Crystal structure, luminescence properties and energy transfer of Eu³⁺/Dy³⁺ doped GdNbTiO₆ broad band excited phosphors

Abstract

GdNbTiO₆ is used as a host material for phosphors for the first time. Lanthanide ion (Eu³⁺/Dy³⁺) doped GdNbTiO₆ phosphors were prepared by solid-state reaction, and the crystal structure, luminescence properties, and relevant luminescence mechanisms were investigated. The crystal structure of Eu³⁺/Dy³⁺ doped GdNbTiO₆ was refined from powder XRD data by the Rietveld method, which is made up of irregular (Gd/Eu/Dy)³⁺O₈¹³⁻ polyhedra and slightly distorted Nb(Ti)O₆ octahedra forming a layered structure. The luminescence properties of the GdNbTiO₆:Eu³⁺/Dy³⁺ phosphors were studied under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The GdNbTiO₆ host shows a broad emission band about the 400–650 nm region centered at 509 nm owing to the Nb(Ti)O₆ octahedral groups, which has spectral overlap with f–f excitation transitions of Eu³⁺/Dy³⁺ in the doped samples. For red phosphor GdNbTiO₆:Eu³⁺, a dominant emission peak at 614 nm was attributed to the ⁵D₀ → ⁷F₂ transition of Eu³⁺, which confirmed that Eu³⁺ ions are located at sites without inversion symmetry. The phosphor GdNbTiO₆:Dy³⁺ shows bright yellow-green emission prevailing at 577 nm upon 273 nm excitation. With increasing activator concentration, the emission derived from the characteristic f–f transitions of Eu³⁺/Dy³⁺ is enhanced while the host emission is weakened, which is due to the energy transfer from the host to Eu³⁺/Dy³⁺. Considering the facile synthesis and excellent Eu³⁺/Dy³⁺ doped luminescence properties of this compound, self-activated GdNbTiO₆ may be a good candidate as a host phosphor for use in various optical devices.

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1. Introduction

Inorganic luminescent phosphors have been a long focus of research due to their good thermal and chemical stability and environmental friendliness. Rare earth ion-doped oxide phosphors are outstanding inorganic luminescent materials, and they can be excellently applied in lighting, display equipment, and solid-state lasers.^1–3 Therefore, more efforts should be paid to the improvement of the existing oxide phosphors and development of new oxide phosphor materials for modern display systems.

As is well known, lanthanide ion involved intra-4f transitions are barely affected by the external environment or crystal field on account of the shielding of the 4f orbital by the filled outer 5s and 5p orbitals.^4,5 The intra-4f emission spectra of lanthanide ion usually present the shape controlled, narrow and characteristic emission peaks forming peculiar spectrum properties. The typical rare earth ions, such as Eu³⁺ and Dy³⁺, have been extensively studied and applied in the luminescent materials. The trivalent europium ion (Eu³⁺) often shows its specific red emitting in the emission spectrum due to the transitions of ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4).⁶ When the Eu³⁺ ions occupy sites without inversion symmetry, the transitions of ⁵D₀ → ⁷F₂ hold the dominant position in their emission spectrum.^7–9 The trivalent dysprosium ion (Dy³⁺) doped phosphors have three peculiar emission bands with the blue band at around 480 nm, the yellow band at around 575 nm and red band at around 625 nm corresponding to ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2, respectively.¹⁰ Furthermore, the yellow emission of Dy³⁺ is especially hypersensitive to the chemical environment surrounding as Dy³⁺ ions are located in the lower symmetric sites without inversion centers, but the crystal field environment has little effect on the blue emission.^11,12 It can be known that the luminescence properties of the materials is not only decided by the active ions but also depend on the host. Thus, it is important to find a suitable host lattice for the Eu³⁺ and Dy³⁺ to research and develop phosphors emitting strongly and colorful.

