Influence of alkaline ions on the luminescent properties of Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors

Journal of Luminescence 192 (2017) 1072–1083

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Influence of alkaline ions on the luminescent properties of Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors

MARK

Junpeng Xuea, Weiguang Rana, Hyeon Mi Noha, Byung Chun Choia, Sung Heum Parka, ⁎ Jung Hyun Jeonga, , Jung Hwan Kimb a b

Department of Physics, Pukyong National University, Busan 608-737, Republic of Korea Department of Physics, Dongeui University, Busan 614-714, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Germanate Mn4+ Luminescence Red-emitting phosphors

Mn4+ -doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors were successfully prepared and characterized. Owing to the different ionic radii and electron configurations among the Li, Na and K ions, the MGe4O9 (M = Li2, LiNa and K2) compounds have different space group and crystal structure, such as different bond lengths, bond angles, crystal fields and nephelauxetic effect. Under the excitation of ultraviolet and blue light, the studied samples emit visible red emissions. The thermal stabilities of phosphors are greatly affected by the alkaline ions, especially when M = LiNa and K2, the thermal stability is significantly enhanced. Furthermore, the crystal field strength (Dq) and Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ ions in MGe4O9 (M = Li2, LiNa and K2) host lattices. The results indicate that the alkaline ions can influence the red emissions of Mn4+ ions in MGe4O9 (M = Li2, LiNa and K2) phosphors and these resultant phosphors may have potential applications for solid state lighting.

1. Introduction In recent years, red-emitting phosphor, which plays an important part in the field of solid-state lighting, display, signal indication, plant cultivation, optical temperature sensing and solar cell, has been extensively studied [1–10]. Furthermore, it is also widely used to improve the correlated color temperature and color rendering index of the phosphor-converted white light-emitting diodes (WLEDs). Up to now, the rare-earth ions, such as Eu3+, Sm3+, Pr3+ and Eu2+, are intensively investigated as the red-emitting activators [11–20]. Nevertheless, these trivalent rare-earth ions activated phosphors usually show narrow 4f-4f excitation lines in the near-UV and blue region, which leads to weak absorption and limits their applications. Among the developed redemitting phosphors, the Eu3+-doped Y2O2S red-emitting phosphors is commercialized. Unfortunately, it is found that the Y2O2S:Eu3+ phosphor has a weak absorption band in the blue region and its practical application is restricted to some degree. Moreover, the sulfide phosphors also have poor thermal stability and are susceptible to moisture, which leads to an environmental threat [21,22]. On the other hand, for other red-emitting phosphors based on Eu2+ ions doped nitrides, a well-known defect is that the synthesis of nitride phosphors needs severe conditions, such as high temperature and high pressure (typically prepared under 1800 °C, 0.5 MPa N2 pressure) [5,23]. Also, the

⁎

Corresponding author. E-mail address: [email protected] (J.H. Jeong).

http://dx.doi.org/10.1016/j.jlumin.2017.08.036 Received 4 July 2017; Received in revised form 15 August 2017; Accepted 18 August 2017 Available online 19 August 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.

absorption of the phosphor covers the spectral range of 200–650 nm. Therefore, serious re-absorption will take place when the nitrides phosphors are mixed with other green-emitting or yellow-emitting phosphors, resulting in unstable emission color and lower luminous efficacy of LED devices [23,24]. Considering the aforementioned issues, Mn4+ ions in octahedral environment can show a strong broad absorption band spanning from 300 to 480 nm and emit a bright red emission ranging from 600 to 760 nm [23]. Up to date, some inorganic materials have been successfully developed for Mn4+ ions, such as fluorides, and oxides [24–29]. Although the Mn4+ ions doped fluoride compounds usually exhibit high luminescence efficiency, they suffer from low thermal stability and low water-resistance as well as a complex synthesis process. In comparison, Mn4+ ions doped oxides, which exhibit better chemical stability and an eco-friendly preparation process, are thought to be promising candidates for blue chip based WLEDs as red-emitting phosphors. It has been reported that Mn4+-doped oxides, including CaMg2Al16O27, BaMgAl10O17, SrGe4O9, BaGe4O9, Li2MgTiO4, La (MgTi)0.5O3, Sr2LaNbO6, and Ba2LaNbO6, show promising applications in WLEDs [10,23,30–36]. In spite of these admirable achievements, much attention still should be paid to improve the luminescent efficiency of Mn4+ doped oxides. By substituting the appropriate elements in the luminescent host is a

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light source for excitation of the samples. All the measurements were performed at ambient temperature except thermal property.

widely strategy to modify the luminescence properties of the phosphors. As is known, various chemical compositions will produce different crystal field environments, which further influence the luminescent properties of the phosphors. Researchers have found that replacing W with Mo can enhance the emission intensity in the Sr2ZnW1-xMoxO6:Eu3+, Li+ phosphors, substituting with smaller cations can shift towards the red region in the emission spectra of Eu2+doping (Ca,Sr,Ba)4MgAl2Si3O14 and substituting of Si4+ and N3− for B3+ and O2− not only results in red shift of the excitation band, but also a significant increase thermal stability in La5(Si2+xB1−x) (O13−xNx):Ce3+ [37–39]. MGe4O9 is an important branch of germinate and M are alkaline ions, including Li2, LiNa, K2, Sr, Ba, etc [1,21,40–42]. For the MGe4O9 compounds, they contain orthorhombic and trigonal crystal system with two kinds of Ge ligands (GeO6 and GeO4). As is well known, the luminescent properties of Mn4+ doped phosphors are largely dependent on the luminescent hosts. Alkaline ions with different radius and electrons can impact the host, which further affect the emission of Mn4 +ions. In this paper, we chose the MGe4O9 (M = Li2, LiNa and K2) as the luminescent hosts and a series of Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors were synthesized by the conventional solid-state reaction method. The photoluminescence (PL) properties, diffuse reflectance spectra, concentration quenching mechanism and fluorescence decay curves of the synthesized materials are studied in detail. In addition, the thermal quenching properties, quantum yield, crystal field analysis and nephelauxetic effect of the final products are also discussed.

