Novel visible phosphors based on MgGa2O4-ZnGa2O4 solid solutions with spinel structure co-doped with Mn2+ and Eu3+ ions

Novel visible phosphors based on MgGa2O4-ZnGa2O4 solid solutions with spinel structure co-doped with Mn2+ and Eu3+ ions

Author’s Accepted Manuscript NOVEL VISIBLE PHOSPHORS BASED ON MgGa2O4-ZnGa2O4 SOLID SOLUTIONS WITH SPINEL STRUCTURE CO-DOPED WITH Mn2+ AND Eu3+ IONS A...

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Author’s Accepted Manuscript NOVEL VISIBLE PHOSPHORS BASED ON MgGa2O4-ZnGa2O4 SOLID SOLUTIONS WITH SPINEL STRUCTURE CO-DOPED WITH Mn2+ AND Eu3+ IONS Andriy Luchechko, Oleh Kravets www.elsevier.com/locate/jlumin

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S0022-2313(16)31815-4 http://dx.doi.org/10.1016/j.jlumin.2017.05.046 LUMIN14770

To appear in: Journal of Luminescence Received date: 8 December 2016 Revised date: 15 May 2017 Accepted date: 16 May 2017 Cite this article as: Andriy Luchechko and Oleh Kravets, NOVEL VISIBLE PHOSPHORS BASED ON MgGa2O4-ZnGa2O4 SOLID SOLUTIONS WITH SPINEL STRUCTURE CO-DOPED WITH Mn2+ AND Eu3+ IONS, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

NOVEL VISIBLE PHOSPHORS BASED ON MgGa 2 O 4-ZnGa 2 O 4 SOLID SOLUTIONS WITH SPINEL STRUCTURE CO-DOPED WITH Mn 2+ AND Eu 3+ IONS Andriy Luchechko*, Oleh Kravets Ivan Franko National University of Lviv, Department of Sensor and Semiconductor electronics, Tarnavskogo str. 107, Lviv 79017, Ukraine *

[email protected]

ABSTRACT Photoluminescence excitation and emission spectra of Mg 1-xZnxGa2O4 solid solutions (x= 0; 0.25; 0.5; 0.75; 1.0) co-doped with 0.05 mol.% Mn2+ and 4 mol.% Eu3+ ions have been investigated at room temperature. Polycrystalline samples were synthesized via high-temperature solid-state reaction technique. XRD measurements confirmed that all ceramic samples are spinel structure compounds. Lattice parameter follows linear dependence on composition changing that confirms the Vegard's law for a Mg1-xZnxGa2O4 solid solution system. Complex broad luminescence band ranging from 350 to 475 nm ascribed to emission from host defects was found at the excitation in "band-to-band" spectral region. Noticeable intense matrix luminescence in ceramic samples with x= 0.25 and 0.5 was observed. Emission of Mn2+ ions is presented by an intense band with a maximum around 505 nm and shifts at different compositions. Intense excitation of Mn2+ ions was found around the fundamental absorption edge. Complex excitation nature of Mn2+ ions was suggested. The charge transfer band and f-f excitation lines were found in excitation spectra of Eu3+ ions. Luminescence of Eu3+ ions is represented by a number of sharp f-f lines in the 575650 nm spectral region. Composition changing from MgGa2O4 to ZnGa2O4 leads to suppression of Eu3+ ions emission which shows a maximum at x = 0.25. Maximum of matrix luminescence and emission of Mn2+ ions were found at x = 0.50. Further increasing of zinc amount in Mg1-xZnxGa2O4 solid solution leads to suppression of intensity of all luminescence bands. The influence of excitation wavelength and composition on visible range luminescence intensity has been shown. Commission 1