Recently, rare-earth ternary niobates with the crystal structure to tuning of emission properties have been attracting interest, meanwhile due to their excellent properties which are strong direct excitation bands and higher activator critical concentration.¹³ For the broad band excitation of self-activated phosphors, the energy within these materials can be efficiently absorbed and transferred to doping ions achieving the effect of matrix sensitization and improving the luminous efficiency of the material.^14,15 The existence and crystallization of the RETiNbO₆ (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) compounds were firstly described in 1963 by A. I. Komkov who made a detailed discussion about the structure of ErTiNbO₆.¹⁶ In 1980, the synthesis and crystal structure of YTiNbO₆ was reported by H. Weitzel and H. Schroecke.¹⁷ With the development of the laser, X. Qi et al. (1996) reported that RETiNbO₆ single crystal functional materials are ideal gain media for the miniature solid laser systems.^18,19 And then, S. Solomon (2000) and Sebastian (2003) et al. reported that RETiNbO₆ ceramic materials has excellent microwave dielectric properties suitable for utilized in the dielectric resonators of microwave communication.^20,21 For the moment, a large number of reports have referred to the microwave dielectric properties of the compounds. Actually, RETiNbO₆ compounds are considered as a valuable, efficient and broadband-excited luminescent host due to excellent chemical properties and optical characteristics. For example, RE³⁺ doped into YNbTiO₆ and LaNbTiO₆ host can offer a variety of transition pathways and broad distributing emission spectrum.^22–25 Moreover, there are many reports found that the luminous performance of the Gd-based compounds is better than other Ln-based compounds (Ln = Y, La) such as M₂RE₈(SiO₄)₆O₂ (M = Mg, Ca; RE = Y, Gd, La),²⁶ RENbO₄:Ln³⁺ (RE = Y, Gd, Lu),²⁷ and LnCa₄O(BO₃)₃:RE³⁺ (Ln = Y, La, Gd).²⁸ Therefore, unceasing efforts to design and obtain euxenite type GdNbTiO₆-based phosphors with good luminescence properties has the very vital significance for potential application. In this work, we report the synthesis, crystal structure and luminescence properties of a novel phosphor Eu³⁺/Dy³⁺-doped GdNbTiO₆, and investigate the relative luminescent mechanism of Eu³⁺/Dy³⁺-doped phosphors.

2. Experimental

The phosphors Gd_1−xNbTiO₆:xRE³⁺ (0 ≤ x ≤ 0.1, RE = Eu, Dy) were synthesized by a high temperature solid-state reaction method in air. The stoichiometric mixtures of Gd₂O₃ (99.99%), Nb₂O₅ (99.99%), TiO₂ (99.99%), Dy₂O₃ (99.99%) and Eu₂O₃ (99.99%) were ground into powders in an agate mortar. The homogeneous mixtures were heat at 1100 °C, 1250 °C, and 1300 °C for 24 h, respectively. Then, the samples were naturally cooled to room temperature.

Their powder X-ray diffraction (XRD) patterns were measured on a Rigaku diffractometer D/MAX-2500 with Cu Kα radiation and graphite monochromator operated at 40 kV and 250 mA. In addition, Materials Date Inc. software Jade 6.0 and Powder Diffraction File were used for phase analysis. Data for crystal structure analysis were collected at room temperature in the step-scanning mode with a step 0.017° (2θ), counting time 2 s per step and 2θ range of 5–130°.

Luminescent properties of Eu³⁺/Dy³⁺ doped GdNbTiO₆ and undoped GdNbTiO₆ were investigated by fluorescence emission and excitation spectra under ultraviolet were recorded by using a Hitachi F-7000 Florescence Spectrophotometer. The obtained data were corrected for emission and excitation with respect to the Xe lamp excitation source. The luminescence spectra under VUV region were measured at Beam line 3B1B in Beijing Synchrotron Radiation Facilities (BSRF) under normal operating conditions (220 mA). The VUV excitation spectrum was corrected by sodium salicylate, whose quantum efficiency is almost constant in the region. XEL spectra for the compound were measured using an X-ray excited spectrometer (Fluormain) in which an F-30 X-ray tube (W anticathode target) was used as an X-ray source. All the measurements were recorded at room temperature.

3. Result and discussion

3.1 XRD patterns and crystal structure

Fig. 1 shows the representative patterns of the GdNbTiO₆ samples annealed at different temperature. For the samples annealed below 1100 °C, the intermediate phase of GdNbO₄ appeared, and a very small amount of TiO₂ was also observed, which were confirmed by JCPDS card no. 22-1104 and JCPDS card no. 83-2248, respectively. With the reaction temperature up to 1250 °C, a pure phase of GdNbTiO₆ was obtained, whose XRD pattern was found to be consisted with that reported in JCPDS card no. 27-0221. Single-phase GdNbTiO₆ was also obtained after annealing at 1300 °C, indicating that the pure samples can be obtained at between 1250 °C and 1300 °C. Fig. 2 shows the XRD patterns of GdNbTiO₆ host, GdNbTiO₆:0.03Eu³⁺ and GdNbTiO₆:0.02Dy³⁺ and all the diffraction peaks of the as-prepared samples are in good match with the reported pattern. No second phase was detected at the current doping level, indicating that the Eu³⁺/Dy³⁺ can be completely dissolved in the GdNbTiO₆ host lattice by substituting Gd³⁺. When one Gd³⁺ is replaced by one Eu³⁺ or Dy³⁺, we do not observe obvious crystal structure change. This may be due to their similar radius (r(Gd³⁺, CN = 8) = 0.9380 Å, r(Eu³⁺, CN = 8) = 0.9470 Å, r(Dy³⁺, CN = 8) = 0.9120 Å), coordination structure and physical chemical properties.

