Growth, Quenching Mechanism, and Luminescent Properties of Mn4+∶K2Ge4O9 Single Crystals

doi:10.16553/j.cnki.issn1000-985x.2026.0037

Abstract

Abstract: Red-emitting luminescent materials constitute core components in solid-state lighting and display technologies，offering distinct advantages for high-color-rendering-index （CRI） white light-emitting diodes （wLEDs） and agricultural illumination. Commercial red phosphors currently rely heavily on rare-earth activators，such as Eu²⁺，Eu³⁺，and Sm³⁺. While these materials exhibit excellent luminescence via 4f-4f and 4f-5d transitions，inherent resource scarcity and complex extraction processes inflate costs，severely restricting their large-scale deployment. Consequently，developing efficient，rare-earth-free alternatives has emerged as a critical research frontier. In facility agriculture，specific light spectra precisely govern plant development. While blue light promotes vegetative growth，deep-red emission （~660 nm） is essential for flowering，fruiting，leaf expansion，and stem elongation，directly dictating crop maturation cycles and ultimate yields. Transition metal Mn⁴⁺ demonstrate exceptional suitability for agricultural applications，as their emission profile perfectly matches the peak absorption of plant phytochromes. Because the luminescent properties of Mn⁴⁺ depend heavily on the local crystal field，selecting an appropriate host lattice is paramount. K₂Ge₄O₉ （KGO） represents an ideal matrix. Here，Ge⁴⁺ perfectly matches Mn⁴⁺ in both ionic radius and valence state. Furthermore，its unique tetrahedral-octahedral composite framework provides the requisite ［GeO₆］ octahedral sites for Mn⁴⁺ incorporation. Despite these structural advantages，traditional Mn∶KGO polycrystalline phosphors suffer from poor photothermal stability，retaining less than 40% of their room-temperature emission intensity at 100 ℃，with underlying thermal quenching mechanisms remaining ambiguous. To address the severe thermal quenching bottleneck commonly observed in traditional Mn⁴⁺-doped polycrystalline oxide phosphors，a novel low-melting-point Mn∶KGO single crystal was successfully grown using the Czochralski method. Through density functional theory （DFT） calculations，a site-dependent dual thermal quenching mechanism within this system is elucidated. Specifically，Mn⁴⁺ occupying the Ge1 sites are restricted by parity selection rules，whereby a carrier autoionization process is triggered. Conversely，Mn⁴⁺ at the Ge2 sites，which serve as the primary luminescent centers，are subjected to strong covalent hybridization with the valence band，forcing the charge transfer state energy to be lowered significantly. Furthermore，it is demonstrated that non-radiative transition pathways are effectively suppressed by the continuous lattice structure of the single crystal. Consequently，the thermal quenching temperature of the crystal is elevated by 6.2 ℃，and the internal quantum efficiency （IQE） is substantially increased by 44%，reaching a remarkable value of 48.3%. In terms of device application evaluations，it was observed that upon the integration of this single crystal into wLED packages，the CRI is improved by 7.5，and the correlated color temperature （CCT） is reduced by over 2 000 K，successfully driving a transition from cool to warm white light. Simultaneously，the 664 nm deep-red emission spectrum of the crystal is found to be closely matched with the characteristic absorption bands of plant phytochromes. Ultimately，a low-melting-point Mn⁴⁺-doped oxide crystal is developed in this study，which provides a highly promising single-crystal red-emitting material for high-power solid-state lighting and plant growth illumination applications.

Key words: Mn:KGO crystal; thermal quenching; DFT; white LED; plant growth illumination

CLC Number:

O734

SONG Qingsong, LIU Jian, ZHANG Fan, ZHANG Chaoyi, WANG Wudi, CAO Xiao, QIAN Xinyu, TANG Huili, WANG Qingguo, ZHANG Chenbo, LIU Bo, XU Xiaodong, XU Jun. Growth, Quenching Mechanism, and Luminescent Properties of Mn⁴⁺∶K₂Ge₄O₉ Single Crystals[J]. Journal of Synthetic Crystals, 2026, 55(6): 910-929.

