A New Family of Fluoroborates Ba5Ln3(BO3)6F: Structure-Property Relationships and Intense Eu3+ Red Photoluminescence

Abstract

A new family of rare-earth fluoroborates, Ba₅Ln₃(BO₃)₆F (Ln = Pr, Nd, Sm, Eu, Gd, and Dy), has been synthesised via a conventional solid-state route and systematically investigated as potential luminescent host materials. Structural analyses reveal that the compounds adopt a monoclinic framework structure of isolated BO₃ units, with lattice contraction observed across the lanthanide series. The fluoroborates exhibit wide optical band gaps (3.84–5.62 eV) and excellent thermal stability, making them suitable hosts for rare-earth activation. Among the investigated compositions, Eu³⁺-activated Ba₅Gd_3-xEu_x(BO₃)₆F displays intense red emission under both near-ultraviolet (near-UV) (394 nm) and blue (465 nm) excitation. The dominant ⁵D₀→⁷F₂ transition and resolved Stark splitting indicate that Eu³⁺ occupies a low-symmetry, non-centrosymmetric environment within the fluoroborate lattice. The emission intensity reaches a maximum at x = 0.7, beyond which concentration quenching occurs through multipolar interactions as the quenching pathway. Furthermore, the phosphor exhibits deep-red chromaticity coordinates and a high color purity of approximately 93%. The combination of structural robustness, strong excitation in the near-UV/blue spectral region, and high-quality red emission highlights Ba₅Gd_3-xEu_x(BO₃)₆F as a promising fluoroborate phosphor for solid-state lighting applications.