Fig. 1 XRD patterns of GdNbTiO₆ samples annealed at different temperature; the standard data for GdNbTiO₆ (JCPDS no. 27-0221) was used as reference.

Fig. 2 XRD patterns of powder GdNbTiO₆ host (a), GdNbTiO₆:0.03Eu³⁺ (b), GdNbTiO₆:0.02Dy³⁺ (c) samples and the reference for the standard data of GdNbTiO₆ (JCPDS no. 27-0221).

According to the similarity of effective cationic radii in the same coordination environment, the Y³⁺ can be substituted by the Gd³⁺ ion. Comparisons between GdNbTiO₆ and YNbTiO₆ in lattice parameters and powder XRD patterns show that the two compounds are probably isostructural. Therefore, taking YNbTiO₆ as the preliminary crystal structural model, we refined the structure parameters of GdNbTiO₆:0.03Eu³⁺ and GdNbTiO₆:0.02Dy³⁺ from the powder XRD data by the Rietveld method^29,30 within the program FullProf_suit. By the way, we assume that Eu³⁺, Dy³⁺ substitute cationic site Gd³⁺ (4c) according to the formula. The profile range of data used for refinement is 5–130° in 2θ, and the pseudo-Voigt function was used as peak shape function. A total of 37 parameters were refined in the refinement, including background parameters, profile parameters and structural parameters. By the way, the practical content of Eu³⁺ and Dy³⁺ have been tested and confirmed near to the nominal composition by ICP. So, the occupation of Eu³⁺ and Dy³⁺ was fixed in the process of refinement. The final agreement factors in the structural refinement converged to R_B = 7.29%, R_p = 2.0%, R_wp = 2.67% with S = 1.88 for GdNbTiO₆:Eu³⁺ and R_B = 7.48%, R_p = 1.97%, R_wp = 2.55% with S = 1.50 for GdNbTiO₆:Dy³⁺, which indicates the structure model is quite right. Fig. 3 shows the final refinement patterns. These results indicate that when Eu³⁺ or Dy³⁺ is doped in the GdNbTiO₆ host, it remains single phase. Details of Rietveld refinement and crystal data are given in Table 1. Position parameters obtained by the Rietveld refinement are listed in Table 2. The structure of GdNbTiO₆:Eu³⁺/Dy³⁺ are depicted in Fig. 4. The compound crystallizes in the space group Pbcn and belongs to typical euxenite-type structure with four formula units per unit cell. There are several different coordination environments that Nb⁵⁺ and Ti⁴⁺ ions share a same crystallographic site, which is coordinated by six neighboring O²⁻ to form a slightly distorted octahedron, and the (Gd/Eu/Dy)³⁺ ion is surrounded by eight fold O²⁻ ions at site with the lower symmetric C_S^xy (4) to form an irregular (Gd/Eu/Dy)O₈¹³⁻ polyhedron.³¹ Each of (Gd/Eu/Dy)O₈¹³⁻ polyhedral connect with four others by shared corner or edge forming a single layers which divides neighboring Nb(Ti)O₆ double layers.

Fig. 3 Final Rietveld refinement patterns of GdNbTiO₆:0.03Eu³⁺ (a) and GdNbTiO₆:0.02Dy³⁺ (b).

Table 1 Details of structure refinement and crystal data for GdNbTiO₆:0.03Eu³⁺ and GdNbTiO₆:0.02Dy^3+a

Formula	GdNbTiO6:0.03Eu^3+	GdNbTiO6:0.02Dy^3+
a Rp = ∑|yio − yic|/∑|yio|, Rwp = [∑wi(yio − yic)^2/∑wiyio^2]^1/2, Rexp = [(N − P)/∑wiyio^2]^1/2, S = Rwp/Rexp.
Sample	Multi-crystal power	Multi-crystal power
Diffractometer	Rigaku D/MAX-2500	Rigaku D/MAX-2500
Radiation type	CuKα	CuKα
Monochromator	Graphite	Graphite
Wavelength (Å)	1.5405	1.5405
Refined profile range (°2θ)	5–130	5–130
Step size (°2θ)	0.017	0.017
Step scan time per step (s)	2	2
RB	7.29%	7.48%
Rp	2%	1.97%
Rwp	2.67%	2.55%
S	1.88	1.50
Symmetry	Orthorhombic	Orthorhombic
Space group	Pbcn	Pbcn
a (Å)	14.6898(9)	14.6898(8)
b (Å)	5.60476(8)	5.6039(2)
c (Å)	5.22478(8)	5.2243(1)
V (Å^3)	430.17(3)	430.07(0)
Z	4	4

