DOI:
10.1039/C5RA25124A
(Paper)
RSC Adv., 2016,
6, 11211-11217
Synthesis, crystal structure and photoluminescence properties of Eu2+-activated RbCaGd(PO4)2 phosphors
Received
26th November 2015
, Accepted 11th January 2016
First published on 14th January 2016
Abstract
A series of novel double-phosphate compounds RbCaGd(PO4)2:Eu2+ were first synthesized by the Pechini-type sol–gel method, and their structure and luminescence properties were characterized in detail. The compound RbCa0.97Gd(PO4)2:0.03Eu2+ crystallizes in the hexagonal system with space group P6222 and the parameters a = 7.006(49) Å, c = 6.358(80) Å, and V = 269.969 Å3, Z = 1. Its crystal structure is similar to that of hexagonal LnPO4, where the Rb atom occupies a large tunnel in the lattice and the Ln position is statistically occupied by both Gd and Ca (Eu) atoms. In addition, RbCaGd(PO4)2:Eu2+ produces an intense broadband emission from 400 to 600 nm with the CIE coordinates near (0.2089, 0.2674) and assembles a broad excitation spectrum from 250 to 425 nm. The concentration quenching, crystal field splitting, critical energy transfers distance and thermally stable luminescence properties of RbCaGd(PO4)2:Eu2+ phosphors are all discussed, which indicate that this material can be considered as a promising bluish green phosphor for application in white-light UV LEDs.
1. Introduction
Rare-earth doped inorganic phosphors based on their 4f–4f or 5d–4f transitions have attracted much attention in displays, optoelectronic devices and white-light-emitting diodes (w-LEDs).1–3 These materials can produce fluorescent radiation efficiently from ultraviolet (UV) to visible light region with good luminescence yield, which makes them a potential application prospect in solid state lighting region, especially the w-LEDs.4 Eu2+ is a kind of important activator ions for its broad-band emission in white-light-emitting phosphors originating from the 4f65d–4f7 transition, and it can show a wide range of emissions from blue to red color dependent on the structural characteristics of host crystal.5–8 Among the various host crystals, the double-phosphate compounds cause extensive concern in many research groups.
Double phosphates, MNR(PO4)2, where M and N are alkali metal and alkaline earth metal respectively, and R is rare earth element, are isotypic with the corresponding orthophosphates LaPO4 which are well-known to exist in three structures viz., hexagonal, monoclinic and tetragonal phases.9 These phases have several potential applications by doping more different activators for fabrication new compounds because of their environmental friendliness, thermal and chemical stability, and easy synthesis. Hence, lots of studies with double phosphate as host have been performed.10–14 For example, Et-Tabirou et al.15 reported the structure of ACaLn(PO4)2 (A = Cs, Rb and K) and confirmed that it was hexagonal LaPO4 type in 1980. Vlasse et al.16 determined the structure of KCaNd(PO4)2 which was similar to that of hexagonal LnPO4 where the Nd and Ca atoms occupied the Ln position and K atom occupied a large tunnel. Shaolong Tie et al.17 first reported the optical properties of Ln3+ ions (Ln = Gd, Eu and Dy) doped hexagonal KCaR(PO4)2 (R = Gd, Y) system with no striking concentration quenching for Eu3+ and Gd3+ activators. Yen-Chi Chen et al.18 reported the KCaGd(PO4)2:Eu3+ phosphor as a down-converting phosphor to improve the power conversion efficiency of single crystalline silicon-based photovoltaic cells. Dongling Geng et al.19 investigated the optical properties of green/green-yellow-emitting KSrGd(PO4)2:Ce3+, Tb3+/Mn2+ phosphors with high quantum efficiency. Kang Geng et al.20 presented a novel blue-emitting double-phosphate phosphor Cs0.72Ca0.72Gd1.28(PO4)2:Eu2+ which could be excited by UV light.
To the best of our knowledge, there is no report on the crystal structure and fluorescence properties of the novel double-phosphate compound RbCaGd(PO4)2. RbCaGd(PO4)2 crystallizes in the hexagonal system with space group P6222 under ambient conditions. Therefore, Eu2+-doped single-phase RbCaGd(PO4)2 are prepared by the Pechini-type sol–gel technique in this work. Crystal structure, crystallographic occupancy of Eu2+ in RbCaGd(PO4)2 matrix and their luminescence properties are investigated systematically, which indicates that the novel RbCaGd(PO4)2:Eu2+ phosphors can act as potential phosphors for application in w-LEDs.
