Nonstoichiometric LaO0.65F1.7 Structure and Its Green Luminescence Property Doped with Bi3+ and Tb3+ Ions for Applying White UV LEDs

Red–green–blue phosphors excited by ultraviolet (UV) radiation for white light LEDs have received much attention to improve the efficiency, color rendering index (CRI), and chromatic stability. The spectral conversion of a rare-earth ion-doped nonstoichiometric LaO0.65F1.7 host was explored with structural analysis in this report. The nonstoichiometric structure of a LaO0.65F1.7 compound, synthesized by a solid-state reaction using La2O3 and excess NH4F precursors, was analyzed by synchrotron X-ray powder diffraction. The crystallized LaO0.65F1.7 host, which had a tetragonal space group of P4/nmm, contained 9- and 10-coordinated La3+ sites. Optical materials composed of La1−p−qBipTbqO0.65F1.7 (p = 0 and 0.01; q = 0–0.2) were prepared at 1050 °C for 2 h, and the single phase of the obtained phosphors was indexed by X-ray diffraction analysis. The photoluminescence spectra of the energy transfer from Bi3+ to Tb3+ were obtained upon excitation at 286 nm in the nonstoichiometric host lattice. The desired Commission Internationale de l’Eclairage (CIE) values of the phosphors were calculated. The intense green La0.89Bi0.01Tb0.1O0.65F1.7 phosphor with blue and red optical materials was fabricated on a 275 nm UV-LED chip, resulting in white light, and the internal quantum efficiency, CRI, correlated color temperature, and CIE of the pc LED were characterized.


Introduction
Phosphor-converted light-emitting diodes (pc LEDs) have been developed as common light sources for various lighting and display industries [1][2][3][4][5]. Yellow garnet phosphors, such as Ce 3+ -doped Y 3 Al 5 O 12 compounds, fabricated on blue LED chips are widely used to generate white light sources. LEDs have excellent advantages, such as low cost and easy fabrication; however, they have several weaknesses such as relatively low efficiency, color rendering index (CRI), and chromatic stability [1][2][3][4][5]. Meanwhile, red-green-blue (RGB) pc LEDs excited by ultraviolet (UV) radiation have strengths, such as higher efficiency and a CRI with higher chromatic stability when subjected to different driving currents, compared with blue-excitable pc LEDs [1][2][3][4][5]. Although RGB tri-color phosphors are intricately blended for fabrication on an LED chip, specific mixtures of RGB phosphors can result in tunable color temperatures. When excited by UV radiation in various host lattices, Bi 3+ ions can emit radiation in the blue-to-green wavelength regions associated with 6s 1 6p 1 to 6s 2 transitions [6][7][8][9][10]. The Bi 3+ ion, behaving as a donor, can facilitate energy transfer and improve the emission of light from acceptors in the host structures such as Eu 3+ or Tb 3+ ions [6][7][8][9][10]. The Bi 3+ and Eu 3+ codoped nonstoichiometric LaO 0.65 F 1.7 phosphor was utilized, as a red emitter, under UV excitation in a previous study [6]. The nonstoichiometric LaO 0.65 F 1.7 host has advantageous optical qualities when compared with stoichiometric LaOF compounds [6,11,12]. Rare-earth ion-doped nonstoichiometric hosts show a broad excitation range with stronger Stark splitting. Furthermore, they have better spectral conversion properties owing to their low nonradiative relaxation caused by the low phonon frequency [6,11,12]. The LaO 0.65 F 1.7 lattice, which is stacked by 9-coordinated LaO 2 F 7 and 10-coordinated LaO 3 F 7 layers along the c-axis with the tetragonal space group P4/nmm, was studied by high-resolution powder neutron diffraction analysis [13]. The single phase of the LaO 0.65 F 1.7 host was synthesized using a high temperature and long reaction period at 1200 • C for 2 d in a nickel-sealed tube [13]. In this study, a LaO 0.65 F 1.7 host was prepared using the flux method at 1050 • C for 2 h in air [6,11]. The nonstoichiometric LaO 0.65 F 1.7 structure was refined using synchrotron powder X-ray diffraction data at room temperature. The luminescence spectra of the La 1−p−q Bi p Tb q O 0.65 F 1.7 (p = 0 and 0.01; q = 0-0.2) phosphors were explored by examining the energy-transfer mechanism from Bi 3+ to Tb 3+ . UV-excitable white light was emitted using a green La 0.96 Bi 0.01 Tb 0.02 Eu 0.01 O 0.65 F 1.7 phosphor with blue and red phosphors on a 275 nm UV-LED chip. The internal quantum efficiency (IQE), CRI, correlated color temperature (CCT), and Commission Internationale de l'Eclairage (CIE) coordinates of the RGB pc LEDs were calculated.