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Table 2 Atomic coordinates, atomic occupancies and isotropic displacement parameters for GdNbTiO₆:0.03Eu³⁺ and GdNbTiO₆:0.02Dy³⁺

Atom	Site	x	y	z	Biso (Å^2)	Occ
GdNbTiO6:0.03Eu^3+
Gd	4c	0	0.23625(33)	0.25	0.155(35)	0.970
Eu	4c	0	0.23625(33)	0.25	0.155(35)	0.030
Nb	8d	0.16913(14)	0.31898(29)	0.82130(31)	0.071(47)	0.500
Ti	8d	0.16913(14)	0.31898(29)	0.82130(31)	0.071(47)	0.500
O1	8d	0.09728(62)	0.43928(135)	0.53447(201)	−0.736(68)	1.000
O2	8d	0.09061(60)	0.10006(159)	0.90759(187)	−0.303(83)	1.000
O3	8d	0.26120(61)	0.13728(149)	0.64002(162)	−0.050(83)	1.000
GdNbTiO6:0.02Dy^3+
Gd	4c	0	0.23672(34)	0.25	0.220(35)	0.980
Dy	4c	0	0.23672(34)	0.25	0.220(35)	0.020
Nb	8d	0.16914(14)	0.31764(30)	0.82110(30)	0.010(46)	0.500
Ti	8d	0.16914(14)	0.31764(30)	0.82110(30)	0.010(46)	0.500
O1	8d	0.09791(64)	0.43924(137)	0.53499(200)	−0.424(277)	1.000
O2	8d	0.09027(55)	0.09985(159)	0.90304(175)	−0.541(256)	1.000
O3	8d	0.26216(58)	0.14399(149)	0.64356(152)	−0.467(216)	1.000

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Fig. 4 Crystal structure of GdNbTiO₆:Eu³⁺/Dy³⁺ (a), the coordination geometry of (Gd/Eu/Dy)O₈ (b) and Nb(Ti)O₆ (c) polyhedral in the host lattice.

3.2 Luminescence characteristics

3.2.1 Luminescence characteristics of GdNbTiO₆. Fig. 5a shows the excitation and the emission spectra of GdNbTiO₆ samples annealed at 1250 °C. The excitation spectrum shows a broad band in the UV range of 200–330 nm with a maximum at 273 nm which is due to the charge transfer band (CTB) from O-ligands to central Nb or Ti atoms in the slightly distorted Nb(Ti)O₆ octahedral groups.³² Upon 273 nm excitation, the corresponding emission spectrum of GdNbTiO₆ displays that a broad emission band centered at 509 nm in the visible range of 400–650 nm can be observed. Generally, a large amount of free electrons in the conduction bands (CBs) and holes in the valence bands (VBs) of GdNbTiO₆ host can be respectively created by the UV radiation. The self-trapped holes (STHs) will be always yielded immediately from the conversion of the as-gained free holes. Then, some unsaturated atoms and unpaired electrons are situated in the surface regions to generate some localized levels in the forbidden gaps. Some electrons are transited to these localized levers under expected energy excitation; thus certain polarons as captured centers will appear and interact with the STHs to generate self-trapped excitons (STEs). Finally, the recombination of these STEs leads to the broad emission band.³¹ The luminescence spectrum of GdNbTiO₆ is to corroborate the energy of the host absorption which is ascribed to the luminescence centre of Nb(Ti)O₆ groups and the Gd³⁺ ion did not show any luminescence in host. The GdNbTiO₆ sample shows an efficient blue-green luminescence under the 273 nm UV radiation excitation. To make thorough inquiry the properties of the host phosphor, the spectroscopic properties of the phosphor were investigated under vacuum ultraviolet (VUV) excitation. As shown in Fig. 5b, the excitation spectrum consists of an intense and broad band which is the host-related absorption band at 274 nm by monitored the emission 509 nm. We could eventually conclude, therefore, that GdNbTiO₆ is a broadband absorption and self-activated phosphor, and it is worthy of further research as a luminescence matrix material.

Fig. 5 The excitation and emission spectra of GdNbTiO₆ under UV (a) and VUV (b) region at room temperature.

