Crystalline (corundum) has each ion coordinated by six ions in an octahedron
PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Crystal structure, bonding and diffraction · Crystal structure, bonding and diffraction
Solid State Physics, Ch Solids without a lattice: glasses and quasicrystals
Problem
Crystalline Al2O3 (corundum) has each Al3+ ion coordinated by six O2− ions in an octahedron, and neighbouring octahedra share edges as well as corners. Quartz and vitreous SiO2, by contrast, have each Si4+ coordinated by four O2− ions in a tetrahedron, and neighbouring tetrahedra share corners only, with every bridging oxygen bonded to exactly two silicons.
((a)) State Zachariasen's rules for a glass-forming oxide network, and give the physical reason behind each one.
((b)) Identify which rule, or rules, a network built from these Al2O3 octahedra violates, and predict whether it will form a glass at an ordinary furnace cooling rate of order 1Ks−1.
((c)) Vitreloy, a zirconium-based metallic glass with no oxide network at all, also vitrifies at a cooling rate of about 1Ks−1. Explain briefly why Zachariasen's rules cannot apply to that case, and what plays their role instead.