Internationale de l'Eclairage chromaticity diagrams are presented for different compositions and excitations. KEYWORDS Mg 1-x Zn x Ga 2 O 4

solid

solution;

spinel

structure;

photoluminescence;

manganese ions; europium ions; chromaticity diagrams. INTRODUCTION Even though the complex oxide based phosphors has been widely investigated so far it didn't lose attention of a high number of researchers in world wide. Among many oxide compounds, MgGa2O4 and ZnGa2O4 phosphors have attracted much attention due to great demand for new types oxide semiconductors with high luminescence output [1-3]. These gallate spinel compounds have various potential applications in high performance displays, plasma display panels, field emission displays, electroluminescent displays and quick response cathode ray tubes that are still highly required [4]. MgGa2O4 and ZnGa2O4 exhibit a few potential advantages, in particular, of the superior chemical and thermal stability as well as electron beam bombardment tolerance with respect to sulfide phosphors that are the basic materials used in applications mentioned above [5]. Through increasing energy expenditures of civilization, a high number of scientists are searching for new methods of power saving. White-light emitting diodes (WLEDs) are a high spot among sources of white light, therefore, zinc and magnesium gallate spinel compounds are possible to be applied. W-LED is a promising solidstate light approach due to their fascinating advantages of energy savings, high efficiency, longer lifetime and environmental safeness in comparison to conventional incandescent and fluorescent lamps [5, 6]. Possible white lighting application can also base on the semiconductor properties which give a possibility to excite emission with applying of electric field directly to the samples with no needs of UV excitation [6]. Spinel compounds have attracted much attention, even though solid solutions of these compounds still aren't investigated, only a few papers were dedicated to this kind of studies. B. S. Tsai et al. reported about MgxZn1-xGa2O4 compounds doped with Eu3+ ions synthesized via sol-gel technique [2]. There was shown that Eu3+ 2

excitation lines become more prominent with increasing of Mg content. Interesting results were presented in the [7] paper that reports about luminescence properties of MgIn2-xGaxO4 solid solutions activated with Eu3+ ions. The main result of this paper is that no charge transfer band between activator ions and lattice was found. Samples with x= 0.2 show most intense luminescence of activator ions. Another remarkable investigation of the spinel solid solution was carried out by Y. M. Moon et al. [8]. This paper reports about luminescence studies of MgGa2O4 together with MgGa2yAlyO4

(y= 0.5÷2.0) doped with Mn2+ ions. Partial substitution of Ga3+ to Al3+ cations

leads to red-shift of the Mn2+ luminescence band. Such substitution also affects a luminescence intensity, which rises with increasing of Al 3+ ions and reaches maximum at y = 1.0. For the first time, properties of co-doped with Mn2+ and Eu3+ ions MgGa2O4 spinel compound were reported by A. Luchechko et al. [9, 10]. These studies showed that co-doping affects relative intensities of matrix luminescence in “blue”, emission of Mn2+ in “green” and sharp luminescence lines of Eu3+ ions in "red" spectral regions. The optimal concentration of Eu3+ activator ions was identified around 4 mol. % in MgGa2O4: Mn, Eu samples. This paper reports about photoluminescence investigations of Mg 1-xZnxGa2O4 (x= 0; 0.25; 0.50; 0.75; 1.0) solid solutions co-doped with 0.05 mol.% Mn2+ and 4 mol.% Eu3+ ions synthesized via solid state reaction method for various phosphor applications. The emission mechanisms of Mg1-xZnxGa2O4: Mn2+, Eu3+ samples are proposed. Meanwhile, there are just a few reports about luminescence of solid solutions with the spinel structure and no papers on Mn2+ and Eu3+ co-doped MgGa2O4-ZnGa2O4 solid solutions were found. EXPERIMENTAL DETAILS Polycrystalline samples of Mg1-xZnxGa2O4: Mn, Eu (x= 0; 0.25; 0.50; 0.75; 1.0) were prepared by high-temperature solid state reaction method. Magnesium oxide (MgO), zinc oxide (ZnO), β-gallium oxide (β-Ga2O3), europium ( I I I ) oxide (Eu2O3) and manganese oxide (MnO) were used as starting materials. All reagents were at least 4N grade of purity. Powders of stoichiometric composition with 4 mol.% of Eu2O3 and 0.05 mol.% of MnO were ground in an agate mortar for 6 h with further 3