2.3. Details of calculation The band structure and density of states (DOS) calculations of Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 were performed using first-principles quantum mechanical program CASTEP of Materials studio based on density functional theory (DFT). The ultrasoft pseudopotential was chosen in the calculations because it is efficient and credible. Firstly, geometry optimization was performed using the Perdew–Burke–Emzerh of general gradient approximation (GGA-PBE) algorithm. Electronic structures and density of states were calculated on the basis of the optimized Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 structure. The planewave basis set energy cutoff was set at 10 eV for Li2Ge4O9, at 370 eV for LiNaGe4O9 and at 340 eV for K2Ge4O9. In the sampling of the Brillouin zone of the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 unit cell, the Monkhorst–Pack k-point mesh 1 × 1 × 1 for Li2Ge4O9, 2 × 5 × 3 for LiNaGe4O9 and 2 × 2 × 4 for K2Ge4O9 were chosen for the electronic structures and band structures. 3. Results and discussion 3.1. Structure properties The representative XRD patterns of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ with the corresponding standard JCPDS cards are shown in Fig. 1. As shown in Fig. 1(a), it is clear that most of the diffraction peaks are in accord with the standard orthorhombic phase of Li2Ge4O9 (JCPDS #37-1363) and some weak impurity peaks (marked as black dots) are coincided with the orthorhombic Li2Ge7O15 (JCPDS #49-0523). The low cooling rate may be the main reason for the formation of Li2Ge7O15 [40]. Owing to the small amount impurity of Li2Ge7O15, it has little influence on the luminescent properties of Mn4+ ions doped Li2Ge4O9 phosphors. Furthermore, the diffraction peaks of synthesized LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ samples match well with those of standard Li2Ge4O9 (JCPDS# 37-1363) and K2Ge4O9 (JCPDS# 40-1188), respectively, indicating that the doping of Mn4+ ions does not cause significant influence on the host crystal structure. As is known, the crystal structure information of host materials is

2. Experimental procedure 2.1. Sample preparation The Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors were prepared through a milder-temperature solid-state reaction technique. According to the appropriate stoichiometric ratio, the starting materials, Li2CO3 (99.997%), Na2CO3 (99.995%), K2CO3 (99.60%), GeO2 (99.999%) and MnCO3 (99.99%), were weighed and ground finely in an agate mortar for 30 min with an appropriate amount of ethanol. Then, the homogeneous mixture was kept in a crucible and sintered in a muffle furnace at 900 °C for 6 h. After cooled to room temperature, the obtained white samples were ground to a fine powder for further characterization. 2.2. Characterization and optical measurements The X-ray diffraction (XRD) measurement was performed to verify the phase purity by a Philips X′Pert MPD (Philips, Netherlands) X-ray diffractometer at 40 kV and 30 mA. The diffraction patterns were scanned within an angular range of 10–70° (2θ). The morphology and particle size of phosphors was characterized using a scanning electron microscope (SEM) system (JSM-6490, JEOL Company). The energy dispersive X-ray system (EDS) and elemental mappings of samples were examined using field emission transmission electron microscopy (TEM) (JEOL JEM- 2010F). UV–vis diffuse reflectance spectra (DRS) were collected by a V-670 (JASCO) UV–vis spectrophotometer. The PL and PL excitation (PLE) spectra were recorded by a Photon Technology International (PTI, USA) fluorimeter with a 60 W Xe-arc lamp as the excitation light source. The temperature dependence of PL spectra was measured by using the FS-2 system and the temperature ranging from 303 K to 443 K was controlled using a homemade temperature controlled system. X-Ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 5000 VersaProbe spectrometer using a monochromatic Al Kα radiation source. The quantum yields (QY) of the samples were measured by means of an integrating sphere and a FLS 920 fluorescence spectrophotometer. The powder samples were placed into the integrating sphere and the 450 W Xe lamp was employed as the

Fig. 1. XRD patterns of (a) Li2Ge4O9:0.002Mn4+, (b) LiNaGe4O9:0.002Mn4+, (c) K2Ge4O9:0.002Mn4+ phosphors and corresponding standard card (JCPDS# 37-363), (JCPDS# 40-1188) and (JCPDS# 49-0523).

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Fig. 2. The crystal structures of (a) Li2Ge4O9, (b) LiNaGe4O9 and (c) K2Ge4O9.

