Structural and spectroscopic characterization of a new series of Ba2RE2Ge4O13 (RE = Pr, Nd, Gd, and Dy) and Ba2Gd2−xEuxGe4O13 tetragermanates

Abstract

A new series of Ba₂RE₂Ge₄O₁₃ (RE = Pr, Nd, Gd, Dy) germanates and Ba₂Gd_2−xEu_xGe₄O₁₃ (x = 0.1–0.8) solid solutions have been synthesized using the solid-state reaction technique and characterized by X-ray powder diffraction. All compounds crystallize in the monoclinic system, space group C2/c, Z = 4. The crystal lattice consists of RE₂O₁₂ dimers, zigzag C₂-symmetric [Ge₄O₁₃]¹⁰⁻ tetramers, and ten-coordinated Ba atoms located in voids between polyhedra. The density-functional theory (DFT) calculations performed on a rich set of Ba₂RE₂Ge₄O₁₃ compounds have confirmed the high thermodynamic stability of monoclinic modification. Under ultraviolet (UV) light excitation Ba₂Gd_2−xEu_xGe₄O₁₃ phosphors exhibit an orange-red emission corresponding to the characteristic f–f transitions in Eu³⁺ ions. The highest intensity of lines at 580 nm (⁵D₀ → ⁷F₀), 582–602 nm (⁵D₀ → ⁷F₁), 602–640 nm (⁵D₀ → ⁷F₂), 648–660 nm (⁵D₀ → ⁷F₃), and 680–715 nm (⁵D₀ → ⁷F₄) is observed for the samples with x = 0.4–0.6. The possibility of their application has been assessed by studying their color characteristics, quantum efficiency, and thermal stability. The obtained data indicate that Ba₂Gd_2−xEu_xGe₄O₁₃ solids can be considered as promising materials for UV-excited phosphor-converted light-emitting diodes (LEDs) if an aluminum nitride substrate (λ_ex = 255 nm) is used as a semiconductor chip.