Bi3+ and Eu3+ Activated Luminescent Behaviors in Non-Stoichiometric LaO0.65F1.7 Structure

Optical materials composed of La1-p-qBipEuqO0.65F1.7 (p = 0.001–0.05, q = 0–0.1) were prepared via a solid-state reaction using La(Bi,Eu)2O3 and NH4F precursors at 1050 °C for two hours. X-ray diffraction patterns of the phosphors were obtained permitting the calculation of unit-cell parameters. The two La3+ cation sites were clearly distinguished by exploiting the photoluminescence excitation and emission spectra through Bi3+ and Eu3+ transitions in the non-stoichiometric host lattice. Energy transfer from Bi3+ to Eu3+ upon excitation with 286 nm radiation and its mechanism in the Bi3+- and Eu3+-doped host structures is discussed. The desired Commission Internationale de l’Eclairage values, including emissions in blue-green, white, and red wavelength regions, were obtained from the Bi3+- and Eu3+-doped LaO0.65F1.7 phosphors.

In this study, Bi 3+ and Eu 3+ were substituted into LaO 0.65 F 1.7 compounds that were synthesized by a solid-state method using NH 4 F flux in air. The unit-cell parameters of the phosphors were calculated. The excitation and emission luminescence spectra of the La 1-p-q Bi p E u q O 0.65 F 1.7 (p = 0.001-0.05, q = 0-0.1) phosphors were investigated with respect to the site dependency of Bi 3+ and Eu 3+ ions in the host Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the phosphors were obtained.

Materials and Methods
Phosphors of La 1-p-q Bi p Eu q O 0.65 F 1.7 (p = 0.005-0.05, q = 0-0.1) were prepared by heating the appropriate amounts of La 2 O 3 (Alfa 99.9%), Bi 2 O 3 (Alfa 99.99%), Eu 2 O 3 (Alfa 99.9%), and NH 4 F (Alfa 99%). Powdered samples with 1:2 molar ratios of La(Bi,Eu)O 3/2 and NH 4 F were used to prepare nonstoichiometric LaO 0.65 F 1.7 :Bi 3+ , Eu 3+ . The precursors were mixed with an agate mortar and pestle and subsequently heated at 1050 • C for 2 h in air [27]. The La 2 O 3 precursor was pre-heated at 700 • C for 3 h to remove hydroxide in the sample. Phase identification of the phosphors was performed using a Shimadzu XRD-6000 powder diffractometer (Cu-Kα radiation, Shimadzu CO., Kyoto, Japan). The Rietveld refinement program Rietica was used for the unit-cell parameter calculations. UV spectroscopy of the excitation and emission spectra of the phosphors was measured using spectrofluorometers (Sinco Fluoromate FS-2, Sinco CO., Seoul, Korea).

Results and Discussion
The crystallographic phase of the La 1-p-q Bi p Eu q O 0.65 F 1.7 (p = 0.001-0.05, q = 0-0.1) powders 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 2A. Figure 2B-F show the XRD patterns of non-stoichiometric La 1-p-q Bi p Eu q O 0.65 F 1.7 phosphors (p = 0.01 and q = 0, p = 0.05 and q = 0, p = 0 and q = 0.05, p = 0 and q = 0.1, and p = 0.01 and q = 0.1, respectively), synthesized by the mixing of  Table 1. The Bi 3+ and Eu 3+ ions, under these conditions, occupy 9-and 10-coordinated La 3+ sites (LaF(1) 3 F(2) 2 O 2 F(3) 2 and LaF(1) 4 F(2)O 3 F(3) 2 ) in the non-stoichiometric LaO 0.65 F 1.7 structure, as shown in Figure 1 [27,28]. The single La 3+ site comprises 56% 9-fold and 44% 10-fold polyhedrons in the LaO 0.65 F 1.7 lattice based on the La(F(1) 0.86 V 0.14 )(F(2) 0.35 O 0.65 )(F(3) 0.49 ) formula. The 9-and 10-coordinated LaO 2 F 7 and LaO 3 F 7 polyhedrons in the non-stoichiometric unit cell are arrayed along the c-axis, as shown in Figure 1. When Bi 3+ ions (r = 1.17 Å for 8 coordination number (CN)) were substituted for La 3+ ions (r = 1.16 Å for 8 CN) in the LaO 0.65 F 1.7 host lattice, gradual shifts in the positions of the various Bragg reflections to lower angles with unit-cell expansion