681.69 Å3 for the Li2Ge4O9, and for LiNaGe4O9, a = 9.32 Å, b = 4.68 Å, c = 15.90 Å, Z = 4 and V = 694.11 Å3. In comparison, the K2Ge4O9 exhibits larger lattice parameters of a = b = 11.79 Å, c = 9.75 Å, Z = 6 and V = 1173.72 Å3. In addition, the three compounds have different bond lengths and bond angles in octahedron, respectively. As described in Fig. 2, the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 comprise four different types of germanium sites, that is, Ge (1), Ge (2), Ge (3) and Ge (4). From Fig. 2(a), for the Li2Ge4O9, the sites of Ge (2), Ge (3) and Ge (43) are tetrahedral environment, while Ge (1) is an octahedral environment. It can be seen that each GeO6 is connected with five GeO4 tetrahedron via sharing common corners and sides and the GeO6 octahedra is isolated with each other. As shown in Fig. 2(b), the sites of Ge (1), Ge (2) and Ge (3) are tetrahedral environment, while Ge (4) is an octahedral environment. It is evident that each GeO6 is connected with three GeO4 tetrahedron via sharing common corners and the GeO6 octahedra is separated with each other. On the other hand, as for the K2Ge4O9, there are six formula units in one-unit cell and four different kinds of germanium sites in the crystal structure (see Fig. 2(c)). The sites of Ge (2), Ge (3) and Ge (4) are octahedral environment, while remanent Ge (1) is tetrahedral circumstance. Ge (1) builds up the basic frame structure and Ge (3) is gathered around Ge (1) by common angle connections. At the same time, Ge (2) is in the internal site of the single cell and Ge (4) is gathered around Ge (2) by

Table 1 The crystallographic data of MGe4O9 (M = Li2, LiNa and K2). Compounds

Li2Ge4O9

LiNaGe4O9

K2Ge4O9

Crystal system Space-group a(Å) b(Å) c(Å) V(Å3) N Average bond lengths of octahedron (Å) Average bond angles of octahedron(°)

orthorhombic P21ca 9.31 4.63 15.82 681.69 4 1.873

orthorhombic Pcca 9.32 4.68 15.9 694.11 4 1.885

trigonal P3 c1 11.79 11.79 9.75 1173.72 6 1.963

176.46

175.07

171.54

closely related to the luminescent properties of Mn4+ ions. The crystal unit cell structures and crystallographic data of Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 are displayed in Fig. 2 and Table 1. Clearly, Li2Ge4O9 possesses orthorhombic phase with the space group of P21ca (No. 29) and LiNaGe4O9 also shows orthorhombic phase with the space group of Pcca (No. 54) at room temperature, while K2Ge4O9 belongs to a trigonal system with the space group of P3 c1 (No. 165) [40,41,43]. Note that, with increasing the ionic radii of alkaline ions, the space group of the host changes as well as the varied phase structure. In particular, the cell parameters are a = 9.31 Å, b = 4.63 Å, c = 15.82 Å, Z = 4 and V = 1074

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Fig. 3. SEM images of prepared phosphors (a) Li2Ge4O9:0.002Mn4+; (b) LiNaGe4O9: 0.002Mn4+; (c) K2Ge4O9:0.002Mn4+. XPS survey spectrum of the (d) Li2Ge4O9: 0.002Mn4+; (e) LiNaGe4O9:0.002Mn4+; (f) K2Ge4O9:0.002Mn4+. The insets show high resolution XPS spectra position of Mn 2p core-level XPS spectra for 2p1/2 and 2p3/2 of Mn4+ in Li2Ge4O9, LiNaGe4O9 and K2Ge4O9.

particle sizes for these three compounds are approximately 1 µm. To examine the element component of these samples, their EDX spectra were detected, as shown in Fig. 4(a)–(c). The peaks of sodium (Na), germanium (Ge), oxygen (O), manganese (Mn) and potassium (K) are clearly seen in the EDX spectra, whereas the Li cannot be detected due to its light element property and the Cu signal in the spectra is attributed to the instrument. Furthermore, the weight percentages and atomic percentages of the elements present in the samples, which are presented in the inset of Fig. 4(a)–(c), further indicating that Mn4+doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors are successfully synthesized. To support the EDX analysis and show the elements distribution in the samples, two-dimensional elemental mappings were done, as shown in Fig. 4(d)–(f). From the elemental mapping (Fig. 4(d)–(f)), one knows that the element compositions of Na, Ge, O, Mn and K are uniformly distributed over the whole particles, revealing

common edge connections as well. According to the exchange charge model, we can predict that there is no obvious preference for Mn4+ ions occupies the different types of octahedral germanium sites in K2Ge4O9 [1]. The sites of GeO6 octahedrons should be ideal sites to be substituted by Mn4+ ions from the point of the radius match and charge balance (RGe4+ = RMn4+ = 0.53 Å) [1]. If the Ge4+ ions are substituted by the Mn4+ ions, there is no need for charge compensation, resulting in the minimize the symmetry distortion of crystal field around the Mn4+ ions as well as superior luminescence performance in these Mn4+ ions doped compounds. Fig. 3(a)–(c) present the SEM images of obtained Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+and K2Ge4O9:0.002Mn4+ red-emitting phosphors, respectively. It can be seen that the Li2Ge4O9:0.002Mn4+ red-emitting phosphors present blocky-granular shape, while both LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors are made of irregular particles. However, the 1075

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Fig. 4. EDX spectra of the (a) Li2Ge4O9:0.002Mn4+, (b) LiNaGe4O9:0.002Mn4+ and (c) K2Ge4O9:0.002Mn4+ phosphors. Elemental mappings of the (d) Li2Ge4O9:0.002Mn4+, (e) LiNaGe4O9:0.002Mn4+ and (f) K2Ge4O9:0.002Mn4+ samples. The insets show the weight percentages (wt%) and atomic percentages (at%) of the elements in the samples.

