Bi³⁺ and Eu³⁺ Activated Luminescent Behaviors in Non-Stoichiometric LaO_0.65F_1.7 Structure

Abstract

Optical materials composed of La_1-p-qBi_pEu_qO_0.65F_1.7 (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 LaO_0.65F_1.7 phosphors.

1. Introduction

Ce³⁺-doped Y₃Al₅O₁₂ (YAG) yellow phosphors are commonly used with blue light-emitting diodes (LEDs) to create white light sources [1,2,3,4,5]. The Ce³⁺ ions emit in the blue to yellow wavelength regions assigned by 5d¹ to 4f¹ transitions when excited by ultraviolet (UV) to visible radiation in various host lattices [6,7,8,9]. The Ce³⁺ ion, as a donor, enables efficient energy transfer, improving the emission from acceptors, such as Tb³⁺ or Mn²⁺ ions in the host structures [10,11,12,13,14,15,16,17,18]. The Bi³⁺ ion is an active luminescent center emitting blue to green light assigned to 6s¹6p¹ to 6s² transitions when excited by UV to near UV wavelength regions in host lattices [19,20,21]. The energy levels of the Bi³⁺ 6s²–6s¹6p¹ transitions consist of ¹S₀ and the triplet ³P_J (J = 0, 1, or 2) and singlet ¹P₁ states. The ¹S₀ to ³P₁, ¹P₁ transitions occur via spin-orbital coupling [19,20,21]. The states of the ¹S₀ to ³P₀ and ³P₂ transitions are forbidden [19,20,21]. Like Ce³⁺ ions, Bi³⁺ ions act as sensitizers to enhance the anticipated emission light from acceptors, such as Eu³⁺ or Tb³⁺ ions in host structures, by facilitating efficient energy transfer [22,23,24,25,26].

The up-conversion properties of Er³⁺- and Yb³⁺-doped LaO_0.65F_1.7 compounds were exploited under 980 nm diode laser excitation in a previous study [27]. This non-stoichiometric LaO_0.65F_1.7 host comprises alternating stacked LaO₂F₇ and LaO₃F₇ layers along the c axis with tetragonal space group P4/nmm, as shown in Figure 1 [27,28]. The 9- and 10-coordinated La³⁺ sites in the LaF(1)₃F(2)₂O₂F(3)₂ and LaF(1)₄F(2)O₃F(3)₂ polyhedrons are located in accordance with the La(F(1)_0.86V_0.14)(F(2)_0.35O_0.65)(F(3)_0.49) lattice of the LaO_0.65F_1.7 host structure [27,28]. Notably, the nine-fold LaF(1)₃VF(2)₂O₂F(3)₂ polyhedron contains a vacancy (V) associated with the F(1) anion.

In this study, Bi³⁺ and Eu³⁺ were substituted into LaO_0.65F_1.7 compounds that were synthesized by a solid-state method using NH₄F flux in air. The unit-cell parameters of the phosphors were calculated. The excitation and emission luminescence spectra of the La_1-p-qBi_pE u_qO_0.65F_1.7 (p = 0.001–0.05, q = 0–0.1) phosphors were investigated with respect to the site dependency of Bi³⁺ and Eu³⁺ ions in the host structure. The energy transfer mechanism from Bi³⁺ to Eu³⁺ in the phosphors was explored. Commission Internationale de l’Eclairage (CIE) chromaticity coordinates of the phosphors were obtained.

2. Materials and Methods

Phosphors of La_1-p-qBi_pEu_qO_0.65F_1.7 (p = 0.005–0.05, q = 0–0.1) were prepared by heating the appropriate amounts of La₂O₃ (Alfa 99.9%), Bi₂O₃ (Alfa 99.99%), Eu₂O₃ (Alfa 99.9%), and NH₄F (Alfa 99%). Powdered samples with 1:2 molar ratios of La(Bi,Eu)O_3/2 and NH₄F were used to prepare nonstoichiometric LaO_0.65F_1.7:Bi³⁺, Eu³⁺. The precursors were mixed with an agate mortar and pestle and subsequently heated at 1050 °C for 2 h in air [27]. The La₂O₃ 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).

