58 Glass Forming at Limit: How Glass Forms when there is Insufficient Network Former ? Vitreous forsterite (v-Mg 2 SiO 4 ) is an important “prototypical” orthosilicate in the earth’s magmas in the upper mantle and a model system for non- equilibrium low-silica-content binary melts. Experimental studies remain difficult due to the high melting point (~2150 K) and instability of forming a bulk glass. The low (33.33 mole%) SiO 2 content in v-Mg 2 SiO 4 effectively prevents the formation of an extended SiO 4 -based network, hence an interpretation based on the model structure of conventional silicate glasses. v-Mg 2 SiO 4 can be considered as an extremely fragile glass as opposed to the strong characteristic of SiO 2 -rich silicate network glasses [1]. Information regarding the structure of v-Mg 2 SiO 4 is important for the development of refractory and laser host materials, formation of interplanetary and interstellar dust, prediction of thermodynamical properties of olivine minerals, and advancement of phenomenological theory of glass formation. Experiments using the shock pressurization of olivine or the splat quenching of forsterite resulted in samples that contained a few percent glass. Only until recently Mg 2 SiO 4 -composition glasses prepared by containerless melting are available for structural characterization. In the present work, combined neutron and high-energy X-ray diffraction data obtained at beamline BL04B2 and reverse Monte Carlo (RMC) modelling [2] were applied to the study of a bulk Mg 2 SiO 4 -composition glass formed by containerless processing of small droplets of a molten precursor [1]. The measured neutron and X-ray structure factors, S(Q) , [3] of bulk Mg 2 SiO 4 -composition glasses are shown in Fig. 1. The higher Q position (~2.2 Å -1 ) of the first sharp diffraction peak than those in vitreous silica and typical non-orthosilicate glasses and melts (below 1.8 Å -1 ) [4] indicates that v-Mg 2 SiO 4 has a unique short-range order structure. The derived total correlation functions, T(r) , are shown in Fig. 2. The well-resolved peak profiles corresponding to the Si-O and Mg-O nearest neighbors reveal details of the short-range order structure. The Si-O peak is sharp and symmetric and the Si-O coordination number, N Si-O , is 4.05 ± 0.10, which indicates the presence of 4-oxygen coordinated silicon atoms as relatively regular SiO 4 tetrahedra typically found in silicate S (Q) 0.0 0 .5 1.0 0.0 0.5 1.0 1.5 10 0 0 0 15 20 25 5 Q (Å -1 ) Fig. 1. Total structure factors S ( Q ) of v-Mg 2 SiO 4 (blue: experimental data; red: RMC simulation). 0 5 10 0 5 10 1 2 3 4 5 r ( Å ) T ( r ) (a) (b) (a) (b) Fig. 2. Total correlation functions T ( r ) of c-Mg 2 SiO 4 (a) and v-Mg 2 SiO 4 (b) . The curves of the crystal were calculated using the PDFFIT computer code according to the forsterite crystal structure. For clarity, they are displaced upward by 4 units from the baseline. The unphysical oscillations below 1.5 Å arose mainly from the Fourier transformation of truncated data at a finite Q . 59 Shinji Kohara a, *, Kentaro Suzuya b and Ken Takeuchi c (a) SPring-8 / JASRI (b) Neutron Science Research Center, Japan Atomic Energy Research Institute (JAERI) (c) Faculty of Industrial Science and Technology, Tokyo University of Science *E-mail: kohara@spring8.or.jp References [1] J.A. Tangeman et al. : Geophys. Res. Lett. 28 (2001) 2517. [2] R. L. McGreevy: J. Phys Condens. Matter. 13 (2001) R877. [3] S. Kohara, K. Suzuya, K. Takeuchi, C.-K. Loong, M. Grimisditch, J.K. Weber, J.A. Tangeman, T.S. Key: Science 303 (2004) 1649. [4] S. Kohara and K. Suzuya: Nucl. Instrum. Meth. B 199 (2003) 23. [5] F.J. Molster et al. : Nature 401 (1999) 563. glasses. The Mg-O peak in T(r), on the other hand, is broad and skewed towards the high- r side, indicating a distribution of Mg-O distances from 1.8 to 2.5 Å due to highly distorted MgO x polyhedra. The average N Mg-O is 4.95 ± 0.20. This Mg-O peak differs markedly from that of crystalline forsterite, c-Mg 2 SiO 4 , (Fig. 2). The shift of the Mg-O peak towards smaller distances in v- Mg 2 SiO 4 clearly implies a general relaxation of the MgO 6 octahedra of the crystalline state to include short Mg-O bonds corresponding to MgO 4 and MgO 5 local coordinations. The secondary shoulder near the nominal Mg-O bond length of crystalline MgO 6 octahedra and the tail at larger distances indicate an extensive distortion of polyhedra that include long Mg-O bonds. To obtain a 3D representation of the glassy structure, particularly the “global” connectivity of SiO 4 and MgO x polyhedra, we supplemented the measured data with computer simulation of diffraction data using the reverse Monte Carlo (RMC) technique. The converged configuration of the ensemble produced the calculated neutron and X-ray total structure factors S(Q) that agree well with the experimental data (see Fig. 1). Figure 3(a) shows a distribution of relatively regular SiO 4 tetrahedra, about 33% of which are isolated units. The remaining SiO 4 tetrahedra are connected to form corner-sharing dimers. There are almost no SiO 4 chains or cross-linked networks, in sharp contrast to the extensive 3D SiO 4 networking features in vitreous silica or in many silica-rich binary glasses. The Mg-O atoms, on the other hand, show a well-connected assembly featuring both edge- and corner-sharing MgO x polyhedra (Fig. 3(b)). We found the presence of MgO x polyhedra of x = 4, 5 and 6 corresponding to approximately the ratio of 3:5:2 according to the RMC snapshot representing the best fit of the data. Figure 3(c) shows a typical network element of MgO x polyhedra. The MgO 4 units are relatively uniform tetrahedra whereas the MgO 5 and MgO 6 units have long mean bond lengths and contain one or two long bonds (observed as the shoulder and tail in Fig. 2). Our findings concerning the glassy structure at the limit of insufficient network former have broad implications in the use of unconventional network former/modifier ratio in glass materials that may form new environments for dopant ions and offer potentials for tailored optical and electronic properties. In addition, the knowledge of the structure of glassy forsterite will aid in understanding the present and past magmatic processes in the earth’s and moon’s mantle. The present study of an unusual glass prepared by the containerless method may provide insights into the highly non-equilibrium material processing that occurs in interplanetary and interstellar environments [5]. Fig. 3 (a) Configuration of SiO 4 tetrahedra obtained from the RMC snapshot. About 33% of tetrahedral are unconnected (purple). The remaining SiO 4 units are joined by an oxygen atom to form dimers (blue). (b) Network configuration of MgO x polyhedra, x = 4 (red), 5 (yellow) and 6 (blue). The majority are MgO 5 units. (c) Typical network element containing MgO 4 , MgO 5 , and MgO 6 units obtained from the RMC snapshot. (a) (b) 2.35Å 1.97Å 2.21Å 1.93Å 2.33Å 2.08 Å 2.32 Å 1.99 Å 2.24Å 2.01 Å 2.17 Å 2.56 Å 1.95 Å 2.03 Å 2.39 Å : 6-coordinatedMg, : O : 4-coordinatedMg, : 5-coordinatedMg (c)
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