Mechanism of thermal quenching of Mn⁴⁺ deep-red luminescence in perovskite titanate phosphors (La₂MTiO₆:Mn⁴⁺, M = Mg, Zn, vs SrTiO₃:Mn⁴⁺)

Mn⁴⁺ (3d³) substituting for Ti⁴⁺ in perovskite titanates is a rare-earth-free deep-red emitter for LED lighting. In 2015, La₂MgTiO₆:Mn⁴⁺ and La₂ZnTiO₆:Mn⁴⁺ were reported to emit near 710 nm at room temperature with internal quantum yields of 58.7% and 24.3%, whereas SrTiO₃:Mn⁴⁺ shows no luminescence at room temperature because of significant thermal quenching already at low temperature (restated as ~100 K by the same group in ref [3], citing ref [1]). The proposers attributed the contrast to a non-radiative charge-transfer channel: in the $^2E_g$ emitting state, the empty Mn-3d $t_{2g}$ spin-orbital lies inside the O-2p-derived valence band in SrTiO₃, so a valence-band electron can transfer thermally onto Mn, and the resulting valence-band hole recombines non-radiatively with the Mn $t_{2g}$ electron of opposite spin; in La₂MgTiO₆ the $t_{2g}$ level sits above the valence-band top and the channel is closed. This inference rests on ground-state density-functional band calculations and steady-state photoluminescence only, and the same evidence underlies an empirical correlation between host band gap and quenching behavior (3.27, 3.89, 4.01 eV for SrTiO₃, La₂ZnTiO₆, La₂MgTiO₆). The problem asks for (i) validation or refutation of the proposed channel in the titanate family and (ii) assignment of the La₂ZnTiO₆ vs La₂MgTiO₆ quantum-yield gap to intrinsic electronic structure or to defect chemistry, with the mechanism result read against the qualitative host-design levers documented in the source lineage.

Background

A phosphor-converted LED (pc-LED) uses a near-UV or blue LED chip covered by one or more luminescent phosphors; its white-light quality depends strongly on the red phosphor, today dominated by Eu²⁺-activated nitrides. Mn⁴⁺ (3d³ electron configuration) on octahedral cation sites is a rare-earth-free alternative whose spin-forbidden internal transition $^2E_g \to {}^4A_{2g}$ gives narrowband deep-red emission; in titanate hosts it falls near 710 nm, suitable for the far-red component of plant-growth lighting and deep-red-assisted white LEDs. In an octahedral crystal field the five Mn 3d orbitals split into lower $t_{2g}$ and higher $e_g$ sets; the labels α and β denote the two spin directions. A double perovskite A₂B′B″O₆ (here La₂MTiO₆ with M = Mg or Zn) is a perovskite derivative in which the two different octahedral B sites order in a rock-salt pattern, so that TiO₆ (here MnO₆) octahedra are isolated from each other. Internal quantum efficiency (IQE) is the ratio of emitted to absorbed photons. Thermal quenching (TQ) is the loss of luminescence efficiency with increasing temperature, commonly quantified by a quenching temperature $T_q$, the temperature at which the integrated emission intensity falls to 50% of a stated low-temperature reference value. The empirical problem: SrTiO₃:Mn⁴⁺ shows no luminescence at room temperature because of significant thermal quenching at low temperature (restated as ~100 K by the same group in ref [3], citing ref [1]), whereas La₂ZnTiO₆:Mn⁴⁺ and La₂MgTiO₆:Mn⁴⁺ emit efficiently near 710 nm at room temperature; the host band gaps are 3.27, 3.89 and 4.01 eV for SrTiO₃, La₂ZnTiO₆ and La₂MgTiO₆, and the IQE of La₂MgTiO₆:Mn⁴⁺, 58.7% (absorption 77.5% at 330 nm, 0.2 at.% Mn vs Ti), contrasts with 24.3% for La₂ZnTiO₆:Mn⁴⁺ (absorption 73.1% at 360 nm). (These values are approximate, from the primary abstract.) The proposed mechanism (Takeda et al. 2015, references [1]–[3]): in the $^2E_g$ excited state one $t_{2g}$ spin-orbital of Mn is singly occupied in the β spin channel, leaving an empty α-spin $t_{2g}$ slot. Ground-state density-functional theory (DFT) supercell calculations found that in SrTiO₃:Mn the filled Mn-3d $t_{2g}$(α) states lie buried inside the O-2p valence band (VB), while in La₂MgTiO₆:Mn they lie above the VB top. If this alignment carries over to the excited state, a valence-band electron in SrTiO₃ can transfer into the empty $t_{2g}$(α) slot with little or no thermal activation, creating a VB hole that recombines non-radiatively with the Mn $t_{2g}$(β) electron, whereas in La₂MgTiO₆ the transfer is energetically uphill and suppressed. The authors also considered photoionization (excitation of the Mn $e_g$ electron into the conduction band) as a second quenching route that a wide band gap suppresses. Two caveats frame this mechanism. First, the electronic-structure evidence is ground-state DFT, which locates filled levels; the argument transfers the alignment to the excited state, and no direct observation of the transient Mn³⁺/VB-hole state has been reported for any Mn⁴⁺ titanate. Second, competing quenching frameworks exist and are not mutually settled for Mn⁴⁺ oxides: the configurational-coordinate (CC) crossover model, in which the $^4T_2$ potential-energy surface crosses the $^2E_g$ surface and the crossing barrier sets the activation energy, is the standard alternative (Adachi 2020 [6]; established quantitatively for Mn⁴⁺ fluorides by Senden et al. 2018 [5]); yet Grinberg and Leśniewski 2019 [7] showed that measured Mn⁴⁺ activation energies are roughly an order of magnitude lower than crossover-model predictions, implying additional non-radiative channels such as charge transfer. Direct photoelectric evidence for a valence-band-hole-type quenching mechanism exists only for Cr³⁺ (also 3d³) in (Ga,In)₂O₃ oxides, from photocurrent-excitation spectroscopy (measuring the current induced in the sample as a function of excitation wavelength; Majewska et al. 2024 [8]); its authors note that such photoelectric studies on transition-metal-activated dielectrics remain largely absent. Excited-state ΔSCF calculations (constrained DFT computing total-energy differences between chosen spin/charge configurations) have been applied to Mn⁴⁺ fluorides (Kurboniyon et al. 2024 [9]) but not to the titanate VB-transfer question. The structural-design lever is separately documented: reducing the connectivity of the MnO₆ octahedra or widening the band gap (layered Sr₂TiO₄, oxyfluoride Sr₂ScO₃F with 5.38 eV and 50.5% IQE, and Ruddlesden–Popper phases, i.e. layered perovskite derivatives with reduced octahedral connectivity) raises quenching temperature, though even Sr₂ScO₃F:Mn⁴⁺ retains only about 20% of its intensity at 373 K (Kato et al. 2018 [3]; Piotrowski et al. 2023 [16]). Emission energy, unlike quenching, is already predictable from crystal-field and covalence parameters (Srivastava et al. [4]; Boutinaud 2024 [18]). What remains missing is exactly the content of the problem statement below.