There's a variety of studies suggesting that some rare earth fluorescence materials have good scintillation performance, such as Li₆Gd(BO₃)₃:Ce,³³ REBa₃B₉O₁₈,³⁴ and so on. In order to explore the scintillation characteristics of GdNbTiO₆ host, the X-ray excited luminescence (XEL) measurements were performed on polycrystalline GdNbTiO₆ sample and Bi₄Ge₃O₁₂ (BGO) at room temperature. Fig. 6 shows a comparison of the X-ray excited luminescence (XEL) spectrum of GdNbTiO₆ and Bi₄Ge₃O₁₂ (BGO) in the same measurement conditions. A broad band in the range of 500–700 nm with peak center at 600 nm is observed in the emission spectra of the GdNbTiO₆ pure sample. Compared with the PL emission spectra of GdNbTiO₆, the broad band shows an obvious red shift and the centre moves from 509 nm to 600 nm. Furthermore, by comparing the integral area of the emission bands for GdNbTiO₆ with that BGO, it is interesting to notice that the light yield is of GdNbTiO₆ about 11% of BGO. Therefore, we can conclude that the compound of GdNbTiO₆ is a potential scintillator for registration of X-radiation. At present, the scintillator mechanism is concerned; there may be an important relationship between their efficient XEL and the structural features for this compound. In addition, lattice defects such as oxygen vacancies, excess negative charges, and excitons among the band gap, probably play an important role on the scintillating properties of the material. Therefore, a detailed research of the scintillation mechanism will be necessary in the future work.

Fig. 6 Comparison of the XEL spectra of BGO and GdNbTiO₆ powders under the same measurement condition.

3.2.2 Luminescence characteristics of GdNbTiO₆:Eu³⁺/Dy³⁺. In order to investigate the effects of rare earth ions to the luminescence of GdNbTiO₆ phosphor, we chose two typical kinds of lanthanide ions to synthesize GdNbTiO₆:Eu³⁺ and GdNbTiO₆:Dy³⁺ phosphors. Fig. 7 reflects the differences of the luminescence characteristics between GdNbTiO₆ (Fig. 7a) and GdNbTiO₆:Eu³⁺/Dy³⁺ (Fig. 7b and c), which illustrates the GdNbTiO₆ is an effective matrix. Fig. 7b exhibits the excitation and the emission spectra of GdNbTiO₆:0.03Eu³⁺ sample. Monitored by Eu^{3+ 5}D₀ → ⁷F₂ transition at 614 nm, the excitation data for GdNbTiO₆:Eu³⁺ shows a broad band from 200 nm to 330 nm with a maximum at 273 nm, which should attribute to the possible overlap between the charge transfer band of O²⁻ → Eu³⁺ and the host lattice absorption,³⁵ and numbers of weak of line excitation peaks from 300 nm to 470 nm assigned to the f–f transition of Eu³⁺ 4f⁶ configuration. According to the Gaussian analysis, the asymmetric excitation band from 200 nm to 330 nm can be divide into two distinct bands peaked at 225 nm (I₁) and 277 nm (I₂),³⁶ which is consistent with the conjecture. In addition, it is observed that the absorption of the host lattice is more intense than those of the f–f transitions of Eu³⁺ ion, showing that the energy transfer from the matrix to Eu³⁺ is very efficient.³⁷ Under excitation at 273 nm, the phosphor GdNbTiO₆:Eu³⁺ exhibits a hypersensitive red emission band peaking at about 614 nm (Fig. 7b) due to the electric dipole ⁵D₀ → ⁷F₂ transition of Eu³⁺, which indicates that the Eu³⁺ ions are located at sites without inversion symmetry.³⁸ There are other line peaks in the range of 580–710 nm, namely, ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₃ (656 nm), and ⁵D₀ → ⁷F₄ (698 and 707 nm), respectively. Fig. 7c shows the excitation and the emission spectra of GdNbTiO₆:0.02Dy³⁺. A strong band peaking at 273 nm was also observed in the excitation spectra, and numbers of weak of line excitation peaks from 325 nm to 500 nm assigned to the characteristic f–f transition of Dy³⁺. The emission spectra of GdNbTiO₆:0.02Dy³⁺ phosphor shows three groups of peaks centered at about 482, 577, and 660 nm under 273 nm UV excitation, which originate from the transitions of ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 of Dy³⁺, respectively. It is well known that ⁴F_9/2 → ⁶H_13/2 transition of Dy³⁺ belongs to a forced electric dipole transitions and can be easily disturbed by the crystal field environment. The GdNbTiO₆:Dy³⁺ phosphor exhibits a predominant luminescence of ⁴F_9/2 → ⁶H_13/2 transition, suggesting that Dy³⁺ ions can occupy the lower symmetric location without the inversion centers in the host structure.³⁹ The photoluminescence properties of GdNbTiO₆:Eu³⁺/Dy³⁺ are consistent with the structure data of GdNbTiO₆:Eu³⁺/Dy³⁺.

Fig. 7 UV excitation and emission spectra of GdNbTiO₆ (a), GdNbTiO₆:0.03Eu³⁺ (b), and GdNbTiO₆:0.02Dy³⁺ (c) at room temperature, and the dotted lines denote the Gaussian fitting of the corresponding excitation peaks centered at 225 nm (I₁) and 277 nm (I₂).