3. Results and Discussion

The crystallographic phase of the La_1-p-qBi_pEu_qO_0.65F_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_0.65F_1.7 (ICSD 40371) structure is shown in Figure 2A. Figure 2B–F show the XRD patterns of non-stoichiometric La_1-p-qBi_pEu_qO_0.65F_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 ½La(Bi,Eu)₂O₃ and NH₄F at 1050 °C in air. The XRD patterns of the obtained phosphors in Figure 2B–F show a single-phase structure without any noticeable impurities indexed to a tetragonal unit cell. The unit cells of La_0.99Bi_0.01O_0.65F_1.7, La_0.95Bi_0.05O_0.65F_1.7, La_0.95Eu_0.05O_0.65F_1.7, La_0.9Eu_0.1O_0.65F_1.7, and La_0.89Bi_0.01Eu_0.1O_0.65F_1.7 phosphors were calculated to be a = 4.0934 (1) Å and c = 5.8336 (2) Å, a = 4.1018 (2) Å and c = 5.8315 (2) Å, a = 4.0833 (2) Å and c = 5.8162 (4) Å, a = 4.0788 (3) Å and c = 5.8095 (5) Å, and a = 4.0993(3) Å and c = 5.7712(6) Å, respectively, using the Rietveld refinement. The unit-cell parameters, including the cell volumes of the phosphors, are summarized in

Table 1. The unit-cell parameters with the cell volumes of the La_0.99Bi_0.01O_0.65F_1.7, La_0.95Bi_0.05O_0.65F_1.7, La_0.95Eu_0.05O_0.65F_1.7, La_0.9Eu_0.1O_0.65F_1.7, and La_0.89Bi_0.01Eu_0.1O_0.65F_1.7 phosphors.

Phosphors	a (Å)	c (Å)	V (Å3)	Rp
La0.99Bi0.01O0.65F1.7	4.0934 (1)	5.8336 (2)	97.75 (1)	9.11
La0.95Bi0.05O0.65F1.7	4.1018 (2)	5.8315 (2)	98.11 (1)	9.98
La0.95Eu0.05O0.65F1.7	4.0833 (2)	5.8162 (4)	96.98 (1)	9.46
La0.9Eu0.1O0.65F1.7	4.0788 (3)	5.8095 (5)	96.65 (1)	8.95
La0.89Bi0.01Eu0.1O0.65F1.7	4.0993 (3)	5.7712 (6)	96.98 (1)	9.62

. The Bi³⁺ and Eu³⁺ ions, under these conditions, occupy 9- and 10-coordinated La³⁺ sites (LaF(1)₃F(2)₂O₂F(3)₂ and LaF(1)₄F(2)O₃F(3)₂) in the non-stoichiometric LaO_0.65F_1.7 structure, as shown in Figure 1 [27,28]. The single La³⁺ site comprises 56% 9-fold and 44% 10-fold polyhedrons in the LaO_0.65F_1.7 lattice based on the La(F(1)_0.86V_0.14)(F(2)_0.35O_0.65)(F(3)_0.49) formula. The 9- and 10- coordinated LaO₂F₇ and LaO₃F₇ polyhedrons in the non-stoichiometric unit cell are arrayed along the c-axis, as shown in Figure 1. When Bi³⁺ ions (r = 1.17 Å for 8 coordination number (CN)) were substituted for La³⁺ ions (r = 1.16 Å for 8 CN) in the LaO_0.65F_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 Eu³⁺ ions (r = 1.066 Å for 8 CN) were substituted for La³⁺ 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 Bi³⁺ ions were doped in the La_0.9Eu_0.1O_0.65F_1.7 phosphors, no further shift to higher angles was observed in the La_0.89Bi_0.01Eu_0.1O_0.65F_1.7 phosphors, as shown in Figure 2F.

