Glasses and glassy crystals from molecular and molecular ionic systems

Some general principles underlying the possibility of vitrifying a randomly chosen molecular substance are reviewed by way of explaining the Tb/Tm > 2 rule (> 1.7 for homogeneously nucleating systems), and a comparable rule, Tb/Tq > 2.5 is shown to have predictive value for plastic crystals. Hydrogen bonded substances provide exceptions in both liquids and plastic crystal cases. Glassforming ability is traced to a crystalline state packing problem. New data on molecular, and molecular ionic isomeric families are used to illustrate the distinction between vitrifying and non-vitrifying liquids, and compounds which form glassy crystal as opposed to ordinary glassy phases.