Preparation, structure and luminescent properties of Eu3+ doped MBPO5 (M = Ca, Sr, Ba) red phosphor for white LED

Preparation of Eu³⁺ phosphors

A series of MBPO₅: Eu³⁺ (M = Ca, Sr or Ba) phosphors were prepared with traditional solid-phase reaction technique. The original materials were CaCO₃ (99.9%), SrCO₃ (99.9%), BaCO₃ (99.9%), H₃BO₃ (99.9%), NH₄H₂PO₄ (99.9%) and Eu₂O₃ (99.99%), and the charge compensator was supplied by Li₂CO₃ (99.9%), Na₂CO₃ (99.9%) or K₂CO₃ (99.9%). All the reagents were obtained from Tianjin Kermel Chemical Reagent Development Center (Tianjin, China). The molar ratio of Eu³⁺ ions to M²⁺ ions varied from 0.01 to 0.19. The staring reagents with appropriate stoichiometric ratio were put into an agate mortar and added 15 mL ethanol for the purpose of mixing the materials homogeneously. Then the mixture was pre-fired at 400°C for 3 h and subsequently at 900°C for 4 h in the air. Ultimately, the obtained red phosphor needed to be ground into powder for the purpose of latter measurement. Under some circumstances, Li₂CO₃, Na₂CO₃ or K₂CO₃ was moderately put in as charge compensator, respectively.

Structure analysis and luminescence measurement

The crystal structures of the samples were collected on a Bruker D₈ x-ray powder diffractometer with a CuKa radiation (40 kv, 40 mA). A Hitachi U-4100 ultraviolet and visible spectrophotometer was used to measure the UV-Vis absorption spectra. A Hitachi F-4600 spectro-photometer with Xe (150 W) lamp excitation source was used to record the fluorescence spectra. The measurement of luminescence lifetime was conducted by utilizing an Edinburgh FLS920 luminescence spectrometer by the excitation source of 375 nm pulse laser radiation (nano-LED). All measurements were operated at ambient temperature (20°C).

X-ray diffraction analysis

The crystal structure of the MBPO: 0.05Eu³⁺ phosphor was analyzed by the XRD. The diffraction patterns of the MBPO₅ samples belonged to a pure hexagonal phase (space group: P6cc), which were consistent with the standard data of CaBPO₅ (JCPDS card No.18-0283), SrBPO₅ (JCPDS card No.18-1270) and BaBPO₅ (JCPDS card No.19-0096). The diffraction peaks of MBPO₅:0.05Eu³⁺ phosphors shifted slightly to the higher angle compared to the standard cards (Fig. 1a). The lattice constant decreased since the Eu³⁺ ions whose radius is smaller (r = 0.0947 nm) substituted for the Ca²⁺(r = 0.099 nm), Sr²⁺ (r = 0.112 nm) and Ba²⁺ ions (r = 0.135 nm), which matched with the diffraction equation 2d sin θ = k λ. According to the peak (110), the lattice constants of CBPO, SBPO and BBPO could be calculated to be 0.67, 0.68 and 0.70 nm, respectively. The increasing of lattice constants was attributed to the ionic radius increasing of matrix cations.

All the samples have similar structures. From the structure diagram of SBPO molecule, it could be seen that B³⁺ ions and P⁵⁺ ions formed BPO₅^2- ionic group via O₂, and Sr²⁺ ions combined with the BPO₅^2- ionic group via O₁ (Fig. 1b).

The electronic structure of SrPBO₅

A plane-wave cutoff energy of 340 eV and a (4 × 4 × 4) Monkhorst-Pack k-point grid for integration were applied in computations. The convergence in energy, maximum force, maximum displacement, and maximum stress tolerances were set as 1.0 × 10⁻⁵ eV/atom, 0.03 eV/Å, 0.001Å, and 0.05 GPa, respectively. First principle calculation was carried out by the software CASTEP (14). A Vanderbilt-type ultra-soft pseudopotential formalism and exchange-correlation function founded on the generalized gradient approximation (GGA) in the scheme of Perdew-Burke-Eruzerhof (PBE) were applied to calculate the band structure and density of state. The calculated band structure of MBPO are shown in Figure 2a, b, c. It can be seen that MBPO belongs to direct optical band gap materials and the gap between the conduction band minimum (CBM) and the valence band maximum (VBM) are approximately 5.481, 5.498 and 5.604 eV for CBPO, SBPO and BBPO, respectively. The UV-Vis absorption spectra have also been studied, and the forbidden band gap has been calculated. The results are given in Supplementary Figure 1 (available online as supplementary material at www.jab-fm.com). For the sake of investigating the forbidden band gap of the prepared samples, the UV-Vis absorption spectra were converted to an absorption-equivalent spectra using Kubelka-Munk function (15). In the absorption-equivalent spectra, the x-axis represents the energy, and the intercept at x-axis is their forbidden bang gap. The intercepting points gave the band gap energies of CBPO, SBPO and BBPO to be 5.499, 5.516 and 5.659 eV, respectively. It could be seen from that the forbidden band gaps are successively broadening, which matches with the result in Figure 2a, b, c. The broad band gap energy indicated that this material can be used as a matrix of phosphor.

According to the report of Xiaocheng Zeng et al (16), the electron-occupied 4f energy levels of Eu located within the band gap of host, while the empty 5d levels are just above the CBM, which indicated that the electronic transition between 4f and 5d levels of Eu induced the luminescence. Thus, MBPO belonged to the category of materials with broad band gaps that could be used as hosts of phosphors. The total and partial densities of states of MPBO showed that the upper valence band was dominated by 2p4 (O), 3p3 (P) and 2p1 (B) states, while the bottom of conduction band was mainly composed of 3p3 (P), 2p1 (B), 3d (Ca), 4d (Sr) and 5d (Ba) states, which indicated that different hosts contained different conduction band components. The dotted lines at x = 0 indicated the position of the Fermi level (Fig. 2d, e, f).