Problem Statement

Establish mechanism-level understanding of thermal quenching in Mn⁴⁺-activated perovskite titanates — with La₂MgTiO₆:Mn⁴⁺, La₂ZnTiO₆:Mn⁴⁺ and SrTiO₃:Mn⁴⁺ as the reference system. Concretely, the problem is to determine the following. (a) Mechanism. Determine whether the proposed non-radiative channel operates: thermally activated transfer of an O-2p valence-band electron into the empty Mn-3d $t_{2g}$ spin-orbital of the $^2E_g$ emitting state (the lowest excited state of the 3d³ configuration) when that orbital is resonant with the valence band (SrTiO₃:Mn⁴⁺), producing a valence-band hole that recombines non-radiatively with the Mn $t_{2g}$ electron of opposite spin, and its suppression when the $t_{2g}$ level lies above the valence-band top (La₂MgTiO₆:Mn⁴⁺). The determination must discriminate this channel against the alternatives — configurational-coordinate crossover (thermal crossing of the $^4T_2$ excited-state potential-energy surface with the $^2E_g$ one), photoionization into the conduction band, and defect-mediated quenching — on at least SrTiO₃:Mn⁴⁺ and La₂MgTiO₆:Mn⁴⁺ (La₂ZnTiO₆:Mn⁴⁺ included where the gap question requires it), and must be consistent with the documented behavior that SrTiO₃:Mn⁴⁺ undergoes significant thermal quenching at low temperature (restated as ~100 K in ref [3] citing ref [1]) and shows no luminescence at room temperature, whereas La₂MgTiO₆:Mn⁴⁺ emits efficiently at ~710 nm at room temperature (IQE 58.7% at 330 nm excitation) and La₂ZnTiO₆:Mn⁴⁺ emits more weakly (IQE reported as 24.3% in ref [1]). (b) Efficiency gap. Assign the origin of the internal quantum efficiency (IQE; emitted divided by absorbed photons) gap between La₂ZnTiO₆:Mn⁴⁺ (24.3%) and La₂MgTiO₆:Mn⁴⁺ (58.7%) — intrinsic electronic-structure difference between the Zn and Mg double perovskites versus defect chemistry (synthesis-related non-stoichiometry, cation volatilization, oxygen-related defects) — by a controlled comparison that varies the candidate cause (firing atmosphere/temperature, Zn non-stoichiometry, flux or co-doping, defect spectroscopy) and either closes the gap by intervention or demonstrates its persistence under defect-controlled conditions. The mechanism result should additionally be read against the qualitative design levers documented in the source lineage — the Mn $t_{2g}$ position relative to the valence-band top, the host band gap, and reduction of octahedral connectivity (layered Sr₂TiO₄, oxyfluoride Sr₂ScO₃F, and Ruddlesden–Popper phases; ref [3]) — as context for host choice. That host-design context is background to the mechanism question; the question is the mechanism determination and the efficiency-gap assignment. The two parts share one objective — a causal, testable account of thermal quenching that informs host choice.

