Atomistic Simulations of Liquid and Vitreous

From the experimental point of view, an overwhelming body of evidences for borox-ols has been built over the years and there is little, if any, room for controversy. However, a resurgence of criticisms appeared in the early eighties with the emergence of atomistic simulations. 

Molecular dynamics (MD) simulations of V-B2O3 using empirical 2-body potentials were pioneered by Soules [90-92] and Amini et al. [93], followed later by others [62, 94-96]. In all cases, a random network of BO3 triangles without any boroxols was obtained. The authors thus challenged the boroxol model though the simulations were at the most confronted either to neutron diffraction only [93] or to limited X-ray diffraction results [96]. At least in one case were the limitations in the simulations clearly pointed out [91], namely the short simulation times (10 ps) and the absence of directional or covalency forces in the interatomic potentials. 

Still with a potential of 3-body type, the first MD simulation for which the presence of boroxols was reported is actually that by Inoue et al. [97]. The predicted B-O-B bond angles distribution (peaked at 120 ) is in much better agreement with the experimental value ( 130 ), derived indirectly from X-ray scattering [51] and from NMR [39], than in the previously mentioned MD [62, 91, 93, 99] ( 150-160°). The 

Cormack used a coordination-dependent potential supplemented by three-body terms [105, 112], slightly modified from Takada s potential. A value of / 15 % was obtained. A discrepancy with the experimental total radial distribution function in the region around 3.7 A was noted and seen as an indication of a too small value of boroxol rings in the simulations. The origin was ascribed to finite-size effects (systems of 1010 atoms at the most were used) and possibly to the glass-forming process [105]. 

Using a potential with three- and four-body terms, Kashchievaet al. [107] intended to explore the effects of cooling rates and of system sizes. They reported results for 10 systems from 100 to 2000 atoms and used, for each of them, two cooling rates differing by 2 orders of magnitude, respectively 3.3 x 10 and 3.3 x lO Kps , between 1300 and 300 K. With the fastest cooling rate, a low value / 10-15 % 

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