The Y2O3-Al2O3 phase diagram [11, 94, 95] has been studied extensively . Large single crystals of YAG required for laser hosts are grown from the liquid phase by the Czochralski technique and a good knowledge of the phase equilibria is required to avoid formation of phases less suited for laser and scintillation applications [11, 97, 98].
There are five crystalline compounds in the Y2O3-Al2O3 binary system (Fig. 4). The two end-members have well-known crystal structures. Y2O3 is cubic with two YO6 environments and a-Al2O3 is trigonal with one aluminum site of AlO6 and one oxygen site of OAl4. The crystalline compounds show two common aluminum coordination environments (IVAl and VIAl) and a range of Y-O units (YO6, YO7, and YO8). There is a large range of oxygen environments. The range in coordination environments for the three ions in the Y-O-Al system have been extensively studied by nuclear magnetic resonance spectroscopy (NMR) using the 27Al, 89Y, and 17O nuclei .
The garnet phase (YAG, composition Y3Al5O12) has a complicated crystal structure (Fig. 5) [17, 18, 99, 100]. The garnet phase is cubic, with 160 atoms per unit cell. There are two aluminum environments in YAG: one coordinated by four oxygen atoms (IVAl) and one coordinated by six oxygen atoms (VIAl). There is an unique yttrium site in YAG (YO8) and a single, distinctive oxygen environment (OY2Al2).
Fig. 4 The Y2O3-Al2O3 phase diagram [96-98]
Fig. 5 The crystal structure of garnet (YAG)
The perovskite phase (YAlO3) is orthorhombic and has a single type of VIAl-site (AlO6) and a single YO8 site. In contrast to the garnet phase, there are two oxygen sites: OY2Al2 and OY3Al2. The monoclininc phase, YAM, has two different AlO4 environments and four yttrium ions: two in YO6 and two in YO7. There are nine oxygen sites in YAM, four OY3Al, two OY2Al, two OY4, and one OY2Al2.
Thermodynamic data from solution calorimetry using molten lead borate [95, 101] for the perovskite and garnet phases have been combined with data for the YAM phase, and heat capacity data from adiabatic calorimetry and differential scanning calorimetry have been used to calculate the phase diagram for the binary system. This recent study shows unequivocally congruent melting of the perovskite phase and that it does not decompose to the YAM phase + liquid .
Studies of the liquid state of Y2O3-Al2O3 liquids close to YAG have revealed complex relations in the liquid state and the existence of a metastable eutectic at 23% Y2O3-77% Al2O3 [96, 98, 102] projected form the YAlO3 composition and from a-Al2O3. The melting temperature at this eutectic composition is 1,702°C, considerably lower than that of the melting point of YAG itself (1,940°C). The presence of this eutectic has important implications for the nucleation behavior of YAG. Under some conditions the Y2O3-Al2O3 liquids can be deeply undercooled and form a eutectic mixture of a-Al2O3 and YAG. This is a reflection of the difficulty in forming YAG nuclei , implying major differences in the local structure of the YAG-liquids and the YAG crystal phase. YAG will only form if composition are heated to a temperature of ~50°C or less above the liquid temperature [98, 104], if heated to higher temperature the liquid can be supercooled considerably below the liquid temperature and depending on conditions form a mixed ceramic or a glass (if under containerless conditions [105-107]).
Undercooled Y2O3-Al2O3 liquids are notable in that they show an unusual form of transition. This is a so-called polyamorphic transition, which is a transition from a high density to low density amorphous phase without a change in composition . This transition has been reproduced by several groups and the resultant samples comprise two glassy phases, which are amorphous. It is also to be noted that the composite samples do not consist of glass and nanocrystalline regions, as suggested by other groups .