2. Experimental
2.1 Materials and synthesis
The RbCaGd(PO4)2:Eu2+ samples were prepared by the Pechini-type sol–gel technique using the following starting materials: Ca(NO3)2·4H2O (A.R.), Rb2CO3 (A.R.), Gd2O3 (99.99%), Eu2O3 (99.99%), (NH4)2HPO4 (A.R.) and C6H8O7 (A.R.) from Beijing Haidian chemical reagent company store. Stoichiometric amounts of Gd2O3 and Eu2O3 were respectively dissolved in HNO3 to obtain Gd(NO3)3 and Eu(NO3)3 solutions. The detailed synthesis process for RbCa1−xGd(PO4)2:xEu2+ where x indicates the content of Eu2+ was as follows. Ca(NO3)2·4H2O and Rb2CO3 were dissolved in distilled water under stirring and the predetermined amounts of Gd(NO3)3, Eu(NO3)3, (NH4)2HPO4 and the citric acid (citric acid/metal ion = 2
:
1 in mole) were added successively into the above solution and stirred at 80 °C for 30 min until homogeneous gels formed. After drying at 120 °C for 12 h in an oven, the precursor was ground, heated to 500 °C for 10 h in air, ground again, and calcinated at 1100 °C in air for 5 h in CO reducing atmosphere.
2.2 Measurements and characterization
The phase identification of the as-prepared samples was carried out on the SHIMADZU model XRD-6000 diffractometer at 40 kV and 30 mA and using monochromatic Cu Kα (λ = 0.15406 nm) radiation. The continuous scanning rate (2θ ranging from 10° to 70°) used for phase identification was 4° (2θ) min−1 and the data for the Rietveld analysis were collected in a stepping mode with a step size of 0.02° and 8 s per step over the 2θ range from 10° to 120°. The diffraction patterns were analyzed by the General Structure Analysis System (GSAS). The Fourier transform infrared (FT-IR) measurements were performed on a Spectrum One Paragon 1000 PC spectrometer over the 1500–400 cm−1 region using KBr pellet as background. The room-temperature excitation and emission spectra were recorded on a HITACHI F-4600 fluorescence spectrophotometer with a photomultiplier at 400 V and 150 W Xe lamp as the excitation lamp. And the temperature-dependent luminescence properties were performed by the same spectrophotometer, which was equipped with a self-made heating apparatus and a computer controlled electric furnace. The luminescence decay curve was obtained from a lifetime and steady state spectrometer (Edinburgh Instruments Ltd, FLS920) with a 335 nm pulse laser radiation (nano-LED) as the excitation source. The quantum efficiency was measured on an absolute photoluminescence quantum yield measurement system (Nanolog® FluoroLog-3-2-iHR320, Horiba Jobin Yvon) with F-3018 integral sphere.
3. Results and discussion
3.1 Phase analysis
The XRD patterns of the synthesized RbCaGd(PO4)2:Eu2+ phosphors and the Joint Committee on Powder Diffraction Standards (JCPDS) reference card of RbCaGd(PO4)2 are depicted in Fig. 1(a). The diffraction patterns of the synthesized samples can be indexed to the standard card of RbCaGd(PO4)2 (JCPDS, no. 34-0091), which is a hexagonal phase with the space group of P6222. No other peaks are observed indicating complete dissolution of the dopants in the RbCaGd(PO4)2 host and no significant changes in the crystal structure.
 |
| Fig. 1 XRD patterns of the as-prepared RbCaGd(PO4)2:Eu2+ phosphors (a), Rietveld structure analysis (b) of the selected RbCaGd(PO4)2:0.03Eu2+ phosphor and crystal structure (c) of RbCaGd(PO4)2. | |
For the first time, the crystal structure of RbCaGd(PO4)2 was determined. The crystal structure of double phosphate of RbCa0.97Gd(PO4)2:0.03Eu2+ host lattice is refined using the computer software GSAS. The Rietveld structure of the RbCa0.97Gd(PO4)2:0.03Eu2+ and the single crystal data of the ICSD-35098-KCaNd(PO4)2 phase model are shown in Fig. 1(b). No impurity phase is detected and the fractional atomic coordinates, occupancy, and isotropic thermal parameters of RbCa0.97Gd(PO4)2:0.03Eu2+ are presented in Table 1. Rb+ and Gd3+ are designed to occupy the sites of K+ and Nd3+, respectively, due to the similar ion radius and equal valence. Table 1 also lists the reflection conditions Rwp (%) = 8.62, Rp (%) = 6.34 and χ2 = 2.01. The parameters suggest that pure RbCaGd(PO4)2:0.03Eu2+ is synthetized and indexed to the hexagonal crystal system and space group P6222, a = 7.006(49) Å, c = 6.358(80) Å, and V = 269.969 Å3. The schematic illustration of the crystal structure of RbCaGd(PO4)2 viewed from the c axis and the coordination environment around Rb+ and Ca2+/Gd3+ are shown in Fig. 1(c). Rb+ is coordinated by 8O located along the c axis whereas Ca2+/Gd3+ is coordinated to 8O with the distorted D2 point symmetry forming the chains of –(Ca2+/Gd3+)–(PO4)–(Ca2+/Gd3+)– running along the c-axis which forms the edge sharing [Ca/GdO8] polyhedra chains interconnected by corner sharing. These interconnected chains leave large open channels, which is occupied by Rb atom with oxygen lined tunnels parallel to the c axis.
