A DFT-Based Aspherical Ion Model for Sodium Aluminosilicate Glasses and Melts

Abstract

Na + ions play important roles on the physical and chemical properties of aluminosilicate glasses. It is known that they strongly modify the network of tetahedral SiO 4 and AlO 4 units, but the microscopic details on how they alter the local structure remain to be fully established. Here we address this issue by performing classical molecular dynamics simulations over a wide range of glasses compositions. The simulations include atomic polarization and deformation effects through the use of a DFT-parametrized aspherical ion model (AIM), which is carefully validated against experimental data (bond lengths, neutron and X-ray diffraction, and NMR spectroscopy). We show that the structure of the glasses is a subtle interplay between the nature of bridging/nonbridging oxides and the arrangement of Na + ions around them. This reflects, in particular, on the oxide instantaneous dipole moments. ■ INTRODUCTION Durability and mechanical properties of oxide glasses at ambient pressure have attracted much attention from the perspective of material chemistry. 1−11 In particular, in the case of aluminosilicates, the mechanical strength can be further improved through the ion-exchanging procedure on the glass surface, such as in the well-known Corning Gorilla glass. 12,13 Since aluminosilicates in the form of polycrystalline ceramics also show excellent optical property thanks to structural disorder, they are practically used as transparent materials in electrical devices. 14 The dense packing structure of oxide glasses also enables to seal radioactive ions for a long period. 15−18 A key component of functional glasses are alkali and alkaline-earth ions. Because of the weaker character of the bonds they form with oxide ions, in a pure silicate network, they generally create nonbridging oxygens and modify the tetrahedral network topology; they are acting as network modifiers. However, when coexisting with Al 3+ or B 3+ , they can also play the role of charge compensators and assist the formation of four-coordinated cations (tetrahedral AlO 4 or BO 4 units) and bridging oxygens. 11 As a consequence, they increase the packing fraction of the network structure. Due to the difference of strengths between the Si−O/Al−O/B−O bonds on the one hand and the Na−O bonds on the other hand, these oxide materials are often called iono-covalent glasses, even if most of the interactions are very well captured by purely ionic models. 19 Recently, structural and mechanical properties of several aluminosilicate glasses have been extensively investigated using experimental techniques 4,5,8,20,21 and computer simulations. 7,13,22 One of the main remaining challenges is to fully understand the effect of network modifiers on the glass properties. Thereby, a precise atomistic understanding of the nature of ionic bonding in aluminosilicate glasses is important both for fundamental studies and because of their widespread usage in material design. The most intuitive approaches for the experimental determination of atomic structures are X-ray and neutron diffraction, together with EXAFS measurements. 23−37 They are indeed useful techniques for obtaining first coordination shell bond lengths and numbers, although it remains difficult to extract information on the second coordination shell. For such a task, NMR spectroscopy has proven to be a very powerful tool that gives access to the short-and intermediate-range