Hydrothermal synthesis, structure, and property characterization of rare earth silicate compounds: NaBa₃Ln₃Si₆O₂₀ (Ln Combining double low line Y, Nd, Sm, Eu, Gd)

A series of new lanthanide (Ln) silicates have been synthesized using high temperature hydrothermal techniques, and structurally characterized using single crystal and powder X-ray diffraction. The compounds have the general formula NaBa₃Ln₃Si₆O₂₀ (Ln Combining double low line Y, Nd, Sm, Eu, Gd), and crystallize in the space group Ama2 (No.40). As a representative example, the unit cell parameters of NaBa₃Gd₃Si₆O₂₀ are a Combining double low line 14.731(3) Å, b Combining double low line 23.864(5) Å, c Combining double low line 5.5449(11) Å and Z Combining double low line 4. The title compounds adopt a three dimensional polar acentric framework made of Ln-O-Si bonding. The framework is comprised of LnO₈ and LnO₇ units forming edge-sharing infinite chains along the c-axis. These oxy-bridged infinite chains are also linked by [Si₄O₁₃] tetrasilicate and [Si₂O₇] disilicate units to form the three-dimensional framework structure, with Ba²⁺ and Na⁺ cations residing inside channels of the framework. The polarity in the structure is imparted by the unusual tetrasilicate arrangement. The luminescence and magnetic properties were investigated on selected compounds. The temperature dependent magnetic susceptibility measurements on the Nd, Sm, and Gd derivatives reveal a Curie-Weiss behavior with an antiferromagnetic coupling parameter. For the Eu-derivative, the temperature dependent magnetic susceptibility deviates significantly from Curie-Weiss behavior. Luminescence properties of NaBa₃Eu₃Si₆O₂₀ and NaBa₃Sm₃Si₆O₂₀ compounds exhibited the characteristic transitions of Eu³⁺ (5D₀ g†' 7F_J, J Combining double low line 0-4) and Sm³⁺ (4G_5/2 g†' 6H_J, J Combining double low line 5/2, 7/2), respectively, leading to strong visible red and orange emissions, respectively.