Comment on “Investigation on Anomalous Thermal Quenching of Mn4 + Luminescence in A2XF6:Mn4+” [ ECS J. Solid State Sci. Technol., 10, 076007 (2021)]

Abstract

Adachi (ECS J. Solid State Sci. Technol., 10, 076007 (2021)) reported that an anomalous enhancement of integrated intensity of Mn4+ luminescence (I PL) in A2XF6:Mn4+ phosphors with increasing lattice temperature was an intrinsic property of the phosphors due to the increased phonon number that makes it possible to gain the parity and spin-forbidden 2 E g → 4 A 2g transitions. He proposed a series of formulas expressing the temperature dependence of intensities for zero-phono line and phonon lines of Mn4+ luminescence. Despite a good agreement of the two sets of experiment data for Rb2GeF6:Mn4+ and Cs2WO2F4:Mn4+ phosphors respectively with the formulas Adachi proposed, I argue in this comment that it seems still unconvincing to ascribe the anomalous increase of I PL with temperature as an intrinsic property of Mn4+-doped fluorides. Since theoretical derivation of the formulas expressing temperature dependence of the intensities was based on an unjustified assumption that the 2 E g → 4 A 2g transition which is parity and spin-forbidden gains intensities with the activation of lattice vibronic modes and becomes observable. It should be spin–orbit coupling which produces a non-zero contribution of the spin-quartet states in the spin-doublet states that allows this spin-forbidden transition to occur.