Fig. 8a exhibits the VUV excitation (monitored the emission 612 nm) and VUV-excited emission spectra (under excitation at 286 nm) of GdNbTiO₆:Eu³⁺. The broad band centering at 281 nm can be assignable to the host absorption band and the charge transfer band of O²⁻ → Eu³⁺. Fig. 8b exhibits the VUV excitation (monitored the emission 576 nm) and VUV-excited emission spectra (under excitation at 286 nm) of GdNbTiO₆:Dy³⁺. A broad band peaking at 284 nm similar to host excitation spectrum appears to be attributed to the CT from the oxygen ligands to the central niobium atom within the NbO₆ octahedral groups. Compared with UV excitation spectrum, a red-shifted of the CT band was observed at VUV excitation spectrum because the resolution of VM504 vacuum monochromator is higher than Xe lamp (VM504 vacuum monochromator is 0.06 nm and Xe lamp is 0.1 nm) and different light source can result in difference.^40,41 For another reason, the different filter have been selected according the two different test conditions. All of the emission peaks under VUV excitation are similar to those under UV excitation. In addition, it is obvious that the intensity of the emission spectra is higher than the excitation spectra in the GdNbTiO₆:Eu³⁺/Dy³⁺ samples, which suggests that there may be the energy transfer between the host and the activated ions.

Fig. 8 VUV excitation and emission spectra of GdNbTiO₆:0.03Eu³⁺ (a) and GdNbTiO₆:0.02Dy³⁺ (b) under 286 nm at room temperature.

Fig. 9a and b depict luminescence intensity decay curves of ⁵D₀ → ⁷F₂ emission (614 nm) for GdNbTiO₆:0.03Eu³⁺ and ⁴F_9/2 → ⁶H_13/2 emission (577 nm) for GdNbTiO₆:0.02Dy³⁺ phosphors after irradiation at 273 nm. The corresponding luminescent decay times can be well fitted by a second-order exponential decay mode with the following equation:^42,43

I = I₀ + A₁ × exp(−t/t₁) + A₂ × exp(−t/t₂) (1)

where I and I₀ is the luminescence intensity at t and 0; A₁ and A₂ are constants; t is the time, and t₁ and t₂ are decay times for exponential components. According to these parameters, the average decay times (t) can be determined by the following formula:

Fig. 9 Decay curves of activators characteristic emissions in representative phosphors GdNbTiO₆:0.03Eu³⁺ (a) and GdNbTiO₆:0.02Dy³⁺ (b), and decay curves of host emission in Gd_(1−x)NbTiO₆:xEu³⁺ (c) and Gd_(1−y)NbTiO₆:yDy³⁺ (d) with different activators concentrations.

The effective lifetime values were calculated to be 0.056 ms (t₁ = 0.178 ms, t₂ = 0.008 ms) and 0.045 ms (t₁ = 0.009 ms, t₂ = 0.115 ms) for GdNbTiO₆:Eu³⁺ and GdNbTiO₆:Dy³⁺ phosphors, respectively. The both decay were be found to be a second-order exponential decay, which also indicates that there are two luminescence centers in GdNbTiO₆:Eu³⁺/Dy³⁺ crystal lattice, implying two mechanisms for the PL processes. It is completely in conformity with the crystal structure information of GdNbTiO₆:Eu³⁺/Dy³⁺. In addition, we investigated the decay curves and lifetimes of host emission with the increasing Eu³⁺/Dy³⁺ (x = 0, 0.01, 0.03, 0.05; y = 0, 0.01, 0.03, 0.05) concentration in GdNbTiO₆:xEu³⁺/yDy³⁺ samples as shown in Fig. 9c and d. The luminescence decay curve can also be fitted successfully by a second-order exponential function. On the basis of eqn (1) and (2), we can observe that the average decay times of host emission is 23 μs without any doping of activation ions. The average lifetimes of host are determined to be 22.76, 21.85, 17.07 μs for Eu³⁺ concentrations of 0.01, 0.03 and 0.05, respectively. For GdNbTiO₆:yDy³⁺ phosphors, we obtain values of 21.64, 21.35, 16.94 μs for y = 0.01, 0.03 and 0.05, respectively. With the increasing concentration of dopant ions, the value of decay time of host emission monotonously decrease in the GdNbTiO₆:Eu³⁺/Dy³⁺ system, which can well certify the existence of energy transfer from host to activators.