were observed, as shown in Figure 2B,C. When Eu 3+ ions (r = 1.066 Å for 8 CN) were substituted for La 3+ ions in the host lattice, gradual shifts in the positions of the various Bragg reflections to higher angles with unit-cell contraction were observed, as shown in Figure 2D,E. When the Bi 3+ ions were doped in the La 0.9 Eu 0.1 O 0.65 F 1.7 phosphors, no further shift to higher angles was observed in the La 0.89 Bi 0.01 Eu 0.1 O 0.65 F 1.7 phosphors, as shown in Figure 2F. to lower angles with unit-cell expansion were observed, as shown in Figure 2B,C. When Eu 3+ ions (r = 1.066 Å for 8 CN) were substituted for La 3+ ions in the host lattice, gradual shifts in the positions of the various Bragg reflections to higher angles with unit-cell contraction were observed, as shown in Figure 2D,E. When the Bi 3+ ions were doped in the La0.9Eu0.1O0.65F1.7 phosphors, no further shift to higher angles was observed in the La0.89Bi0.01Eu0.1O0.65F1.7 phosphors, as shown in Figure 2F.  Figure 3aA-E show the photoluminescence (PL), excitation (EX), and emission (EM) spectra of the Bi-doped La1-pBipO0.65F1.7 phosphors (p = 0.001, 0.005, 0.01, 0.025, and 0.05, respectively). The excitation band centered near 278 and 286 nm in the La0.99Bi0.01O0.65F1.7 PL spectra is attributed to the 1 S0  3 P1 transition of Bi 3+ ions because the 1 S0  3 P0 and 1 S0  3 P2 transitions are forbidden from ground 1 S0 [19][20][21][22][23][24][25][26]. The blue emission spectra of the LaO0.65F1.7:Bi 3+ phosphors revealed a broadband range from 350 to 650 nm, centered at approximately 497 nm, which is attributed to the intense 3 P1  1 S0 transitions of the Bi 3+ ions, as shown in Figure 3a. When the Bi 3+ concentration in the host lattice was 1 mol %, the maximum emission intensity of the obtained phosphors was observed at the excitation wavelength of 278 nm, as shown in Figure 3aC. After the Bi 3+ concentration was increased 2.5 mol % in the phosphors, the centered excitation peak shifted to a higher wavelength region from 278 to 286 nm, as shown in Figure 3aD,E. Thus, as the Bi 3+ content in the LaO0.65F1.7 host lattice was increased and the excitation center of the 1 S0  3 P1 transition of Bi 3+ ions underwent a shift to a longer wavelength. The La 3+ ion is coordinated by seven F − and two O 2− anions (LaF(1)3F(2)2O2F(3)2), or seven F − and three O 2− anions (LaF(1)4F(2)O3F(3)2) in the LaO0.65F1.7 host structure [27,28]. As depicted in Figure 1, there was a vacancy associated with the F(1) anion in the LaF(1)3F(2)2O2F(3)2 polyhedron. Based on the ratios of oxygen and fluoride to lanthanum, the LaF(1)3F(2)2O2F(3)2 polyhedron had a lower oxygen ion covalency than LaF(1)4F(2)O3F(3)2 polyhedrons in the structure. This observation   spectra is attributed to the 1 S 0 → 3 P 1 transition of Bi 3+ ions because the 1 S 0 → 3 P 0 and 1 S 0 → 3 P 2 transitions are forbidden from ground 1 S 0 [19][20][21][22][23][24][25][26]. The blue emission spectra of the LaO 0.65 F 1.7 :Bi 3+ phosphors revealed a broadband range from 350 to 650 nm, centered at approximately 497 nm, which is attributed to the intense 3 P 1 → 1 S 0 transitions of the Bi 3+ ions, as shown in Figure 3a. When the Bi 3+ concentration in the host lattice was 1 mol %, the maximum emission intensity of the obtained phosphors was observed at the excitation wavelength of 278 nm, as shown in Figure 3aC. After the Bi 3+ concentration was increased 2.5 mol % in the phosphors, the centered excitation peak shifted to a higher wavelength region from 278 to 286 nm, as shown in Figure 3aD,E. Thus, as the Bi 3+ content in the LaO 0.65 F 1.7 host lattice was increased and the excitation center of the 1 S 0 → 3 P 1 transition of Bi 3+ ions underwent a shift to a longer wavelength. The La 3+ ion is coordinated by seven F − and two O 2− anions (LaF(1) 3 F(2) 2 O 2 F(3) 2 ), or seven F − and three O 2− anions (LaF(1) 4 F(2)O 3 F(3) 2 ) in the LaO 0.65 F 1.7 host structure [27,28]. As depicted in Figure 1, there was a vacancy associated with the F(1) anion in the LaF (1) Figure 3b shows the excitation and emission PL spectra of the La 0.95 Eu 0.05 O 0.65 F 1.7 phosphors. The charge-transfer bands (CTBs) and the f-f transitions of the Eu 3+ activator in the host lattice were observed at 220-350 and 350-540 nm, respectively. Two CTBs centered at 290 and 320 nm were found in the excitation spectra because there were two La 3+ sites associated with the LaF(1) 3 F(2) 2 O 2 F(3) 2 and LaF(1) 4 F(2)O 3 F(3) 2 polyhedrons in the host structure. When Eu 3+ ions were doped in the nine-coordinated La 3+ site of the LaF(1) 3 F(2) 2 O 2 F(3) 2 polyhedron, the center of the Eu 3+ CTB transitions occurred at 290 nm. Additional energy was required to excite an electron from the Eu 3+ ions in seven F − and two O 2− containing lattices, compared to seven F − and three O 2− polyhedrons.
The Eu 3+ transitions of the emission spectra in the La 0.95 Eu 0.05 O 0.65 F 1.7 phosphors exhibited both the 5 D 0 -7 F 1 magnetic dipole and the 5 D 0 -7 F 2 electric-dipole transitions, centered at 592 and 610 nm, respectively [29,30]. When the Eu 3+ ions were substituted in no inversion site of the nine-coordinated polyhedron in the host lattice, the 5 D 0 -7 F 2 transition dominates. When the Eu 3+ activators were doped into symmetric inversion site of the 10-fold polyhedron, the 5 D 0 -7 F 1 transition dominates. Figure 3c shows the excitation spectra of the  Figure S1). The blue-green emission of the La 1-p Bi p O 0.65 F 1.7 phosphors centered at 497 nm reached a maximum intensity for a Bi 3+ content (p = 0.01), as shown in Figure 3a. After increasing the Bi 3+ content, concentration quenching of the relative emission intensity was observed. The increase in the Bi 3+ content of the phosphors enhanced energy transfer up to some critical value, whereas after this value was reached subsequent increase of Bi 3+ levels decreased the emission intensity by reducing the critical distance between the Bi 3+ ions. This resulted in non-radiative energy transfer between Bi 3+ ions from the electric multipole interactions. The critical distance (R c ) is expressed by the following formula: where V is the volume of the La 0.99 Bi 0.01 O 0.65 F 1.7 unit cell, N is the number of available La 3+ sites for the dopant in the unit cell, m c is the critical concentration of Bi 3+ , and R c is the critical distance for energy transfer [10,[22][23][24]32]. When N and V are 1 and 97.75 Å 3 , respectively, for La 0.99 Bi 0.01 O 0.65 F 1.7 , R c (m c = 0.01) is 26.53 Å. The energy transfer mechanism designated an electric multipole interaction because the critical distance is greater than 5 Å. Figure 4a shows the emission spectra of La 0.  Figure 4c. The efficiency was gradually enhanced from 23% to 97% as the Eu 3+ content in the phosphors increased from q = 0.01 to 0.1. concentration of Bi 3+ and Eu 3+ ions, with α = 6, 8, or 10, corresponding to dipole-dipole, dipolequadrupole, and quadrupole-quadrupole interactions, respectively, in accordance with Dexter theory [10,[22][23][24]32]. In Figure 4b, when α = 6, 8, and 10, the linear plots showed energy transfer from the Bi 3+ to Eu 3+ ions with R 2 = 0.9635, 0.9894, and 0.9982 in the La0.94Bi0.01Eu0.05O0.65F1.7 phosphors, respectively. As the value of α is 10, a closer linear plot is determined for the phosphor, the