Photoluminescence properties, crystal structure and electronic structure of a Sr₂CaWO₆:Sm³⁺ red phosphor

Abstract

A novel Sm³⁺-doped Sr₂CaWO₆ (SCWO) red phosphor, synthesized though a solid-state reaction, was reported. Its crystal structure was analyzed and refined via the Rietveld full-pattern fitting method based on XRD patterns. The CASTEP module of Materials Studio was used to investigate the band structure and density of states of the SCWO. The optical band gap was calculated through the UV-vis diffuse reflectance spectrum and compared with the value predicted by the DFT method. Raman spectra were recorded to confirm the substitution of cations by Sm³⁺ ions. The broad W–O charge transfer band and narrow excitation band at 406 nm from Sm³⁺ ions have comparable intensities in SCWO:Sm³⁺. After the introduction of charge compensators Li⁺, Na⁺ or K⁺, the intensity of the near-UV excitation band at 406 nm was almost twice as much as before. The profiles of the emission spectra under excitation into the charge transfer absorption and f–f transition of Sm³⁺ are different, and low-symmetry Sm³⁺ centres are preferentially excited via f–f absorption transitions. The intensity ratios of electronic, magnetic dipole-allowed transitions under different radiations show that the charge compensators can influence the chemical environment around Sm³⁺.

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Introduction

Solid-state lighting devices based on white light-emitting diodes (LEDs) are now taking off as a potential industry, and phosphor-converted LEDs are regarded as a new lighting source for the next generation.^1–3 White LEDs are not just highly efficient, but are also energy-saving, compact, mercury-free and have long lifetimes.^4,5 The application of LEDs is now expanding from point light sources to general illumination, demanding a high-quality white light source.⁶ Phosphors play a key role in producing white light with satisfactory colour rendering index (CRI) as well as high efficiency. Currently, the most common commercial white LEDs employ a blue LED coated with YAG:Ce³⁺ yellow phosphors, because of their simplicity and cost-saving in fabrication.^7,8 However, these white LEDs have a low CRI because of their deficient emission in the red spectral region. In attempts to improve their performance, usually red phosphors have been incorporated.^9–11 Another approach to white light is combining near-UV LEDs with red/green/blue tri-color phosphors.^12–14 Therefore, it is urgent to search for stable and highly-efficient red phosphors to meet the requirements for white LEDs with better performance.

Recently, there has been interest in tungstate compounds as potential hosts for rare-earth-activated red phosphors, due to their good stability under various physical conditions.^15–17 The general formula for tungstates with double perovskite structure is A₂BWO₆. The B^II and W⁶⁺ ions in the three-dimensional network are ordered in such a way that every WO₆ octahedron has only B^IIO₆ octahedra as nearest neighbors, and the A cations are located in the spaces between them.^18,19 Cation site A has various coordination numbers from eight to twelve due to the tilt and rotation of B^IIO₆ and WO₆ octahedra along their crystallographic axes.²⁰ The luminescence properties of phosphors are closely related to the composition and local environment of the host lattice. Tungstate compounds can absorb ultraviolet (UV) light efficiently and give blue luminescence because they possess O–W charge transfer transitions.^21,22 The trivalent Sm³⁺ (4f⁵) ion is a common activator which has been used in many kinds of host compounds for red phosphors.^23–26 The Sm³⁺ ion has narrow excitation bands in the near-UV and blue region and emission bands in the reddish-orange light region, due to intrinsic 4f–4f transitions.^23,25

In this paper, SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) were synthesized by solid-state reactions. The crystal structure analysis and refinement of SCWO were carried out based on XRD patterns. Band structure and density of states calculations were performed using the CASTEP mode of Materials Studio, and the optical band gap was also estimated from the UV-vis diffuse reflection spectrum. Raman spectra were measured to confirm the sites substituted by Sm³⁺ ions. The photoluminescence properties, charge compensation and luminescence lifetimes of the phosphors were investigated and discussed in detail. The results indicated that SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) can absorb near-UV light efficiently and give red emission, and that it is a promising red phosphor for near-UV LEDs.

Experimental

Sample preparation

The samples SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) were prepared by a solid-state reaction technique. The starting materials were SrCO₃ (Aldrich, 99.9%), CaCO₃ (Junsei chemicals, 99.5%), WO₃ (Aldrich, 99.995%), Sm₂O₃ (Aladdin, 99.5%), Li₂CO₃ (Aldrich, 99.997%), Na₂CO₃ (Aldrich, 99.995%) and K₂CO₃ (Yakuri Pure Chemicals Co. LTD., Tokyo, Japan, 99.60%). The raw materials were weighed in stoichiometric proportions and then mixed in an agate mortar. After 30 minutes of grinding, the uniformly mixed materials were firstly pre-fired at 850 °C for 5 h, and then the powders were reground to improve their homogeneity. After that, the precursors were calcined at 1200 °C for 12 h.

