Y3Al5O12 Garnet Research Articles

Er3+:Y3Al5O12/Pt/ZnO material for photocatalytic hydrogen production under UV–Vis light

Hydrogen is a promising energy source, requiring increasingly efficient photocatalysts for production under UV–Vis light. For the first time, this study evaluated the photocatalytic generation of H2 using the Er3+:Y3Al5O12/Pt/ZnO material. The effect of the Er3+:Y3Al5O12 garnet on the photocatalytic activity of Pt/ZnO under UV–Vis light was evaluated. Firstly, both the ZnO and the Er3+:Y3Al5O12 garnet were synthesized by a sol-gel method, and the Pt-coating of the ZnO was performed using a sonochemical method. The samples were characterized through XRD, SEM-EDX, Raman, UV–Vis-DRS, N2 adsorption-desorption, XPS, and fluorescence spectroscopy. The H2 production under UV–Vis light was measured using mass spectrometry, and the effect of methanol as a sacrificial agent was evaluated. The synthesis method allowed obtaining ZnO with a defined spherical morphology and high crystallinity, with the H2 generation of 676 μmolh−1g−1. In the meantime, the Pt-coating reduced oxygen vacancies and the recombination rate of e−/h+ pairs, and the garnet addition in a mass ratio of Pt/ZnO:garnet at 1.0:0.3 led to the highest H2 production (1304 μmolh−1g−1), with only a 20 % decrease after 5 production cycles. Finally, adding 10 % v/v methanol significantly enhanced the H2 generation (3026 μmolh−1g−1). Altogether, the Er3+:Y3Al5O12/Pt/ZnO material with the sacrificial agent (10 % v/v methanol) improved H2 production from ZnO by a factor of 4.5. Thus, this study demonstrates that the Er3+:Y3Al5O12 garnet is an efficient alternative for significantly increasing H2 production from Pt/ZnO.

A competitive trivalent-europium-activated phosphor: Achieving highly efficient red light emission in a sodium-rich garnet host

The reaction temperature is key factor for light-emitting phosphors synthesized by solid state method because it directly determines the crystallinity and further affects the luminescence property of the phosphors. While high temperature certainly benefits to the solid diffusion in reaction, it would waste enormous energy during long time heating. Herein, a novel Eu3+-activated and highly efficient red phosphor, NaGd2Ga3Ge2O12:Eu3+, is reported. By the reconstruction of the traditional Y3Al5O12 garnet host into the sodium-rich NaGd2Ga3Ge2O12 one, huge decrease of the synthesis temperature is successfully achieved. More importantly, such decrease of the synthesis temperature does not dissatisfy the luminescence property of the NaGd2Ga3Ge2O12:Eu3+ phosphors. Instead, the Eu3+ in the NaGd2Ga3Ge2O12 exhibits extremely high quantum yield up to 96.78% upon 394 nm excitation, showing great competitiveness among other iso-structral garnet hosts. This work indicates that the introduction of sodium in the eight-coordinated rare earth site of the garnet host can not only reduce the energy consumption during the stage of phosphor synthesis but also avoid the loss of excitation energy during the light converting stage of the Eu3+ to a large extent. By fabricating the as-synthesized red phosphor into a near-UV chip together with commercial green and blue phosphors, quality white light-emitting diodes with satisfactory color rendering index (as high as 90) and correlated color temperature (as low as 3986 K) can be easily obtained.

Nd3+→ Cr4+ energy transfer in co-doped Nd,Cr:Y3Al5O12 garnet transparent ceramics

Herein, we quantitatively analyze the Nd3+ → Cr4+ energy transfer efficiency in the co-doped transparent ceramic YAG host and determine that it can be maintained at less than a 20 % loss of Nd3+ excited states when the Cr4+ absorption coefficient at 1064 nm is less than 1.0 cm−1. Furthermore, we conclude that the energy transfer mechanism can be fully accounted for on the basis of classic Förster-Dexter theory, as the transfer efficiency deduced from the emission kinetics is consistent with that independently determined from the spectroscopic properties of Cr4+ absorption and Nd3+ emission. Systematic studies of trends for Cr3+ → Cr4+ conversion are reported as a function of the oxygen annealing temperature, initial Cr3+ concentration, and incorporation of divalent (Mg2+ and Ca2+) charge compensators. Lastly, no effect of energy migration among the Nd3+ ions was observed.

