Novel Red-Orange Phosphors Na₂BaMg(PO₄)₂:Pr³⁺: Synthesis, Crystal Structure and Photoluminescence Performance

3.1 Crystal Structure

Figure 1 shows the XRD patterns of Pr³⁺-doped and undoped Na₂BaMg(PO₄)₂. The main diffraction peaks are indexed well with the space group of P3̅m1 which is isostructural to the mineral glaserite K₃Na(SO₄)₂ [12]. The structure can be depicted as the stacking along the [001] direction of equivalent [MgP₂O₈]⁴⁻ mixed layers of corner sharing MgO₆ octahedron which is connected to six PO₄ tetrahedra while each tetrahedron is linked to only three octahedra. The Na⁺ ions are sited in large cavities bound to the layers and the Ba²⁺ ions are found within the interlayer space. There is a charge compensation problem with the Pr³⁺ ions doped Na₂BaMg(PO₄)₂. Based on the effective ionic radii and electronegativity of cations with different coordination numbers, it might be proposed that Pr³⁺ ion occupies preferably the Ba²⁺ site. The required charge compensations for the occupation of Pr³⁺ ions could be more complicated due to three kinds of cations (Na⁺, Ba²⁺ and Mg²⁺) in the lattices. This can probably be obtained by the following possible mechanism: the positive charge due to Pr³⁺ ion substitute for Ba²⁺ may be combined with the cation vacancy to form the dipole complexes of [(Pr_Ba³⁺)^*−V′_Na]. In fact, because of the reaction at high temperature, the Pr³⁺ ions are not excluded into the Na⁺ or Mg²⁺ sites, thus forming the dipole complexes of [(Pr_Na³⁺)^*−V″_Mg] and [(Pr_Mg³⁺)^*−V′_Na], respectively. This is very common in other components because of the charge compensation mechanism. For example, trivalent rare-earth (RE³⁺) ions doped apatite structure phosphate [13]. In addition, the possible negative charge compensation related to interstitial oxygen here is difficult because this mechanism usually occurs in the case of the high RE-doping [14]. And usually, in oxides, the reaction energy to create the interstitial oxygen is higher than that of cation vacancy [15]. Moreover, it should be noted that due to Pr³⁺ and host cations having different electrical charges, ionic radii and coordination number, the diffraction intensities, diffraction peak positions and lattice constants of Na₂BaMg(PO₄)₂:Pr³⁺ should be changed with the increasing Pr³⁺ content, but it is hard to find these differences from Figure 1. The crystal data are collected in Table 1 which are calculated by using the Jade software and XRD experimental data. From Table 1, the cell angles (α, γ) are invariable. The lattice parameters a, c and cell volume of the Na₂BaMg(PO₄)₂:Pr³⁺ are reduced slightly with the increase of doping concentration of Pr³⁺. Thus, the crystalline density will increase a little. Even so, the change of the cell parameters is very small. The reason is that the doping concentration is very low and is not enough to cause significant change in lattice properties.

Figure 1:

XRD patterns of Na₂BaMg(PO₄)₂:Pr³⁺ phosphors.

3.2 SEM Image of Na₂BaMg(PO₄)₂:Pr³⁺

The typical SEM image is shown in Figure 2. From Figure 2, it is revealed that the surface morphology of the representative sample (0.5 mol% of Pr³⁺ doping case) is irregular. It is made of numerous micron-sized particles. These particulate sizes of Na₂BaMg(PO₄)₂:Pr³⁺ are mainly in the range of 10–60 μm.

Figure 2:

Typical SEM image of Na_1.995BaMg(PO₄)₂:0.005Pr³⁺ phosphors.

3.3 Photoluminescence Properties

Figure 3 presents the excitation and emission spectra of the undoped and doped samples. The excitation spectra of Na₂BaMg(PO₄)₂:Pr³⁺ powders indicate two broad charge transfer bands at 200–280 nm and 310–420 nm, respectively. This is due to the charge transfer from Pr³⁺, Na⁺, Ba²⁺, Mg²⁺, P⁵⁺ to O²⁻. There are three adjacent peaks at around 449, 468, and 480 nm in the excitation spectra of Na_2−x BaMg(PO₄)₂:xPr³⁺, corresponding to the transitions from the ground state ³H₄ to the multiplets ³P₂, ³P₁, and ³P₀, respectively. Considering 449 nm as the strongest peak, it could be used as a potential red-orange phosphor for the fabrication of white LED with a commercial blue LED chip (430–480 nm).

