Review—On the Origin of Diminishing Radiative Lifetime of Mn4+ in Complex Fluoride Phosphors with Temperature

Temperature dependence of the radiative lifetime of Mn4+ 2 E g→4 A 2g luminescence in oxides and complex fluorides is reviewed. Unlike other phosphors, where a decrease in lifetime with increasing temperature was due to thermal quenching (TQ), here the decrease in lifetime of Mn4+ 2 E g→4 A 2g luminescence in complex fluorides occurred at low temperatures where luminescence TQ had not occurred. Various explanations and models were proposed to phenomenologically account for or to fit the temperature-dependent experimental results. Validity of these explanations and theoretical models is assessed. It is argued that diminishing radiative lifetime of Mn4+ in complex fluorides or oxides with temperature seems irrelevant to progressive mixing of 3d orbitals of Mn4+ and 2p orbitals of ligands (F- or O2-) or to enhanced transition probabilities of the vibronic emission. It is suggested that an increase in 4 T 2g–2 E g spin-orbit mixing along with in thermal population of the 4 T 2g manifold from the 2 E g manifold with increasing temperature arising from lattice thermal expansion induced reduction in 4 T 2g–2 E g energy separation could contribute to the diminishing radiative lifetime of Mn4+ 2 E g → 4 A 2g luminescence with temperature. Large thermal expansivity of host compounds should be the root cause for the special temperature-dependent decay behavior of Mn4+ luminescence in complex fluoride phosphors.