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Meiting Li, Bingxin Yuan, Xiangyang Zheng, Bo Chen, Chi Zhang, Xuejiao Wang, [Ji-Guang Li](https://orcid.org/0000-0002-5625-7361)

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[Phase tailoring and color-tunable luminescence of LaNbO4:Tb3+,Eu3+ nanophosphors for deep UV-pumped pc-WLEDs application](https://mdr.nims.go.jp/datasets/93fe997c-026f-4397-a0f7-f348415b1adc)

## Fulltext

1 Phase tailoring and color-tunable luminescence of LaNbO4:Tb3+,Eu3+ nanophosphors for deep UV-pumped pc-WLEDs application  Meiting Lia,b*, Bingxin Yuana, Xiangyang Zhenga, Bo Chena, Chi Zhanga, Xuejiao Wangc, Ji-Guang Lib* aSchool of Materials Science and Engineering, Liaoning University of Technology, Jinzhou, Liaoning 121001, China bResearch Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan cCollege of Chemistry and Materials Engineering, Bohai University, Jinzhou, Liaoning 121007, China      *Corresponding author Dr. Meiting Li Liaoning University of Technology  Tel: +86-416-4198730 E-mail: limeiting@aliyun.com   Dr. Ji-Guang Li National Institute for Materials Science Tel: +81-29-860-4394 E-mail: LI.Jiguang@nims.go.jp mailto:limeiting@aliyun.commailto:LI.Jiguang@nims.go.jp 2 Abstract A series of LaNbO4:RE3+ (RE=Tb and Eu) phosphors were successfully synthesized via a calcination-assisted hydrothermal reaction, where the vital roles of solution pH, calcination temperature and NbO43-/La3+ molar ratio in phase/morphology evolution of LaNbO4 nanocrystals were elaborated. It was unambiguously demonstrated that monoclinic LaNbO4 can be crystallized at solution pH=8-13 after calcination at 900 ºC. With pH=10 precursor for example, higher calcination temperature led to phase transformation from orthorhombic to monoclinic structure. LaNbO4 nanoparticles exhibit self-activated broad-band excitations (1A1 → 1T1,2) and emissions (3T2,1 → 1A1) within the NbO43- ligand. Intriguingly, Tb3+/Eu3+ incorporations allow for phase tailoring and full-visible-spectrum color-tunable luminescence, where the latter benefits from the effective NbO43- → Tb3+ → Eu3+ energy transfer. LaNbO4:0.01Tb3+,0.01Eu3+ phosphor exhibits stronger white-light emission, which gradually drifts to yellowish green at high heating temperature. This phosphor is an attractive candidate for optical thermometers and warm pc-WLED owning to excellent thermal stability, a low correlated color temperature of ~5694 K and a high color rendering index of ~90.   Keywords: LaNbO4:Tb3+,Eu3+ WLEDs; phase tailoring; muticolor luminescence; NbO43- → Tb3+ → Eu3+ energy transfer; optical thermometers     3 1. Introduction White light emitting diode (WLEDs) are regarded as an indispensable solid-state light-emitting device, which has desirable merits of high luminous efficiency, energy saving, long lifetime and environmental protection, and so forth [1]. They can cover the full-visible light in the spectral range of ~400-750 nm and exhibits high color rendering index, which is recognized as a fourth-generation light source to replace traditional incandescent lamp and fluorescent lamp. Currently, WLEDs have two kinds of preparation processing, that is, multi-chip and single-chip modules [2]. In the former case, white light can be obtained by integrating single blue, green and red LED chips, however, the practical development is rather limited by the sophisticated and expensive LED circuitry [3]. The commercial single-chip WLEDs are constituted of blue GaN/InGaN chip and yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor, nevertheless, the loss of red component results in a high correlated color temperature (CCT) and a low color rendering index (CRI), and constant cold white light will bring about asthenopia and brain damage [4]. Therefore, previous studies proposed that red/green/blue (RGB) tri-phosphors were accurately executed by introducing the sensitizer/activator combinations into host and their energy transfer (CT) under NUV LED chip excitation. Though this phosphor-converted (pc-) WLEDs exhibit lower CCT and higher CRI, the thermal quenching is detrimental to color balance between components [5]. Meanwhile, the inevitable reabsorption of phosphors lowers the luminescence efficiency of white light [6]. In this regard, the integration of high-quality pc-WLED devices are still confronted with challenges.   4 Up to date, considerable efforts have devoted to design advanced single-component white phosphor depositing on a UV or near-UV (NUV) LED chip, because it has the merits of better color stability, less current drooping and binning for high-power WLED application, compared with blue-triggered one [7]. The effective strategies include (1) “host + activator” combinations; (2) codoping of multi-colored activators, such as Tm3+/Dy3+, Tb3+/Sm3+, Tm3+/Tb3+/Eu3+, Yb3+/Er3+/Tm3+; (3) sensitizer/activator pairs and their energy transfer, including Ce3+ → Eu2+, Ce3+ → Mn2+, Ce3+ → Tb3+→ Mn2+, Eu2+ → Mn2+, Bi3+ → Eu3+, etc.; (4) defect-related optical materials [8]. The highly-charged transition metal ions with d0 electronic configuration, such as TiO42-, WO42-, WO66-, MoO42-, VO43-, NbO43-, TaO43-, have been conceived as satisfactory fluorescence centers owning to their self-activated broadband emissions upon UV excitation and host sensitization of ligand → RE3+ energy transfer [9]. Wang et al., for example, hydrothermally synthesized thermodynamically stable t-(Gd0.94Eu0.06)(P0.01V0.01)O4:Eu3+ nanocrystallites, and 1 at% VO43- separately pronounced the orange and red emissions by ~1.4 and 6 times via cooperative energy transfer of VO43-/Gd3+ → Eu3+ [10].  