Mn–Si synergistically co-doping adjusting microstructures and microwave-millimeter-wave dielectric properties of Y3Al5O12 garnet ceramics: Lattice occupancy and ligand distortion perspective

Y3-xMnxAl5-xSixO12 (x = 0.1–0.5) ceramics were fabricated through the high-temperature reaction method. The effects of Mn2+-Si4+ co-doping on lattice occupancy, ligand distortion and microwave dielectric properties of Y3Al5O12 garnet were inquired. Mn2+ ions successively occupy the lattice sites of Y3+ dodecahedron and Al3+ octahedron along with the increase of distortion degree of [AlO6] octahedron. In Y3-xMnxAl5-xSixO12, εr predominantly correlates with ionic polarizability, average bond ionicity (△fi) and Mn2+ ionic lattice occupation. The Q × f is primarily influenced by factors like relative density, lattice energy (U) and atomic packing fraction. Meanwhile, τf is associated with the distortion of octahedra, bond energy (E), and the bond valence between Y/Mn and O. Ultimately, the microwave and millimeter-wave dielectric properties of the Y2.8Mn0.2Al4.8Si0.2O12 ceramics excel with values of εr = 10.89, Q × f = 56,135 GHz@11 GHz (Q × f = 88200 GHz@25 GHz), and τf = -39.35 ppm/°C.

Structural Damage and Recrystallization Response of Garnet Crystals to Intense Electronic Excitation

AbstractThe susceptibility to irradiation‐induced damage, the fundamental mechanism of the relaxation kinetics, and the corresponding recrystallization effect related to the cation radius ratio are comprehensively investigated for Y3Al5O12 and Gd3Ga5O12 garnet crystals under 645 MeV Xe35+ irradiation with different fluences of 5 × 1011–3 × 1012 ions cm−2. Regarding different lattice distortion and swelling levels, the observed microstructure transformations to disordered and amorphous phases, and corresponding hillock dimensions, consistently confirm that Gd3Ga5O12 has a higher susceptibility to radiation damage than Y3Al5O12. Combined with iTS model calculations, although Y3Al5O12 has higher atomic temperature and energy deposition than Gd3Ga5O12 under the same ion velocity and electronic energy loss, the relatively high thermal conductivity and specific heat coefficient of Y3Al5O12 crystals enhance the conduction and dissipation of deposition energy, and Gd3Ga5O12, with a higher cation‐radius‐ratio (rA/rB), is more easily damaged to amorphous phase due to the less favorable kinetics of ordering and recovery of a melted track region to the crystalline phase. Additionally, the significant bandgap modification in spectral ranges of 5.88–6.75 eV for Y3Al5O12 and 4.83–5.41 eV for Gd3Ga5O12, and the enhancement of defect‐assisted‐related luminescence are achieved, providing a basis to design novel optoelectronic devices in microelectronics fields.

Basic Characteristics of Dose Distributions of Photons Beam for Radiotherapeutic Applications Using YAG:Ce Crystal Detectors.

Thermostimulated luminescence (TSL) dosimetry is a versatile tool for the assessment of dose from ionizing radiation. In this work, the Ce3+ doped Y3Al5O12 garnet (YAG:Ce) with a density ρ = 4.56 g/cm3 and effective atomic number Zeff = 35 emerged as a prospective TSL material in radiotherapy applications due to its excellent radiation stability, uniformity of structural and optical properties, high yield of TSL, and good position of main glow peak around 290-300 °C. Namely, the set of TSL detectors produced from the YAG:Ce single crystal is used for identification of the uniformity of dose and energy spectra of X-ray radiation generated by the clinical accelerator with 6 MV and 15 MV beams located in Radiotherapy Department at the Oncology Center in Bydgoszcz, Poland. We have found that the YAG:Ce crystal detects shows very promising results for registration of X-ray radiation generated by the accelerator with 6 MV beam. The next step in the research is connected with application of TSL detectors based on the crystals of much heavier garnets than YAG. It is estimated that the LuAG:Ce garnet crystals with high density ρ = 6.0 g/cm3 and Zeff = 62 can be used to evaluate the X-rays produced by the accelerator with the 15 MV beam.