Experimental Section
Synchrotron powder X-ray diffraction data of the nonstoichiometric LaO 0.65 F 1.7 host were collected at the PLS-II 6D UNIST-PAL beamline of the Pohang Accelerator Laboratory (PAL). The powdered sample was loaded into a quartz capillary (diameter: 200 µm) and rotated during the data collection to eliminate the preferred orientation effect. Monochromatic X-rays (λ = 0.65303 Å, 18.986 keV) and a charge-coupled device detector (MX225-HS, Rayonix, Evanston, IL, USA) were used in these experiments [14]. The LaO 0.65 F 1.7 structure was refined using the Rietveld refinement program FullProf Suite [15,16]. Phosphors of La 1−p−q Bi p Tb q O 0.65 F 1.7 (p = 0 and 0.01; q = 0-0.2) were prepared by heating the appropriate stoichiometric molar amounts of La 2 O 3 (Alfa 99.9%), Bi 2 O 3 (Alfa 99.99%), and Tb 4 O 7 (Alfa 99.9%) with excess NH 4 F (Alfa 99%) precursor [6,11]. Powdered samples with 1:2 molar ratios of La(Bi,Tb)O 3/2 and NH 4 F were used to prepare the LaO 0.65 F 1.7 :Bi 3+ and Tb 3+ phosphors. The La(Bi,Tb)O 3/2 and NH 4 F precursors were mixed using an agate mortar and pestle and, subsequently, heated at 1050 • C for 2 h in air [6,11]. The La 2 O 3 precursor was used after preheating at 700 • C for 3 h to remove hydroxide from the acquired sample. The phase identification of the La 1−p−q Bi p Tb q O 0.65 F 1.7 phosphors was performed using a Shimadzu XRD-6000 powder diffractometer (Cu-Kα radiation). The excitation and emission photoluminescent spectra of the phosphors were measured using a spectrofluorometer (Sinco Fluoromate FS-2, Seoul, Korea). The pc LED was fabricated by packing the green La 0.96 Bi 0.01 Tb 0.02 Eu 0.01 O 0.65 F 1.7 , blue Ba 4 Ca 5 Al 2 Si 6 O 24 :Ce,Na, and red LaO 0.65 F 1.7 :Bi,Eu phosphors in-between quartz glasses on a 275 nm LED chip (Seoul Semiconductor, Ansan-si, Korea). A spectrometer (USB4000, Ocean Optics, Dunedin, FL, USA) was used to measure the CRI, CCT, CIE coordinates, and IQE of the white-light pc LEDs.