pressing in а steel mold under the pressure of 150 kg/cm2. Obtained tablets were annealed at 1200 °C for 8 hours in the air. These samples were 4 mm in diameter and 1 mm thick. X-ray diffraction measurements were carried out in "Interfaculty scientificeducational laboratory of X-ray structure analysis" of Ivan Franko National University of Lviv. XRD analysis was performed on STOE STADI P diffractometer with linear position-sensitive PSD detector using X-ray tube with Cu anode (Kα1radiation, λ= 1.5406 Å). XRD measurements were performed with 0.005° scanning step. Analysis of diffraction peaks was realized with STOE WinXPOW software package. Photoluminescence measurements were carried out on spectrofluorometer CM2203 in the 220-820 nm spectral range at room temperature. All excitation and luminescence spectra were obtained with a spectral resolution of 0.5 nm. Excitation of luminescence was performed with 150 W xenon lamp. A Hamamatsu R928 photomultiplier was used as luminescence detector. All photoluminescence and excitation spectra were automatically corrected to the photomultiplier sensitivity and lamp intensity, respectively. RESULTS AND DISCUSSIONS X-ray diffraction measurements were performed for all investigated Mg1-xZnxGa2O4 (x= 0; 0.25; 0.5; 0.75; 1.0) solid solution samples co-doped with Mn2+ and Eu3+ ions. The XRD patterns were compared with standard powder diffraction data file ICSD №37359. All investigated samples are spinel type structure compounds with Fd3m space group (No. 227). The typical XRD patterns of Mg1-xZnxGa2O4: Mn2+, Eu3+ (x= 0; 0.5; 1.0) samples are shown on Fig. 1. Average cell parameters of MgGa2O4, Mg0.5Zn0.5Ga2O4 and ZnGa2O4 samples co-doped with Mn2+ and Eu3+ ions were determined and equal 8,2675 Å, 8,3069 Å, 8,3408 Å, respectively. An additional diffraction line is observed around 32.275 due to presence of -Ga2O3 phase from 1 to 3%. The additional phase can appear as a result of evaporation of starting materials during annealing. It should be noted that Eu3+ doped spinels show a higher value of cell parameter with respect to pure MgGa2O4 and ZnGa2O4 [1, 11]. It is known that Mn2+ and Eu3+ ions occupy tetrahedral (Td point symmetry) and octahedral (D3d point 4

symmetry) sites, respectively [2, 12]. Thus, it can be explained by distortion of spinel lattice due to ionic radius mismatch of activator ions (0.66 Å for Mn2+,0.95 Å for Eu3+) and host atoms (0.66, 0.74 Å, 0.62 Å for Mg2+, Zn2+, Ga3+), respectively [4, 5, 13].

Figure 1 XRD pattern of MgGa2O4, Mg0.5Zn0.5Ga2O4, ZnGa2O4 samples co-doped with Mn2+ and Eu3+ ions synthesized via high-temperature solid state reaction method at 1200 C and Standard diffraction pattern ICSD №37359.

Lattice parameter depends on changing of solid solution composition, usually, follows the Vegard's law. It is evidently seen that lattice parameter increases with changing of composition from magnesium to zinc gallate spinel compound. Obtained data for magnesium gallate (MgGa2O4) and zinc gallate (ZnGa2O4) solid solutions is depicted in Fig. 2. Linear dependence of lattice parameter on composition has been confirmed. Photoluminescence excitation spectra of Mg1-xZnxGa2O4: Mn2+, Eu3+ (x= 0; 0.25; 0.50; 0.75; 1.0) solid solutions at 430, 505 and 617 nm registrations are shown on Fig. 3. A broad excitation band in the 230-270 nm spectral region with a maximum around 235 nm was found at 430 nm registration for all samples Fig. 3(a). This band is in 5

fundamental absorption spectral range and corresponds to the excitation of matrix luminescence. Matrix emission excitation band of x= 0.25 and 0.50 samples shows relatively strong intensity. The excitation spectra of x=0, 0.75 and 1.0 samples were smoothed and 10 times increased due to low signal with respect to x= 0.25 and 0.50 samples. Together with that, a weak broad excitation band in the 300-360 nm spectral region was found in excitation spectra of Mg1-xZnxGa2O4: Mn2+, Eu3+ samples. Y. Zhang et al. reported that this band is responsible for the excitation of oxygen vacancies in spinel structure [1].