the formation of MGe4O9:Mn4+ (M = Li2, LiNa and K2) red-emitting phosphors matches well with result achieved from the EDX spectra. To further investigate the surface compositions and the valence state of manganese in the synthesized phosphors, the XPS measurements were carried out. As shown in Fig. 3, the signals of Li, Na, Ge, O, K and C species were clearly detected. The C1s peak at around 284.6 eV is attributed to the signal from carbon contained in the instrument which was used for calibration or the absorption of CO2 [18,34]. Meanwhile, the relatively weak manganese (Mn) signals are also observed and the bonding energy of Mn4+ 2P2/3 is estimated at 642.42 eV (see inset of Fig. 3). It is known that the peaks of Mn2+ 2p3/2 (MnO), Mn3+ 2p3/2 (Mn2O3) and Mn4+ 2p3/2 (MnO2) located at 641.7, 641.8 and 642.4 eV, respectively [44]. Therefore, we can confirm that the valence of the Mn ions is +4.

3.2. Luminescence properties The DR spectra of pure host and Mn4+ ions doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors are shown in Fig. 5(a)–(c). The Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 host show a platform of high reflection in the wavelength range of 400–800 nm and then slightly increase from 400 to 350 nm, after that, they dramatically decrease from 350 nm to 200 nm. When Mn4+ ions are doped into the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 host, the reflection spectra of the studied samples show two strong broad absorption bands over the range of 250–520 nm. Especially, two obvious dips located at about 330 and 470 nm are assigned to strong spin-allowed transitions of Mn4+ ions corresponding to 4A2g → 4T1g and 4A2g → 4T2g transitions, respectively. Besides, the broad dip at in the wavelength range of 280–360 nm is considered to be composed of the Mn4+-O2- charge transfer transition 1076

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Fig. 5. UV–vis DRS of (a) Li2Ge4O9 and Li2Ge4O9:0.002Mn4+; (b) LiNaGe4O9 and LiNaGe4O9:0.002Mn4+; (c) K2Ge4O9 and K2Ge4O9:0.002Mn4+. Insets show the band gap of pure Li2Ge4O9, LiNaGe4O9 and K2Ge4O9. PL and PLE spectra of (d) (e) LiNaGe4O9: Li2Ge4O9:0.002Mn4+; 0.002Mn4+; (f) K2Ge4O9:0.002Mn4+.

and 4A2g → 4T1g transition of Mn4+ ions. The relatively strong broad absorption at 330 and 470 nm in the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ phosphors matched well with their excitation spectra, as shown in Fig. 5(d)–(f). By means of following equation, the band gaps of the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 are estimated [45]:

[hvF (R∞)]n /2 = A (hv − Egap)

F(R∞) = (1−R)2 /2R = K / S

(2)

where R, K and S are the reflection, absorption and scattering coefficient, respectively. From the linear extrapolation of [hvF (R∞)] 2 = 0 (see Fig. 5), the Egap values of Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 were estimated to be about 5.34, 5.50 and 5.46 eV, respectively. Base on the crystal structure of studied samples, the density functional theory calculations of MGe4O9 (M = Li2, LiNa and K2) were performed and the corresponding results are shown in Fig. 6. The local density approximation (LDA) was chosen as the theoretical basis of the density function. As shown in Fig. 6, the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 compound possess a direct band gap of 2.946 eV, 2.940 eV and 3.321 eV, respectively, with the valence band (VB) maximum at the G point and the conduction band (CB) minimum at the G point of the

(1)

where hv is the photon energy, A is a proportional constant, Egap is the value of the band gap, n = 1 for a direct transition or 4 for an indirect transition and F(R∞) is a Kubelka–Munk function defined as follows [46]: 1077

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Fig. 6. Band structure of (a) Li2Ge4O9, (c) LiNaGe4O9 and (e) K2Ge4O9; total and partial density of states for (b) Li2Ge4O9, (d) LiNaGe4O9 and (f) K2Ge4O9.

d-d inner transitions of Mn4+ ions. As exhibited in Fig. 5(d)–(f), the PLE spectrum can be fitted by three Gaussian curves, leading to three distinguished bands centered at around 296 (band A, 33,727 cm−1), 344 (band B, 29,028 cm−1) and 444 (band C, 22,522 cm−1) nm for Li2Ge4O9:0.002Mn4+, and 299 (band A, 33,444 cm−1), 350 (band B, 28,571 cm−1) and 447 (band C, 22,371 cm−1) nm for LiNaGe4O9:0.002Mn4+, and 303 (band A, 33,003 cm−1), 320 (band B, 31,250 cm−1) and 449 (band C, 22,271 cm−1) nm for K2Ge4O9:0.002Mn4+, which are in good agreement with those in the diffuse reflection spectra. The excitation band C is assigned to the spinallowed (4A2g → 4T2g) transitions of Mn4+ ions [10,35]. The broad band, which is composed of band A and B, is due to the overlap between the transitions of Mn4+-O2- and the spin-allowed transitions of Mn4+ (4A2g → 4T1g) [27,35]. Furthermore, the narrow red emission peaks, which are observed in the synthesized samples, are originated from the spin forbidden 2Eg →4A2g transition of 3d3 electrons in the [MnO6]8octahedral complex [29]. For the Mn4+ ions doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 phosphors, the emission peaks are centered at 669 (14,948 cm−1), 661 (15,129 cm−1) and 664 (15,060 cm−1) nm, respectively. From Fig. 5(d)–(f), it can be found that the positions of the