Table 1 Fractional atomic coordinates, occupancy, and isotropic thermal parameters of RbCaGd(PO4)2 obtained by GSAS Rietveld refinement using powder X-ray diffraction data at room temperaturea
Atom |
x |
y |
z |
Occupancy |
Uiso/Å2 |
Multiplicity |
Space group: P6222 – hexagonal; a = 7.006(49) Å, c = 6.358(80) Å, V = 269.969 Å3; α = 90°, γ = 120°; 2θ-interval = 10–120°; Rwp (%) = 8.62, Rp (%) = 6.34, χ2 = 2.01. |
Rb |
0 |
0 |
1/2 |
0.5 |
0.071(91) |
3c |
Ca |
1/2 |
0 |
0 |
0.483(12) |
0.007(97) |
3c |
Eu |
1/2 |
0 |
0 |
0.016(63) |
0.007(97) |
3b |
Gd |
1/2 |
0 |
0 |
0.5 |
0.007(97) |
3c |
P |
1/2 |
0 |
1/2 |
1.0 |
0.011(79) |
3d |
O |
0.4348(45) |
0.1356(84) |
0.3529(56) |
1.0 |
0.028 (01) |
12k |
The FT-IR spectra of the double phosphates are shown in Fig. 2(a). Due to small amounts of the dopant ions, the spectra of the doped and undoped samples are similar, therefore only one representative spectrum for RbCaGd(PO4)2:Eu2+ is shown in this figure. The singlet strong absorption band centered at 1074 cm−1 attributes to the asymmetric stretching of PO4 groups, while the symmetric stretching of the same groups manifests at the 1001 cm−1. Two bands centered at 609 and 503 cm−1 are due to the asymmetric deformation modes of PO4 groups, and a single band centered at 414 cm−1 is due to symmetric deformation modes of PO4 groups. The FT-IR results show that all vibrations originate from the anion groups of PO4 units, and one of the characteristic feature of the phosphates is the low number of absorption bands. Some similar results have been observed in the other types of hexagonal phosphates.21,22
 |
| Fig. 2 The FT-IR spectrum (a), PL and PLE spectra (b) of the RbCaGd(PO4)2:0.03Eu2+ phosphor. | |
3.2 Photoluminescence properties of the RbCaGd(PO4)2:Eu2+ phospors
The PLE and PL spectra of the RbCaGd(PO4)2:0.03Eu2+ are displayed in Fig. 2(b). The PLE spectrum monitored at 476 nm exhibits two obvious bands from 250 to 425 nm, which originate from the electron transition from the ground state 4f7 to the crystal-field split 4f65d levels of Eu2+ in the host lattice. Upon excitation at 334 nm, all of the emission spectra of RbCaGd(PO4)2:xEu2+ show a similar board band from 435 to 600 nm peaking at 476 nm attributable to the transitions between the 5d excited state and 4f ground state. In order to further understand the origin of the emission centered at 476 nm, the experiential equation of Van Uitert is adopted to fit the emission peak and excitation edge data23 and the crystallographic site substituted by Eu2+ in RbCaGd(PO4)2 is investigated theoretically: |
 | (1) |
where E is the energy of the d-band edge for rare earth ions (cm−1) equivalent to the wavelength of 470 nm, V is the valence of the activator (Eu2+) with V = 2, Q represents the energy of the lower d-band edge of the free ions (34
000 cm−1 for Eu2+), n is the coordination number of the “active” ion (Eu2+), r is the effective radius of the host replaced by the “active” ion, Ea is the constant electron affinity of the atoms that form anions in the same host. Here, Ea is 2.5 eV as reported from other phosphate systems and as discussed above, Eu2+ occupies the Ca2+ site in the RbCaGd(PO4)2 host and the effective ionic radii of Ca2+ is 1.12 Å. The calculated n is 7.9 which means that Eu2+ has eight coordination elements.