Figure 2 Variation of lattice parameter for different composition of Mg1-xZnxGa2O4: Mn, Eu solid solutions. The solid line represents the Vegard's law.

An intense broad excitation band in the 230-320 nm spectral range was found in excitation spectra of Mn2+ ions in Mg1-xZnxGa2O4: Mn, Eu (x= 0÷1.0) at 505 nm registration Fig. 3 (b). Excitation intensity increases with composition changing of solid solution ceramics and reaches maximum for x= 0.25. Further increasing of ZnGa2O4 amount leads to decreasing of the excitation intensity. It should be noted that maximum of this band shows slight red-shift with composition changing from MgGa2O4 to ZnGa2O4. It can be easily explained by the difference of band gap of those 6

compounds [14, 15]. This intense band with maximum around 240 nm belongs to "band-to-band" transitions. A shoulder on long-wavelength side of these excitation bands was found. It is caused by band with charge-transfer from O2- anions to Mn2+ activator ions. B. Yasoda et al. elucidate this band with energy transfer from host to activator that is associated with charge-transfer transitions including intergap transition and ionization of activator [3]. Thus, excitation nature of Mn2+ ions is complex (see also [9, 10]). It can be suggested, that excitation involves both recombination mechanism and direct excitation of manganese activator ions.

7

Figure 3 Photoluminescence excitation spectra of Mg1-xZnxGa2O4: 0.05 mol.% Mn2+, 4 mol.% Eu3+ polycrystalline samples at different registrations.

A broad asymmetrical band in the 250-350 nm spectral region was found in excitation spectra of Mg1-xZnxGa2O4: Mn, Eu ceramic samples at 617 nm registration Fig. 3 (c). This band corresponds to charge transfer from 2p orbital of O2- to 4f vacant 8

orbital of Eu3+ ions. J. S. Kim et al. reported that asymmetry of this band is caused by Eu3+ ions in different positions of spinel host [16]. Thus, this band has complex nature and consists of two overlapped sub-bands corresponding to Eu3+ ions in octahedral sites substituting Ga3+ ions and tetrahedral sites of Mg2+/Zn2+ ions. A slight red-shift of the charge transfer band was observed due to redistribution of Eu3+ ions in different lattice sites. The phenomenon of Eu3+ occupying both tetrahedral and octahedral sites can be explained by the fact that Ga3+ ionic radius is close to the ionic radius of magnesium and zinc ions. It also leads to inversion of the spinel structure of magnesium gallate compound [3]. Besides of broad band, f-f excitation lines in the 350-475 nm spectral region were found in excitation spectra of Eu3+ ions Fig. 3(c). The most intense excitation line was observed at 393 nm in all investigated Mg1-xZnxGa2O4 ceramics. It corresponds to transition from 7F0 ground state to 5L6 energy level. Less intense lines at 318 nm (7F05H4), 361 nm (7F05D4), a doublet line at 375, 381 nm (7F05L7), 412 nm (7F05D3) and 463 nm (7F05D2) were found as well. These results are in good agreement with [2, 9]. Changing of composition between MgGa2O4 and ZnGa2O4 gallates co-doped with Mn2+ and Eu3+ ions simultaneously affects excitation intensity of the charge transfer band (CTB) and f-f excitation lines. As it is seen from the Fig. 3(c), maximum of CTB excitation intensity was found in Mg0.75Zn0.25Ga2O4: Mn, Eu sample. Further increasing of ZnGa2O4 content is followed by suppression of the charge transfer band. In the case of f-f excitation band, the strongest emission was found in MgGa2O4: Mn, Eu. The addition of ZnGa2O4 results to diminishing of f-f excitation lines intensity down to x= 0.50. Then a slight increase of excitation intensity observed in Mg0.25Zn0.75Ga2O4 and ZnGa2O4 samples co-doped with Mn2+ and Eu3+ ions due to the better crystallinity of these samples. It should be noted that only in the case of Mg1-xZnxGa2O4: Mn, Eu (x= 0, 1.0) samples the f-f excitation lines are predominant over

the CTB by intensity.

