来源：Science Direct
The use of rare earth compounds as ceramic coatings to protect engine turbines blades from chemical degradation has been widely studied in the last two decades. After interaction with CMAS (CaO-MgO-Al2O3-SiO2) type liquid silicates, these materials lead to the formation of silicate phases (e.g. apatite, cyclosilicate, garnet, etc.). Since 2022, the cyclosilicate Ca3RE2(Si3O9)2 structure is only proven with RE=Y, Nd, Sm, Gd, Dy and Yb. Special attention is given here to the case of an equimolar dual rare earth cyclosilicate. Indeed, its existence is proven here for the first time with two rare earth elements with very different ionic radii, i.e. Nd and Yb. High-purity Ca3NdYb(Si3O9)2 phase was prepared by solid state synthesis and characterized through complementary methods. Single rare earth cyclosilicates Ca3RE2(Si3O9)2 (RE = Nd, Gd, Y and Yb) and the dual Ca3NdGd(Si3O9)2 were prepared for comparison. Their compositions were determined thanks to EPMA. Refinement of X-ray and electron diffraction data provided structural characterization of the dual (Nd,Yb) phase and cation partitioning within the structure. TEM analysis was performed on a single crystal to verify the sample's crystallinity and homogeneity. Raman and FTIR spectroscopy analysis were proposed for the first time for all the phases. The cyclosilicate structure perfectly accommodates these two rare earth elements: localization, size, and occupancy of the three cationic sites are accurately determined. The cationic partitioning within the structure is linked to the cations’ ionic radius as the occupancy of Yb3+ decreases from M1 (CN=6) to M3 (CN=8), while Ca2+ follows the opposite trend.