quadrupole-quadrupole interaction was involved in the energy transfer mechanism of the La0.94Bi0.01Eu0.05O0.65F1.7 phosphors. The efficiency of the energy transfer from Bi 3+ to Eu 3+ in La0.94Bi0.01Eu0.05O0.65F1.7 (EX = 286 nm) phosphors is shown in Figure 4c. The efficiency was gradually enhanced from 23% to 97% as the Eu 3+ content in the phosphors increased from q = 0.01 to 0.1. concentration of Bi 3+ and Eu 3+ ions, with α = 6, 8, or 10, corresponding to dipole-dipole, dipolequadrupole, and quadrupole-quadrupole interactions, respectively, in accordance with Dexter theory [10,[22][23][24]32]. In Figure 4b, when α = 6, 8, and 10, the linear plots showed energy transfer from the Bi 3+ to Eu 3+ ions with R 2 = 0.9635, 0.9894, and 0.9982 in the La0.94Bi0.01Eu0.05O0.65F1.7 phosphors, respectively. As the value of α is 10, a closer linear plot is determined for the phosphor, the quadrupole-quadrupole interaction was involved in the energy transfer mechanism of the La0.94Bi0.01Eu0.05O0.65F1.7 phosphors. The efficiency of the energy transfer from Bi 3+ to Eu 3+ in La0.94Bi0.01Eu0.05O0.65F1.7 (EX = 286 nm) phosphors is shown in Figure 4c. The efficiency was gradually enhanced from 23% to 97% as the Eu 3+ content in the phosphors increased from q = 0.01 to 0.1.    As shown in Figure 5a, the chromaticity coordinates, x and y, are in accordance with the desired CIE (Commission Internationale de l'Eclairage) values from the blue-green to white and red wavelength regions for La0.99-qBi0.01EuqO0.65F1.7 (q = 0-0.1) phosphors (EX = 286 nm). The CIE values are summarized in the inset of Figure 5a, along with the values obtained for the phosphors. The CIE coordinates near the blue-green, white, orange, and red regions of the CIE diagram from the phosphors were observed to be x = 0.240 and y = 0.334, x = 0.328 and y = 0.348, x = 0.466 and y = 0.354, and x = 0.591 and y = 0.353, for values of q = 0, 0.02, 0.05, and 0.1, respectively. When the concentration of Eu 3+ ions in the La0.99-qBi0.01EuqO0.65F1.7 phosphors increased from q = 0 to 0.02 and 0.1, the emission colors exhibited a significant shift from blue-green to white, and red emission regions, respectively. These tunable emission lights are appropriate for a high color-rendering index to apply phosphor converted UV-LEDs. This indicates that there was effective energy transfer from Bi 3+ to Eu 3+ in the La0.99-qBi0.01EuqO0.65F1.7 phosphors. Emission of the La0.99-qBi0.01EuqO0.65F1.7 (q = 0-0.1) phosphors under 254, 312, and 365 nm hand-lamp excitation was exhibited blue-green, white, orange, and red colors, as shown in Figure 5b.

Conclusions
Non-stoichiometric tetragonal La 1-p-q Bi p Eu q O 0.65 F 1.7 (p = 0.001-0.05, q = 0-0.1) phosphors were prepared via a solid-state method using a heat treatment at 1050 • C for two hours using NH 4 F flux. The site dependency of the Bi 3+ and Eu 3+ ions in the LaF(1) 3 F(2) 2 O 2 F(3) 2 and LaF(1) 4 F(2)O 3 F(3) 2 polyhedrons of the host structure was analyzed using the PL spectra of the phosphors. The maximum luminescence intensity of the blue-green La 1-p Bi p O 0.65 F 1.7 phosphors was obtained when p = 0.01. The critical distance (Rc) value for the La 0.99 Bi 0.01 O 0.65 F 1.7 phosphor was determined to be 26.53 Å. As the Eu 3+ concentration was increased in La 0.99-q Bi 0.01 Eu q O 0.65 F 1.7 (q = 0-0.1) phosphors under 286 nm excitation, an efficient energy transfer from Bi 3+ to Eu 3+ occurred, involving quadrupole-quadrupole interactions in the phosphors. The CIE coordinate values attributed to the emissions from blue-green, white, and red for La 0.99-q Bi 0.01 Eu q O 0.65 F 1.7 (q = 0-0.1) phosphors were successfully obtained.