Distribution coefficients for rare-earth doping in Y3Ga5O12 garnet: Comparison with Y3Al5O12

We present the distribution coefficient k in Y3Ga5O12 garnet (YGG) for a series of rare-earth dopants (RE = La, Ce, Nd, Eu, Tb, Er, and Lu), and compare the results with our previous data on RE-doped Y3Al5O12 garnet (YAG). Under our condition of flux growth, single crystals of undoped YGG show the shape of pure trapezohedron with {211} faces, whereas RE doping stabilizes {110} faces. This is to be compared with the case of YAG, where {110} was the dominant face in both undoped and RE-doped crystals. In RE-doped YGG, k remains near unity when the size mismatch between the dopant and host Y3+ ions is within about 3–4%, and drops off rapidly for larger dopants. This is in contrast to RE-doped YAG, where k decreases continuously from the smallest (Lu3+) to the largest (La3+) dopant. We find such distinct behaviors to be correlated with the reported solution energies for the series of RE ions in YAG and YGG.

Achieving Ultraviolet C and Ultraviolet B Dual‐Band Persistent Luminescence by Manipulating the Garnet Structure

AbstractPersistent luminescence (PersL) at the high‐energy spectral end (ultraviolet, UV) is generally hard to achieve, although a long‐lasting UV PersL can play an irreplaceable role in outdoor imaging and other UV‐triggered applications. No PersL with dual ultraviolet C (UVC) and ultraviolet B (UVB) bands from a single material has previously been attained until the authors have designed Pr3+‐doped (Ca1.5Y1.5)(Al3.5Si1.5)O12 (CYAS‐Pr) through cosubstituting Ca2+–Si4+ into Y3Al5O12 garnet structure in this study. The CYAS‐Pr exhibits lattice shrinkage and distortion caused by the Ca2+–Si4+ cosubstitution, which boosts the creation of trapping centers essential to the unprecedented UVC and UVB dual‐band PersL with maxima at 266 and 311 nm upon being charged by UV irradiation at 254 nm. Both UVC and UVB PersL decay over 12 h with the existence of two different Pr3+ environments at Ca2+ and Y3+ sites, respectively. Moreover, the CYAS‐Pr shows efficient dual information storage and precise long‐distance self‐sustained “solar‐blind” imaging capabilities in a wide field of view. It is expected that this dual UVC and UVB PersL in the CYAS‐Pr will trigger further exploration toward persistent sterilization and persistent photocatalysis along with other applications.

Distribution coefficient of rare-earth dopants in Y3Al5O12 garnet

We present the distribution coefficient for the entire series of rare-earth dopants (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in flux-grown Y3Al5O12 garnet crystals. When the distribution coefficient is plotted in a logarithmic scale as a function of dopant radius, it shows a smooth parabolic drop from the smallest dopant (Lu3+) at 1.5 to the largest dopant (La3+) at 0.004. This behavior can be interpreted by the lattice strain model, with Lu3+ having close to the ideal radius for the 8-fold coordination site of the aluminum garnet structure.

Electroluminescent Y3Al5O12 nanofilms fabricated by atomic layer deposition on silicon: using Yb as the luminescent dopant and crystallization impetus.

Silicon-based Yb-doped Y3Al5O12 garnet nanofilms are fabricated by atomic layer deposition, which are polycrystalline after annealing at 1150 °C. The sub-nanometer compositional regulation and the Yb2O3 cladding layers, which also work as the luminescent dopants, are critical for the crystallization. Characteristic Yb3+ luminescence at 1030 nm and 970 nm is identified under electrical injection, exhibiting the external quantum efficiency of 0.65% and the fluorescence lifetime of 80-200 µs. The doped Yb3+ are impact-excited by hot electrons stemming from Fowler-Nordheim tunneling mechanism within the Y3Al5O12 matrix, with the excitation cross section of 0.7×10-15 to 6.4×10-15 cm2. This work certifies the manipulation of multi-oxide nanofilms with designed composition and crystallinity, revealing the possibility of developing Si-based optoelectronic devices from crystalline garnet films.