Figure 3aA–E show the photoluminescence (PL), excitation (EX), and emission (EM) spectra of the Bi-doped La_1-pBi_pO_0.65F_1.7 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 La_0.99Bi_0.01O_0.65F_1.7 PL spectra is attributed to the ¹S₀ → ³P₁ transition of Bi³⁺ ions because the ¹S₀ → ³P₀ and ¹S₀ → ³P₂ transitions are forbidden from ground ¹S₀ [19,20,21,22,23,24,25,26]. The blue emission spectra of the LaO_0.65F_1.7:Bi³⁺ phosphors revealed a broadband range from 350 to 650 nm, centered at approximately 497 nm, which is attributed to the intense ³P₁ → ¹S₀ transitions of the Bi³⁺ ions, as shown in Figure 3a. When the Bi³⁺ 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 Bi³⁺ 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 Bi³⁺ content in the LaO_0.65F_1.7 host lattice was increased and the excitation center of the ¹S₀ → ³P₁ transition of Bi³⁺ ions underwent a shift to a longer wavelength. The La³⁺ ion is coordinated by seven F⁻ and two O²⁻ anions (LaF(1)₃F(2)₂O₂F(3)₂), or seven F⁻ and three O²⁻ anions (LaF(1)₄F(2)O₃F(3)₂) in the LaO_0.65F_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)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 indicated that Bi³⁺ ions are preferentially substituted in the nine-fold La site and subsequently doped into the 10-fold La site in the host structure. Figure 3b shows the excitation and emission PL spectra of the La_0.95Eu_0.05O_0.65F_1.7 phosphors. The charge-transfer bands (CTBs) and the f–f transitions of the Eu³⁺ 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 La³⁺ sites associated with the LaF(1)₃F(2)₂O₂F(3)₂ and LaF(1)₄F(2)O₃F(3)₂ polyhedrons in the host structure. When Eu³⁺ ions were doped in the nine-coordinated La³⁺ site of the LaF(1)₃F(2)₂O₂F(3)₂ polyhedron, the center of the Eu³⁺ CTB transitions occurred at 290 nm. Additional energy was required to excite an electron from the Eu³⁺ ions in seven F⁻ and two O²⁻ containing lattices, compared to seven F⁻ and three O²⁻ polyhedrons.

The Eu³⁺ transitions of the emission spectra in the La_0.95Eu_0.05O_0.65F_1.7 phosphors exhibited both the ⁵D₀–⁷F₁ magnetic dipole and the ⁵D₀–⁷F₂ electric-dipole transitions, centered at 592 and 610 nm, respectively [29,30]. When the Eu³⁺ ions were substituted in no inversion site of the nine-coordinated polyhedron in the host lattice, the ⁵D₀–⁷F₂ transition dominates. When the Eu³⁺activators were doped into symmetric inversion site of the 10-fold polyhedron, the ⁵D₀–⁷F₁ transition dominates. Figure 3c shows the excitation spectra of the La_0.95Eu_0.05O_0.65F_1.7 (EX_EM=610nm and EX_EM=592nm) and the emission spectrum of La_0.99Bi_0.01O_0.65F_1.7 (EM_EX=286nm) phosphors. The efficiency of energy transfer from Bi³⁺ to Eu³⁺ was estimated by the spectral overlap between the excitation of the Eu³⁺ transition and the emission band of Bi³⁺ ions in the host lattice [31]. The excitation spectrum of the La_0.95Eu_0.05O_0.65F_1.7 (EX_EM=610nm) phosphor and the emission spectrum of the La_0.99Bi_0.01O_0.65F_1.7 (EM_EX=286nm) phosphor exhibited considerable overlap, as shown in the top of Figure 3c. This indicated that effective energy transfer from Bi³⁺ to Eu³⁺ ions occurs in the nine-coordinated La³⁺ site of the LaF(1)₃F(2)₂O₂F(3)₂ polyhedron in the LaO_0.65F_1.7 host structure. The individual transitions of Bi³⁺ and Eu³⁺ ions with the energy transfer from Bi³⁺ to Eu³⁺ ions in the phosphors can simultaneously occur under approximately 290 nm excitation wavelength. However, the energy transfer was effectively observed rather than the individual transitions because the integrated emission intensity of Eu³⁺ transition was enhanced by approximately 91% from La_0.95Eu_0.05O_0.65F_1.7 to La_0.94Bi_0.05O_0.65F_1.7 phosphors (Figure S1). The blue-green emission of the La_1-pBi_pO_0.65F_1.7 phosphors centered at 497 nm reached a maximum intensity for a Bi³⁺ content (p = 0.01), as shown in Figure 3a. After increasing the Bi³⁺ content, concentration quenching of the relative emission intensity was observed. The increase in the Bi³⁺ content of the phosphors enhanced energy transfer up to some critical value, whereas after this value was reached subsequent increase of Bi³⁺ levels decreased the emission intensity by reducing the critical distance between the Bi³⁺ ions. This resulted in non-radiative energy transfer between Bi³⁺ ions from the electric multipole interactions. The critical distance (R_c) is expressed by the following formula:

