Thermodynamic and structural aspects of the polyamorphic transition in yttrium and other rare-earth aluminate liquids

A first-order transition between two liquids has been reported in yttrium–aluminate liquids close to Y3Al5O12 (YAG) composition. This transition is seen as the nucleation and growth of a low-density phase in a matrix of a higher density liquid when Y2O3–Al2O3 liquids are cooled below the liquidus. Both liquids are quenched to glass before the transition is complete. Analysis of the resulting composite samples shows that the two glasses are identical in composition although the two glasses differ in density by 4%. There are also mechanical differences between the two glasses; the low-density glass is more resistant to polishing, while the high-density glass becomes highly scratched. Following the initial observation, recent research has focused on establishing the thermodynamic and structural features of Y2O3–Al2O3 liquids which lead to liquid–liquid transitions. In addition, studies of other rare-earth aluminate compositions have been used to determine what, if anything makes the yttrium-bearing liquids exceptional. Differential scanning calorimetry (DSC) has been used to establish the onset of glass transition and, when combined with the heat capacity of crystalline phases, the jump in heat capacity at the point of glass transition. The DSC data enable the liquid fragility to be established and show the high-density liquid to be characterized by a non-Arrhenian viscosity–temperature relation. DSC studies of La–aluminates also show fragile liquid behavior, although liquid–liquid transitions have not been reported for these compositions. A combination of DSC and solution calorimetry has been used to evaluate the energetic differences between the two liquid phases of yttrium–aluminates and these data can also be used to estimate the changes in entropy. The entropy differences between the two phases also indicate differences in liquid rheology with the high- to low-temperature liquid transition also being a transition from a fragile to strong liquid. The structural differences between the two liquid polymorphs of Y2O3–Al2O3 are difficult to discern. The stable crystalline phase is garnet-structured YAG but this is difficult to nucleate and a metastable assemblage of a-Al2O3 and YAlO3 perovskite tend to crystallize. This has led to the suggestion that the high temperature liquid is dominated by octahedral aluminum, which is consistent with a higher density. Neutron diffraction studies of Y- and La–aluminate glasses show that the liquid structure is very different from that of the crystalline phases, the aluminate liquids are dominated by a tetrahedral aluminate framework and there is little change in the mean Al–O coordination number either at the liquid–liquid transition or when La(III) is substituted for Y(III). The main differences in structure are seen in the ordering of the rare earth coordination polyhedra and network topology, the La- and the low-density Y–aluminate glasses showing increased rare-earth oxygen distances and more mid-range order. Nuclear magnetic resonance studies also show little change in Al(III) coordination although there are differences in the degree of disorder.