Microstructure evolution and mechanical properties of YAG/YAG joint using bismuth-borate glass

Joining of Y3Al5O12 garnet single crystal (YAG) was achieved by using a bismuth-borate based glass filler. The thermal properties of glass filler were experimentally determined and the wettability of molten glass on YAG was investigated. The YAG reacted with glass to form ZnAl2O4 particles and Y2O3 nanowires successively as joining temperature is above 625℃. ZnAl2O4 preferentially nucleated and grew on Y2O3 nanowires. The cooperative growth of the two phases accelerated YAG decomposition, which in turn led to cluster growth of Y2O3 nanowires and aggregation of ZnAl2O4 particles at elevated temperature. The microstructure evolution and reaction mechanism were studied. The highest shear strength of 29.6 ± 5.2 MPa was obtained for the joint brazed at 650℃. The fracture morphology demonstrated that the dispersive strengthening of ZnAl2O4 and stress relief by Y2O3 nanowire network contributed to the superior mechanical performance.

Effects of MgO doping on microwave dielectric properties of yttrium aluminum garnet ceramics

In this study, the crystal structures and microwave dielectric properties of the Y3Al5O12 garnet (YAG) doped with Magnesium (Mg) were investigated. The samples were prepared by a method that the isostatic pressing technique was introduced into the conventional preparation of microwave dielectric ceramics. From XRD patterns of resulted compound, we see Y3Al5−xMgxO12–1/2x (x = 0–0.045) samples form a solid solution with MgO. The lattice parameters of the Y3Al5−xMgxO12–1/2x samples have been refined by the Rietveld method, and the unit cell volume of samples swell up slightly as thexvalue increases. The SEM images demonstrate that the porosity in the system has been effectively eliminated and the values of the relative density significantly increase due to the addition of Mg2+, which is conducive to the improvement of Q×f value. The TEM results present the high crystallinity of Y3Al5−xMgxO12–1/2x samples. The Raman spectra study illustrates the relationship between the vibrational FWMH values corresponding to the internal loss and the Q×f value. Finally, the Q×f value has been dramatically promoted from 133,209 GHz at x = 0–218,168 GHz at x = 0.035.

Volatile impurities in the structure of Y3Al5O12 garnet synthesized in water fluid

The mechanism of synthesis of fine-crystalline yttrium aluminum garnet Y3Al5O12 (YAG) in sub- and supercritical conditions of water medium was studied. It was found that, under subcritical conditions, the initial oxides are hydroxylated to form yttrium hydroxide, boehmite, and agglomerates of nanoscale YAG crystals and, under supercritical conditions, the nanoscale crystals are recrystallized yielding micron YAG crystals with well-formed faces. Two-stage synthesis in sub- and supercritical water fluid allows obtaining cerium-doped YAG crystals with high photoluminescence intensity. The gas content, gas release, degassing kinetics, and water diffusion coefficients in YAG at high temperatures were determined using the method of kinetic thermodesorption mass spectrometry. Water, carbon-containing substances and oxygen play a significant role in degassing. It was concluded that the ordering of the crystal structure occurs due to the appearance of solid-phase mobility of the oxide structure and changes in the state of hydroxyl groups. Knowing the content of volatile impurities and the temperature dependence of the diffusion coefficient allows us to develop the technology of high-temperature sintering of YAG crystals in order to obtain high-quality transparent ceramics.

Up-Converting Lanthanide-Doped YAG Nanospheres

The development of lanthanide-doped Y3Al5O12 (Ln:YAG) garnet nanostructures is a hot topic in the field of inorganic nanophosphors due to the current interest in developing small nanoparticles for solid-state lighting (SSL), displays, lasers and scintillation applications. In this study, we report the preparation of homogeneous Ln:YAG (Ln: Ho/Yb ions) nanospheres through a combined two-steps coprecipitation-solvothermal synthesis at low temperature. The crystal growth takes place in ethylene glycol, which is an inexpensive, non-toxic and easily available solvent. Monodisperse and crystalline spherical YAG particles of 80 nm in diameter were obtained. Furthermore, the protocol can be extended to other compositions (Tb/Yb, Tm/Yb…) to explore different luminescent properties, without affecting the morphology of the material, indicating the robustness and practical utility of the reported methodology. Thermal treatment of the nanogarnets at 1200oC is necessary for making materials optically active upon both UV and NIR excitation. The spherical morphology of annealed samples is preserved, what helps their further dispersion in solvents, barbotines, inks or printing vehicles. The lanthanide-doped nanogarnets exhibited the characteristic blue, green and red emissions from lanthanide upconversion photoluminescence (UCPL) upon NIR excitation. The UCPL mechanism was studied and CIE chromate coordinates were obtained. These nanogarnets were further evaluated as functional ceramic phosphors by incorporating them into commercial glazes. The materials exhibited an exceptional chemical stability in a harsh medium such as a fused glaze. Consequently, the visible emissions of the nanoparticles were transferred to the whole glass matrix, thus providing a functional glaze that emits intense blue and green light upon NIR excitation. These luminescent nanogarnets have promising applications in smart enamels, but can also be useful for lighting displays (white LEDs…), smart paintings or plastics, and anti-counterfeiting systems.