Brillouin zone. The values of band gap suggested that the MGe4O9 (M = Li2, LiNa and K2) host provide a suitable band gap for Mn4+ ions acting as an emission center. The calculated band gap of Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 were much smaller than the band gap gained from DRS, due to the underestimation in the LDA calculation. The total density of states and projected density of states of Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 are described in Fig. 6(b), (d) and (f), respectively. The conduction band of MGe4O9 (M = Li2, LiNa and K2) are composed mostly of Ge 4s states and a slight contribution of the O 2p state, while the valence band of them are different. The valence band of Li2Ge4O9 is mainly composed of Li 2s, Ge 4s, O 2s and O 2p states and LiNaGe4O9 is made up of Li 2s, Na 2s, Na 2p, Ge 4s, O 2s and O 2p states, whereas K2Ge4O9 originates predominantly from K 4s, K 3p, Ge 4s, O 2s and O 2p states. The results suggested that different alkaline ions (Li+, Na+ and K+) have an effect on the band gap values of compounds, the total density of states and projected density of states. Fig. 5(d)–(f) show the PLE and PL spectra of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors, respectively. It can be seen that the PLE spectra demonstrate several excitation bands between 200 and 500 nm originating from the 1078

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Fig. 8. Decay curves of Li2Ge4O9:0.002Mn4+, K2Ge4O9:0.002Mn4+ phosphors under different excitation. Fig. 7. CIE 1931 chromaticity diagram of the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+, K2Ge4O9:0.002Mn4+ phosphors, commercial Y2O3:Eu3+, 3.5MgO·0.5MgF2·GeO2:Mn4+ phosphors and standard red light.

red phosphor Y2O2S:Eu3+ (0.647,0.343) [48,49]. Upon UV light (365 nm) excitation, luminescent images of the resultant samples are presented in the inset of Fig. 7. Since the color purity of phosphor is an important feature to evaluate the phosphor chromatic property, the color purity of studied materials was calculated by using the following equation [14,50]:

absorption band of Li2Ge4O9 and LiNaGe4O9 are almost similar, and those of the K2Ge4O9 are different, especially the position of band B. In addition, the emission peak positions and intensity of Mn4+ ions doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 phosphors are different. It is well known that the crystal field strength and nephelauxetic effect is related with bond lengths and bond angles between the Mn4+ ions and ligands and can influence the optical properties of Mn4+ ions. It is precisely because the alkaline ions have different radius to cause different crystal environments, including bond lengths and bond angles, which result in the different excitation and emission positions and intensity for Li2Ge4O9, LiNaGe4O9 and K2Ge4O9. In addition, the position of the emission peaks of samples at 450 nm excitation is the same as the position of the emission peaks under UV excitation, except the intensity. As illustrated in Fig. 7 and Table 2, the CIE 1931 chromaticity coordinates of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors, which were estimated from their PL spectra under UV light excitation, are (0.723, 0.277), (0.723, 0.278) and (0.719,0.280), respectively, approaching to that of the commercial red-emitting phosphor 3.5MgO·0.5MgF2·GeO2:Mn4+ (0.711, 0.289) [23,47]. In comparison, when excited at 450 nm, the CIE 1931 chromaticity coordinates of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ phosphors are (0.618, 0.369), (0.664, 0.329) and (0.685, 0.310), respectively, which approach to the standard red light (0.67, 0.33) as well as the commercial

Color purity =

Samples

Excitation wavelength

CIE 1931 coordinate

Color purity

Decay time (μs)

a1 a2 a3 b1 b2 b3

Li2Ge4O9:0.002Mn4+ LiNaGe4O9:0.002Mn4+ K2Ge4O9 :0.002Mn4+ Li2Ge4O9:0.002Mn4+ LiNaGe4O9:0.002Mn4+ K2Ge4O9 :0.002Mn4+

330 nm 330 nm 300 nm 450 nm 450 nm 450 nm

(0.723,0.277) (0.723,0.278) (0.719,0.280) (0.618,0.369) (0.664,0.329) (0.685,0.310)

97.8% 98.1% 97.3% 71.7% 83.2% 88.7%

1148.34 1300.24 1010.29 742.58 755.29 753.22

(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2

(3)

where (x, y) is the CIE 1931 chromaticity coordinate of the sample, (xi, yi) is the CIE of an equal-energy illuminant with a value of (0.310, 0.316), and (xd, yd) is the CIE chromaticity coordinate of the dominant wavelength [50]. As displayed in Table 2, the color purity of the studied samples is largely dependent on the excitation wavelength. Under the excitation of UV light, the color purity of the resultant compounds is much higher than that of excited at 450 nm light. Clearly, the color purity of LiNaGe4O9:0.002Mn4+ red-emitting phosphors is as high as 98.1%. The luminescence lifetime of the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ samples monitoring at 669, 661 and 664 nm under different excitation wavelengths are investigated, as depicted in Fig. 8. As demonstrated, the decay curves can be well fitted with the following a single-exponential decay model [33,51]:

I(t ) = I0 + A1 exp (−t / τ )

