Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba2RE2Si4O12F2 (RE = Er3+-Lu3+) and Ba2RE2Si4O13 (RE = La3+-Ho3+)

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

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.

Chemistry of Metal Silicates and Germanates: The Largest Metal Polygermanate, K11Mn21Ge32O86(OH)9(H2O), with a 76 Å Periodic Lattice

An examination of manganese silicates and germanates revealed unusual structural motifs and extremely different chemistries, with identical hydrothermal reactions forming K₂Mn₂Si₃O₉ versus K₁₁Mn₂₁Ge₃₂O₈₆(OH)₉(H₂O). The germanate is exceptional in both its c-axis length (exceeding 76 Å) and unit cell volume (nearly 18000 ų), the largest known polygermanate structure to our knowledge.