9

Figure 4 Photoluminescence emission spectra of Mg1-xZnxGa2O4: 0.05 mol.% Mn2+, 4 mol.% Eu3+ polycrystalline samples at different excitations.

Photoluminescence emission spectra of Mg1-xZnxGa2O4: Mn, Eu solid solution ceramic samples at 240, 280 and 393 nm excitation are shown in Fig 4. An intense broad emission band with a maximum around 430 nm was found in photoluminescence spectra of Mg0.75Zn0.25Ga2O4 and Mg0.5Zn0.5Ga2O4 co-doped with 10

Mn2+, Eu3+ samples at 240 nm excitation Fig. 4(a). The emission in 350-475 nm spectral region of x= 0; 0.75; 1.0 samples were found negligibly small. This 430 nm intense broad band corresponds to the matrix luminescence of gallate spinels and has complex nature (see, e.g., [10]). Authors of papers [15, 17] reveal matrix luminescence with antisite defects and emission of Ga3+-O2- complexes with disordered octahedral sites. The strongest matrix luminescence was found in Mg0.75Zn0.25Ga2O4: Mn, Eu samples. It can be assumed that such strong matrix emission is related to a difference of Mg2+ and Zn2+ ionic radii and as a result of MgGa2O4 and ZnGa2O4 lattice parameters. It leads to lattice distortion and produces a high number of host defects. An intense Mn2+ luminescence band with a maximum at 505 nm was also found in the photoluminescence spectra of all samples at 240 nm excitation Fig. 4(a). The mission band of Mn2+ ions decreases with increasing of Zn/Mg ratio. A minor band shift from 502 nm (x= 0) to 506 nm (x= 1.0) was observed. Manganese ions are sensitive to the crystal field strength which results in splitting of energy levels. In particular, Mn2+ emission band is slightly asymmetrical in the long-wavelength side due to splitting into two sub-bands as a result of the influence of weak crystal field in Mg2+/Zn2+ tetrahedral sites occupied by Mn2+ ions [17]. Furthermore, band shift is also a consequence of matrix luminescence overlapping Mn2+ emission. Thus, Mn2+ emission band can appear both as from 4T16A1 spin forbidden transitions of Mn2+ ions [17] and recombination of charge carriers on Mn2+ related centers [10]. Three different types of emission in 350-475, 475-575 and 575- 640 nm spectral regions were found in the photoluminescence spectra at 280 nm excitation Fig. 4(b). Emission band in the spectral region 350-475 nm corresponds to matrix luminescence. The maximum intensity of matrix luminescence at 280 nm excitation was found in Mg0.5Zn0.5Ga2O4: Mn, Eu samples. A weak band with a maximum around 385 nm in 370-400 nm spectral region for ZnGa2O4: Mn, Eu sample (x = 1.0) was observed. A band in 475-560 nm region with a maximum around 505 nm associates with Mn2+ ions emission Fig. 4(b). Emission of manganese ions at 280 nm excitation increases with composition changing and reaches maximum for x= 0.50. Band shift of Mn2+ ions emission from 502 to 506 nm was observed as well as at 240 nm excitation.