(4)

where I0 and I(t) are the emission intensity at initial time and t, respectively. A1 is constant, τ is the decay time. Based on the decay curves, the fluorescent lifetimes are calculated and listed in Table 2. Under UV light excitation, the decay times for Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ phosphors are determined to be 1148.34, 1300.24 and 1010.29 μs, respectively. However, when excited at 450 nm, the decay times are found to be 742.58, 755.29 and 753.22 μs, respectively. The decay time of the prepared phosphors located in the range of microseconds which is mainly caused by the forbidden transition character of Mn4+ ion intra-d-shell transitions [42,51]. The same phenomenon was found in other compounds, CaMg2Al16O27 [23], Li2MgTiO4 [34], Sr2MgAl22O36 [52], CaAl12O19 [52] and Ca14Al10Zn6O35 [53]. It is well-known that the fluorescence

Table 2 The CIE 1931 chromaticity coordinates, color purity and life time of Mn4+-doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 phosphors. Point

LiNaGe4O9:0.002Mn4+,

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Fig. 9. Temperature-dependent PL emission spectra of the (a) Li2Ge4O9:0.002Mn4+; (b) LiNaGe4O9:0.002Mn4+; (c) K2Ge4O9:0.002Mn4+ (d) energy level diagram of Mn4+-doped Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 phosphors. Insets present the normalized PL emission intensity of the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ as a function of temperature.

found that these three compounds have different thermal stability, so it can be concluded that the different alkaline ions with various ionic radii, resulting in different crystal structures as well as the inconsistent thermal stability. The same phenomenon was also found in other compounds, for example La5(Si2+xB1−x)(O13−xNx):Ce3+ [39]. The energy level diagram of Mn4+ ions in MGe4O9 (M = Li2, LiNa and K2) host lattices is presented in Fig. 9(d). Under UV or blue light excitation, the electrons are excited from the ground state 4A2g to the 4T1g or 4T2g state. After that, most of the electrons radiative decay to the ground state and the bright red emissions are generated. Apart from the thermal stability, the quantum yield (QE) of the phosphors is another indispensable factor to evaluate their practical applications in WLEDs. Under the excitation of UV light, the quantum yields of the phosphors were measured by the integrated sphere method at room temperature, are shown in Table 3. It is clear that the quantum yields of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors were measured to be 30.7%, 58.9% and 57.8%, respectively. Note that, these obtained QE values are comparable with other Mn4+-doped red-emitting phosphors, such as Ca14Al10Zn6O35:Mn4+, LiAlO2:Mn4+ and Gd2ZnTiO6:Mn4+ [53,54,57,58]. These results indicate that the MGe4O9:Mn4+ (M = Li2, LiNa and K2) red-emitting phosphors with good CIE chromaticity coordinate, high color purity and high quantum are promising candidates for WLEDs as red-emitting phosphors. In the ideal octahedral crystal field, the splitting of the Mn4+ energy

lifetime is proportional to the luminous intensity of the phosphor, so different alkaline ions exhibit different effect on the decay time of the Mn4+ ions in MGe4O9 (M = Li2, LiNa and K2) phosphors [54]. Thermal stability of the studied phosphor is an indispensable factor for their practical application in solid state lighting. The temperature-dependent PL spectra of the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors in the temperature range of 303–443 K were recorded and depicted in Fig. 9(a)–(c), respectively. It can be clearly seen that the PL emission intensity decreases gradually with increasing the temperature from 303 to 443 K. As disclosed in the inset, the PL emission intensity of the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ dropped to about 10%, 45% and 50% compared with that of at 303 K when the temperature was raised up to 363 K, respectively. These results indicate that the Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors possess relatively poor thermal stability, which can be enhanced by mixing phosphor in glass or using hydrophobic organic skin as a protective shield [55,56]. Generally, with the increasing temperature, the PL emission intensity decreases gradually can be attributed to the thermal quenching effect caused by the nonradiative transition. With increasing the temperature, the nonradiative transition probability will be enhanced, and the PL emission intensity is decreased. According to the previous report, the CTB of Mn4+-O2+ provides an easy way for electrons to return to the ground state, leading to the lower thermal stability [21]. However, it can be 1080

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field is followed by a reduction of the Racah parameters from the free ion values as a result of the nephelauxetic effect. Owing to the formation of chemical bonds between impurity 3d ion and ligands, the nephelauxetic effect is formed. The hybridization between the Mn4+ ions and ligands is maximized, and the value of the Racah parameters is decreased strongly. Therefore, the degree of reduction of the B and C parameters even for the same Mn4+ ion also differs from host to host, leading to a wide variation in the energy of the 2Eg → 4A2g emission transition in solids. The crystal field strength (Dq) of Mn4+ ions can be roughly estimated by using the 4A2g → 4T2g transition energy gap [10,23]:

Table 3 The QE values of Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ phosphors and some other reported red-emitting phosphors. Samples

QE (%)

CIE 1931 coordinates

Ref.

