Lett. 93, 115502 (2004).
[2] M. Murakami and J. D. Bass, Spectroscopic evidence for ultrahigh-pressure polymorphism in SiO2
glass. Phys. Rev. Lett. 104, 025504 (2010).
[3]T. Sato and N. Funamori, High-pressure structural transformation of SiO2 glass upto 100 GPa. Phys.
Rev. B 82, 184102 (2010).
[4] T. Sato and N. Funamori, Sixfold-coordinated amorphous polymorph of SiO2 under high pressure.
Phys. Rev. Lett. 101, 255502 (2008).
[5] J. Badro, D. M. Teter, R. T. Downs, P. Gillet, R. J. Hemley and J.-L. Barrat, Theoretical study of a
five-coordinated silica polymorph. Phys. Rev. B 56, 5797 (1997).
[6] T. Demuth, Y. Jeanvoine, J. Hafner and J. G. Angyan, Polymorphism in silica studied in the local
density and generalized-gradient approximations. J. Phys.: Condens. Matter 11, 3833 (1999).
[7] A. M. Teter and R. J. Hemley, High pressure polymorphism in silica. Phys. Rev. Lett. 80, 2145
(1998).
[8] Y. Liang, C. R. Miranda and S. Scandolo, Tuning oxygen packing in silica by nonhydrostatic
pressure. Phys. Rev. Lett. 99, 215504 (2007).
[9] C. Goughoussis, M. Calandra, A.P. Settsonen and F. Mauri, First Principles calculations of x-ray
absorption in a scheme based on ultrasoft pseudopotenitals: From α-quartz to high Tc compounds, Phys.
Rev. B, 80, 075102 (2009).
[10] G. Giannozzi et.al., http://wwww.quantum-espresso.org.
[11] http://exciting-code.org/electronic-structure.
[12] J.A. Soininen, J.J. Rehr, and E.L. Shirley, Final-state rule vs the Bethe-Salpeter equation for deep-
core x-ray absorption spectra, Physica Scripta T115, 243 (2005).
[13] H. Fukui, M.N., Kanzaki,.N. Hiraokaand Y. Cai,. Coordination environment of silicon in silica
glass up to 74 GPa: an x-ray Raman scattering study at the silicon L edge. Phys. Rev. B 78, 012203
(2008).
[14] D. Li, G.M., Bancroft, M., Kasrai, M.E., Fleet,.R,A., Secco, X.H., Feng, K.H., Tan.and B.X. Yang,
X-ray-absorption spectroscopy of silicon dioxide (SiO2) polymorphs-The structural characterization of
opal. Am. Mineral. 79, 622 (1994)




                                                      5

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:5
posted:7/4/2012
language:English
pages:5