Editors' Choice—Thermal Quenching Analyses of Eu²⁺-Activated Sr-Containing Sialon Phosphors Using the Thermally Activated Cross-Over Model

Abstract

To investigate the relation between the luminescence properties and the crystal structure, the thermal quenching of three kinds of Eu 2+ -activated Sr-containing sialon phosphors are analyzed using the Struck-Fonger model. The inherent frequency of the host lattice ω is calculated from the measured internal quantum efficiency at various temperatures, and the results confirm that the inherent frequency of the host lattice ω dominates the thermal quenching of these phosphors. Moreover, clarification of the relationship between the inherent frequency of the host lattice ω and the host crystal structure reveals that the covalence and density of the host crystal structure mainly influence the inherent frequency and therefore influence the extent of the thermal quenching. 17,18 Despite being composed of the same elements and activated with the same ion, the divalent Eu ion, these three phosphors emit different color and their luminescence properties are not the same. The differences between their crystal structures and compositions are thought to result in these different luminescence properties. In an attempt to investigate the relation between the compositions, structures of the host crystals, and their luminescence properties, the thermal quenching of these phosphors is analyzed. The research about the thermal quenching of phosphors have increased in these days, [19][20][21][22][23][24][25][26][27][28] and two processes of the thermal quenching are mainly considered. One is the thermally activated cross over from the excited state to the ground state;19-24 the other is the thermal ionization from the excited state to the bottom of the conduction band (CB). [25][26][27][28] Struck and Fonger proposed a quantitative method to describe the radiative and non-radiative relaxation from excited states using a quantum mechanical single configurational coordinate (QMSCC) model.19 Parameters for this model are depicted in Fig. 1. In this figure the horizontal coordinate is the single configurational coordinate r, and the vertical coordinate is the total energy E of the system. The parabolas u and v are the potential energy wells of the electronic ground state and the relevant excited state, respectively. The u and v parabola quantum numbers, wavefunctions, and phonon energies are denoted n, u n , ω u and m, v m , ω v , respectively. r 0 measures the Franck-Condon (FC) offset and is expressed in terms of S u and S v : r 0 2 = 2(S u + S v ). S u ω u and S v ω v represent the relaxation energies after emission and excitation, respectively. The QMSCC model is taken in the thermal equilibrium and the v m → u n rate is proportional to the squared overlap integral u n |v m 2 . 29 These overlap integrals are evaluated using the recursion formulas given by Manneback,20 offering the possibility of a quantitative treatment of radiative and nonradiative processes. Struck and Fonger applied QMSCC-calculation to the luminescence properties of ruby and emerald. 21 Bleijenberg and Blasse performed QMSCC calculations on model phosphor syste...

The Inductive Effect of Neighboring Cations in Tuning Luminescence Properties of the Solid Solution Phosphors

“…Since it is involved in the electron redistribution of the coordinated anions, an influencing factor is the number of anions X shared by the central cation M and the counter (neighboring) cation T to be replaced by the doping cation. 42 The more shared anions X there are, the stronger the inductive effect, which is supported by the fact that an anomalous shift is observed in those oxides with a large ratio between the cation to be replaced and the other cations, such as (Sr 3−x Ba x )SiO 5 :Eu 2+ (3/1), 24 Ca 3−x Sr x (PO 4 ) 2 :Eu 2+ (3/2), 26 and Sr 3−m Ca m B 2 O 6 :Eu 2+ (3/2). 27 Another is the sensitivity of the covalency of the M−X bond to the countercation T. While this is difficult to quantify, it can be gauged by the type of anion X on this point.…”

“…Below we give general discussions about where the inductive effect of neighboring cations may obviously take effect and play a key role in shifting the lowest 5 d level. Since it is involved in the electron redistribution of the coordinated anions, an influencing factor is the number of anions X shared by the central cation M and the counter (neighboring) cation T to be replaced by the doping cation . The more shared anions X there are, the stronger the inductive effect, which is supported by the fact that an anomalous shift is observed in those oxides with a large ratio between the cation to be replaced and the other cations, such as (Sr 3– x Ba x )­SiO 5 :Eu 2+ (3/1), Ca 3– x Sr x ­(PO 4 ) 2 :Eu 2+ (3/2), and Sr 3– m Ca m ­B 2 O 6 :Eu 2+ (3/2) .…”

“…It has been demonstrated that centroid shift as well as Stokes shift depend strongly on the covalency of the bonds between the central cation M and the anions X. ,,, To better understand the above peculiar changes in ε c and Δ S with cation substitution, we introduce the well-known inductive effect in solid-state chemistry to highlight the key role of neighboring cations in controlling centroid shift and Stokes shift simultaneously. ,− Since the anions X should be bonded with a second cation T other than M, the property of the M–X bond will be modified by the T–X bond. If T is more electronegative than M, the anions X will tend to share more electrons with T, and thus, fewer electrons will be available for the M–X bond, resulting in a less covalent M–X bond.…”

Influence of composition and structure on the thermal quenching of the 5d–4f emission of Eu2+ doped M–Si–N (M = alkali, alkaline earth, rare earth) nitridosilicates

“…20 In literature, both these models have been used to explain the differences in thermal quenching behaviour of the Eu 2+ 5d-4f emission for the nitridosilicates. Thermal quenching via a thermally activated cross-over was considered by for example Fukuda, 21 who concluded that the thermal quenching in various Eu 2+ doped Sr-containing sialon phosphors is influenced by the phonon frequency. On the other hand, thermal quenching via thermal ionization to the conduction band was, for example, considered by Tolhurst et al, 22 using X-ray absorption spectroscopy to show that the lowest Eu 2+ 5d level is closer to the bottom of the conduction band in SrMg 3 SiN 4 :Eu 2+ than in Li 2 Ca 2 Mg 2 Si 2 N 6 :Eu 2+ , explaining the stronger thermal quenching for SrMg 3 SiN 4 :Eu 2+ as compared to Li 2 Ca 2 Mg 2 Si 2 N 6 : Eu 2+ .…”

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

“…For Eu 2+ -doped phosphors, depending on the bandgap and rigidity of the host compound as well as the doping concentration of Eu 2+ , the quenching temperatures (T 0.5 ) of the specific phosphors were different. 9,12,24,28,32 Meanwhile, there had been an argument about the TQ mechanism of Eu 2+ -doped phosphors originating either from a thermally assisted 4 f−5d crossover in the configuration coordinate diagram, 14,15,17,37 or from a thermally assisted photoionization of 5d electron to the conduction band of the host lattice. 12,24,26,28,30 Nevertheless, no one doubts that the Eu 2+ -doped phosphor suffers from a lattice-dependent certain degree of luminescence TQ.…”