Figures/Tables 29

Fig.1 Phase diagram of the K2O-GeO2 system［26］

Fig.2 Physical photo of the Mn∶KGO sample prepared during the exploration stage and its luminescence image under ultraviolet light. （a） Samples formed by the molten drops from the feed rods in the optical floating zone furnace （transparent and orange parts）；（b） transparent sample obtained from the molten drops via the floating zone method；（c） luminescence images of the transparent and orange samples under UV irradiation；（d） pre-sintered 0.1%Mn∶KGO powder；（e） crystal formed by melting the pre-sintered powder （Fig.（d）） in a muffle furnace；（f） luminescence images of the samples in Fig.（d） under UV irradiation；（g） luminescence images of the samples in Fig.（e） under UV irradiation

Fig.3 Photographs of KGO crystals with varying Mn-doping concentrations grown by the Czochralski method. （a） 0.1%Mn∶KGO crystal，where the inset illustrates the sampling regions for spectroscopy and ICP measurements alongside their corresponding designations；（b） pure KGO crystal；（c） 0.05%Mn∶KGO crystal；（d） 0.05%Mn，0.5%Si∶KGO crystal

Table 1 Densities of the KGO crystal samples

Sample	Average dry weight/g	Average buoyant weight/g	Density/（g·cm^-3）
KGO-as-grown crystal	33.441 7	25.650 0	4.292
KGO-crucible residue	71.448 4	54.647 7	4.253
0.1%Mn∶KGO-E2	0.767 6	0.588 7	4.287
0.1%Mn∶KGO-I1	0.357 2	0.274 5	4.315
ICSD#3169 KGO （calculated）			4.292

Fig.4 XRD patterns of the KGO samples

Table 2 Lattice parameters of the KGO crystal samples

Sample	a/Å	c/Å
KGO-FZ-T	11.765 2	9.786 0
KGO-FZ-O	11.812 5	9.795 1
KGO-TGFM	11.812 5	9.798 9
ICSD#31969 KGO	11.84	9.80

Fig.5 Crystal structure （a） and supercell models （b）~（d） of KGO

Table 3 Optimized crystal structure parameters of the KGO model

KGO112-K₂₄Ge₄₈O₁₀₈

Ge1 1.947 2

Ge2 1.917 6

9.837 9

9.392 7

1.565 1

2.388 9

0.005 9

Fig.6 Room-temperature Raman spectra of Mn∶KGO crystal and phosphor

Fig.7 XPS of the Mn∶KGO crystal

Fig.8 EPR spectrum of the Mn∶KGO crystal

Fig.9 Optical properties of different regions in the 0.1%Mn∶KGO crystal

Fig.10 Optical properties of the Mn∶KGO crystal

Fig.11 DFT-calculated electronic structures and optical properties of pure and Mn-doped KGO

Table 4 Transmittance and absorption coefficients of the KGO crystal samples

Sample	T@650 nm/%	$α a b s$ @460 nm/cm^-1	UV cutoff edge/nm	Optical band gap/eV
0.1%Mn∶KGO-E1	85.2	0.83	233.0	4.95
0.05%Mn∶KGO	86.6	0.28	229.5	5.31
0.05%Mn，0.5%Si∶KGO	61.1	1.89	231.5	5.00

Table 4 Transmittance and absorption coefficients of the KGO crystal samples

Sample	T@650 nm/%	$α a b s$ @460 nm/cm^-1	UV cutoff edge/nm	Optical band gap/eV
0.1%Mn∶KGO-E1	85.2	0.83	233.0	4.95
0.05%Mn∶KGO	86.6	0.28	229.5	5.31
0.05%Mn，0.5%Si∶KGO	61.1	1.89	231.5	5.00

Fig.12 Luminescence spectra and fluorescence lifetime of different regions in the 0.1%Mn∶KGO crystal

Fig.13 Gaussian fitting of the excitation spectra for different samples of the 0.1%Mn∶KGO crystal

Table 5 Spectroscopic parameters of the Mn∶KGO crystal

Sample	CTB/cm^-1	⁴A_2g→⁴T_1g，a/cm^-1	⁴A_2g→²T_2g/cm^-1	⁴A_2g→⁴T_2g/cm^-1	²E_g→⁴A_2g/cm^-1	R²
KGO-E1	34 444	28 933	24 972	21 601	15 337	0.997 4
KGO-E2	34 795	28 913	24 222	21 565	15 337	0.997 6
0.1%Mn∶KGO	34 667	29 213	24 966	21 623	15 337	0.998 1
0.05%Mn∶KGO	34 610	29 662	24 530	21 606	15 337	0.997 8
0.05%Mn，0.5%Si∶KGO	34 599	29 272	25 352	21 580	15 337	0.999 1

