Garnet phases

Table 5.16 Elements present at major, minor, and trace levels in the garnet phase.

Table 5.18 Chemistry and terminology of main components of the garnet phase.

On the basis of these principles, Novak and Gibbs (1971) proposed a map of compatible and ry dimensions for the formation of a garnet phase. This map (figure 5.15) shows that the commonly observed components plot in the low-volume zone of the compatibility area. Theoretically, compounds with higher volumes may form and remain stable at appropriate P and T conditions. 

Table 5.22 Bulk modulus and thermal expansion for various compositional terms of the garnet phase. The experimental range for bulk moduli is relative to a common value of K = 4. The calculated values are from Ottonello et al. (1996). References for thermal expansion are as follows (1) Ottonello et al. (1996) (2) Skinner (1956) (3) Suzuki and Anderson (1983) (4) Isaak et al. (1992) (5) Armbruster and Geiger (1993). ...

By applying the ionic solution model, we may express the relationship between ionic fraction and activity in the garnet phase as... 

For auother possible reactiou mechauism for the formatiou of mixed oxides, binary alkoxide (glycoxide) may be formed prior to the formation of the mixed oxide. In fact, various binary glycoxides have been prepared and their crystal structures have been elucidated. Formation of BaTiOj by the glycothermal reaction may be explained by this mechanism, because Ba-Ti binary alkoxide is well established. However, formation of garnet phases (Sections III.C.l through III.C.3) cannot be explained by this mechanism because addition of seed crystals, in some cases, gives a product with a chemical composition completely different from that of the product obtained without the addition of the seed crystals. 

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 [95],... 

Forsterite itself can be altered by V2O5 to form a garnet phase that exhibits liquid formation. 

Shaula, AX., Kharton, V.V, and Marques, FA4J5. (2004) Mixed conductivity of garnet phases based on gadolinium ferrite. /. Eur. Ceram. Soc., 24 1309 1312. 

Figure 3.23 The complex structure of the garnet phase Ca3Al2Si30i2 contains AlOg and Si04 units shown as octahedra and tetrahedra respectively. The relatively large Ca " cations are shown as large black spheres ...

In the case of alumina scales formed on alumina-forming materials, the presence of the garnet phases is explained by the diffusion of RE from the underlying metal to the oxide surface via the oxide grain boundaries. This theory, called dynamic segregation , has been proposed by Pint and is based on ihe oxygen potential gradient across the oxide scale [128], The consequences of the diffusion of the RE towards the gas-oxide interface are, on the one hand, to suppress the diffusion of metal ions, and on the other hand, to form a mixed Al-RE oxide at the oxide surface. 

---