Impact of crystallization method on the strain, defect formation, and thermoluminescence of YAG:Ce crystals

Numerous studies have addressed the excellent properties of Ce-doped Y3Al5O12 garnet (YAG) crystals for utilization in lighting applications. However, the differences in the photoluminescence and defect formation of YAG crystals obtained by different methods have not been elucidated. In this study, the effects of the crystallization path of YAG:Ce on the local structure of its emission site and defect formation were investigated. Ce-doped YAG crystals were prepared in three ways: (1) solid-state reaction, (2) heat-treatment of YAG glass, and (3) crystallization from a supercooled YAG melt by a controlled cooling process from a deep supercooled state. For the sample obtained by the crystallization of a supercooled melt, the photoluminescence peak red-shifted by 3 nm relative to those of the other samples, and the thermoluminescence increased by a factor of 50–5000. As the thermoluminescence intensity has been associated with the defects formed, positron annihilation lifetime spectroscopy was performed to investigate the defects. The positron lifetimes of all crystallized samples exhibited two components, and the crystallized sample obtained by melt-cooling showed the longest lifetime among the samples.

Inhomogeneous broadening and splitting of lines in spectra of YAG : Tm

The shape and fine structure of lines due to Tm3+ f −f electronic transitions in multifunctional Y3Al5O12 garnet crystals have been studied by high-resolution spectroscopy. The observed inhomogeneously broadened lines have a Lorentzian shape, suggesting that point defects make a predominant contribution to the inhomogeneous broadening. Moreover, YAl antisite defects, which are formed during high-temperature melt growth, produce spectral satellites near the main lines.

First-principles study of the formation energies and positron lifetimes of vacancies in the Yttrium-Aluminum Garnet Y3Al5O12

Lattice vacancies are a major concern for the use of the Y3Al5O12 garnet (YAG) in optical applications. They are known to trap charge carriers preventing them from reaching luminescence centers. This reduces useful photon emission and deteriorates performance. Recent efforts to characterize such defects include experimental works by positron-annihilation spectroscopy (PAS) where extensive positron trapping was reported and attributed to defects made up of both cation and oxygen vacancies. The present study reports first-principles calculations for monovacancy and divacancy defects in YAG by means of conventional and two-component density-functional theory. The defect formation energies and corresponding charge-transition levels in the gap were initially determined. The ability of the lower-energy defects to act as positron-trapping centers was then examined. Corresponding positron lifetimes and binding energies to defects were calculated and compared to the experimental PAS data. The lifetimes of aluminum monovacancies agreed well with experiment if gradient corrections are included in the electron-positron correlation. Association of oxygen with aluminum vacancies was found to lead to stable negatively-charged divacancy complexes which also trap positrons. These defects are characterized by longer positron lifetimes, in agreement with the experimental observations.

Spectroscopic characteristics of Dy3+-doped Y3Al5O12 (YAG) and Y3ScAl4O12 (YSAG) garnet single crystals grown by the micro-pulling-down method

Dy3+-doped Y3Al5O12 (YAG) and Y3ScAl4O12 (YSAG) single crystals have been successfully grown by the micro-pulling-down (μ-PD) technique for the first time. The room temperature absorption spectra, fluorescence spectra and fluorescence decay curves were measured. Dy:YAG and Dy:Y3ScAl4O12 crystals exhibit strongest emission centered at 583 nm, corresponding to the 4F9/2 → 6H13/2 transition. The decay lifetime of the 4F9/2 level was measured to be 634 μs and 667 μs, respectively. All these results indicate that the Dy:YAG and Dy:Y3ScAl4O12 crystals are promising candidates for yellow laser operation.