LaNbO4 exhibits low phonon frequency, high ionic and electronic conductivity, excellent mechanical/chemical stability and optical properties, which may find potential applications in optical storage [11], light-emitting diodes [12], 3D display solid-state lasers [13], fuel cell anodes [14,15], gas sensors [16]. A solid-state reaction is commonly employed in LaNbO4 synthesis because monomeric NbO43- is hardly dissociated in aqueous solution, but a harsh high temperature (>1000 °C) and  5 uncontrollable shape/size for coarse aggregates seriously restrict the functionalization applications [17]. Hydro-/solvo-thermal synthesis routes can comprehensively regulate phase structure and morphology of a compound via rational design of reaction parameters, and even govern photoluminescence performance. With citrate as a chelating agent, Li et al. produced β-NaYF4:Tb3+ octadecahedron, microrod and hexagonal microprism, and found that octadecahedron exhibited the strongest Tb3+ emission [18]. It was reported that compared with bulk materials, quasi-equiaxial nanocrystals allowed more activator ions to reside onto or near the surface regions owning to their virtues of small particle size, large specific surface area and high surface activity, and thus, remarkably changed relative intensity of the electric dipole and magnetic dipole transition of RE3+ (RE=Eu and Dy), and also luminous colors [19].  Inspired by the fact that the inherently blue-emitting characteristics of NbO43- ligand can fulfill emission-color tailoring combined with appropriate concentration and types of activators. We thus adopted calcination-assisted hydrothermal synthesis strategy for full color-emitting LaNbO4:Tb3+,Eu3+ nanocrystals in this work, and the underlying mechanisms of crystallization kinetics, NbO43- → Tb3+ → Eu3+ cooperative energy transfer and thermal quenching were also deciphered. The optical properties of pc-WLED fabricated by LaNbO4:0.01Tb3+,0.01Eu3+ white-emitting phosphor in combination with 275 nm UV chip were also investigated in detail, and this warm WLED device possesses a high CRI (~90) and a low CCT (5694 K).     6 2. Experimental section 2.1 Reagents The starting chemicals of La(NO3)3·6H2O (>99.99% pure), Eu(NO3)3·6H2O (>99.99% pure), Tb(NO3)3·5H2O (>99.9% pure), Nb2O5 (>99.9% pure) and La2O3 (>99.99% pure) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.. NbCl5 (>99.0% pure) and NaOH (>97.0% pure) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd.. Nitric acid (HNO3, ultrahigh purity, analytical grade) was purchased from Jinzhou Gucheng Chemical Reagent Co., Ltd.. Milli-Q filtered water (resistivity ~18.2 MΩ cm) was used throughout the experiments. All of the reagents were used without further purification.  2.2 Preparation of LaNbO4:Tb3+,Eu3+ phosphors In a typical procedure of calcination-assisted hydrothermal synthesis of LaNbO4 micro/nanocrystals, a certain amount of NbCl5 was dispersed into aqueous solution, followed by magnetic stirring for 30 min at room temperature, addition of 2 mmol of La3+, and pH adjustment with dilute HNO3 and/or NaOH solution. After continuously stirring with 30 min, 60 mL of resultant homogeneous suspension was immediately transferred into 100 mL Teflon-lined stainless steel autoclave for 24 h of reaction at 200 ºC in a preheated electric oven. After natural cooling to ambient temperature, the white precipitate was collected via centrifugation, washed with distilled water three times and absolute ethanol once to the removal of byproducts, followed by air drying at 70 ºC for 24 h. Calcination of the precursor was performed in stagnant air at a predetermined temperature for 2 h, using heating and cooling rate of 5 ºC/min at the ramp stage. The synthesis parameters including NbO43-/La3+ molar ratio (R=0.75, 1, 1.2, 1.5, 2, 3 and 4),  7 solution pH (2-13) and calcination temperature (300-900 ºC) were systematically varied to identify reaction kinetic and phase/morphology evolution of the products. A series of LaNbO4:xTb3+,yEu3+ (x=0-0.05, y=0-0.07) phosphors were prepared by the above protocol at the fixed NbO43-/RE3+ molar ratio R of 1, solution pH of 10, 24 h hydrothermal reaction of 200 ºC, and subsequent calcination at 900 ºC.  The LaNbO4 counterpart was prepared via a conventional solid-state reaction for comparison, and the synthesis process is briefly described as follows. 