New phase within the SrO–RЕ2O3–GeO2 (RЕ = Dy–Lu) systems: Synthesis and quantum-chemical modeling

The phase formation of Sr3RЕ2(Ge3O9)2 (RE = Dy, Ho, Er, Tm, Yb, Lu) cyclogermanates was studied at 1000 and 1100 °C in air. It was shown that these compounds did not exist under these conditions, in contrast to cyclogermanates with large RE3+ ions, such as La, Pr, Nd, Sm, Eu and Gd. Using the thulium-based system SrO–Tm2O3–GeO2 as an example, it was established that compounds with Sr/RE atomic ratio other than 3/2 were formed instead of the expected Sr3RЕ2(Ge3O9)2. Single phase Sr4TmGe5.667O16.834 was prepared and characterized by X-ray powder diffraction. The quantum-chemical calculations confirmed the thermodynamic instability of strontium germanates Sr3RЕ2(Ge3O9)2 with RE3+ cations smaller than Gd3+. It is suggested that the prototypic structure of mixed strontium and Dy, Ho, Er, Tm, Yb and Lu rare earth germanates stems from the crystal structure of walstromite BaCa2Si3O9.