Fig. 10 exhibits the corresponding CIE chromaticity diagram of the GdNbTiO₆, Gd_(1−x)NbTiO₆:xEu³⁺ and Gd_(1−y)NbTiO₆:yDy³⁺ samples under 273 nm UV excitation. It can be found that the GdNbTiO₆ samples can emit bright cyan and the chromaticity coordinate is x = 0.258, y = 0.398 (point 0). The CIE chromaticity coordinates change from x = 0.397, y = 0.422 (point 1) to x = 0.534, y = 0.379 (point 5) by changing the doping concentration of Eu³⁺ from x = 0.01 to x = 0.1 in Gd_(1−x)NbTiO₆:xEu³⁺ phosphors and the corresponding luminescence color can change from yellow to red. The obtained Eu³⁺ ions doped GdNbTiO₆ phosphors emerge the advantage of multicolor emission in the visible region. The CIE chromaticity coordinates of Gd_(1−y)NbTiO₆:yDy³⁺ are just gathered around point 10 and the luminescence color have no obvious change. All of obtained chromaticity coordinates excited at 273 nm are list in Table 3.

Fig. 10 The corresponding CIE chromaticity diagram of the GdNbTiO₆, Gd_(1−x)NbTiO₆:xEu³⁺ and Gd_(1−y)NbTiO₆:yDy³⁺ samples under UV excitation.

Table 3 Variations of CIE chromaticity coordinates for GdNbTiO₆:xEu³⁺/yDy³⁺ phosphors

No. of points	GdNbTiO6:xEu^3+/yDy^3+	CIE (x, y)
0	x = 0	(0.258, 0.398)
1	x = 0.01	(0.397, 0.422)
2	x = 0.02	(0.444, 0.407)
3	x = 0.03	(0.467, 0.399)
4	x = 0.04	(0.511, 0.384)
5	x = 0.05	(0.534, 0.379)
6	x = 0.07	(0.554, 0.371)
7	x = 0.1	(0.588, 0.359)
8	y = 0.01	(0.348, 0.436)
9	y = 0.02	(0.357, 0.443)
10	y = 0.03	(0.361, 0.443)
11	y = 0.04	(0.364, 0.442)
12	y = 0.05	(0.343, 0.449)

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3.3 Energy transfer mechanism

In our experiment, as shown in Fig. 7, there are overlaps between the host emission spectra and the excitation spectrum of Eu³⁺/Dy³⁺. Obviously, there is the energy transfer between the host and the doped ions. The different energy transfer mechanism determines the luminescence properties of oxide phosphors doped with activators, including radiation reabsorption transfer, exchange interaction, and multipolar interactions.^44,45 For rare earth ions, the exchange interaction become effective only when the distance between activators is about 5 Å or shorter.⁴⁶ The multipolar interaction have three kinds including dipole–dipole (d–d), dipole–quadrupole (d–q) and quadrupole–quadrupole (q–q) interaction.⁴⁷ Doping concentration is a considerable factor affecting luminescence performance and luminescence mechanism. With Eu³⁺ content from 1 mol% to 10 mol%, the Eu³⁺ concentration dependence of the emission intensity of ⁵D₀ → ⁷F₂ transition (614 nm) under 273 nm excitation increases in Fig. 11a. The Dy³⁺ concentration dependence of the emission intensity of ⁴F_9/2 → ⁶H_15/2 transition (482 nm) and ⁴F_9/2 → ⁶H_13/2 transition (577 nm) under 273 nm excitation are shown in Fig. 11b. The effect of two emission peaks influenced by Dy³⁺ concentration is very similar. The emission intensities of GdNbTiO₆:Dy³⁺ phosphors constantly increase with increasing Dy³⁺ ion concentration, however, the emission intensities of host clearly decrease, which also suggests that the energy transfer from the host to the doped ions. In order to further confirm the type of energy transfer mechanism in the host, the relationship of doped ions concentration and photoluminescence quantum efficiencies is discussed with a suitable method according to Dexter's energy transfer expressions of multipolar interaction and Reisfeld's approximation. The model can be described as follows:⁴⁸where η_S0 and η_S stand for the photoluminescence quantum efficiencies of the host with the absence and presence of the Eu³⁺/Dy³⁺ ions, respectively. C is the doped ions concentration, and θ is a variance related to the type of energy transfer. The value for θ = 6, 8 and 10 correspond represent d–d, d–q, q–q interaction, respectively. However, the value of η_S0/η_S is difficult to acquire so eqn (3) can be transformed to the relationship of doped ions concentration and luminescence intensities as follows:where I_S0 and I_S are the photoluminescence intensity of the host without and with the Eu³⁺/Dy³⁺ ions, respectively. Fig. 11c and d exhibit the corresponding plots of I_S0/I_S and C of GdNbTiO₆:Eu³⁺ and GdNbTiO₆:Dy³⁺, respectively. According to the curves, only when θ = 6 does it display a linear relation, which indicates that the dipole–dipole interaction is the dominant energy transfer mechanism for the host to Eu³⁺/Dy³⁺ ions.