5 mmol of La2O3 and Nb2O5 were mixed and adequately grinded in an agate mortar for 6 h with the aid of appropriate ethanol. After air drying at 70 ºC for 24 h, the powder was air-calcined in a muffle furnace at 800, 900, 1000 and 1200 ºC for 6 h with heating and cooling rate of 5 ºC/min, respectively. 2.3 Encapsulation of LED devices The as-synthesized white light-emitting LaNbO4:0.01Tb3+,0.01Eu3+ phosphor was dispersed into UV curing agent (Leaftop 6300) under the mass ratio of 1:1. The mixture was stirred thoroughly and aged for 15 min to eliminate bubbles, and then uniformly casted on the surface of a commercial 275 nm LED chip (0.2 W, San'an Optoelectronics Co., Ltd.) followed by 6 h drying at 70 ºC for further measurements. 2.4 Characterization techniques Phase identification was detected by X-ray diffractometry (XRD, Model D/max-2500 PC, Rigaku, Tokyo, Japan), using nickel filtered Cu-Kα radiation (λ=0.15406 nm, 40 kV/100 mA) and a scan rate of 6.0 º/min. Rietveld refinement of the XRD pattern was conducted using the TOPAS 4.2 software [20]. Morphology and microstructure were characterized by field emission scanning electron microscopy (FE-SEM, Model  8 Sigma 500, Carl Zeiss, Oberkochen, Germany) under an acceleration voltage of 10 kV and transmission electron microscopy (TEM, Model JEM-2100F, JEOL, Tokyo) under 200 kV. Thermogravimetry/differential scanning calorimeter (TG/DSC, Model STA449F3, Jupiter, NETZSCH, Germany) was measured in flowing simulated argon (80 mL min-1) at a heating rate of 10 ºC min-1, and the furnace temperature ranged from R.T. to 1100 ºC. The photoluminescence, absolute quantum yield and fluorescence decay curves were obtained by identical instrument with a Model FLS-1000 fluorescence spectrometer (Edinburgh Instruments, Ltd. Livingston, UK) equipped with a 450 W Xe lamp for excitation and a TAP-02 high temperature controller for the temperature-dependent luminescence and decay kinetics. Electroluminescence performances of LEDs were evaluated using an OHSP-350M LED Fast-Scan Spectrophotometer (Hangzhou Hopoo Light&Color Technology Co., Ltd, China) under a current range of 20-120 mA in the spectral range of 350-1050 nm. 3. Results and discussion 3.1 Phase structure and morphology evolution of LaNbO4 nanocrystals  Fig. 1 XRD patterns for the products hydrothermally synthesized under various solution pH after calcination at 900 ºC. The vertical bars denote the standard diffractions of monoclinic LaNbO4, orthorhombic LaNb5O14 and orthorhombic Nb2O5 for comparison.  9 Fig. S1 shows the powder XRD patterns of the precursors obtained via hydrothermal reaction at 200 ºC for 24 h with the solution pH values increasing from 2 to 13. It is clear that a phase mixture of hexagonal (h-) La(OH)3 (JCPDS No. 01-083-2034) and orthorhombic (o-) LaNb5O14 (JCPDS No. 01-076-0263) was found in strong alkaline solutions (pH=~10-13), while pH=2-9 precursors are all amorphous mass. The phase selectivity conforms to the complicated solution chemistries of lanthanum cation and niobate. Like vanadate, tungstate and molybdate, niobate undergoes strong polymerization and protonation to Nb6O19-z(OH)z(8-z)- polyoxoniobates in solution, which may simultaneously dissociate to form polyoxoanions of [Nb12O36]12- for pH=4-7, [Nb10O28]6- for pH=5.5-10.8, and [Nb6O19]8- at sufficient alkalinity (pH≥11) [21-23]. Another indispensable consideration is the hydrolysis of La3+ to [La(OH)x(NO3)y(H2O)z]3-x-y complex under alkaline conditions [21,24]. Increasing solution pH promoted hydrolysis to yield more OH- (larger x) and less NO3- (smaller y) and also improved reactivity of hydrolyzed La species, ultimately led to direct precipitate as La(OH)3. Hence, the free [Nb10O28]6- and [Nb6O19]8- species showed the limited reactivity towards La species to produce LaNb5O14. XRD analysis of the precursors after calcination at 900 ºC (Fig. 1) illustrated that the amorphous precursors at pH=2 and 3 directly transformed into o-Nb2O5 (JCPDS No. 00-019-0862) and o-LaNb5O14, and pH=4 and 5 ones exclusively yielded o-LaNb5O14. The targeted monoclinic-structured (m-) LaNbO4 (JCPDS No. 01-083-1911) may be crystallized up to solution pH≥6, and becomes the only one at pH = 8-13. The above results further manifested hydrothermal products contain the amorphous mass of lanthanum cations  10 and niobate. Furthermore, increasing solution pH led to more split and sharper (-121) and (121) diffraction peaks, implying a higher crystallinity and/or larger crystallite size of the product. It is noteworthy that solution pH of 11 unexpectedly weakened the whole diffraction peaks, which may be related to the quantitative transformation from [Nb10O28]6- to [Nb6O19]8- [22].  