11

Luminescence of Eu3+ ions is presented by several intense emission lines peaking at 581, 595 and 617 nm Fig. 4(c). These lines are associated with 5D07Fj transitions of 4f6 configuration of Eu3+ ions. The most intense line corresponds to forced electricdipole 5D07F2 transitions, expected by the Judd–Ofelt selection rule, was observed at 617 nm. Increasing of x from 0 to 0.25 results to insignificant enhance of the emission intensity of 5D07F2 line. Further increase of x leads to suppression of f-f emission lines. The weak emission line with a maximum at 581 nm is related to 5D07F0 transitions of Eu3+ ions that are strictly forbidden in inverse symmetry due to the J–J mixing by the crystal field effect [18]. It indicates that some of the Eu3+ ions are into octahedral low symmetry sites. Thus, it's possible to suggest that the local point group of Eu3+ in the spinel structure is Cs, Cn or Cnv. While emission line peaking at 595 nm is related to 5D07F1 magnetic dipole transitions. The lowered crystal symmetry results to the remarkable three-fold superfine structure of this line [11]. The emission spectra of Eu3+ ions at 393 nm excitation for Mg1-xZnxGa2O4: Mn, Eu (x= 0÷1.0) solid solution samples are presented in Fig. 4(c). The superfine structure of 5

D07F1 line consists of three peaks at 589, 595 and 600 nm. The 617 nm emission

line also has a complex structure and consists at least of three sub-bands of the 5

D07F2 transition due to crystal field splitting. The most intense emission was

observed for MgGa2O4: Mn, Eu sample. Increasing of ZnGa2O4 component in solid solution results in suppression of f-f emission lines. Minimum intensity was observed in Mg0.5Zn0.5Ga2O4 samples. Further increasing of ZnGa2O4 component leads to enhancement of luminescence emission intensity. Moreover, it is clearly seen from the Fig. 4(c) that all f-f emission lines are better resolved in MgGa2O4: Mn, Eu than in the case of the other investigated solid solutions. The 4f orbitals mix with the opposite parity orbitals due to the absence of symmetry center that results in the appearance of electric-dipole transitions [2]. All emission lines of Eu3+ ions with an odd number of j belong to electric-dipole transitions that strongly depend on the local symmetry near Eu3+ ions. At the same time, emission lines with an even number of j correspond to magnetic-dipole transitions which aren't affected with the symmetry of surrounding. As it was already said, Eu3+ ions occupy octahedral sites substituting Ga3+ ions and tetrahedral sites of Mg2+ or Zn2+ ions. Thus, intensity ratio 12

I617/I595 gives important information about positions of Eu3+ ions in the lattice and local symmetry around these ions [4]. In this investigation, the average I617/I595 = 3.4 derived from emission spectra at λexc = 393 nm confirms low symmetry around Eu3+ ions. More detail analysis of obtained results should be given in order to elucidate the effect of composition changing. One of the key aspects is that zinc gallate is normal spinel structure compound, while magnesium gallate is partly inversed spinel. The inversion index of MgGa2O4 depends on preparation condition and varies from 0.15 to 0.27 [17]. Thus, changing composition towards ZnGa2O4 improves crystal structure and significantly affects matrix luminescence. It's expected that improvement of crystal structure from MgGa2O4 to ZnGa2O4 will gradually suppress emission of the matrix. Indeed, zinc gallate spinel exhibits less intense matrix luminescence with respect to MgGa2O4. However, even the smallest addition of ZnGa2O4 (x=0.25, 0.5) increases matrix luminescence (Fig. 4a). It was shown above, the cell parameter of zinc gallate spinel is substantially higher and this implies much higher interatomic distances. It leads to strong structure disorder and produces a high number of host defects in Mg1-xZnxGa2O4 that excite around the fundamental absorption edge (Fig. 3a). Approaching to ZnGa2O4 spinel suppresses matrix luminescence as a result of the more ideal structure and less number of host defects. From excitation spectra of Mn2+ ions are clearly seen that changing of composition from magnesium gallate to zinc gallate shifts excitation band to long-wavelength side of spectra (Fig. 3b). The red-shift takes place due to difference of band-gap of both spinel compounds [2]. The band-gap dependence of Mn2+ excitation is an evidence of recombination nature of Mn2+ ions. As was mentioned above, a slight shift of the Mn2+ emission band was observed as a result of difference of crystal field strength. M. Yu et al. reported about ZnGa2O4: Mn2+ synthesized by citrate–gel process and observed strong charge transfer band, weak d-d excitation lines and relatively weak host excitation band. Following that, it can be assumed that preparation conditions strongly affect mechanism of Mn2+ ions excitation. Thus, CTB excitation is predominant in more structurally ideal nanosized materials. The solid-state synthesis provides highdefective spinel host with dominant recombination mechanism of excitation. Taking into account results obtained by V. T. Gritsyna et al., this leads to the creation of 13