Fig.14 Emission spectra of the Mn∶KGO crystal at room temperature and 100 K

Fig.15 Luminescence spectra and fluorescence lifetime of the Mn∶KGO crystals

Table 6 Crystal field parameters Dq，B，C，and β1 of selected Mn4+-doped germanate phosphors

Sample	D_q	B	C	D_q /B	β₁
K₂SrGe₈O₁₈ phosphor^［34］	2 336.4	800.1	3 064.4	2.92	0.991
K₂BaGe₈O₁₈ phosphor^［46］	2 164.5	654.0	3 345.5	3.31	0.960
LiBaGe₄O₉ crystal^［47］	2 175.2	813.0	3 235.7	2.68	1.028
KGO-E1	2 160.1	701.0	3 347.2	3.08	0.985
KGO-E2	2 156.5	703.3	3 342.5	3.07	0.985
0.1%Mn∶KGO	2 162.3	731.4	3 280.1	2.96	0.989
0.05%Mn∶KGO	2 160.6	788.5	3 156.1	2.74	1.000
0.05%Mn，0.5%Si∶KGO	2 158.0	744.1	3 252.7	2.90	0.991

Fig.16 Energy level diagram and related parameter dependencies of the Mn∶KGO crystals

Fig.17 Quantum efficiency of the Mn∶KGO crystal

Table 7 Internal and external quantum efficiencies （IQE/EQE） of Mn4+-doped germanate materials

Sample	λ_ex/nm	IQE/%	AE/%	EQE/%
K₂SrGe₈O₁₈ phosphor^［34］	287	35.50	52.96	18.80
K₂BaGe₈O₁₈ phosphor^［46］	365	32.90
Ba₂GeO₄ phosphor^［53］	290	22.53		—
（Mg，Ba）₃Al₂GeO₈ phosphor^［54］ Mg₃Al₂GeO₈ phosphor^［54］	284 284	— —	— —	49.35 29.44
BaGe₄O₉ phosphor^［55］ SrGe₄O₉ phosphor^［55］	440 430	50.00 46.00
LiNaGe₄O₉ phosphor^［56］	310 450	78.00 48.00
LiNaGe₄O₉ glass-ceramic^［57］	—	37.50
Mn∶KGO phosphor^［52］	300 460 470	45.00 33.50 33.80
0.05%Mn∶KGO	460	46.10	17.70	8.16
0.05%Mn，0.5%Si∶KGO	460	27.10	25.50	4.34
0.1%Mn∶KGO-E1	460	48.30	37.00	17.87
E2	460	41.60	60.60	25.21
E3	460	38.90	60.60	23.57
I1	460	46.70	43.60	20.36
I2	460	35.10	40.70	14.29
I3	460	41.10	47.00	19.32

Fig.18 Temperature-dependent luminescence properties of the Mn∶KGO crystal at low temperatures. （a） Low-temperature emission spectra；（b） temperature dependence of the integrated PL intensity and zero-phonon line （ZPL） peak intensity；（c） temperature-dependent fluorescence decay curves

Fig.19 Comparison of the temperature-dependent luminescence properties between the Mn∶KGO crystal and phosphor at elevated temperatures. Temperature-dependent emission spectra of the crystal （a） and phosphor （b），respectively； temperature-dependent fluorescence decay curves of the crystal （c） and phosphor （d），respectively；（e） temperature dependence of the integrated PL intensity

Table 8 CIE chromaticity coordinates and color purity parameters of Mn∶KGO crystals

Sample	CIE 1931 chromaticity coordinates （X，Y）	Color purity/%
0.1%Mn∶KGO	（0.724 1，0.275 8）	98.24
0.05%Mn∶KGO	（0.724 0，0.276 0）	98.21
0.05%Mn，0.5%Si∶KGO	（0.724 2，0.275 8）	98.26

Fig.20 Spectral performance and chromaticity coordinates of LED devices packaged with the Mn∶KGO crystal. Electroluminescence spectra of the plant-growth LED device （a） and the white LED device （b）；（c） CIE chromaticity diagram of the Mn∶KGO crystal and the LED devices

Fig.21 Comparison of garlic growth conditions under sunlight，dark field，and plant growth LED illumination

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Growth, Quenching Mechanism, and Luminescent Properties of Mn⁴⁺∶K₂Ge₄O₉ Single Crystals

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