The lattice framework of fergusonite-type m-LaNbO4 (space group: I2/c) based on the data of structural refinement is depicted in Fig. S2 [25]. The unit cell is composed of distorted LaO8 dodecahedron (six) and NbO6 octahedron (six), and both La and Nb reside at the 4e sites of C2 symmetry. Each of LaO8 are linked with adjacent NbO6 (or LaO8) by edge-sharing to form zig-zag chains. There are two types of coordinating oxygen atoms of O1 and O2, with four O1 (O1´ crystallographically equivalent to O1) and four O2 (O2´ equivalent to O2) for LaO8, and four O1 and two O2 for NbO6. La-O1 bond length is fairly close to La-O1´ one, while the bond length of La-O2 is remarkable larger that of La-O2´, the similar situation can be observed for Nb-O1/ Nb-O1´ bond length. As a result, LaO8 dodecahedron are interlinked in the whole 3D crystallographic direction, and simultaneously isolate 1D zig-zag chain of NbO6 octahedron. Rietveld refinement of the XRD pattern for the pH=10 LaNbO4 after calcination at 900 ºC using the existing crystallographic data of LaNbO4 as initial model yielded acceptable reliability factors (Rwp=10.107%, Rp=7.56%, χ2=3.68) (Fig. S3).  FE-SEM observation of the hydrothermal products (Fig. S4) revealed that solution pH significantly affects coordination chemistry and ionic mobility [26], and thus results in the multiform morphologies of poorly-crystallized granular agglomerates (~50 nm;  11 pH=2-5 and 9, Fig. S4a-d and h), mixed shapes of microplate-like assemblies (lateral size ~1-1.5 μm, thickness ~50 nm) and granular agglomerates (~50 nm) (pH=6-8, Fig. S4e-g), and microbelt-like assemblies (length ~1-3 μm, width up to ~500 nm) of rounded nanoparticles (~50 nm) and/or nanorods (length ~0.2-1 μm) (pH=10-13, Fig. S4i-l). For pH=10-13 samples, raising pH promoted the one-dimensional growth to yield a mass of discrete nanorods and microbelts.                Fig. 2 FE-SEM (a-i and m-o) and TEM (j) morphologies for the products hydrothermally synthesized under pH 2 (a), 3 (b), 4 (c), 5 (d), 6 (e), 7 (f), 8 (g), 9 (h), 10 (i and j), 11 (m), 12 (n) and 13 (o) after calcination at 900 ºC. The insets in (d) and (f) are for magnified views. Part (k) and (l) show the HR-TEM lattice fringes and SAED patterns of the pH=10 one.  12 Apart from the changes of phase composition, calcination collapsed the shapes and facilitated the inter-crystallite sintering to the extent (Fig. 2). SEM and TEM morphologies (Fig. 2i and j) demonstrated that LaNbO4 obtained under pH=10 consists of agglomerates interconnected with well-defined nanoparticles (~200-500 nm). High-resolution TEM analysis (HR-TEM, Fig. 2k) taken from the rectangle region in Fig. 2j better resolved the lattice fringes with interplanar spacing of ~0.249 nm, which well correspond to the (220) plane of monoclinic LaNbO4 (JCPDS No. 01-083-1911: d220=0.25045 nm). Selected-area electron diffraction (SAED, Fig. 2l) pattern yielded a series of well-arranged spots which can be indexed to (220), (141) and (-121) planes, and the measured dihedral angle of ~103.94º for (220)/(-121) planes approximately coincides with the calculated value of 104.04º. The results also confirmed single crystalline nature of an individual nanoparticulate.  Fig. 3 XRD profiles for the hydrothermally crystallized pH=10 precursor before and after  13 calcination at different temperatures with a holding period of 2 h.  Fig. 3 investigated calcination kinetic process of as-synthesized pH=10 precursor, where a phase mixture of La(OH)3 and LaNb5O14 can be retained up to 300 ºC. Annealing at 500 ºC additionally hastened the chemical reaction of h-La(OH)3 and o-LaNb5O14 (2La(OH)3 + LaNb5O14 = 5LaNbO4 + 6H2O) to yield trace orthorhombic (o-) LaNbO4 (JCPDS No. 00-054-0001). Elevating the temperature to 700 ºC accelerated the generation of o-LaNbO4 (major), and also induced a phase transformation from poorly crystallized o-LaNbO4 to m-LaNbO4 (minor), with the latter being single phase up to 800 ºC. Calcining at even higher temperature of 900 ºC caused no changes to the monoclinic crystal structure, but obviously strengthened the diffractions of m-LaNbO4. Previous studies demonstrated that high-temperature calcination facilitated luminescence by eliminating quenching defects such as surface dangling bonds [27]. TG/DSC curves of pH=10 precursor includes two major stages of weight loss up to ~800 ºC (Fig. S5). The first stage (~1.41% up to 370 ºC) accompanied by a endothermic peak at ~318 ºC is ascribed to the removal of surface adsorption water and intermolecular hydration water in La(OH)3. The second stage (~0.66%) within the ~370-700 ºC temperature range and an endothermic peak at 539 ºC may be derived from the formation of o-LaNbO4 via o → m phase transition. SEM observation (Fig. 4) found that microbelt-like architectures (Fig. 4a and b) disintegrated into loosely-agglomerated nanorods (Fig. 4c and d), and eventually into nanoparticles (~200 and 500 nm, Fig. 4e and f). In contrast, phase-pure LaNbO4 can only be crystallized via  14 solely high-temperature solid-state reaction at 1200 ºC for 6 h, and its crystallinity is inferior to that of pH=10 one. The as-obtained rough aggregates (~2-3 μm sized irregular objects) with inter-crystallite sintering (Fig. S6) are detrimental to photoluminescence when compared with calcination-assisted hydrothermal nanocrystallites. This may be owning to the more surface/structural defects and less RE3+ located at/near crystallite surface, and thus led to the for scattering and quenching of excitation light [28]. Huang et al. compared the photoluminescence properties of NaLa(WO4)2:0.05Tb3+ microspindles and irregular blocks synthesized via hydrothermal process and solid-state reaction, and found that the former exhibits the higher Tb3+ emissions owning to the smooth surface [29]. The 800-1000 ºC calcined products are of h-La2O3, o-LaNb5O14 and m-LaNbO4, and raising temperature promoted the crystallization of m-LaNbO4.          