Multiscale investigations of nanoprecipitate nucleation, growth, and coarsening in annealed low-Cr oxide dispersion strengthened FeCrAl powder

A major challenge in the design of oxide dispersion strengthened (ODS) FeCrAl alloys is the optimization of the fine-scale particle size distribution that provides both beneficial mechanical properties and irradiation resistance. To address this obstacle, the nucleation, growth, and coarsening of the fine-scale (Y,Al,O) nanoprecipitates within an ODS FeCrAl powder was studied using atom probe tomography (APT) and small-angle neutron scattering (SANS). Mechanically alloyed Fe10Cr-6.1Al-0.3Zr + Y2O3 wt.% (CrAZY) powders were heated in-situ from 20 to 1000 °C to capture the nucleation and growth of the nanoprecipitates using SANS. Furthermore, CrAZY powders were annealed at 1000 °C, 1050 °C, and 1100 °C for ageing times from 15 min to 500 h followed by either APT or magnetic SANS to study the structure, composition, and coarsening kinetics of the nanoprecipitates at high temperature. In-situ SANS results indicate nanoprecipitate nucleation and growth at low temperatures (200–600 °C). APT results revealed compositions corresponding to the cubic Y3Al5O12 garnet (YAG) stoichiometry with a possible transition towards the perovskite YAlO3 (YAP) phase for larger precipitates after sufficient thermal ageing. However, magnetic SANS results suggest a defective structure for the nanoprecipitates indicated by deviations of the calculated A-ratio from stoichiometric (Y,Al,O) phases. Particle coarsening kinetics follow n = 6 power law kinetics with respect to particle size, but the mechanism cannot be explained through the dislocation pipe diffusion mechanism. The potential effect of precipitate coarsening during pre- and post-consolidation heat treatments on the irradiation resistance of ODS FeCrAl alloys is discussed with respect to sink strength maximization.

(Invited) Investigation of Thermal Quenching Process for 5d-4f and 3d-3d Luminescence

White light LEDs (light-emitting diodes) are rapidly replacing incandescent lamps and (compact) fluorescent tubes in all lighting markets (consumer, automotive, and professional). The most widely used type of white LEDs (w-LEDs) is a phosphor-converted w-LED (PC-LED), which is composed of an InGaN-based blue LED and visible light emitting inorganic phosphors, such as oxides and nitrides doped with Ce3+, Eu2+ , Mn4+ as an active center. These phosphors are required to have excellent thermal quenching behavior because the LED chip reaches temperatures up to ∼200 °C in recent high-power w-LED. In these phosphors, two processes of thermal quenching are considered generally. One is the thermally activated cross over from the excited state to the next lower state. The other is the thermal ionization from the excited state to the bottom of the conduction band (CB). In this study, we investigated the thermal quenching mechanism for the 5d-4f and 3d-3d luminescence in w-LED phosphors doped with Ce3+ or Mn4+. Ce3+-doped Y3Al5O12 garnet (yttrium aluminum garnet,YAG) is the most prominent phosphor in w-LEDs. The excellent thermal quenching behavior of the Ce3+ luminescence in YAG:Ce3+ is well established, but the mechanism for thermal quenching remains unclear. To elucidate the mechanism of thermal quenching of YAG:Ce3+, thermoluminescence excitation (TLE) spectra were recorded at room temperature and 300 °C. At room temperature, the lowest 5d1 band at 450 nm does not contribute to the charging process for TL while at 300 °C, a temperature corresponding to the onset of thermal quenching of the Ce3+ luminescence, the excitation in 5d1 band gives rise to a TL signal. This result indicates that thermal ionization is responsible for thermal quenching of the Ce3+luminescence. We also investigated the thermal quenching process for calcium aluminum magnetoplumbite, CaAl12O19, doped with Mn4+ (CAO:Mn4+), which is narrow red luminescence phosphor. We tried to understand the quenching mechanism of Mn4+ compared with the luminescence quenching properties of Mn2+. From the temperature dependence of Mn4+: 2E-4A2 red luminescence and Mn2+:4T1- 6A1 green luminescence intensities in CaAl12O19, the quenching temperatures of Mn4+ and Mn2+ are 330K and 800K, respectively. There is a big difference between the quenching temperatures of Mn4+ and Mn2+ in spite of the same host material and the similar luminescence transtion energy. The Mn4+:2E-4A2 red luminescence is quenched by the thermally activated crossover through the 4T2 excited state whose potential curve possesses a large configurational offset in a configurational coordinate diagram. Mn2+ green luminescence is also quenched by the thermally activated crossover, but the activation energy for non-radiative process is much larger than that for Mn4+ because there are no other excited states related with the quenching process.