A New Family of Fluoroborates Ba 5 Ln 3 (BO 3 ) 6 F: Structure‐Property Relationships and Intense Eu 3+ Red Photoluminescence

ABSTRACT A new family of rare‐earth fluoroborates, Ba 5 Ln 3 (BO 3 ) 6 F (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 BO 3 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, Eu 3+ ‐activated Ba 5 Gd 3‐ x Eu x (BO 3 ) 6 F displays intense red emission under both near‐ultraviolet (near‐UV) (394 nm) and blue (465 nm) excitation. The dominant 5 D 0 → 7 F 2 transition and resolved Stark splitting indicate that Eu 3+ 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 Ba 5 Gd 3‐ x Eu x (BO 3 ) 6 F as a promising fluoroborate phosphor for solid‐state lighting applications.