Fig. 4 FE-SEM images for the hydrothermally crystallized pH=10 precursor before (a) and after  15 calcination at 300 (b), 500 (c), 700 (d), 800 (e) and 900 ºC (f).  The effect of NbO43-/La3+ molar ratio R on phase structure and morphology of the pH=10 product obtained under otherwise identical synthesis conditions was investigated in Fig. S7 and Fig. S8. It is seen that m-LaNbO4 purity can only be retained at R=1, while R=0.75 one contains m-LaNbO4 and trace La2O3 owning to the insufficient La source. For LaNbO4, the relatively low crystallinity and an extraordinarily strong (130) diffraction indicated largely exposure of (130) plane. FE-SEM analysis found that R=0.75 product has smaller and uniform-sized granular aggregates (~50-200 nm) when compared with R=1 one (Fig. S8a). A slight excess of NbO43- at R=1.2 prevented o → m phase transformation resulting from kinetic reasons, and ultimately led to the sufficient crystallization of m-LaNbO4 with the weak (-121) and (121) diffractions and the appearance of o-LaNbO4 and cubic (c-) La0.33NbO3 impurity (JCPDS No. 00-036-0126). Hence, both of short nanorods (~200-400 nm in length, ~50-100 nm in diameter) and nanoparticles (~200-500 nm) were witnessed from the SEM micrographs in Fig. S8c. Superfluous NbO43- tends to react with relatively low stoichiometric amount of La3+ for more c-La0.33NbO3, and as a consequence, the strong diffractions of c-La0.33NbO3 and almost vanished diffractions of large-sized m-LaNbO4 particulates and h-La2O3 were the result up to R=4 (Fig. S8g).     16 3.2 Photoluminescence of LaNbO4:Yb3+,Eu3+ nanophosphors            Fig. 5 Photoluminescence excitation (PLE, a) and photoluminescence (PL, b) spectra of LaNbO4 phosphors synthesized by calcining the pH=10 precursor at 900 ºC for 2 h. Part (c) shows deconvolution of the ~397 nm emission by Gaussian fitting. Part (d) is semi-log plot of fluorescence decay for the 460 nm emission of NbO43- in LaNbO4.  Fig. 5a-c present the photoluminescence excitation (PLE) and emission (PL) of LaNbO4 phosphor obtained through calcining the pH=10 precursor at 900 ºC. It is thus clear that PLE spectrum has two well-separated and broad sub-bands centered at 260 and 300 nm separately corresponding to the parity-allowed 1A1 → 1T2 and 1A1 → 1T1 transitions from O2- → Nb5+ charge transfer (CT) within the NbO43- ligand in terms of ligand field theory, where 1A1 → 1T2 belongs to spin-allowed electric-diople transition, while 1A1 → 1T1 transition will be enhanced with increasing deviation from a cubic symmetry [30]. Exciting the LaNbO4 phosphor with the dominant 260 nm UV light produced a broad emission peaked at 397 nm, which is assignable to the intrinsic  17 emission of distorted NbO43-. Gaussian deconvolution of the asymmetric PL band gave two overlapping peaks located at 20943 and 25017 cm-1, which is originated from intrinsic 3T2 → 1A1 and 3T1 → 1A1 transitions via radiative decay, respectively. Fluorescence decay kinetics for the 397 nm emission of NbO43- can be well-fitted by a bi-exponential equation as I(t) = A1exp(-t/τ1) + A2exp(-t/τ2) + C (Fig. 5d), where I(t) is the emission intensity at time t, τ1 and τ2 denote exponential components of the decay times, and A1, A2 and C are constants, and the average lifetime can be determined by the function τ* = (A1τ12 + A2τ22)/(A1τ1 + A2τ2) being ~6.90 μs [31].        Fig. 6 PLE (a) and PL (b) spectra for the 900 ºC calcined LaNbO4:xTb3+ (x=0.005-0.05) phosphors.  The luminescence behaviors of the calcined LaNbO4:Tb3+ phosphors (x=0-0.05) synthesized under the identical condition of the aforesaid LaNbO4 phosphor were discussed as follows to elaborate the energy interaction mechanism between RE3+ and NbO43- host. XRD profiles of LaNbO4:xTb3+ products (x=0-0.05) synthesized under the identical condition of pH=10 sample (Fig. S9) testified that single-phased m-LaNbO4 can only be retained up to x=0.03. Further increasing Tb3+ concentration to 4 at% additionally produced tetragonal (t-) LaNbO4 (JCPDS No. 00-050-0919; space group: I41/a) [32], as demonstrated by the appearance of (200) and extraordinarily strong (112) diffractions in the t-LaNbO4 standard (JCPDS No. 00-050-0919), implying the change  18 of crystallization habit. Such a sequence phase transition of m → t-LaNbO4 commonly occurred at high temperature in the range of ~490-525 ºC [33]. However, this phenomenon observed in our results could be due to more distorted crystal structure by smaller Tb3+, which further affected sintering kinetics, though yet needs further clarification. It is also evident that Tb3+ incorporation weakened and broadened the diffractions, and meanwhile gradually