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 Mn4+-doped fluoride phosphors with a formula A2XF6:Mn4+ (A = K, Na, Rb or Cs; X = Si, Ti, Ge, Sn, Zr or Hf) is overviewed. Some researchers reported that the A2XF6:Mn4+ phosphor showed a normal TQ behavior. On the contrary, numerous researchers claimed that the A2XF6:Mn4+ phosphor showed an anomalous (or negative) TQ behavior, denoting that integrated photoluminescence intensity (I PL) of a given A2XF6:Mn4+ 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 Mn4+ luminescence. The creditability of anomalous TQ of Mn4+ luminescence in A2XF6:Mn4+ phosphors and relevant explanations are assessed. It is argued that the anomalous TQ should not be an intrinsic attribute of A2XF6:Mn4+ phosphors. It is suggested that the I PL enhancement with temperature for A2XF6:Mn4+ 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 A2XF6:Mn4+ 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·0.5MgF2·GeO2:Mn4+ phosphor in literature.

Luminescence and Formation of Cubic and Hexagonal (K,Rb)2SiF6:Mn4+

“…The thermal quenching behavior is therefore crucial. Indeed, previously Mn 4+ -doped fluorides have been found where the lower local symmetry also resulted in the desired observation of a strong ZPL and shorter emission lifetime, but the poor thermal quenching behavior made these phosphors unfit for application in wLEDs. ,, The thermal quenching behavior of h-KRSF:0.1% Mn 4+ was, therefore, measured and compared with those of cubic KRSF:0.1% Mn 4+ and KSF:0.5% Mn 4+ . The temperature dependence of the integrated emission intensities in the relevant high temperature region 373–700 K is shown in Figure a.…”

“…The strictly forbidden zero-phonon line (ZPL) is not observed, and the luminescence lifetime of the 2 E state is long. Later, other Mn 4+ -doped fluoride hosts were found with hexagonal or trigonal crystal structures. − The lower local symmetry for the Mn 4+ ion results in the appearance of a ZPL (induced by static odd-parity crystal field components) in addition to vibronic lines . The emission lifetime for Mn 4+ in these hosts is shorter, and the higher eye sensitivity at the ZPL wavelength is also beneficial for the efficacy.…”

On The Validity of the Defect- Induced Negative Thermal Quenching of Eu2+-Doped Phosphors

“…This is because most inorganic host lattices expand upon increasing the temperature. The optical quartz lid of the sample holder (or sample chamber) for masking phosphor powder samples is an elastic material 33 that could hardly prohibit the phosphor sample from expanding upon heating at ambient pressure. Therefore, even though the excitation source, sample holder of the spectrometer could maintain static at fixed positions at elevated temperatures, the distance between the excitation source, top surface of the phosphor sample is expected to diminish with continuously increasing volume of the phosphor.…”

White‐light emission and red scintillation from Mn2+ ions single‐doped aluminum‐silicate glasses

“…The radiative transition probability of discrete center luminescence may be interrelated with phonon energy. 54 In 2018, Tang et al proposed related equations 55 :…”