Swapping sodium for magnesium makes glass harder and more heat-resistant

<p>Physical-property alterations were probed in three series of aluminosilicate (AS) glasses with 𝑥MgO-(25-𝑥)Na₂O-(20 + 𝑦)Al₂O₃-(55 - 𝑦)SiO₂ stoichiometries (in mol%), where  0 ⩽ 𝑥 ⩽ 25 and 𝑦 = {0, 5, 10}. The members of each glass branch feature progressively increasing MgO-for-Na₂O substitutions but share a fixed Al₂O₃ content of 20, 25, or 30 mol% along with a constant net amount of 25 mol% of network-modifier oxides. We established composition-structure-property relationships for several measured physical properties, encompassing the nanohardness (𝐻), reduced elastic modulus (𝐸_r), crack resistance (CR), glasstransition temperature (𝑇_g), and the density-related molar volume (𝑉_m) and glass compactness (𝐶; atom-packingefficiency). All mechanical properties increase for an increasing number density of O (𝜌^𝑁_O ) and/or the ²⁷AlNMR-derived average coordination number of Al (𝑍_Al), where formation of high Al-coordination AlO₅ and AlO6 polyhedra boost 𝑍_Al that accompanies the replacements of Na₂O by MgO. Both the hardness and reducedelastic modulus increase linearly with each of 𝑍_Al and 𝜌^𝑁_O . A key finding is that the observed mechanical-property enhancements for increasing MgO-for-Na₂O substitution do not stem directly from the Mg²⁺ cationsper se, but rather from the concurrently increased number of strong bonds forming between O and Al of thenetwork-forming AlO₅ and AlO₆ polyhedra, along with the increased O number density accompanying each 2Na⁺ → 1Mg²⁺ replacement. We also discuss property–composition trends in other AS glass systems subjectto systematic oxide substitutions, encompassing Al₂O₃-for-SiO₂ replacements.</p>