3.1 Phase Purity and crystal structure
Figure 1 shows the powder XRD pattern of synthesized Mg3(PO4)2 phosphors for pure and doped with Eu3+. The XRD pattern matches in good agreement with the ICSD Ref code 98-000-9849. The absence of extra peaks indicates that Eu3+ has incorporated into the host Mg3(PO4)2 host without any change of phase. X-ray diffraction (XRD) confirmed the successful formation of Mg3(PO4)2:Eu3+ phosphors with monoclinic phase. As per comparative ionic radii rules, it was expected that Eu3+ take place Mg2+ sites. We have also seen that with the doping of Eu ions crystallinity of the samples are improved.
Figure 2 represents crystal structure of the Mg3(PO4)2 phosphor. The Mg3(PO4)2 phosphor has monoclinic crystal structure with P1 21/n1 space group. The lattice parameters are α = 90o, β = 94.05 o, γ = 90o, a = 7.59, b = 8.23, c = 5.07 and volume of unit cell is 316.63 Å3. From the crystal structure, we have seen that Mg atom bounded with 6 oxygen atoms, the bond length between Mg and oxygen atom is 2.41 Å. Whereas, P atom bounded with 4 oxygen atoms and their bond length is around 2.08 Å.
3.3 Photoluminescence (PL) Properties:
The PL excitation spectra of Mg3(PO4)2 phosphors when activated by Eu3+ is as shown in Fig. 4. The spectra were recorded in the wavelength range of 220 nm to 500 nm when monitored under 615 nm wavelength. Spectra reveals absorption bands at 260 nm, 395 nm and 466 nm respectively. Sharp and intense band is observed at 260 nm and it is the CTB charge transfer band. The excitation bands centered 395 nm and 460 nm were attributed to the (7F0→5D3), (7F0→5D2), (7F0→5D1) transitions of Eu3+ ions in the host [14, 15]. High intense peaks were observed at 260 nm and 395 nm and hence emission spectra have been taken for these two excitation wavelengths. The excitation band 395 nm and 466 nm matched the emission wavelength of near-UV and red LED chips suitable for the WLEDs.
Figure 5 (a) shows the PL emission spectra of Mg3(PO4)2 :xEu3+ (x = 0.1–1 mol%) phosphor recorded in the range of 560 nm to 640 nm for excitation wavelength of 260 nm. The emission spectra exhibits two sharp and intense peaks at 595 nm and 615 nm attributed due to (5D0→7F1) and (5D0→7F2) transitions of Eu3+ ions. Also, the peak at 595 nm is divided into two peaks the other having wavelength 588 nm and this is attributed due to crystal field splitting. 5D0→ 7F2 transition having peak around 615 nm is hypersensitive transition, also known as forced electric dipole (ED) transition. The transition 5D0→ 7F1 transition around 595 nm is known as magnetic dipole (MD) transition. Figure 5 (b) shows the PL emission spectra of Mg3(PO4)2 :xEu3+ (x = 0.1–1 mol%) phosphor recorded in the range of 540 nm to 660 nm for excitation wavelength of 395 nm. The emission spectra exhibits two sharp and intense peaks at 595 nm and 615 nm. As per literature survey, if intensity of Magnetic Dipole transition is greater than Electric Dipole transition, the Eu3+ ions occupy inversion symmetry site and if intensity of Electric Dipole transition is greater than Magnetic Dipole transition, then the activator Eu3+ occupies without inversion symmetry in host lattice [6, 15, 16]. From Fig. 5(a) and Fig. 5(b) we see that intensity Magnetic Dipole transition around 595 nm is greater that of Electric dipole transition around 615nm (5D0→ 7F2) and thus the activator ion Eu3+ occupies inversion symmetry sites in the Mg3(PO4)2 host lattice. Figure also shows that nature of transition is same for both emission wavelengths and the intensity changes with changing concentration of Eu3+ ions.