drifts the (040) and (200) diffractions toward larger angle side by ~0.22º and 0.24º, respectively. This complies with the fact that smaller Tb3+ ions would substitute larger La3+ (rTb3+=1.04 Å, rLa3+=1.16 Å under 8-fold coordination (CN=8)) to occupy asymmetric C6 2h of m-LaNbO4 and/or C6 4h sites of t-LaNbO4 without an inversion center owning to lanthanide contraction, and thus enlarged the interplanar spacing [34,35]. Monitoring at 544 nm green emission of Tb3+ produced a broad and strong band in the ~200-315 nm spectral region by O2- → Nb5+ CT in NbO43- overlapped with spin-allowed interconfigurational 4f8 → 4f7d1 of Tb3+ and intra-4f8 narrow-band transitions of Tb3+ (Fig. 6a). The excitation intensities of NbO43- and Tb3+ were found to be remarkably enhanced by increasing Tb3+ contents, which also resulted in an appreciable red-shift from 262 nm to 273 nm. This is primarily attributed to a stronger energy transfer (ET) of CTB of NbO43- with f-d and/or 4f states of Tb3+ via resonant interaction, more distorted NbO6 octahedron caused by short Tb-O bond and stronger f-d transition of Tb3+. Upon UV excitation at 270 nm, PL spectra exhibit two components, that is, negligibly weak self-activated broad-band emission of NbO43- in the ~300-475 nm region and characteristic 5D4 → 7FJ (J=2-6) transition of Tb3+ at ~488 nm (J=6), 544 nm (J=5), 589 nm (J=4), 620 nm (J=3) and 656 nm (J=2) (Fig. 6b). Tb3+ doping did not bring about any change of peak position, but drastically deteriorated the NbO43- emission, further verified the efficient NbO43- → Tb3+ ET. The  19 best luminescent LaNbO4:0.03Tb3+ phosphor presents integral intensities ~10 times than LaNbO4:0.005Tb3+ counterpart for the dominant ~544 nm green emission. Accordingly, the optimal Tb3+ content is 3 at% due to the localized concentration quenching, which has been designated as the initial Tb3+ content to accomplish luminescence tailoring and enhancement by codoping of Eu3+.                      Fig. 7 PLE (λem=612 nm, a) and PL (λex=270 nm, b) spectra for the LaNbO4:0.03Tb3+,yEu3+ (y=0.005-0.07) phosphors after calcination at 900 ºC. Part (c) plots the asymmetry factor of luminescence against the Eu3+ contents. Part (d) is a schematic diagram showing the excitation, emission and possible NbO43- → Tb3+ → Eu3+energy transfer processes.   XRD analysis of LaNbO4:0.03Tb3+,yEu3+ (y=0-0.07, Fig. S10) indicated a pure m-LaNbO4 for y=0-0.03, a phase mixture of m- and t-LaNbO4 for y=0.05 and 0.07, and ~0.04 and 0.08° right-shift as shown by an amplified view of the diffractions at 2θ=30.5-32.5° due to size difference (rEu3+=1.066 Å, CN=8). Fig. 7a and b shows the PLE and PL spectra of ternary LaNbO4:Tb3+,Eu3+ phosphors. In each case, PLE spectra  20 contain a broad excitation of O2- → Nb5+ CT, O2- → Eu3+ CT and 4f8 → 4f7d1 transition of Tb3+ by monitoring the 5D0 → 7F2 transition of Eu3+ at 612 nm, which is hardly distinguished owning to apparent spectral overlap. Besides the relatively weak intra-4f8 transitions of Tb3+, the strong intra-4f6 excitations of Eu3+ are also identified at 362 nm (7F0,1 → 5D4), 382 nm (7F0,1 → 5L7), 394 nm (7F0,1 → 5L6), 428 nm (7F0,1 → 5D3) and 465 nm (7F0,1 → 5D2). The PL spectra taken under 270 nm excitation of NbO43- disclosed the luminescent characteristic of NbO43-, Tb3+ and Eu3+, and have the strong 5D0 → 7F2 electric dipole transition of Eu3+ instead of the 5D0 → 7F1 magnetic dipole allowed transition, since Eu3+ ions reside an asymmetric C6 2h  site without inversion center in terms of Judd-Ofelt theory [36]. It can be inferred from the spectral overlap between NbO43- excitation and RE3+ emission, intense Eu3+ and monotonically reduced NbO43- and Tb3+ emissions that an efficient NbO43- → Tb3+ → Eu3+ ET successively occurred in the LaNbO4:Tb3+,Eu3+ phosphors. Simultaneously, the symmetric tetragonal structure may be more superior to monoclinic structure for more energy interaction of NbO43- with RE3+. As a result, both of LaNbO4:0.03Tb3+,0.05Eu3+ and LaNbO4:0.03Tb3+,0.07Eu3+ phosphors have stronger broad-band excitation and Eu3+ emission across the series of counterparts, and their substantially intensity ratio (asymmetry factor of luminescence, R) of 5D0 → 7F2 to 5D0 → 7F1 is lower that of y=0.03 counterpart (Fig. 7c).  The cooperative ET mechanism of NbO43- → Tb3+ → Eu3+ can be reasonable elucidated as below, and an energy level diagram is simplified in Fig. 7d. Upon 270 nm excitation, electrons in 1A1 ground state are jumped to 1T2 and 1T1 excited state of  21 NbO43- groups, followed by non-radiative relaxation (NR) to lower-lying double-split 3T2 and 3T1 excited state and also ET to the neighboring 5D1 higher level of Tb3+ via resonant interaction. The 3T2 and 3T1 energies have two destinations: radiative relaxation to the 1A1 ground state to produce 394/440 nm intrinsic blue emissions of NbO43- group; ET to 5D3 and 5D4 state of Tb3+, respectively. The electrons in 5D1 and 5D3 of Tb3+ then populate into 5D4 energy level of Tb3+ via NR, which is followed by ET to adjacent 5D4 (only arising from 5D1 of Tb3+) and 5D1 level of Eu3+. A part of the 5D4 electrons then fall back to 7FJ (J=3-6) of Tb3+ for the observed 623 nm (red), 582 nm (orange), 544 nm (green) and 488 nm (blue) emissions. After directly absorbing the energies from 5D4 and 5D1 of Tb3+, Eu3+ electrons nonradiatively relax to lowest-lying 5D0 excited state, and then radiative relaxation to 7FJ (J=1-4) ground states for the Eu3+ emissions. The more matched 1T1 level of NbO43- with 5D1 state of Tb3+ and 3T1 level of NbO43- with 5D4 state of Tb3+ suggested that Tb3+ serves as a “bridge” for more efficient NbO43- → Tb3+ → Eu3+ ET, and thus the conspicuously intense Eu3+ emissions were the result.         