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This work advances the understanding of the influence of rare earth (RE) ion radius on the stability and extent of the garnet solid solution phase in the (ytterbia/yttria/gadolinia)‐calcia‐magnesia‐alumina‐silica systems. Guided by the crystal chemistry and charge neutrality constraints, selected compositions in the notional garnet stability field were synthesized, equilibrated at 1400°C, and characterized to determine the equilibrium phases and their compositions. The results show a significant reduction in the stability of the silicate garnet relative to apatite with increasing RE ion radius. Apatite was not observed for any composition in the Yb‐containing system, the Y‐containing system formed both garnet and apatite, and there was no evidence of silicate garnets in the Gd‐containing system. However, despite the apparent differences in stability relative to apatite, the extent of the garnet solid solution increases only slightly for the Yb‐ compared to Y‐containing systems. The quantitative microchemical analysis suggests that Mg2+ prefers the octahedral site over the dodecahedral site in the garnet structure, and that the solubility of Mg2+ in the dodecahedral site increased in the system containing Yb3+ compared to Y3+. The results are discussed for their relevance to reactions between RE‐containing thermal and environmental barrier coatings and CMAS‐type silicate deposits.

Mixtures of rare‐earth zirconates and aluminates containing Y or Y + Gd that form a two‐phase garnet–fluorite mixture exhibit much slower sintering than pure fluorite at 1400°C. An equivalent Y‐free, Gd‐containing composition that forms a perovskite aluminate instead of garnet showed faster densification after the metastable garnet decomposes. At 1500°C, the Y‐free sample also showed the fastest initial sintering rate, whereas there was more divergence in the sintering rate for the samples containing Y + Gd. The zirconate–aluminate with equimolar Y + Gd shows the slowest densification at 1500°C and retains ∼25% porosity after 250 h. The results highlight possibilities for designing compliant thermal barrier coatings that can retain significant porosity at 1400°C or higher.

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