defect-Mn2+ ion complexes which serve as energy channels for emission of Mn2+ ions [17]. However, excitation nature of Mn2+ ions in mixed spinel compound doesn't give a significant rise in luminescence intensity at 240 nm excitation due to relatively low concentration of Mn2+ ions and redistribution of excitation energy between matrix luminescence and emission of Mn2+ ions (Fig. 4a). It seems that mixing of MgGa2O4 with ZnGa2O4 provides better Mn-O bonding due to higher luminescence intensity with excitation near CTB (Fig 3b, 4b). MgGa2O4 spinel compound exhibits more intense charge transfer band and f-f excitation lines with respect to ZnGa2O4. It can be caused by lower crystal field strength and inverse MgGa2O4 structure. It provides better incorporation of Eu3+ ions in spinel host and enhances the oscillator strength of these lines [19]. Eu3+ ions are strongly affected by composition changing. A small addition of ZnGa2O4 significantly enhances the intensity of charge transfer band from O2- to Eu3+ ions. Mixing of magnesium gallate with zinc gallate leads to strong structural disorder and produces porous in spinel host. It leads to the better Eu3+ entrance and Eu-O bonding, that enhances the excitation intensity in CTB region. However, this effect is also responsible for suppression of f-f excitation lines of Eu3+ ions. It was observed that f-f excitation lines of all solid solutions are much weaker with respect to CTB. All mixed spinel materials contain highly disordered regions. In these regions the crystal field strength is raised due to distortion of the spinel host as a result of cell parameter mismatch. Thus, it results to opposite effect that was observed for enhanced Eu3+ emission in MgGa2O4: Mn2+. Eu3+ compound.

14

Figure 5 Luminescence intensity of matrix (430 nm), manganese (505 nm) and europium (617 nm) at 280 nm excitation versus changing of Mg1-xZnxGa2O4 composition. Emission intensities of all registrations were normalized.

It should be noted, that one of the most important results is that excitation bands of all three luminescence types overlap in the UV-region of spectra. Thus, it is possible to obtain required color by composition changing of Mg1-xZnxGa2O4 phosphors with a single wavelength pumping. It is clearly seen from emission spectra, for example at 280 nm excitation. This effect is clearly represented in Fig. 5, for all types of luminescence in full range of solid solutions. Maximum of matrix luminescence was found around x= 0.50. The most intense manganese luminescence was found in between x= 0.25 and 0.50. Further increasing of x leads to suppression of Mn2+ emission. The brightest emission of Eu3+ lines was found in MgGa2O4: Mn, Eu and Mg0.75Zn0.25Ga2O4: Mn2+, Eu3+ samples. Finally, for these three types of luminescence suppression of intensities was observed with increasing of x above 0.50. Fig. 6 shows Commission Internationale de l'Eclairage (CIE) chromaticity diagrams for Mg1-xZnxGa2O4: 0.05 mol.% Mn2+, x Eu3+ solid solution at 240 and 280 nm excitations. A solid blue line represents the emission of Black Body. A, B, C, D65 points are Standard illuminants with calibrated color temperature and E is a white 15

achromatic point with (0.3333; 0.3333) coordinates [20]. As it is seen from the Fig. 6(a), both x= 0.5 and 0.25 samples show "deep blue" emission color. While the other samples are located in "green" and "yellowish-green" region of the visible spectrum. Emissions of all samples at 280 nm excitation wavelength are shifted to the middleright part of diagram Fig. 6 (b). Samples with x= 1.0 and 0.75 have yellow emission color. While samples with x = 0.0, 0.25 and 0.5 are located around the Black Body Curve and can be considered as almost colorless white. It can be seen, that MgGa2O4, Mg0.75Zn0.25Ga2O4 co-doped with Mn2+, Eu3+ ions show the closest emission color to the Black Body emission at about 2800 and 3700 K, respectively. Mg 0.5Zn0.5Ga2O4: Mn2+, Eu3+ shows the closest emission color to the achromatic E point with 5400 K color temperature.