Fig. 8 TEM micrograph (a), HR-TEM lattice fringes (b), SAED pattern (c) and elemental mapping  22 images (d) taken for the LaNbO4:0.01Tb3+,0.01Eu3+ phosphor.  In view of the fascinating bluish green emission of LaNbO4:0.01Tb3+, the white light-emitting diode can be attainable through precisely modulating the emission color of LaNbO4:0.01Tb3+,yEu3+ phosphors (y=0-0.02). XRD analysis (Fig. S11) indicated that y=0-0.015 products exhibited a phase-pure m-LaNbO4, while concentrated Eu3+ up to y=0.017-0.02 induced partial crystallization of t-LaNbO4. Moreover, the diffraction intensity of products dramatically decreased when the Eu3+ contents exceed 1 at%, and the crystallinity gets better at y=0.02 without any obvious peak shifting, which may be caused by the phase transformation and rather limited content of Eu3+ in different lattice. With the LaNbO4, LaNbO4:0.01Tb3+ and LaNbO4:0.01Tb3+,0.01Eu3+ for example, analysis of the full-range XRD patterns with the Jade 6.5 software and applying Scherrer formula indeed found that 1 at% Tb3+ and 1 at% Eu3+ co-substitutions reduced the cell parameter and cell volume (a=5.576(2) Å, b=11.550(4) Å, c=5.212(1) Å, β=94.06(4) Å, V=334.86 Å3 for LaNbO4; a=5.554 (1) Å, b=11.536 (3) Å, c=5.206(1) Å, β=94.03 (3) Å, V=332.84 Å3 for LaNbO4:0.01Tb3+; a=5.555(1) Å, b=11.518(2) Å, c=5.195(1) Å, β=94.06(2) Å, V=331.67 Å3 for LaNbO4:0.01Tb3+,0.01Eu3+). Fig. 8 shows the results of TEM for the representative LaNbO4:0.01Tb3+,0.01Eu3+, where it is seen that the product comprises ~200-300 nm sized nanoparticles (Fig. 8a). HR-TEM analysis (Fig. 8b) also confirmed the distinct exposure of (220) facet, but codoping of smaller Tb3+ and Eu3+ slightly reduced d-spacing to be ~0.240 nm. SAED pattern (Fig. 8c) unambiguously identified the diffraction spots arising from (220), (141) and (-121), and the dihedral angle of (220)/(-121) planes decreased to ~102.81º. Elemental  23 mapping (Fig. 8d) via STEM manifested that La, Nb, O, Tb and Eu elements are homogeneously distributed throughout the whole granular aggregates, which further proved the formation of solid solution.        Fig. 9 PLE (λem=612 nm, a) and PL (λex=270 nm, b) spectra for LaNbO4:0.01Tb3+,yEu3+ (y=0.002-0.02) phosphors.   Fig. 9 shows the PLE and PL spectra for LaNbO4:0.01Tb3+,yEu3+ (y=0.002-0.02) phosphors. The excitation and emission intensities of Eu3+ exclusively increase with the increase in y to 0.01, beyond which the intensity goes down until y=0.017, whereas an opposite trend observed for NbO43- and Tb3+. LaNbO4:0.01Tb3+,0.02Eu3+ phosphor shows the undoubtedly strongest excitation and Eu3+ emission, which indicated the favorable resonant ET of NbO43- → Tb3+ → Eu3+. The above result also illustrates the quenching concentration of Eu3+ in m-LaNbO4 is determined to be ~1 at%, and tetragonal lattice favors the photoluminescence. The Commission international de l´éclairage (CIE) chromaticity diagram (Fig. 10) and CIE coordinates (Table S1) exhibits full-visible-spectrum emitting LaNbO4:xTb3+,yEu3+ phosphors including deep blue (x=0, y=0), deep green (x=0.05, y=0), white (x=0.01, y=0.01) and deep red (x=0.03, y=0.07) regions, which is in good accordance with the vivid multicolor luminescence  24 of the phosphors under 254 nm UV irradiation from a hand-held UV lamp. The red and white light-emitting LaNbO4:0.03Tb3+,0.05Eu3+ and LaNbO4:0.01Tb3+,0.01Eu3+ phosphors were calculated to have absolute quantum efficiencies of 20.43% and 33.07%, respectively.                Fig. 10 CIE chromaticity diagram (upper) and appearances of multicolor luminescence under a 254 nm UV lamp irradiation (bottom) of LaNbO4:xTb3+ (x=0-0.05), LaNbO4:0.01Tb3+,yEu3+ (y=0.002-0.02) and LaNbO4:0.03Tb3+,yEu3+ (y=0.005-0.07) phosphors.    25 3.3 Thermal stability and UV-excited pc-WLEDs performance of LaNbO4:0.01Tb3+,0.01Eu3+                  Fig. 11 Temperature-dependent emission spectra (a), the normalized emission intensity of NbO43-, Tb3+ and Eu3+ (b) and the ln[I0/I(T)-1] versus 1/(kT) plot (c) for LaNbO4:0.01Tb3+,0.01Eu3+ phosphor under 270 nm excitation within 298-548 K. Parts (d-f) show the FIR (IEu/ITb), absolute sensitivity (Sa) and relative sensitivity (Sr), CIE chromaticity diagram for the temperature-course luminescence of LaNbO4:0.01Tb3+,0.01Eu3+ phosphor under 270 nm excitation, respectively.  