Luminescence thermal quenching (TQ) is classically considered to be a common feature of most phosphors, since enhanced electron−phonon interactions and/or activation of non-radiative decay pathways at elevated temperatures are expected to decrease radiative transition probability of the excited electrons. Recently, a drastically increasing number of papers have been published reporting zero-TQ phenomenon which denotes that integrated intensity of photoluminescence (I_PL) of a given phosphor maintains nearly 100% at a specified elevated temperature compared with that at room temperature(RT), and I_PL of the phosphor is obviously larger than that at RT in a temperature range between RT and zero-TQ temperature (anomalous TQ). This zero- TQ (and anomalous TQ) phenomenon was reportedly observed for a wide variety of phosphors.

In this contribution, defects induced zero-TQ phenomenon of Eu²⁺-doped phosphors is overviewed. The zero-TQ phenomenon of Eu²⁺ luminescence was reportedly observed largely in some phosphate, fluorophosphate or fluoroborate phosphors. It was proposed by the researchers that the defects originating from either polymorphic modifications of the host compound, non-equivalent substituting for the cationic sites by Eu²⁺, or mismatching in ionic sizes between Eu²⁺ and the sites it occupies could absorb the incident excitation light at RT, and upon heating (thermal stimulation) the energy captured in the defects of phosphor could be transferred to 5d levels of the Eu²⁺, resulting in zero-TQ and anomalous TQ of these phosphors. A close inspecting a collection of the relevant data indicates that the variations of I_PL with temperature for those Eu²⁺-doped phosphors with a similar trap depth were often disparate.It is argued that it is still an open question whether zero-TQ (and anomalous TQ) could be an intrinsic property of Eu²⁺-doped phosphors.Since the magnitudes of I_PL enhancement for a given phosphor (e.g., Na₃Sc₂(PO₄)₃:Eu²⁺ ) in a similar temperature span between RT and 150 ^oC reported by the different research groups or even by the same research group in main text and supplementary information of the article were different obviously. Despite the existence of defects in the phosphors being proved by thermoluminescence experiments, it does not mean that energy transfer from the defects to the 5d levels of Eu²⁺ could naturally take place. The energy transfer between donor and acceptor centers in a phosphor could take place only when the rigorous conditions could be satisfied for the specific mechanism, i.e., a resonance interaction, an exchange interaction between the two centers or through the conduction (or valance) band of the host.Furthermore, once the transfer of energy from the defect levels to the 5d levels of Eu²⁺ has occurred, apart from the increase in I_PL of Eu²⁺, lifetime and internal quantum efficiency (IQE) of the phosphors should give corresponding signals simultaneously. 5d→4f transition of Eu²⁺ is a parity-allowed electric dipole transition, it is expected that a change in I_PL with temperature should be accompanied with a change in lifetime correspondingly. Given the fact that the whole process including energy capturing by the defects, de-trapping of the stored energy via thermal stimulation, and energy transferring from the defects to Eu²⁺ and eventually light emitting by Eu²⁺ was a time-consuming process in comparison to a direct decay from the 5d to 4f manifolds of the Eu²⁺, afterglow of Eu²⁺ luminescence which refers to a delayed radiant recombination of electrons and holes due to trapping of electrons or holes is expected if the energy transfer from the defects to Eu²⁺ has occurred. Moreover, if an increase in I_PL with temperature arises from transfer of energy stored in the defects to 5d levels of the Eu²⁺, an increase in IQE is expected as well, since the I_PL is proportional to the product of absorbed photons and IQE of the phosphor under consideration. However, corresponding changes in temperature-dependent lifetime or IQE were unavailable in these publications.

Though the measurement method of temperature-dependent spectra is widely used by researchers in the filed of luminescent materials. Given the reasons elaborated in the text, temperature dependent I_PL results alone seem not to be a reliable parameter for evaluating TQ property of Eu²⁺-doped phosphors unless special caution has been taken to guarantee the stability and consistence of the measuring conditions.To substantiate defects induced zero-TQ phenomenon to be an intrinsic property of the Eu²⁺-doped phosphors,merely an enhancement of I_PL with increasing temperature is insufficient and unconvincing. Only when an increase in IQE and/or lifetime of the phosphor in question with temperature is in accordance with that predicted from an enhanced I_PL, could the conclusion that the defects related energy transfer resulted in zero-TQ and anomalous TQ of Eu²⁺ luminescence hold.