The amorphous aluminium silicate (Al 2 O 3 )·2(SiO 2 ) (abbreviated as AS2) is investigated using a molecular dynamics simulation with the Born-Mayer potential. The models of amorphous AS2 are constructed in a wide pressure range. The results show that the structure of AS2 mainly consists of the basic structural units TO x (T is Al or Si; x = 4, 5, 6). The topology of basic structural units at different pressures is identical. Two adjacent units TO x are linked to each other through common oxygen atoms and form a continuous random network of basic structural units TO x . The different aluminium silicate states result from the difference of the fraction of units TO x and their spatial distributions. The coordination units (triclusters) OAl 3 and OSi 3 and (tetraclusters) OAl 4 and OSi 4 result in AlO x -rich and SiO x -rich regions, and this is the origin of the microphase separation. Regarding the polymorphism, it can be seen that the structure of AS2 comprises three structural phases: TO 4 , TO 5 , and TO 6 structural phases. The size of TO 4 structural phase regions decreases and the size of TO 6 structural phase regions increases as pressure increases. Inversely, the size of TO 5 structural phase regions increases to a maximum value and then decreases as pressure increases. In the considered pressure range, with increasing pressure, there is a transformation from TO 4 structural phase (at low pressure) to TO 6 structural phase (at high pressure). PACS Nos.: 61.43.Bn, 61.43.Fs. Résumé : Nous étudions le silicate d'aluminium amorphe (Al2O3)·2(SiO2) (abréviation AS2), en utilisant une simulation de dynamique moléculaire avec le potentiel de Born-Mayer. La modélisation est faite pour un large domaine de pression. Les résultats indiquent que la structure du AS2 est principalement constituée des unités structurelle de base TO x (T est Al ou Si et x = 4, 5, 6). La topologie des unités structurelles de base aux différentes pressions est identique. Deux unités adjacente de TO x sont liées ensemble via des atomes d'oxygène communs et forment un réseau aléatoire d'unités structurelles de base TO x . Les différents états du silicate d'aluminium résultent de la différence des fractions d'unités TO x et de leur distribution spatiale. Les unités de coordination triple amas OAl 3 , OSi 3 et quadruple amas OAl 4 , OSi 4 résultent en régions riches en AlO x et SiO x et ceci est à l'origine de la séparation en micro-phases.En ce qui concerne le polymorphisme, on constate que la structure de AS2 comprend trois phases structurelles : TO 4 , TO 5 et TO 6 . Lorsque la pression augmente, la grandeur des régions de phase structurelle TO 4 décroit, alors que la grandeur des régions TO 6 augmente. Inversement, la grandeur des régions de phase structurelle TO 5 augmente d'abord avec la pression jusqu'à un maximum, pour diminuer ensuite si la pression continue d'augmenter. Dans le domaine de pression considéré ici, il y a une transformation de la phase structurelle TO 4 (à basse pression) vers la phase structurelle TO 6 (à haute pression)...

Molecular dynamics (MD) simulations and visualizations were explored to investigate the changes in structure of liquid aluminosilicates. The models were constructed for four compositions with varying Al2O3/SiO2 ratio. The local structure and network topology was analyzed through the pair of radial distribution functions, bond angle, bond length and coordination number distributions. The results showed that the structure of aluminosilicates mainly consists of the basic structural units TO[Formula: see text] (T is Al or Si; y = 3, 4, 5). Two adjacent units TO[Formula: see text] are linked to each other through common oxygen atoms and form continuous random network of basic structural units TO[Formula: see text]. The bond statistics (corner-, edge- and face- sharing) between two adjacent TO[Formula: see text] units are investigated in detail. The self-diffusion coefficients for three atomic types are affected by the degree of polymerization (DOP) of network characterized by the proportions of nonbridging oxygen (NBO) and Q[Formula: see text] species in the system. It was found that Q4 and Q3 tetrahedral species (tetrahedron with four and three bridging oxygens, respectively) decreases, while Q0 (with four nonbridging oxygen) increase with increasing Al2O3/SiO2 molar ratio, suggesting that a less polymerized network was formed. The structural and dynamical heterogeneities, micro-phase separation and liquid–liquid phase transition are also discussed in this work.

No abstract

No abstract

No abstract