Phosphor is rather susceptible to surrounding environment temperature, and  26 thereby, the thermal quenching behavior is essential for thermal stability in WLED applications. Herein, the temperature-dependent luminescence of LaNbO4:0.01Tb3+,0.01Eu3+ phosphor was investigated in the range of 298-548 K (Fig. 11a). Raising temperature did not cause Stokes shift, but rapidly weakened the integral emission intensity of Eu3+ and especially NbO43-, which can only maintain ~57% and 10% of its initial value at 423 K (Fig. 11b). It is well-documented that the luminescence thermal quenching was caused by a strong electron-phonon coupling and thermally activated crossover process between the excited state and ground state [37-39]. Nevertheless, Tb3+ emission was extraordinarily intensified by ~20% at 423 K and then slowly dropped down to 90%, which may be due to the rapidly thermal quenching of NbO43- emissions and strong thermal coupling effect by resonant NbO43- → Tb3+ ET toward temperature increment [40]. The quenching activation energy (ΔE) for NbO43- and Eu3+ luminescence can be assayed from the following Arrhenius equation [41,42]: ln [I0I(T)-1]=lnA-ΔEkT                          (1) Where I0 and I(T) denote the PL intensities at 298 K and absolute temperature T, respectively. A and k are separately a pre-exponential constant and Boltzmann constant of 8.617×10-5 eV/K. Fig. 11c shows the plot of ln[I0/I(T)-1] versus 1/(kT) plot. ΔE is thus calculated by the slope of the linear fitting to be ~0.348±0.007 eV for NbO43- and 0.15±0.01 eV for Eu3+. Consequently, it is plausible to speculate that LaNbO4:0.01Tb3+,Eu3+ phosphor is a promising candidate for optical thermometers, among which Tb3+ can be employed as a built-in calibration signal to prevent environmental effect, while NbO43- and Eu3+ with thermal quenching is considered as a temperature detection [43]. Fig. 11d depicted temperature-dependent fluorescence  27 intensity ratio (abbreviated as FIR) between Eu3+: 5D0 → 7F2 (IEu) and Tb3+: 5D4 → 7F5 (ITb). FIR (IEu/ITb) monotonously dropped upon elevating the temperature, and the experimental results are well-fitted with the linear function as FIR (IEu/ITb)=843.835/T-1.091, implying an attractive luminescent thermometer applied for temperature sensing. The absolute (SA) and relative (SR) sensitivities can be expressed as follows against temperature to evaluate the performance of temperature sensing [42].  SA= |d(FIR)d(T)|                             (2) SR= |d(FIR)d(T)1FIR|×100%                       (3) It is observed that the maximum values of SA and SR are ~0.01 K-1 (at 298 K) and 0.63% K-1 (at 548 K), respectively, and the former is higher than ~0.0046 and 0.0073 K-1 for NaYF4:Ce3+,Tb3+,Eu3+ and YNbO4:Yb3+,Er3+ microcrystals, respectively [44,45]. CIE chromaticity diagram (Fig. 11f) for the LaNbO4:0.01Tb3+,0.01Eu3+ phosphor found that the luminescent colors shift from white to yellowish green under 270 nm excitation, which is associated with the weakened NbO43- (major) and Eu3+ (minor) emissions and almost insusceptible Tb3+ emissions.            Fig. 12 The electroluminescence spectra of 275 nm pumped WLED via combining with the LaNbO4:0.01Tb3+,0.01Eu3+ phosphor under 20-120 mA current driving (a), the appearances of the WLED device with the current on and off (b), CIE chromaticity diagram (c) for the as-fabricated WLED lamp under 20 mA driving. The inset in (c) shows color rendering index Ra and R1-R15 factors.  28 The as-fabricated LaNbO4:0.01Tb3+,0.01Eu3+ phosphor was embedded on a commercial 275 nm deep UV chip to integrate a pc-WLED device. The characteristics of electroluminescence spectra (Fig. 12a) are almost identical to PL spectrum of LaNbO4:0.01Tb3+,0.01Eu3+ (Fig. 9b) without considering the change of driving current. The white-emitting intensity continuously increased when the driving current increased from 20 to 120 mA. The CIE coordinate of around (0.32, 0.34) is close to ideal white light of (0.33, 0.33), and the CCT of ~5694 K and CRI value of ~90 under current of 20 mA are more beneficial than the commercial YAG:Ce3+ (CCT~7750 K, CRI<75.0) (Fig. 12c), and obtained prospective R parameters (R1-R15) [46].  4. Conclusion The full-color emitting LaNbO4:xTb3+,yEu3+ (x=0-0.05, y=0-0.07) phosphors were crystallized at 24 h of hydrothermal reaction at 200 ºC and pH=10 after calcination at 900 ºC. Phase composition and morphology analyses demonstrated that calcining the pH=10 precursor at 500 ºC induced the formation of orthorhombic LaNbO4, and the 800 ºC calcined products exclusively crystallized in monoclinic structure. Superfluous Tb3+ and Eu3+ additionally induced a phase transition from monoclinic to tetragonal LaNbO4. Upon 270 nm excitation, the LaNbO4: Tb3+,Eu3+ phosphors showed the blue NbO43-, green Tb3+ and red Eu3+ emissions. Codoping of 0.01Tb3+ and 0.01Eu3+ produced warm white emission with desired color coordinates of (0.36,0.35), and also exhibited favorable resistance to thermal quenching with activation energy of NbO43- and Eu3+ being ~0.348±0.007 eV and 0.15±0.01 eV, respectively. 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