References

1. Y. Takeda, H. Kato, M. Kobayashi, H. Kobayashi, M. Kakihana, Photoluminescence Properties of Mn4+-activated Perovskite-type Titanates, La2MTiO6:Mn4+ (M = Mg and Zn), Chemistry Letters 44 (2015) 1541–1543. DOI: 10.1246/cl.150748. https://doi.org/10.1246/cl.150748
2. Y. Takeda, H. Kato, M. Kobayashi, S. Nozawa, H. Kobayashi, M. Kakihana, Photoluminescence Properties of Double Perovskite Tantalates Activated with Mn4+, AE2LaTaO6:Mn4+ (AE = Ca, Sr, and Ba), The Journal of Physical Chemistry C 121 (2017) 18837–18844. DOI: 10.1021/acs.jpcc.7b06280. https://doi.org/10.1021/acs.jpcc.7b06280
3. H. Kato, Y. Takeda, M. Kobayashi, H. Kobayashi, M. Kakihana, Photoluminescence Properties of Layered Perovskite-Type Strontium Scandium Oxyfluoride Activated With Mn4+, Frontiers in Chemistry 6 (2018) 467. DOI: 10.3389/fchem.2018.00467. https://doi.org/10.3389/fchem.2018.00467
4. A. M. Srivastava, M. G. Brik, H. A. Comanzo, W. W. Beers, W. E. Cohen, T. Pocock, Spectroscopy of Mn4+ in Double Perovskites, La2LiSbO6 and La2MgTiO6: Deep Red Photon Generators for Agriculture LEDs, ECS Journal of Solid State Science and Technology 7 (2018) R3158–R3162. DOI: 10.1149/2.0191801JSS. https://doi.org/10.1149/2.0191801JSS
5. T. Senden, R. J. A. van Dijk-Moes, A. Meijerink, Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors, Light: Science & Applications 7 (2018) 8. DOI: 10.1038/s41377-018-0013-1. https://doi.org/10.1038/s41377-018-0013-1
6. S. Adachi, Review—Mn4+-Activated Red and Deep Red-Emitting Phosphors, ECS Journal of Solid State Science and Technology 9 (2020) 016001. DOI: 10.1149/2.0022001JSS. https://doi.org/10.1149/2.0022001JSS (published online 16 August 2019; issue dated 2020)
7. M. Grinberg, T. Leśniewski, Non-radiative processes and luminescence quenching in Mn4+ doped phosphors, Journal of Luminescence 214 (2019) 116574. DOI: 10.1016/j.jlumin.2019.116574. https://doi.org/10.1016/j.jlumin.2019.116574
8. N. Majewska, M.-H. Fang, S. Mahlik, Photoelectric Studies as the Key to Understanding the Nonradiative Processes in Chromium Activated NIR Materials, Journal of the American Chemical Society 146 (2024) 22807–22817. DOI: 10.1021/jacs.4c08011. https://doi.org/10.1021/jacs.4c08011
9. M. S. Kurboniyon, A. M. Srivastava, B. Lou, D. D. Nematov, A. Burhonzoda, T. Yamamoto, C.-G. Ma, M. G. Brik, Thermal Quenching Mechanism of Mn4+ in Na2SiF6, NaKSiF6, and K2SiF6 Phosphors: Insights from the First-Principles Analysis, Inorganic Chemistry 63 (2024) 21212–21221. DOI: 10.1021/acs.inorgchem.4c03589. https://doi.org/10.1021/acs.inorgchem.4c03589
10. G. Xing, Y. Feng, M. Pan, Y. Wei, G. Li, P. Dang, S. Liang, M. S. Molokeev, Z. Cheng, J. Lin, Photoluminescence tuning in a novel Bi3+/Mn4+ co-doped La2ATiO6:(A = Mg, Zn) double perovskite structure: phase transition and energy transfer, Journal of Materials Chemistry C 6 (2018) 13136–13147. DOI: 10.1039/C8TC05171B. https://doi.org/10.1039/C8TC05171B
