Densely packed glass structure caused by seven-coordinated Zr in high elastic modulus Al2O3–SiO2–ZrO2 glasses

Al₂O₃–SiO₂–ZrO₂ ternary glasses were fabricated using a levitation technique. The addition of ZrO₂ to the Al₂O₃–SiO₂ glasses strongly affected their thermal, mechanical, and structural properties. Several compositions were partially vitrified at the laser-melted area without levitation although their melting required temperatures higher than 2000 °C. With increasing ZrO₂ content, the elastic moduli linearly increased, and the 50Al₂O₃–20SiO₂–30ZrO₂ glass exhibited a Young′s modulus of 166 GPa and Vickers hardness H_V of 10.6 GPa. Conversely, crack resistance significantly decreased with the addition of ZrO₂. Density measurements, Zr L_2,3-edge and K-edge X-ray absorption fine structure analyses, and ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance spectroscopy were performed to investigate the local structure around Zr, Al, and Si in the glasses. Zr formed distorted ZrO₇ as in monoclinic ZrO₂, which has been rarely found in conventional oxide glasses. The highly oxygen-coordinated Al atoms such as AlO₅ and AlO₆, were the main components in the glasses rather than AlO₄. The majority of Si atoms form SiO₄ with four bridging oxygen (Q⁴). Among the four bridging oxygens, the number of oxygens connected to Al or Zr clearly increased with decreasing SiO₂ content. The high packing density of the ternary glasses that resulted in high elastic moduli originated from the highly oxygen-coordinated Zr and Al and their close bonding with SiO₄ without generating nonbridging oxygens.