Structural and spectroscopic characterization of Ba3RE2(Ge3O9)2 (RE = La, Pr, Nd) and Ba3La2-xEux (Ge3O9)2 cyclogermanates

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Abstract Ba3RE2(Ge3O9)2 (RЕ = La, Pr, Nd) and Ba3La2‑xEux(Ge3O9)2 germanates were prepared via solid-state reaction for the first time. The crystal structure of the compounds was identified using X-ray powder diffraction (XRPD). The obtained results indicate that all germanates are isostructural and crystallize in a monoclinic crystal system with the space group С2/c, Z = 4. The crystal lattice consists of two kinds of layers, formed from [Ge3O9]6- rings or from Ba2+/RE3+ cations. The density functional theory (DFT) calculations performed on a rich set of Ba3RE2(Ge3O9)2 compounds have approved a high thermodynamic stability of monoclinic modification and chair conformation of [Ge3O9]6- rings. The luminescence properties of Ba3La2‑xEux(Ge3O9)2 phosphors have been studied under 239 nm and 393 nm excitation. The spectra contain narrow lines in the orange-red spectral region, associated with the intraconfiguration f‒f transitions in Eu3+. The most intensive lines are located at 603 – 635 nm and are caused by 5D0 → 7F2 transition in Eu3+. The concentration dependence indicates an effective energy transfer due to dipole-dipole interaction. The Ba3La2‑xEux(Ge3O9)2 phosphors possess a high color purity and thermal stability of luminescence that allows one to predict that they may be successfully used in solid state lighting.