SUN Yanan ^1,2, ZHAO Xiaojun ², ZHOU Yanchun ¹

(1. Advanced Functional Composite Laboratory, Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China; 2. School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, China)

Abstract: With RE₂O₃︰Al₂O₃=2︰1, single phase RE₄Al₂O₉ (RE=La, Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu and Y) can be formed in the RE₂O₃-Al₂O₃ system. These rare earth aluminates have isomorphous crystal structures. They all have monoclinic structure with the space group of P2₁/c. In RE₄Al₂O₉, the Al atoms are coordinated by four O atoms, forming AlO₄ tetrahedra, while two of them share an apical O atom to form the Al₂O₇ group. For RE, three of them are 7-coordinated by O, forming RE-O decahedra, whereas the other one is 6-coordinated by O, forming RE-O octahedra. This article is aimed to summarize the progress in synthesis, crystal structures, properties and applications of RE₄Al₂O₉ in the areas of thermal/environmental barrier coatings, solid-state lasers, bioluminescence labeling and so on. The main methods to synthesize RE₄Al₂O₉ powders include solid state reaction, sol-gel, co-precipitation and combustion. The lattice parameters and atomic positions can be obtained by using X-ray diffraction (XRD) and Rietveld refinement. Meanwhile, elastic modulus, thermal conductivity, thermal expansion coefficient can be predicted through first-principles calculations. Among various RE₄Al₂O₉ aluminates, Y₄Al₂O₉ has been studied most comprehensively. Both theoretical prediction and experimental results demonstrate that Y₄Al₂O₉ has low thermal conductivity, high temperature stability, low Young's modulus and strong damage tolerance. Nd₄Al₂O₉, Gd₄Al₂O₉ and Tb₄Al₂O₉ are mainly studied in terms of their thermal and mechanical properties, while very few works have been carried out on Yb₄Al₂O₉, La₄Al₂O₉, Lu₄Al₂O₉ and Sm₄Al₂O₉. In addition, RE₄Al₂O₉ usually experiences reversible phase transitions with volume shrinkages during heating and cooling. However, details of the phase transition process are still remained to be clarified.
Key words: rare earth aluminates; crystal structure; first-principles calculations; mechanical properties; thermal properties