Impact of ZnO on the structure and properties of sodium aluminosilicate glasses: Comparison with alkaline earth oxides Morten M. Smedskjaer a , Randall E. Youngman b , John C. Mauro b, ⁎ a Section of Chemistry, Aalborg University, DK-9000 Aalborg, Denmark b Science and Technology Division, Corning Incorporated, Corning, NY 14831, USA abstract article info Article history: Received 16 July 2013 Received in revised form 15 September 2013 Available online 13 October 2013 Keywords: Zinc oxide; Aluminosilicate; Properties; Structure The structural role of ZnO in aluminosilicate glasses is not yet well understood. However, this understanding is requisite for explaining the impact of ZnO on the macroscopic properties of these glasses. In this work, we present a quantitative analysis of the impact of ZnO on the structure and properties of sodium aluminosilicate glass. The properties under study include density, molar volume, refractive index, coefficient of thermal expansion, elastic moduli, isokom temperatures, and liquid fragility. The impact of ZnO is compared with that of four alkaline earth oxides (MgO, CaO, SrO, and BaO) in the same family of aluminosilicate glasses. Our study is performed on a series of glasses with varying Si/Al ratios to quantify the impact of each divalent cation oxide on aluminum speciation and the resulting impact on macroscopic properties. Our results reveal pronounced changes in the scaling of most measured properties around the compositions with [Al 2 O 3 ] = [Na 2 O], particularly for the glass series with MgO or ZnO. Based on 27 Al MAS and 3QMAS NMR experiments at high field, the structural origin of this change is ascribed to the change in aluminum speciation. We thus demonstrate that the impact of Zn 2+ on aluminosilicate glass structure and properties is similar to that of Mg 2+ , following their similarity in ionic radius, despite their differences in electronic structure. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The addition of ZnO to silicate and borosilicate glasses improves the chemical durability and mechanical properties [1], while its addition to phosphate glasses results in lower softening point [2]. Moreover, ZnO acts as a nucleating agent when present at low concentrations in both alkali silicate and aluminosilicate glasses [3]. These findings have been explained based on the structural role of Zn 2+ in the glass network, where it has been reported to adopt either tetrahedral or octahedral configuration [4]. The former tetrahedral configuration is typical of a glass network-former, whereas the latter octahedral configuration is typical of a network-modifier. Thus, ZnO can be classified as an interme- diate oxide [5]. In alkali-containing silicate glasses, there is a preference for Zn 2+ to be in tetrahedral sites, with the alkali cation (e.g., Na + ) compensating for the charge deficit of the ZnO 4 tetrahedra [3]. These ZnO 4 tetrahedra are corner-sharing with the network-forming SiO 4 tetrahedra and thus stabilize the glass structure [6]. With increasing concentration of ZnO, the proportion of tetrahedral zinc decreases and that of octahedral zinc increases. In the total absence of alkali cations, e.g., in alkaline earth silicate glasses, Zn 2+ is reported to have a nucleating role and occur in octahedral configuration [3]. In alkali aluminosilicate and alkali borosilicate glasses, Zn 2+ in four-fold coordination competes for charge-compensation by alkali cations with tetrahedral boron and tetrahedral aluminum species [6]. The presence of ZnO 4 tetrahedra therefore decreases the effective modifier cation concentration. The structural role of Zn 2+ in silicate glasses thus appears to be somewhat different from that of the alkaline earth cations (Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) despite the equality of charge and similarity of ionic radius to Mg 2+ in particular. The ionic radius of Zn 2+ in octahedral configuration is 0.74 Å, while those of Mg 2+ and Ca 2+ are 0.72 and 1.00 Å, respectively [7]. The coordination number of Ca 2+ is believed to be around six to seven [8], that of Sr 2+ to be around seven [9], and that of Ba 2+ to be around eight [10]. The coordination number of Mg 2+ is smaller, with reports of four-, five-, and six-fold coordination [11–13]. An important variable controlling the network speciation of alkaline earth containing silicate glasses is the Dietzel's field strength of the alkaline earth cations [14–17]. Field strength is defined as the ratio of the formal valence to the square of the cation–oxygen bond dis- tance [14]. Generally, alkaline earth cations with higher field strength (such as Mg 2+ ) promote formation of five-fold coordinated aluminum (Al V ) and three-fold coordinated boron in aluminosilicate and boroaluminosilicate glasses [10,18–26]. According to its field strength, the role of Zn 2+ in the silicate glass network should be between that of Mg 2+ and Ca 2+ . The majority of the prior work regarding the role of ZnO in silicate glasses has focused on characterizing the local atomic structure around Journal of Non-Crystalline Solids 381 (2013) 58–64 ⁎ Corresponding author. E-mail address: mauroj@corning.com (J.C. Mauro). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.09.019 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol