Critical Review—On the Anomalous Thermal Quenching of Mn⁴⁺ Luminescence in A₂XF₆:Mn⁴⁺ (A = K, Na, Rb or Cs; X = Si, Ti, Ge, Sn, Zr or Hf)

Abstract

Herein, luminescence thermal quenching (TQ) of Mn 4+ -doped fluoride phosphors with a formula A 2 XF 6 :Mn 4+ (A = K, Na, Rb or Cs; X = Si, Ti, Ge, Sn, Zr or Hf) is overviewed. Some researchers reported that the A 2 XF 6 :Mn 4+ phosphor showed a normal TQ behavior. On the contrary, numerous researchers claimed that the A 2 XF 6 :Mn 4+ phosphor showed an anomalous (or negative) TQ behavior, denoting that integrated photoluminescence intensity ( I PL ) of a given A 2 XF 6 :Mn 4+ phosphor increases when the sample temperature rises from a cryogenic- or room-temperature to certain elevated values. Various explanations were proposed for anomalous TQ of Mn 4+ luminescence. The creditability of anomalous TQ of Mn 4+ luminescence in A 2 XF 6 :Mn 4+ phosphors and relevant explanations are assessed. It is argued that the anomalous TQ should not be an intrinsic attribute of A 2 XF 6 :Mn 4+ phosphors. It is suggested that the I PL enhancement with temperature for A 2 XF 6 :Mn 4+ phosphors observed by some researchers was likely a pitfall caused by diminishing in optical-path lengths of the spectrofluorometer stemming from lattice thermal expansion. An increase in absorption of the excitation light also contributed to the I PL enhancement in case that wavelength of blue excitation light used in temperature-dependent spectra measurements corresponds to the low-energy tail of the low-temperature 4 A 2g → 4 T 2g excitation band of the A 2 XF 6 :Mn 4+ phosphor. The credibility of the explanation for the I PL enhancement with temperature was verified by application to the analysis of a similar anomalous TQ phenomenon reported for the 3.5MgO·0.5MgF 2 ·GeO 2 :Mn 4+ phosphor in literature.