Results and Discussion
Rietveld refinement fitting of the synchrotron powder X-ray diffraction data of the nonstoichiometric LaO 0.65 F 1.7 structure at room temperature is shown in Figure 1. Unidentified small impurities at approximately 14 • and between 15 and 18 • , which are marked, were observed in the experimental XRD data. The refinement and crystal data are presented in Table 1. The nonstoichiometric LaO 0.65 F 1.7 host crystallized in the tetragonal space group P4/nmm, which has a and c cell parameters of a = 4.10058 (6) Å and c = 5.8468 (1) Å. The refined atomic coordinates, equivalent isotropic displacement parameters with Wyckoff positions, and site occupation factors are listed in Table 2.   (2)
The crystallographic phase of the La 1−p−q Bi p Tb q O 0.65 F 1.7 (p = 0 and 0.01; q = 0-0.2) phosphors was identified using powder X-ray diffraction (XRD) patterns. The calculated XRD pattern of the tetragonal LaO 0.65 F 1.7 (ICSD 40371) structure is shown in Figure 3a [13].    [6]. Previously, the photoluminescence properties of Bi 3+doped LaO 0.65 F 1.7 phosphors were explored. The energy levels of the Bi 3+ ions comprise 1 S 0 , 3 P J (J = 0, 1, or 2), and 1 P 1 states. It is known that the 1 S 0 → 3 P 1 and 1 P 1 transitions arise from spinorbital coupling, but the 1 S 0 → 3 P 0 and 3 P 2 transitions are forbidden [6][7][8][9][10]. The blue-emitting spectra of the Bi 3+ -doped LaO 0.65 F 1.7 phosphors, attributed to the 3 P 1 → 1 S 0 transitions of the Bi 3+ ions, revealed the emission band range from 350 to 650 nm under UV excitation as shown in Figure 4a. The maximum emission intensity was observed in a previous study when the Bi 3+ concentration in the host lattice was 1 mol% [6]. The La 0.9 Tb 0.1 O 0.65 F 1.7 phosphor was monitored from 400 to 700 nm while under UV excitation. The intense green emission of 5 D 4 -7 F 5 transition from the Tb 3+ ions was observed near 542 nm as shown in Figure 4a. The individual transitions of Bi 3+ and Tb 3+ ions, with energy transfer from Bi 3+ to Tb 3+ ions in the La 0.99−q Bi 0.01 Tb q O 0.65 F 1.7 (q = 0-0.2) phosphors, occurred under an excitation wavelength of 286 nm as shown in Figure 4b. The energy transfer from Bi 3+ to Tb 3+ operated as a sensitizer and activator, respectively, in the La 0.99−q Bi 0.01 Tb q O 0.65 F 1.7 (q = 0-0.2) phosphors. The emission of Tb 3+ transitions was maximized when the Tb 3+ content in the La 0.99−q Bi 0.01 Tb q O 0.65 F 1.7 (q = 0-0.2) phosphors was q = 0.1, which was compared with commercially available green LaPO 4 :Ce 3+ /Tb 3+ phosphor in Figure 4a. The green emission of the La 0.89 Bi 0.01 Tb 0.1 O 0.65 F 1.7 phosphor under 286 nm excitation significantly increased by >4 times compared to that of the La 0.9 Tb 0.1 O 0.65 F 1.7 phosphor because of the energy transfer from Bi 3+ to Tb 3+ ions as shown in Figure 4b. The energy-transfer efficiency (η T ) was estimated using the following formula: η T = 1 − I S /I SO where I S and I SO are the luminescence intensities of the Bi 3+ sensitizer in the presence and absence of a Tb 3+ activator, respectively [6][7][8][9][10][17][18][19][20][21]. The energy-transfer mechanism can be represented by linear plots of I SO /I S versus C Bi-Tb α/3 , where C Bi-Tb is the concentration of Bi 3+ and Tb 3+ ions, with α = 6, 8, or 10, corresponding to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively, in accordance with the Dexter theory [17][18][19][20][21][22]. In Figure 4c, when α = 6, 8, and 10, the linear plots showed energy transfers from the Bi 3+ to Tb 3+ ions in the La 0.99−q Bi 0.01 Tb q O 0.65 F 1.7 (q = 0-0.2) phosphors with an R 2 = 0.9954, 0.998, and 0.9906, respectively. When the value of α was 8, a closer linear plot was determined for the phosphor, and the dipole-quadrupole interaction was involved in the energy-transfer mechanism of the phosphors. The differences on the R-factors between dipole-dipole and dipole-quadrupole mechanisms are quite small when the concentrations of C Bi-Tb and C Tb were selected (Supplementary Materials, Figure S1). Possibly, there are some arguments that determine the energy-transfer process as the dipole-quadrupole interaction mechanism in the phosphors, because the concentration quenching selectively depends on whether it is a donor-donor or donor-acceptor for C 8/3 concentration [23]. The efficiency gradually enhanced from 30 to 89% as the Tb 3+ content in the phosphors increased from q = 0.01 to 0.1 as shown in Figure 4d.  Figure 6, respectively [8,24]. The pc UV LED was prepared at 3.2 V and 20 mA after the phosphors were packaged on a 275 nm LED chip (inset). A CRI (R a ) of 94.9 at CCTs of 4373 K with CIE coordinates of x = 0.36 and y = 0.37 was determined for the 275 nm chip fabricated with the white-light pc UV LED.
The three phosphors could generate white light in the 275 nm UV-executable LED chip. The IQE of the phosphors is expressed as η QE using the following equation [25][26][27][28]: where L S , E S , and E R are the luminescence integrated emission, excitation spectra of the phosphors, and the integrated excitation spectrum without phosphors in the sphere, respectively. The IQE of the white pc UV LED was approximately 40% under a 275 nm excitation. Color-tunable red, green, and blue phosphors can be diversely fabricated into UV-LED chips, which is an advantage for UV-executable LED applications.

Conclusions
The nonstoichiometric LaO 0.65 F 1.7 structure was determined as a tetragonal unit cell (P4/nmm) with the cell parameters a = 4.10058 (6) Å and c = 5.8468 (1) Å using synchrotron X-ray powder diffraction. The LaO 0.65 F 1.7 host lattice contained 9-and 10-coordinated La 3+ sites in the 2c Wyckoff position. Optical materials composed of La 1−p−q Bi p Tb q O 0.65 F 1.7 (p = 0 and 0.01; q = 0-0.2) exhibited photoluminescence spectra characteristic of an energy transfer from Bi 3+ to Tb 3+ upon excitation with 286 nm involving dipole-quadrupole interactions in the phosphors. The desired Commission Internationale de l'Eclairage values of the blue-green phosphors were calculated. The green La 0.96 Bi 0.01 Tb 0.02 Eu 0.01 O 0.65 F 1.7 phosphor was fabricated with blue and red phosphors on a 275 nm UV-LED chip, resulting in white light. The IQE was determined to be approximately 40% under a 275 nm excitation with a CRI (Ra) of 94.9 at CCT of 4373 K and CIE coordinates of x = 0.36 and y = 0.37.