Importance of rings on the microscopic properties of a strong glass

Abstract

An optimized interparticle potential has been developed for its use in computer molecular dynamics simulations of vitreous boron trioxide ͑v-B 2 O 3 ) which includes four-particle interactions. This is required to force the formation of planar six-membered ͑boroxol͒ rings in such simulations. As a result, a better agreement with the static structure factor is achieved, even though the percentage of atoms taking part in such structures is relatively small. ͓S0163-1829͑96͒02934-7͔The presence of a substantial amount of regular structures in strong glasses such as vitreous silica or boron trioxide has been a matter of heated debate for a number of years.1 The presence of such structures was postulated as a means to explain the origin of sharp peaks in the Raman spectra on the basis of calculations involving a few particles, 2 or more refined approaches which still involved idealized models of the amorphous solid such as the Bethe lattice.3 However, attempts to relate the presence of such structures with discernible features in the diffraction patterns lead to some recent controversies. In fact, results from calculations involving short-range structures only which portray most atoms as being involved in boroxol rings ͑about eighty percent͒ are confronted with attempts to reconstruct the structure of the glassy solid by means of Reverse Monte Carlo approaches, 4 where the best results are obtained if the percentage of boroxol rings is bounded below thirty percent ͑i.e., even the absence of any of those structures gives a better fit to experiment than those assuming a fraction of fifty percent or above͒. In fact, attempts to calculate the structure of vitreous B 2 O 3 using relatively small three-dimensional clusters of atoms forming boroxol rings have often encountered considerable difficulties when trying to reproduce the correct macroscopic density. This contrasts with results from molecular dynamics ͑MD͒ simulations 5,6 which, while satisfactorily accounting for the macroscopic density, have evidenced difficulties in reproducing the presence of a large number of atoms involved in planar, regular rings. On the contrary, such simulation results have shown the presence of cyclic structures albeit severely deformed ͑i.e., nonplanar͒ and of varied ring sizes.A recurrent critique against MD results on the structure and dynamics of v-B 2 O 3 has always been the lack of such structures in the computed equilibrium structure 2 of the glass. Such a shortcoming was rationalized in terms of ͑a͒ the huge cooling rates employed in the computer quench from the equilibrium liquid structure or ͑b͒ the lack of a more realistic potential function which including four-body interactions stabilizes such planar structures.Although little can be done to overcome the first of the difficulties referred above since quench rates comparable with experiment would imply vast resources of computer time, we set our endeavors to explore the feasibility of supplementing the potential function described in Refs. 6 and 7 with a four-body term to provi...

Molecular dynamics studies of vitreous boron oxide

“…Wright and Sinclaire [5], Bray [6] and Zwanziger [7], found that 80% of boron atoms are included in the boroxol rings. Most molecular modeling studies [8,9] of B 2 O 3 came to different conclusions but generally the models could not reproduce the abundance of structural units building the borate network. Some improvements in the modeling of the borate network are the modifications introduced in the MD interactions by Cormack [10] leading to a relatively high fraction of 15-30% of boron in boroxol rings.…”

Fraction of boroxol rings in vitreous boron trioxide

“…Although it is generally accepted that the molecular building block of vitreous B 2 O 3 is the planar BO 3 group, the manner in which the BO 3 triangles are connected has long been a matter of controversy. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The most widely accepted model for the structure of vitreous B 2 O 3 was for many years a random network of corner-linked triangles as suggested by Zachariasen. 9 Later, Krogh-Moe combined the results of many spectroscopic investigations and came to the conclusion that the BO 3 triangles prefer to form planar B 3 O 6 rings ͑see Fig.…”

“…[16][17][18] However, other interpretations of diffraction data have come to different conclusions. [19][20][21][22][23][24][25][26] The radial distribution functions for simple models containing a mixture of boroxol rings and BO 3 triangles with a variable fraction of boron atoms in boroxol rings were computed by Johnson et al 16 and later by Hannon et al 17 In the work of Johnson et al the results of the models were compared with the experimental real-space correlation function T(r) obtained from neutron diffraction in the range of 0-3 Å. The best agreement was obtained for a model containing 60% of the boron atoms in boroxol rings.…”

“…However, at least the earlier results have been much questioned, mainly because of the unrealistic bond angle distribution and the inability to reproduce experimental dynamical results and the measured density. It should be noted that recent ab initio MD simulations 23,24 and ordinary MD simulations using three-or four-body potentials 25,26 overcome most of these problems without introducing any appreciable amount of boroxol rings.…”

Migration of Atomic and Molecular Hydrogen in SiO2: A Molecular Dynamics Study

“…In fact a disordered network may be characterized in term of its ring distribution by shortest-path analysis [15]. Ring statistics has become the generally accepted measure of medium-range order in amorphous materials and it has been used to explain some physical properties of a-Si0 2 [16] as well as others common amorphous materials [17]. For Si0 2 a ring is defined as a closed path which is given by a sequence of O-Si-O atoms belonging the same tetrahedral unit (Si0 4 ).…”