β-LaTeBO5 and RETeBO5 (RE = Y, Gd, Tb): explorations of new optical materials in the RE(iii)–Te(iv)–B–O system†

Exploring new materials and enhancing their optical properties is an endeavor of great significance. Combining different types of optical functional groups, such as borate and tellurite anions, into the same compound can produce a new material of novel structure with enhanced optical performance. In the RE(III)–Te(IV)–B–O system, a borate tellurite, namely, β-LaTeBO₅ (β-LTBO) and a series of lanthanide borotellurites, namely, RETeBO₅ (RE = Y, Gd, Tb), have been successfully synthesized by using a high-temperature solution method. β-LTBO crystallizes in the space group of P2₁/c (no. 14) and its structure features [LaTeO₃]_∞ layers composed of La³⁺ cations and [TeO₃]²⁻ anions, where these layers are interconnected by ¹[BO₂]_∞ chains consisting of [BO₃]³⁻ planar triangles to form a novel 3D structure. RETeBO₅ (RE = Gd, Tb, Y) features 0D [Te₂B₂O₁₀]⁶⁻ clusters in which a central [Te₂O₆]⁴⁻ dimer connects with two [BO₃]³⁻ groups on both sides of the dimer via Te–O–B bridges, and these 0D [Te₂B₂O₁₀]⁶⁻ clusters are interconnected by lanthanide ions to form the 3D structure. A phase transition from the β phase to the α phase was observed at 700 °C for LaTeBO₅ based on DSC and temperature-dependent XRD studies. Excitingly, the birefringence of β-LTBO of 0.134@546 nm is much larger than that of RETeBO₅ (RE = Y, Gd) (0.08, 0.074@546 nm), which is the largest in the borate tellurite system. Theoretical calculations indicate that the ¹[BO₂]_∞ chains make a considerable contribution to the enhanced birefringence of β-LaTeBO₅. Furthermore, the luminescent and magnetic properties of RETeBO₅ (RE = Y, Gd, Tb) were also studied. The present study proposes that the investigation of the metal–Te(IV)–B–O system may potentially result in the identification of numerous novel multifunctional materials.