Li2Ge4O9:0.002Mn4+ LiNaGe4O9:0.002Mn4+ K2Ge4O9:0.002Mn4+ Sr2MgAl22O36:Mn4+ CaAl12O19:Mn4+ Ca14Al10Zn6O35:Mn4+ LiAlO2:Mn4+ Gd2ZnTiO6:Mn4+ CaMg2Al16O27:Mn4+ Li2MgTiO4:Mn4+ K2TiF6:Mn4+ (NH4)2SiF6: Mn4+ ZnTiF6:Mn4+ (NH4)2TiF6: Mn4+

30.7 58.9 57.8 80.0 63.0 50.7 48.0 39.7 35.6 34.0 98.0 64.6 26.4 16.4

Deep Deep Deep Deep Deep Deep Deep Deep Deep Deep Red Red Red Red

This work This work This work [52] [52] [53] [57] [58] [23] [34] [24] [59] [60] [59]

red red red red red red red red red red

Dq = E( 4T 2g → 4 A2g )/10

(5)

On the basis of the peak energy difference between the 4A2g → 4T1g and 4A2g → 4T2g transitions, the Racah parameter B can be calculated by the following equation [10,23]:

Dq B

= 15(x − 8)/(x 2 −10x )

(6)

where parameter x is defined as

x=

E (4 A2g → 4T1g ) − E (4 A2g → 4T 2g ) Dq

(7)

According to the peak energy for the Eg → A2g transition of Mn , the Racah parameter C is evaluated by the following equation [10,23,33]: 2

4

4+

E (2E g → 4 A2g )/ B = 3.05C / B + 7.9 − 1.8B / Dq

(8)

The calculated values of Dq, B, and C for resultant red-emitting phosphors are shown in Table 4. Clearly, the values of Dq/B for Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors are found to be 3.71, 3.92, 2.47, respectively, indicating that Mn4+ ions possess a strong crystal field in Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 host lattices. The energy of the 2Eg → 4A2g transition of Mn4+ ions is determined mainly by the nephelauxetic effect, which is correlated with the overlap of the wave functions of the impurity ion and ligands. Brik et al. [61], established a new parameter that quantitatively predicts the nephelauxetic effect in the spectroscopy of the Mn4+ ion in different hosts, the nephelauxetic ratio (β1). The nephelauxetic ratio, β1, can be defined as below: Table 4 Spectroscopic parameters and calculated β1 values of Mn4+ ions in various hosts.

Fig. 10. (a) Tanabe-Sugano energy-level diagram of Mn4+ in an octahedral crystal field. (b) Relationship between the 2Eg energy level of Mn4+ and the calculated nephelauxetic ratio β1 in different hosts.

levels can be interpreted described by the well-known Tanabe-Sugano diagram (Fig. 10(a)). The crystal field includes strong crystal field (Dq/ B ≥ 2.2) and weak crystal field (Dq/B < 2.2) [10]. It is well known that the energy of the free ion electrostatic terms is determined by the Racah parameters of B and C. The introduction of the ion into the crystalline 1081

Host

Dq (cm−1)

B (cm−1)

C (cm−1)

β1

E (2Eg) (cm−1)

Ref.

Gd2ZnTiO6 Li2MgTiO4 SrTiO3 Mg2TiO4 BaTiO3 BaMgAl10O17 YAl3(BO3)4 CaAl12O19 Sr4Al14O25 La3GaGe5O16 SrGe4O9 CaZrO3 Ba2LaNbO6 Li2Ge4O9

1980 2101 1818 2096 1780 2136 1890 2146 2222 2141 2362 1850 1780 2252

639 724 719 700 738 828 755 750 790 900 832 754 670 608

3132 3122 2839 3348 2820 3012 3015 3245 3192 2858 3024 3173 3290 3423

0.913 0.957 0.905 0.985 0.913 0.980 0.956 0.993 1.007 1.020 1.004 0.983 0.960 0.953

14,224 14,793 13,827 15,267 13,862 15,151 14,620 15,243 15,337 15,174 15,267 15,054 14,679 14,948

LiNaGe4O9

2237

571

3566

0.964

15,129

K2Ge4O9

2227

900

2821

1.016

15,060

[58] [34] [62] [63] [64] [32] [65] [66] [67] [68] [34] [69] [70] This work This work This work

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2

β1 =

2

⎛B⎞ +⎛C⎞ ⎝ B0 ⎠ ⎝ C0 ⎠ ⎜

⎟

⎜

Table 5 CIE 1931 chromaticity coordinates of K2Ge4O9:xMn4+ phosphors with different concentrations.