Excitation and emission spectra of MBPO: 0.05Eu³⁺

The near UV and UV excitation spectra of the three samples were similar, and the CBPO: 0.05Eu³⁺ was selected to be investigated for the excitation and emission spectra. All spectra contained a broad band from 200 to 300 nm and a battery of narrow bands from 300 to 450 nm. The former was ascribed to the charge transfer transition from the O²⁻ (2p⁶) to the empty state of 4f⁷ in Eu³⁺ ion, while the latter excitation peaks at 317, 359, 379, 394 and 413 nm were ascribed to the transition of ⁷F₀-⁵H₅, ⁷F₀-⁵D₄, ⁷F₀-⁵L₇, ⁷F₀-⁵L₆, and ⁷F₀-⁵D₃, respectively. The strongest excitation peak lay at 394 nm, indicating that this phosphor could be excited by near UV light and well matched with the emission of near UV chips (Fig. 3a).

The emission spectra of MBPO: 0.05Eu³⁺ phosphor excited by 394 nm showed that the characteristic emission bands of the samples located in the region of 550-700 nm, which was corresponding to the transitions of ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) of Eu³⁺ (4f⁶) (17). The main emission peaks located at 590 nm (⁵D₀-⁷F₀), 594 nm (⁵D₀-⁷F₁), 614 nm (⁵D₀-⁷F₂), 654 nm (⁵D₀-⁷F₃), 687 and 700 nm (⁵D₀-⁷F₄), respectively. The emission intensity excited by 394 nm was the strongest, which indicated that the phosphor could be excited by near UV light (Fig. 3b).

It also can be seen that the emission induced by the transition from ⁵D₀ to ⁷F₁ (594 nm) was strongest among that of others (Fig. 3b). The transition of ⁵D₀-⁷F₁ was permitted for the magnetic dipole transition and faintly influenced by the symmetry of the Eu³⁺ site. However, the ⁵D₀-⁷F₂ transition was parity-forbidden electric dipole transition and sensitive to the site symmetry of Eu³⁺ ions (18). In this case, the intensity ratio of the orange (⁵D₀-⁷F₁) to the red (⁵D₀-⁷F₂) transition (O/R) for CBPO:Eu³⁺, SBPO:Eu³⁺ and BBPO:Eu³⁺ phosphors were 3.58, 3.4 and 2.78, respectively, which suggested that Eu³⁺ ions occupied the inversion center sites in these three kinds of phosphors and the phosphors with different hosts rendered different colors.

The impact of Eu³⁺ concentration on the emission intensity of MBPO: xEu³⁺ phosphor

The relationship between emission intensity of M_1-x BPO₅:xEu³⁺ phosphor and molar fraction of Eu³⁺ ions is revealed in inset of Figure 3b, in which the experimental data were denoted with pentagrams and the fitting curve was marked with red line. The intensity of emission increases with the concentration of Eu³⁺ ions. However, the concentration quench does not appear till the molar fraction of Eu³⁺ ions reach 0.19, which is due to the distance of the Eu³⁺ ions, is enough far that the reaction between them could be ignored. In addition, the emission intensity increases very slowly and still has no concentration quench when the molar fraction of Eu³⁺ ions exceed 0.19.

The decay curve of MBPO:0.05Eu³⁺ phosphor

The decay curve for ⁵D₀-⁷F₁ transition (594 nm) of Eu³⁺ ions was measured. As shown in Figure 3c, the decay curves consisted of two straight lines with different slopes and could be matched by a second-exponential function:

where A₁ and A₂ are pre-exponential factors, and τ₁ and τ₂ are the decay times of Eu³⁺ ions in two different sites, respectively. The average decay time (τ) of the three samples could be calculated by the following formula:

The average decay time was similar to other phosphors doped with Eu³⁺ ions, which indicated that Eu³⁺ ions occupied the same sites in the three crystal lattices (Tab. I) (19, 20).

The CIE constant of MBPO: 0.05 Eu³⁺ phosphor

Figure 3d gives the CIE constant of MBPO: 0.05Eu³⁺ phosphor. Excited at 394 nm, the chromaticity coordinates of the CBPO:0.05Eu³⁺, SBPO:0.05Eu³⁺ and BBPO:0.05Eu³⁺ phosphors are (0.619, 0.380), (0.610, 0.371), (0.621, 0.386), respectively. The results prove that these phosphors are similar to those of Sr_1-xCa_xSiN₃:Eu²⁺ red materials (0.647, 0.347) (21) and the National Television System Committee (NTSC) for the red phosphor (0.670, 0.330), which could be considered as red phosphor for near-UV LEDs or make up for the blue chip-based LEDs.

The effect of charge compensator on the intensity of emission of MBPO:0.05Eu³⁺ phosphor

Take the valence and electro-negativity into consideration, Eu³⁺ ions would substitute for partial M²⁺ ions. Due to the imbalance charge between Eu³⁺ ions and M²⁺ ions, adding alkali ions as charge compensators is essential. The alkali ions (Li⁺, Na⁺ and K⁺) would affect the emission intensities. The emission intensity of MBPO: 0.05Eu³⁺, 0.05Li⁺ was the strongest (Fig. 4a), which maybe because Li⁺ ion radius (0.102 nm) is less than those of Na⁺ and K⁺ ions, and could enter the crystal lattice easily. The microscopic mechanism of charge compensation including the defect reaction resulted from the elimination of the lattice distortion and the reduction of the lattice center symmetry, which enhanced the red emission.