The PL spectra of the RbCaGd(PO4)2:xEu2+ are displayed in Fig. 3(a). Excited by 334 nm irradiation, all of the emission spectra of RbCaGd(PO4)2:xEu2+ show a similar spectral profiles due to the 4f–5d transition of Eu2+. But the peak red-shifts as the concentration of Eu2+ increases gradually. It originates from the change in the crystal field strength as shown in the following:24
|
 | (2) |
where Dq is the crystal field strength,
e is the charge of an electron,
r is the radius of the central ions,
d is the wave function,
z is the charge or valence of the anion, and
R is the bond distance between the central ion and ligands. Based on this equation, a shorter bond length increases the crystal field strength when the crystal environment is homologous. When Ca
2+ is substituted by the larger Eu
2+, the distance between Eu
2+ and O
2− becomes shorter and the crystal field strength increases. As a result, the emission wavelength red-shifts with increasing Eu
2+ concentrations.
 |
| Fig. 3 PL spectra (a) and relationship (b) of log(x) vs. log(I/x) for the RbCaGd(PO4)2:xEu2+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) phosphors, emission spectra (c) of RbCaGd(PO4)2:0.03Eu2+ excitation under 334 nm at different temperature and (d) activation energy of thermal quenching. | |
The concentration dependence of the emission intensity of the phosphors on Eu2+ is demonstrated in the inset of Fig. 3(a). The PL emission intensity increases, reaches a maximum at the Eu2+ concentration of 0.03, and decreases afterwards due to concentration quenching. Concentration quenching is due to the energy transfer from one activator to another. So it is necessary to calculate the critical transfer distance (Rc) of the activator and quenching site. The optimum Eu2+ concentration can be used to determine concentration quenching caused by energy transfer mechanisms such as exchange interaction, radiation reabsorption, or multipole–multipole interaction and Rc can be estimated with geometrical consideration according to the following formula:25
|
 | (3) |
where
N is the number of host ions in the unit cell,
V is the unit cell volume of the unit cell, and
xc is the atom fraction of the activator at which quenching occurs. In the RbCaGd(PO
4)
2 host, the crystallographic parameters are
V = 269.980 Å
3,
N = 1, and
xc = 0.03. According to
eqn (3), the critical distance
Rc is determined to be 28.81 Å indicating that the nonradiative concentration quenching takes place among two nearest Eu
2+ via electric multipolar interactions based on the Dexter theory. In order to further investigate the energy transfer mechanism, the interaction type between the sensitizers or between the sensitizer and activator is confirmed by
eqn (4):
26 |
I/x = k[1 + β(x)θ/3]−1
| (4) |
In this equation, I is the emission intensity, x is the activator concentration which is not less than the critical concentration, k and β are constants under the same excitation conditions for a given host crystal, and θ = 6, 8, 10 for the dipole–dipole (d–d), dipole–quadrupole (d–q), or quadrupole–quadrupole (q–q) interactions, respectively. To obtain the correct θ, the dependence between log(I/x) and log(x) based on eqn (4) is plotted in Fig. 3(b). The slope of the straight line is determined to be −1.78 and θ is calculated to be 5.34 that is close to 6, indicating that the dipole–dipole interaction is the major concentration quenching mechanism in RbCaGd(PO4)2:Eu2+.
The temperature dependence of luminescence from the RbCaGd(PO4)2:0.03Eu2+ phosphor under 334 nm excitation is presented in Fig. 3(c). As the temperature goes from 30 °C up to 150 °C, the PL intensity diminishes to 37% of the initial value. In general, the configurational coordinate diagram is used to explain thermal luminescence quenching as the excited luminescence center is thermally activated through the crossing point between the ground state and the excited state. This nonradiative transition depends strongly on the temperature resulting in reduction in the emission intensity. When the temperature is increased, the intensity decreases and the full-width at half-maximum (FWHM) increases as described by the Boltzmann distribution function as follows:27
|
 | (5) |
|
 | (6) |
In the above equations, hw is the energy of the lattice vibration that interacts with the electronic transition, W0 is the FWHM at 0 K, k is Boltzmann's constant, and S is the Huang–Rhys–Pekar parameter showing the strength of the electron–lattice coupling. It is postulated that hw is the energy of the vibrational levels of the 4f7 ground state or 4f65d excited state of Eu2+. As the temperature is increased, the electron–phonon interaction becomes dominant and a higher temperature increases the FWHM of the emission line. The activation energy (E) of thermal quenching can be estimated by the Arrhenius equation:28
|
 | (7) |
where
I0 is the initial PL intensity of the phosphor,
I(
T) is the intensity at the given temperature,
c is a constant for a certain host,
k is Boltzmann's constant (8.62 × 10
−5 eV), and
E is the activation energy of thermal quenching.