⎟

−1

(9) −1

where B0 (1160 cm ) and C0 (4303 cm ) represent the Racah parameters for free Mn4+ ions. With the help of Eq. (9), the values of β1 for Mn4+ ions in the Li2Ge4O9, LiNaGe4O9 and K2Ge4O9 host are calculated to be 0.953, 0.964, 1.016, respectively. It is note that the β1 value of K2Ge4O9 is slightly different from the previously reports, which may be due to different preparation conditions [1,21,41]. A summary of the spectroscopic data and calculated β1 values for a number of classical and newly reported Mn4+ ions doped red-emitting phosphors are listed in Table 4. The relationship between the energy of the emission (2Eg → 4 A2g) and β1 parameter is displayed in Fig. 10(b). A fairly linear fitting can be obtained and the linear equation E(2Eg) = 14,846.85β1 + 483.70 can be used to predict the emission position of Mn4+ ions on the basis of β1 parameter. Clearly, the calculated 2Eg energy level location for Li2Ge4O9:0.002Mn4+, LiNaGe4O9:0.002Mn4+ and K2Ge4O9:0.002Mn4+ red-emitting phosphors match with the experimental data. The data quantitatively show that different alkaline ions form different structures, resulting in different crystal environment and nephelauxetic effect that affect the emission and absorption of Mn4+ ions. It also can be seen that the 2Eg level in fluorides is at considerably higher energy than in oxides, due to the more ionic Mn4+–F− bonding than Mn4+–O2− bonding [23]. As a rule of thumb, it can be stated that in more ionic hosts, taking fluorides for example, the weak nephelauxetic effect will result in a smaller decrease of the B and C values and will lead to higher energy position of the 2Eg level. In more covalent hosts, for instance oxides, covalent interaction between the Mn4+ ions and nearest neighbor ligand anions is enhanced with the result that the B and C parameters are strongly decreased. Therefore, compared with that of in fluorides, the 2Eg level of Mn4+ ions in oxides is shifted to lower energy. Upon 300 and 450 nm excitation, the PL emission spectra of K2Ge4O9:xMn4+ phosphors with different Mn4+ ion concentrations are recorded, as presented in Fig. 11. It can be seen that the emission spectra exhibit no distinct difference except for emission intensity and it increases gradually with increasing the Mn4+ ion concentration, reaching its maximum value at x = 0.002. After that, it begins to decrease when the Mn4+ ion concentration over 0.002 owing to the concentration quenching effect. Since the d electron wave functions of transition metals are much extended than the 4f electrons of rare earths, the critical concentration for Mn4+ ions are lower than that of rare earth ions doped phosphors. Therefore, the interactions between transition-metal ions are more intense even at low concentration. The same phenomenon was also found in SrGe4O9, BaGe4O9, La3GaGe5O16 and Li2MgTiO4 [33,34,42,68]. In order to better understand the concentration quenching mechanism in the K2Ge4O9:xMn4+ red-emitting phosphors, the critical distance (Rc) between Mn4+ ions should be investigated and it can be estimated using the following formula given by Blasse [71]:

Samples

K2Ge4O9 K2Ge4O9 K2Ge4O9 K2Ge4O9 K2Ge4O9

:0.0005Mn4+ :0.001Mn4+ :0.002Mn4+ :0.004Mn4+ :0.007Mn4+

R c = 2[3V /4πXc N ]1/3

CIE 1931 coordinates (under 300 nm)

CIE 1931 coordinates (under 450 nm)

(0.719,0.281) (0.720,0.280) (0.720,0.281) (0.720,0.282) (0.719,0.280)

(0.676,0.323) (0.677,0.323) (0.685,0.310) (0.699,0.301) (0.699,0.300)

(10)

In which, Xc stands for the critical concentration, V is the unit cell volume, and N is the number of cation sites that doped ions can occupy per unit cell. Hence, the Rc value is calculated to be 57.17 Å for K2Ge4O9:xMn4+ red-emitting phosphors. Furthermore, the non-radiative energy transfer mechanisms among the dopants can be divided into two different interaction models: electric multipolar interaction and exchange interaction. It is known that the exchange interaction is possible only when the Rc is smaller than 5 Å [72]. Since the calculated Rc is much larger than 5 Å, it can be concluded that the energy migration mechanism is attributed to the electric multipolar interaction. Based on the PL spectra, the CIE 1931 chromaticity coordinates of K2Ge4O9:xMn4+ with different Mn4+ ion concentrations are calculated and shown in Table 5. It is evident that the obtained CIE 1931 chromaticity coordinates are close to that of the standard red light (0.67,0.33) when the samples are excited at 450 nm, and are close to that of the commercial red-emitting 3.5MgO·0.5MgF2·GeO2:Mn4+ (0.711,0.289) phosphors when the resultant compounds excited at 300 nm. 4. Conclusion In summary, the Mn4+-doped MGe4O9 (M = Li2, LiNa and K2) redemitting phosphors have been synthesized via a milder temperature solid-state reaction method. The three compounds have different space group (P21ca, Pcca and P3 c1 for Li2Ge4O9, LiNaGe4O9 and K2Ge4O9, respectively) and crystal structure (orthorhombic and trigonal phase), deriving from alkaline ions (Li, Na and K) with different ionic radii and electron configurations. Furthermore, they also possess different bond lengths, bond angles, crystal fields and nephelauxetic effect. The emission band positions of the Mn4+ ions in the MGe4O9 are largely dependent on the alkaline ions. Moreover, the thermal stabilities of the studied phosphors are also found to be dependent on the alkaline ions, especially when M = LiNa and K2, the thermal stabilities are greatly enhanced. Combined with photoluminescence and theoretical calculation, the crystal field strength (Dq) and Racah parameters (B and C) are estimated to evaluate the nephelauxetic effect of Mn4+ ions in MGe4O9

Fig. 11. (a) PL (λex = 300 nm) spectra of K2Ge4O9:xMn4+ with different Mn4+ concentration. (b) PL (λex = 450 nm) spectra of K2Ge4O9:xMn4+ with different Mn4+ concentration. The insets show the relationship between the emission intensity and Mn4+ ion concentration.

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(M = Li2, LiNa and K2) host lattices. Meanwhile, the relationship between nephelauxetic ratio β1 and emission energy is established which helps for describing the relationship between alkaline ions and emission energy of Mn4+ in different hosts. In addition, under UV and blue light excitation, all the phosphors exhibit an intense red emission with good color coordinates, high color purity and high quantum yield. These results suggest that Mn4+ ions doped MGe4O9 (M = Li2, LiNa and K2) red-emitting phosphors may have potential applications in solid state lighting. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015060315). The Mn4+ doped MGe4O9 (M = Li2, LiNa and K2) phosphors were supplied by the Display and Lighting Phosphor Bank at Pukyong National University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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