Fig. 11 The relationship of the emission intensity and activator concentration: the intensity of 614 nm as a function of Eu³⁺ concentration (a); and the intensity of 577 nm as a function of Dy³⁺ concentration (b). The corresponding plots of ln(I/C_x) as a function of ln C_x are also shown in (c) and (d), respectively. Energy transfer efficiencies from host to activators in Gd_(1−x)NbTiO₆:xEu³⁺ (e) and Gd_(1−y)NbTiO₆:yDy³⁺ (f).

On the other hand, in Eu³⁺/Dy³⁺-doped GdNbTiO₆ phosphors, the energy transfer efficiencies (η_ET) from host to the activators can be calculated using the formula η_ET = 1 − I_S/I_S0, where η_ET is the energy transfer, and I_S and I_S0 are the corresponding luminescence intensities of the host in the with and without the activators, respectively.⁴⁹ The η_ET values constantly increase to reach maximums at 98% depicted in Fig. 11e, which reveals the energy transfer rate from the Nb(Ti)O₆ group in the host to the Eu³⁺ becomes more efficient with the increasing concentration of Eu³⁺ ion. When the doping concentrations of Dy³⁺ increase from 0.01 to 0.05, the energy transfer rate from phosphors the Nb(Ti)O₆ group to Dy³⁺ in the host increases from 60.88% to 83.26% shown as Fig. 11f.

Fig. 12 shows a simple model describing the intrinsic emission process in Nb(Ti)O₆ group, the energy transfer from Nb(Ti)O₆ group charge transfer band to Eu³⁺ and Dy³⁺ ions and the characteristic emission energy levels of Eu³⁺ and Dy³⁺ ions in GdNbTiO₆. For GdNbTiO₆ sample, under 273 nm UV radiation, the GdNbTiO₆ host absorb UV radiation and then a charge transfer from O²⁻ ligands to Nb⁵⁺ or (Ti⁴⁺) ions occurs in the crystal lattice giving its broad emission band with the center at 509 nm. When doping with Eu³⁺/Dy³⁺ ions in GdNbTiO₆ host lattice, the excitation energy is transferred from O²⁻ ligands to Nb⁵⁺ or (Ti⁴⁺) ions charge transfer band to the excited states of Eu³⁺ or Dy³⁺ ions, resulting in the increases of characteristic emission intensities of activators.⁵⁰

Fig. 12 A simple model illustrating the intrinsic emission process in Nb(Ti)O₆ group, the energy transfer from host to Eu³⁺ and Dy³⁺ ions, and the characteristic of Eu³⁺ and Dy³⁺ ions in GdNbTiO₆.

4. Conclusion

In conclusion, we have successfully synthesized self-activated GdNbTiO₆ and Eu³⁺/Dy³⁺ doped GdNbTiO₆ phosphors by solid-state reaction. Structural refinement indicates that Eu³⁺/Dy³⁺ ion in the GdNbTiO₆ host lattice is preferred to occupy the site of Gd³⁺. The luminescence properties of undoped and Eu³⁺/Dy³⁺ doped GdNbTiO₆ phosphors were investigated under UV and VUV excitation in detail. Under the UV excitation of 273 nm, GdNbTiO₆ phosphors have a broad band peaking at 509 nm, which is due to the luminescence centre of Nb(Ti)O₆ group in the host lattice. Eu³⁺ and Dy³⁺ ions in the matrix of GdNbTiO₆ emitted intense red and yellow-green light under UV excitation. In the meantime, the energy transfer mechanisms from the Nb(Ti)O₆ group of the host to Eu³⁺ and Dy³⁺ ions have been confirmed to be a resonant type through a dipole–dipole interaction. Furthermore, the VUV spectrum of GdNbTiO₆, GdNbTiO₆:Eu³⁺, and GdNbTiO₆:Dy³⁺ are similar to their PL spectrum, which can also illustrates the existence of energy transfer from the host to Eu³⁺/Dy³⁺ ions. In addition, XEL measurements showed the light yield of GdNbTiO₆ is about 11% as large as that of BGO and the compound of GdNbTiO₆ is a potential scintillator for registration of X-radiation. Considering the chemical stability, emission wavelength range and luminescence intensity of the self-activated GdNbTiO₆ and Eu³⁺/Dy³⁺-activated GdNbTiO₆, these materials are good candidates for the further study and lighting application system.

Acknowledgements

Financial supports by the National Natural Science Foundation of China (Grant number 51472273), BSRF, and Department for Science and Technology of Hunan province (Grant number 2014FJ4099) are gratefully acknowledged. The work was also supported by grants from the Project of Innovation-driven Plan in Central South University (Grant number 2015CX004).

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