“…The emission spectrum consists of several sharp emission peaks at around ∼626 nm, corresponding to the spin-forbidden 2 Eg → 4 A2 transitions of Mn 4+ . That is, emission peaks at 596, 605, 610, 618, 626.5, 630.5, and 643.5 nm are attributed to the anti-Stokes ν3(t1u), ν4(t1u), and ν6(t2u), zero phonon line (ZPL), Stokes ν6(t2u), ν4(t1u), and ν3(t1u) vibrionic modes, respectively [24,25]. In this work, the ZPL peak is stronger compared to previously observed Mn 4+ activated fluoride such as K2(Si,Ge,Ti)F6 due to the relatively lower symmetry of the substituted distorted octahedral Al 3+ site in α-K3AlF6 [27].…”