Synthesis of Mn⁴⁺ activated Na₂SiF₆ red-emitting phosphors using an ionic liquid

For the first time, the synthesis of Na₂SiF₆ powders was achieved through an ionothermal reaction using an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, without using toxic hydrogen fluoride (HF). Next, these powders were used as a host material to produce the red-emitting phosphor, Na₂Si_1-xMn_xF₆ (x = 0.007–0.200), that was synthesized using a low concentration HF solution with dissolved NaMnO₄ as the Mn⁴⁺ source. The phosphors exhibited red-emission at 610 nm, 618 nm, 629 nm, and 644 nm from the ²E_g → ⁴A_2g energy levels of Mn⁴⁺ under 460 nm excitation. The Na₂SiF₆ particles were non-agglomerated rod-shaped (~560 nm in length and ~230 nm in diameter), while the Na₂Si_1-xMn_xF₆ particles were spherically-shaped (~500 nm in diameter). Both of those powders were found to be much smaller than typical fluoride powders, which enables them to potentially improve the efficiency of phosphor-converted white-light-emitting diodes.