R_c = 2[3V/4πm_cN]^1/3

(1)

where V is the volume of the La_0.99Bi_0.01O_0.65F_1.7 unit cell, N is the number of available La³⁺ sites for the dopant in the unit cell, m_c is the critical concentration of Bi³⁺, and R_c is the critical distance for energy transfer [10,22,23,24,32]. When N and V are 1 and 97.75 ų, respectively, for La_0.99Bi_0.01O_0.65F_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_0.99-qBi_0.01Eu_qO_0.65F_1.7 (q = 0–0.1) phosphors under 286 nm excitation. Co-doping of Eu³⁺ into the Bi³⁺-doped LaO_0.65F_1.7 host structure allowed effective energy transfer from Bi³⁺ to Eu³⁺ under excitation at 286 nm. The energy transfer from Bi³⁺ to Eu³⁺ acted as a sensitizer and an activator, respectively, in the La_0.99-qBi_0.01Eu_qO_0.65F_1.7 (q = 0–0.1) phosphors, which was activated through the absorption from Bi³⁺ transitions. The energy transfer efficiency (η_T) was evaluated using the following formula:

η_T = 1 − I_S/I_SO

(2)

where I_S and I_SO are the luminescence intensities of the Bi³⁺ sensitizer in the presence and absence of a Eu³⁺ activator, respectively [10,22,23,24,32]. The emission of Eu³⁺ transitions was maximized when the Eu³⁺ content in the La_0.99−qBi_0.01Eu_qO_0.65F_1.7 (q = 0–0.1) phosphors was q = 0.05. The energy transfer mechanism could be represented by linear plots of I_SO/I_S versus C_Bi-Eu^α/3, where C_Bi-Eu is the concentration of Bi³⁺ and Eu³⁺ ions, with α = 6, 8, or 10, corresponding to dipole–dipole, dipole–quadrupole, 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 Bi³⁺ to Eu³⁺ ions with R² = 0.9635, 0.9894, and 0.9982 in the La_0.94Bi_0.01Eu_0.05O_0.65F_1.7 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 La_0.94Bi_0.01Eu_0.05O_0.65F_1.7 phosphors. The efficiency of the energy transfer from Bi³⁺ to Eu³⁺ in La_0.94Bi_0.01Eu_0.05O_0.65F_1.7 (EX = 286 nm) phosphors is shown in Figure 4c. The efficiency was gradually enhanced from 23% to 97% as the Eu³⁺ 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 La_0.99-qBi_0.01Eu_qO_0.65F_1.7 (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 Eu³⁺ ions in the La_0.99-qBi_0.01Eu_qO_0.65F_1.7 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 Bi³⁺ to Eu³⁺ in the La_0.99-qBi_0.01Eu_qO_0.65F_1.7 phosphors. Emission of the La_0.99-qBi_0.01Eu_qO_0.65F_1.7 (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.

4. Conclusions

Non-stoichiometric tetragonal La_1-p-qBi_pEu_qO_0.65F_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 NH₄F flux. The site dependency of the Bi³⁺ and Eu³⁺ ions in the LaF(1)₃F(2)₂O₂F(3)₂ and LaF(1)₄F(2)O₃F(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-pBi_pO_0.65F_1.7 phosphors was obtained when p = 0.01. The critical distance (Rc) value for the La_0.99Bi_0.01O_0.65F_1.7 phosphor was determined to be 26.53 Å. As the Eu³⁺ concentration was increased in La_0.99-qBi_0.01Eu_qO_0.65F_1.7 (q = 0–0.1) phosphors under 286 nm excitation, an efficient energy transfer from Bi³⁺ to Eu³⁺ 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-qBi_0.01Eu_qO_0.65F_1.7 (q = 0–0.1) phosphors were successfully obtained.