Fig. 3(d) plots the relationship of ln(
I0/
IT) − 1
vs. 1/
kT, where the straight slope is equal to −
E and the activation energy
E of RbCaGd(PO
4)
2:0.03Eu
2+ is 0.3117 eV. A red shift behavior is observed as the temperature goes up and it can be explained by the semi-empirical Varshni equation:
29 |
 | (8) |
where
E0 is the energy difference between the excited and ground states at 0 K,
E(
T) is the energy difference at temperature
T, and
a and
b are fitting parameters. The covalent bond between the luminescent center and ligand ions becomes stronger and the interaction between electrons is reduced due to the expansion of the electron clouds, resulting in the decreased crystal field as the temperature is increased. Moreover, the symmetry of the luminescent center is distorted so that the Jahn–Teller effect is dominant. Splitting of the degenerate excited state or ground state causes these two effects which drive the emission peak shift to lower energies.
Fig. 4 presents the decay curves of the Eu2+ luminescence in RbCaGd(PO4)2:Eu2+ with different Eu contents (0.1–0.10 mol%) upon excitation at 335 nm. In this work, the decay time of all the samples are characterized by average lifetime (τ*), which can be calculated based on the following expression:30
|
 | (9) |
where
I(
t) is the luminescence intensity at a time
t. According to the equation, the effective lifetime
τ* is determined to be 0.978, 0.927, 0.884, 0.845 and 0.767 μs for RbCaGd(PO
4)
2:
xEu
2+ with
x = 0.01, 0.03, 0.05, 0.08 and 0.10, respectively. Those results are reasonable for the allowed 5d–4f transition of Eu
2+ ions, which is known occurring in less than a microsecond. The lifetime for the Eu
2+ ions decrease monotonically with increasing Eu
2+ concentration due to the increase of the nonradiative and self-absorption rate of the internal doped ions when activators cross the critical separation between activator ion and quenching site.
 |
| Fig. 4 Decay curves of Eu2+ in RbCaGd(PO4)2:xEu2+ phosphors (excited at 335 nm, monitored at 467 nm). | |
The x and y values of CIE chromaticity coordinates for RbCaGd(PO4)2:xEu2+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) were calculated and presented in Table 2. The results obtained is shown in CIE chromaticity diagram of Fig. 5. Besides, the images of phosphors excited by 365 nm are also given in the inset of Fig. 5. The emission light can be clearly observed by the naked eye. The chromaticity coordinates for RbCaGd(PO4)2:xEu2+ could be tuned from greenish blue (0.2169, 0.2543) to bluish green (0.2639, 0.3097) position by changing the Eu2+ concentration. Additionally, the measured internal quantum efficiency (QE) of RbCaGd(PO4)2:0.03Eu2+ phosphor is determined as 13.32% under 334 nm excitation which is not high enough. But the QE can be improved by adjustment the particle size, morphology, size distribution and crystalline defects through optimization of the processing conditions.
Table 2 CIE chromaticity coordinates of RbCaGd(PO4)2:xEu2+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) phosphors
x(Eu2+) |
(x, y) |
0.01 |
(0.2169, 0.2543) |
0.03 |
(0.2089, 0.2674) |
0.05 |
(0.2172, 0.2790) |
0.08 |
(0.2423, 0.2951) |
0.10 |
(0.2639, 0.3097) |
 |
| Fig. 5 CIE chromaticity diagram and the digital photograph for RbCaGd(PO4)2:xEu2+ (x = 0.01, 0.03, 0.05, 0.08 and 0.10) excited by 365 nm. | |
4. Conclusion
In summary, a novel blue-emitting phosphors composed of RbCaGd(PO4)2:Eu2+ are first reported. The crystal structure of RbCaGd(PO4)2:0.03Eu2+ matrix is discussed based on Rietveld refinements results. The series of phosphors show a board band at 476 nm due to the 4f–5d transition. The calculated Rc for the energy transfer from one Eu2+ to another Eu2+ in the phosphor, which is electrostatic multipolar interaction, is 28.81 Å. Concentration quenching resulting from the dipole–dipole interaction is observed. With increasing of the Eu2+ concentration, the CIE coordinates slightly shift from greenish blue (0.2169, 0.2543) to bluish green (0.2639, 0.3097) position. These results suggest that the RbCaGd(PO4)2:Eu2+ exhibits a potential application in the solid-state lighting field.
Acknowledgements
The work was jointly supported by the Fundamental Research Funds for the Central Universities (2-9-2013-49) and City University of Hong Kong Strategic Research Grant 7004188 and Guangdong – Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ.
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