11. Z. Yang, L. Yang, C. Ji, D. Xu, C. Zhang, H. Bu, X. Tan, X. Yun, J. Sun, Studies on luminescence properties of double perovskite deep red phosphor La2ZnTiO6:Mn4+ for indoor plant growth LED applications, Journal of Alloys and Compounds 802 (2019) 628–635. DOI: 10.1016/j.jallcom.2019.06.199. https://doi.org/10.1016/j.jallcom.2019.06.199
12. Y. W. Seo, D. Kim, W. Ran, S. H. Park, B. C. Choi, J. H. Jeong, Luminescence properties and energy transfer of Mn4+-doped double perovskite La2ZnTiO6 phosphor, Optical Materials 106 (2020) 109980. DOI: 10.1016/j.optmat.2020.109980. https://doi.org/10.1016/j.optmat.2020.109980
13. M. Hu, C. Liao, L. Xia, W. You, Z. Li, Low temperature synthesis and photoluminescence properties of Mn4+-doped La2MgTiO6 deep-red phosphor with a LiCl flux, Journal of Luminescence 211 (2019) 114–120. DOI: 10.1016/j.jlumin.2019.03.034. https://doi.org/10.1016/j.jlumin.2019.03.034
14. H. Yuan, Z. Huang, L. Xu, H. Jia, X. Sun, K. Liu, La2MgTiO6:Bi3+/Mn4+ photoluminescence materials: Molten salt preparation, Bi3+ → Mn4+ energy transfer and thermostability, Journal of Luminescence 224 (2020) 117290. DOI: 10.1016/j.jlumin.2020.117290. https://doi.org/10.1016/j.jlumin.2020.117290
15. D. Zheng, X. Li, J. Chen, J. Zhang, K. Gao, Y. Zhu, D. Hou, R. Huang, J.-Y. Li, J. Dong, Deep-red to NIR emission of Mn4+ and Cr3+ doped La2MgTiO6 phosphor with multifunctional applications, Journal of Luminescence 257 (2023) 119778. DOI: 10.1016/j.jlumin.2023.119778. https://doi.org/10.1016/j.jlumin.2023.119778
16. W. M. Piotrowski, P. Bolek, M. G. Brik, E. Zych, Ł. Marciniak, Frontiers of Deep-Red Emission of Mn4+ Ions with Ruddlesden–Popper Perovskites, Inorganic Chemistry 62 (2023) 21164–21172. DOI: 10.1021/acs.inorgchem.3c03113. https://doi.org/10.1021/acs.inorgchem.3c03113
17. Y. Wang, F. Ding, J. Wu, J. Ke, X. Yuan, X. Wang, Z. Qiu, W. Zhou, J. Zhang, S. Lian, Site Preference-Driven Mn4+ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion, Inorganic Chemistry 61 (2022) 3631–3640. DOI: 10.1021/acs.inorgchem.1c03756. https://doi.org/10.1021/acs.inorgchem.1c03756
18. P. Boutinaud, The nephelauxetic function he: A tool to locate the emission of Mn4+ in oxides and fluorides, Optical Materials: X 22 (2024) 100310. DOI: 10.1016/j.omx.2024.100310. https://doi.org/10.1016/j.omx.2024.100310 (the nephelauxetic function quantifies metal–ligand covalence)
19. Y. Xu, Y. Wen, Y. Li, X. Cui, Y. Huang, F. Wang, The incorporation of Ge4+ enhances the performance of La2MgSnO6:Mn4+ double perovskite phosphors, CrystEngComm 28 (2026) 901–911. DOI: 10.1039/d5ce00996k. https://doi.org/10.1039/d5ce00996k
20. V. Altunal, V. Guckan, A. Ozdemir, O. Yigit, K. Kurt, Z. Yegingil, A new double perovskite host material La2MgTiO6 in luminescence dosimetry, Journal of Alloys and Compounds 1044 (2025) 184469. DOI: 10.1016/j.jallcom.2025.184469. https://doi.org/10.1016/j.jallcom.2025.184469