Figure 3:

Photoluminescence excitation and emission spectra of Na₂BaMg(PO₄)₂:Pr³⁺ phosphors.

The characteristic emission peaks of Pr³⁺ are observed at 528 nm (³P₁→³H₅), 552 nm (³P₀→³H₅), 606 nm (¹D₂→³H₄), 611 nm (³P₀→³H₆), 644 nm (³P₀→³F₂) and 722 nm (³P₀→³F₃) for Na₂BaMg(PO₄)₂:Pr³⁺ phosphors. As shown in Figure 3, the emission intensity at 606 nm increases with the increasing doped concentration from 0 to 0.005, and decreases with the increasing doped concentration from 0.005 to 0.015. The maximum of luminescent intensity lies at x=0.005 in the range of 0~0.015, and then increases until x=0.02. The concentration quenching for the fluorescence of Na₂BaMg(PO₄)₂:Pr³⁺ occurs when x value is higher than 0.02. The quenching of the emission of Pr³⁺ in Na₂BaMg(PO₄)₂:Pr³⁺ might be due to the following reason: the distances between the activators are shortened, and the energy transfer between the adjacent Pr³⁺ ions in the host Na₂BaMg(PO₄)₂ occurs with the higher probability of non-radiation transition, which enables the excitation energy to be consumed and results in the quenching of the emission of Pr³⁺.

3.4 The Commission Internationale d’Eclairage Chromaticity Coordinates

As shown in Figure 4, the Commission Internationale d’Eclairage (CIE) average chromaticity coordinates (0.52, 0.46) of Na₂BaMg(PO₄)₂:Pr³⁺ with different concentrations (x=0.2 mol%, 0.5 mol%, 1.0 mol%, 1.5 mol%, 2.0 mol%, 2.5 mol%, and 3.0 mol%) of xPr-doped Na₂BaMg(PO₄)₂ phosphors are obtained from the calculations of optical spectra and intensity data. It is clearly observed that the colour of the as-prepared samples is in the red-orange region. The chromaticity coordinates (x, y) and correlated colour temperatures data of all Na₂BaMg(PO₄)₂:Pr³⁺ samples are shown in Table 2.

Figure 4:

CIE average chromaticity diagram for Pr-doped Na₂BaMg(PO₄)₂ phosphors.

3.5 The Decay Curve

Figure 5 shows the typical fluorescence decay pattern in Na_1.995BaMg(PO₄)₂:0.005Pr³⁺. The fluorescence decay is often given by the multi-exponential or non-exponential decay equation. In this work, the decay curve is non-exponential and well fitted into a three-exponential function [16], [17]

Figure 5:

Typical fluorescence decay curve of 606 nm emission of Na_1.995BaMg(PO₄)₂:0.005Pr³⁺ under excitation into the ³H₄ at 449 nm.

in which τ₁, τ₂ and τ₃ are time constants and A₁, A₂ and A₃ are coefficients. The average lifetime can be calculated by using the equation,

As can be seen, the average lifetime in Na_1.995BaMg(PO₄)₂:0.005Pr³⁺ is 73.32 μs which is caused by the radiation relaxation channels and reduction of the ¹D₂ lifetime in the Na_1.995BaMg(PO₄)₂:0.005Pr³⁺ phosphor. Interestingly, in the experiment, the Pr³⁺ ions instead of the Na⁺ sites are designed, but in the high temperature synthesis condition, a small amount of Pr³⁺ may replace other ions (such as Ba²⁺, Mg²⁺ sites) or occupy the interstitial sites. The three-exponential function indicates that the Pr³⁺ ions occupy two additional positions besides the Na⁺ site. The conclusion here is to verify the correctness of the previously mentioned defect model.