The relationship between PL emission peak intensity and concentration is seen in Fig. 6. The intensity goes on increasing with the increasing concentration of Eu3+ ions and it reaches maximum for 0.5mol% concentration of Eu3+ ions. Due to the concentration quenching effect, the intensity continues to decline beyond 0.5 mol%. The non-radiative energy transfer mechanism is responsible for this. As large number of rare earth ions are increased in the host, they don’t contribute to intensity and hence concentration quenching takes place [17]. Blasse noted the critical distance (RC), which is equal to twice the radius of a sphere with a unit cell volume. This can be written as the following equation [17]
$${R}_{c}=2{\left(\frac{3V}{4\pi {X}_{c}N}\right)}^{\frac{1}{3}} \dots \dots \dots \dots .. \left(1\right)$$
The critical distance (Rc) between the activated ions is evaluated by using Eq. (1) [17]. Here, V for the unit cell's volume, Xc for the optimum concentration and N for the number of cations that activator ions occupy in a unit cell. For Mg3(PO4)2 phosphors, Xc = 0.5, N = 4, and V = 317.01 Å3. The estimated value of Rc is about 12.66 Å. Previous studies suggested that if the value of Rc is greater than 5 Å then the energy transfer occurs due to the electric multipole interaction whereas if the value of Rc is less than 5 Å then the energy transfer occurs through the multipole-multipole interaction. Our estimation revealed that value of Rc is higher than 5 Å therefore nonradiative energy transfer between Eu3 + ions can take place through the electric multipole interaction [18, 19]. It is evident from above equation that the distance between the Eu3+ ions decrease with the increase of the doping concentration. This results in non-radiative energy transfer and is responsible for concentration quenching.
The multipolar interaction strength can be calculated by following equation [20]
Above Eq. (2) is from Dexter’s theoretical model and is used to determine which type of multipolar interaction for the Eu3+ - Eu3+ ions in the host matrix is responsible for effect of concentration quenching. Here, K and β are constants and I is the intensity. θ is the multipolar interaction and X is the concentration of Eu3+ ion greater than the optimum concentration. Interaction type is determined by θ value i.e. 3 (exchange interaction), 6 (dipole-dipole interaction), 8 (dipole-quadrupole interaction) and 10 (quadrupole- quadrupole interaction) [21, 22].
Figure 7 represents a plot of log (I/x) along on y-axis verses log (x) on the x-axis for emission wavelength at 595 nm. The graphs are linearly fitted with slope − 1.76 and − 1.74 at emission wavelength 595 nm. The value of θ is approximately equal to 5.28 ≈ 6 and 5.22 ≈ 6 for 595 nm emission wavelength at different excitations 260 nm and 395 nm. Above values indicate that concentration quenching effect is due to dipole – dipole interaction throughout the Eu3+ ion in Mg3(PO4)2 phosphor.
Substitution of Phosphate ions with molybdate, tungstate and sulfate ions
To investigate the effect of molybdate, tungstate and sulfate ions in Mg3(PO4)2:0.5mol%Eu3+ phosphor, we replaced phosphate group with molybdate, tungstate and sulfate group. Figure 9 represents the PL excitation and emission spectrum of the Mg3(PO4)2−y(MoO4)y, Mg3(PO4)2−y(WO4)y and Mg3(PO4)2−y(SO4)y phosphor under different excitation bands. The PL excitation spectra shows a charge transfer band in the range of 220 nm to 550 nm, centered at 295 nm corresponded due to charge transition from oxygen 2p orbital (O2−) to vacant europium 4f orbital (Eu3+) ions. In addition, two sharp excitation band also observed at 396 nm and 466 nm due to 7F0 → 5L6, 7F2→ 5D2, transition of Eu3+ ions. Under these excitations (295 nm, 396 nm and 466 nm), PL emission spectrums are recorded, which shows orange and red color emission bands similar as Mg3(PO4)2:0.5mol%Eu3+ phosphor. However, with the substitution with molybdate, tungstate and sulfate ions, we have found that emission of colors is similar as Mg3(PO4)2:0.5mol%Eu3+ phosphor but their emission intensities are varying. We have observed that with doping of molybdate and tungstate ions, emission intensity increases under 295 nm and 466 nm. Whereas, with replacement of sulfate ions, it is found that emission intensity decreased compared to Mg3(PO4)2:0.5mol%Eu3+ phosphor. Therefore, we have suggested that Mg3(PO4)2−y(MoO4)y, Mg3(PO4)2−y(WO4)y phosphors are also suitable for future investigation of lighting application.