Rare Earth Aluminates
Rare earth aluminates are extremely important as laser host materials. Most interest is in the system Y2O3-Al2O3, and of the three crystalline phases that are important in this system, the garnet phase (YAG, Y3Al5O12) is the most important laser host. Laser hosts require highly transparent single crystals, and crystal growth studies of YAG were preformed at various laboratories in the 1950s and 1960s. YAG is optically isotropic and transparent from 300 nm to 4 pm and can accept trivalent laser activator ions. In 1964, the Bell Telephone Laboratories reported the lasing of YAG doped with Nd3+ . Single crystals of YAG can be polished to form durable optical components of uniform refractive index and good thermal conductivity. Because of their robustness, YAG-based lasers have a wide variety of military, industrial, and medical applications.
Although Nd:YAG requires large, defect-free single crystals , polycrystalline ceramics are cheaper to manufacture and there is increasing use of YAG ceramics  as scintillators for radiation detection, for example Ce-doped YAG ceramics [91, 92]. In this case, the luminescence comes both from the activation of the Ce ion, with additional UV contribution, and from the YAG host itself.
Polycrystalline rare earth aluminates can also be used as advanced ceramic materials because of their refractory nature and chemical and mechanical durability. Y2O3-Al2O3 coatings are used in crystalline fibers, and Eu-doped Y-Al powders are used for phosphors and scintillation applications. Yttria and alumina are also used as additives for liquid phase synthesis of silicon nitride ceramics, often forming glassy coatings to the nitride phase and therefore having an important role in forming nitride-glass composites.
Because of the applications in laser and other optical devices, the Y2O3-Al2O3 system has been particularly well studied. There are three important crystal phases in theY2O3-Al2O3 system: in addition to the cubic garnet phase, YAG (Y3Al5O12), there is a perovskite phase (YAlO3) and a monoclinic phase (Y4Al2O9). Not all rare earth aluminate binaries have a stable garnet phase. Not all rare earth aluminate systems have a stable garnet phase. The stability of the garnet phase depends on the identity of the rare earth ion , for larger ionic radii (e. g., La(III) ), the garnet phase is not stable and only pervoskites are present.