text: Yttria-alumina melts are another system reported to exhibit polyamorphism. Observation of a liquid–liquid phase transition in the supercooled liquid has been reported. Though this is disputed in the literature. Polyamorphism has also been reported in Yttria-Alumina glasses. Yttria-Alumina melts quenched from about 1900 °C at a rate ~400 °C/s, can form glasses containing a second co-existing phase. This happens for certain Y/Al ratios (about 20–40 mol% YO). The two phases have the same average composition but different density, molecular structure and hardness. However whether the second phase is glassy or crystalline is also debated.
  Continuous changes in density were observed upon cooling silicon dioxide or germanium dioxide. Although continuous density changes do not constitute a first order transition, they may be indicative of an underlying abrupt transition.