Figure 6 CIE 1931 chromaticity diagrams for Mg1-xZnxGa2O4: 0.05 mol.% Mn2+, x Eu3+ solid solution for 240 nm (a) and 280 nm (b) excitations. The blue line shows emission of Black Body at different temperatures. A, B, C, D65, E are Standard illuminants.

At the same time, emission of all samples exited with 393 nm wavelength shows a small deviation of average point with (0.65; 0.35) coordinates (not presented) which is in "reddish-orange" part of the diagram. CONCLUSIONS XRD measurements confirmed that all polycrystalline samples of Mg1xZnxGa2O4:

0.05 mol.% Mn, 4 mol.% Eu (x= 0÷1.0) solid solutions have a spinel

structure with small amount of -Ga2O3 phase. Linear dependence of lattice 16

parameter on composition confirmed Vegard's law for Mg1-xZnxGa2O4: Mn2+, Eu3+ solid solution system. Two excitation bands which result in matrix emission were found in the 230-360 nm spectral region of the excitation spectra monitored at 430 nm. Samples with x= 0.25 and 0.5 composition index show relatively strong matrix emission. Emission band of Mn2+ ions with a maximum at about 505 nm excites in fundamental absorption edge and has recombination mechanism of excitation. A slight shift of Mn2+ emission band from 502 to 506 nm was found at different x values due to changing of crystal field. The emission band of Mn 2+ ions decreases at 240 nm excitation with increasing of Zn content. The maximum of Mn 2+ emission at 280 nm excitation was found in x= 0.5 sample. The charge transfer band from O2- to Eu3+ ions together with f-f excitation lines were observed in the excitation spectra of Eu3+ ions. The emission of Eu3+ ions is presented by the sharp lines that correspond to f-f transitions in 4f

6

configuration of Eu3+ ions. Changing composition between

MgGa2O4 and ZnGa2O4 ceramics co-doped with Mn2+ and Eu3+ ions simultaneously affects the excitation intensity of CTB and f-f excitation lines. Maximum of CTB excitation intensity was found in x= 0.25 sample. In the case of f-f excitation lines, the strongest excitation was found in MgGa2O4: Mn, Eu sample. The maximum emission of 617 nm line (5D07F2 electric-dipole transitions) at 280 nm excitation was found in x= 0.25 sample and further increasing of x leads to suppression of Eu 3+ emission lines. Consequently, mixing ZnGa2O4 in MgGa2O4 spinel causes strong structural disorder which affects matrix luminescence and emission of Mn2+ and Eu3+ ions. The structural disorder generates high number of host defects which in order increases matrix luminescence. Also, this effect was observed for emission of Mn 2+ ions due to the possible existence of defect related channels that enhance emission of Mn2+ ions. Although, the magnitude of this effect is much weaker because relatively low concentration of Mn2+ ions and redistribution of excitation energy in UV region. Strong disorder provides better incorporation of Eu3+ ions in spinel host. This increases the excitation intensity of CTB, but suppresses f-f excitation lines of Eu3+ ions. 17

Changing of excitation wavelength and composition lead to the redistribution of luminescence intensities in three spectral regions that are related to the matrix luminescence as well as emissions of Mn2+ and Eu3+ ions. Combination of different composition of Mg1-xZnxGa2O4: Mn2+, Eu3+ solid solution and excitation wavelength gives an opportunity to obtain emission in the whole visible spectral range. MgGa2O4, Mg0.25Zn0.75Ga2O4 and Mg0.5Zn0.5Ga2O4 compounds co-doped with 0.05 mol.% Mn2+ and 4 mol.% Eu3+ ions show advantages over the rest samples due to possible application as phosphors of white light in a single material and single wavelength pumping. REFERENCES: [1] Y. Zhang, Zh. Wu, D. Geng, X. Kang et al., Full Color Emission in ZnGa2O4:

Simultaneous

Control

of

the

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