A neutron diffraction study of yttrium- and lanthanum-aluminate glasses

Neutron diffraction data for three aluminate glasses have been collected for a range of scattering vector from 0.3 to 25 Å−1. A single phase glass, representing the high-density glass polymorph of yttrium-aluminate (AY25) composition has a structure resembling that of levitated YAG liquid. This glass has a first-neighbor Al–O distance of 1.82 Å and a first-shell coordination number of 4.16±0.21. The Y–O distance is represented by a single Gaussian peak centered at 2.28 Å which yields a first-neighbor coordination number of 6.64±0.33. These values indicate an increase in volume on melting of YAG solid solution. An equivalent lanthanum-aluminate glass (La25), also produced from the stable high-temperature aluminate liquid, has an Al–O distance of 1.79 Å and a coordination number of 4.55±0.23. The La–O distance is increased due to an increase in ionic radius for La(III). The magnitude of this La–O peak is lower than expected and there is evidence for a second La–O distance at 2.79 Å. The T(r) for the lanthanum-aluminate glass shows stronger features than the yttrium-aluminate glass at radial distances greater than 3 Å; these suggest a higher degree of order in the lanthanum-aluminate liquid. A two-phase yttrium-aluminate (AY20) glass, which represents a partly complete transition between a high-density liquid and a lower density polymorph, has an Al–O distance of 1.81 Å and a mean Al–O coordination number of 4.38±0.22. There is an increase in the Y–O radial distance on transition and also additional mid-range ordering. This mid-structure is similar to that seen in the La25 glass. The data show that rare earth aluminate liquids are dominated by a tetrahedral aluminate framework and that the liquid–liquid (transition) in the yttrium-aluminate liquids results from differences in packing among AlO4 and Y–O polyhedra and not from a change in the mean Al–O coordination number.