Two high terbium content apatites: Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃

Graphical abstract: Two terbium apatites Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃ may be promising magneto-optical materials due to their high terbium content. - Highlights: • Two terbium apatite Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃ crystals were grown by flux method. • Structures of both compounds were determined via single-crystal diffraction. • Both compounds show paramagnetic behaviours down to 2 K. • Both compounds may be promising magneto-optical materials due to high Tb³⁺ contents. - Abstract: Two high content terbium apatites, Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃, have been successfully synthesized by sol-gel method and have been crystallized from high temperature flux. Their structures were identified by single crystal X-ray diffraction and were found to belong to the hexagonal system with space group P6₃/m and unit cell parameters of a = 9.2569(10) Å, c = 6.8297(12) Å and Z = 2 for Tb₅Si₂BO₁₃ and a = 9.493(4) Å, c = 6.852(5) Å and Z = 2 for Tb_4.66Si₃O₁₃, respectively. The diffuse optical reflection spectra clearly indicate that the two crystals have good optical transparency in the range of 500-1500 nm. The paramagnetic behaviours of Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃ from 2 K to 300 K are confirmed by magnetic susceptibility measurements. In comparison with other known Tb containing magneto-optical (MO) glass and crystals, Tb₅Si₂BO₁₃ and Tb_4.66Si₃O₁₃ are found to be promising magneto-optical materials in the visible-near IR range due to the high Tb³⁺ concentrations in both compounds