Characterization and calculation

X-ray diffraction (XRD) measurements of the as-prepared samples were carried out using a Philips X’Pert/MPD diffraction system with Cu-Kα₁ irradiation (λ = 1.5406 Å), and high-resolution X-ray diffraction was recorded using a Bruker D8 Advance X-ray diffractometer over an angular (2θ) range of 10–125° with a 0.02° scanning step. UV-vis diffuse reflectance spectra (DRS) were obtained using a V-670 (JASCO) UV-vis spectrophotometer. Raman spectra were measured using a Raman/PL spectrometer (Horiba Jobin-Yvon, LabRAM HR). Photoluminescence excitation (PLE) and emission (PL) spectra were collected using a Photon Technology International (PTI) spectrofluorimeter with a 60 W Xe arc lamp, and lifetimes were measured using a phosphorimeter attached to the main system with a Xe flash lamp (25 W).

In the present band structure and density of state (DOS) calculations of SCWO, the plane wave pseudo-potential approach based on density functional theory (DFT) was used. The CASTEP program was employed to perform geometry optimization using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm. The convergence thresholds in geometry optimization cycles for the energy difference, the maximal ionic Hellmann–Feynman force, the maximum stress and the maximal displacement were set as 5.0 × 10⁻⁷ eV per atom, 0.01 eV Å⁻¹, 0.02 GPa and 5.0 × 10⁻⁴ Å, respectively. The generalized gradient approximation (GGA) by the Perdew–Burke–Ernzerhof (PBE) formulation was chosen. The cutoff energy was 480 eV, and Brillouin zone integration was represented using the K-point sampling scheme of a 5 × 6 × 6 Monkhorst–Pack grid. Ultrasoft pseudopotentials were used to approximate the core electrons.

Results and discussion

Crystal structure analysis and refinement

The phase purities of the Sm³⁺-doped SCWO and the as-prepared samples with Li⁺, Na⁺ and K⁺ as charge compensators were investigated, and their XRD patterns are given in Fig. 1. As indicated in Fig. 1, all the diffraction peaks of the samples can be indexed into the standard SCWO (JCPDS file no. 76-1983). It can be observed that the diffraction peaks of all samples shifted toward higher angles compared with the standard pattern. This can be attributed to the replacement of Sr²⁺ ions (CN = 12, 0.144 nm) with Sm³⁺ (CN = 12, 0.124 nm), Li⁺ (CN = 6, 0.076 nm), Na⁺ (CN = 12, 0.139 nm) or K⁺ (CN = 12, 0.164 nm) ions. The lattice parameter refinements of the samples were performed using MDI Jade 5.0 based on the given XRD patterns, and the corresponding results are shown in Table 1. The pure SCWO, which presents a double-perovskite structure, has an orthorhombic lattice with a space group of Pmm2 and lattice constants of a = 8.2033 Å, b = 5.7676 Å and c = 5.8489 Å, with Z = 2 and cell volume V = 276.73 ų. Compared with the pure SCWO crystals, the cell parameters and volumes of the doped samples were reduced. The lattice volume shrinkage with charge compensators is not consistent, considering the ionic sizes. The reason may be that the lattice parameter refinements, based on low-resolution XRD patterns using MDI Jade 5.0, were not very accurate, and can only be used to analyze qualitatively.

Fig. 1 The X-ray diffraction patterns of SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K).

Table 1 The refined lattice parameters of SCWO doped with Sm³⁺ and charge compensator Li⁺, Na⁺ or K⁺

Samples	a (Å)	b (Å)	c (Å)	Volume (Å^3)
SCWO	8.2033	5.7676	5.8489	276.73
SCWO:2%Sm^3+	8.15687	5.74478	5.82314	272.87
SCWO:2%Sm^3+, 2%Li^+	8.16213	5.7447	5.82431	273.10
SCWO:2%Sm^3+, 2%Na^+	8.15721	5.73899	5.82387	272.64
SCWO:2%Sm^3+, 2%K^+	8.16244	5.73817	5.81908	272.55

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Structure refinement was performed for the Sm³⁺-doped SCWO using the Le Bail and Rietveld methods using the software General Structure Analysis System (GSAS). The initial structural models were SCWO with space group symmetry Pmm2 (25) and SrWO₄ with space group I4₁/a (88), and the corresponding fitting patterns are shown in Fig. 2. The reliability factors of the refinement are R_wp = 12.15%, R_p = 8.53% and χ² = 5.643. The refined cell parameters of SCWO are a = 8.1963 Å, b = 5.7680 Å and c = 5.8507 Å, with Z = 2 and cell volume V = 276.60 ų, which are consistent with the shrinkage of the unit cell deduced from the low-resolution XRD patterns.

Fig. 2 Experimental (black circles) and calculated (red solid line) XRD patterns and their difference (blue solid line) for the fit to the data collected for SCWO:2%Sm³⁺, by the GSAS program. The short magenta and green vertical lines show the positions of Bragg reflections of the calculated patterns.

Electronic structure and optical band gap

Before the band structure and density of states calculations, geometry optimization was first performed. Fig. 3(a) shows the structure of the SCWO 2 × 2 × 2 super cell, and it can be seen that both Ca²⁺ and W⁶⁺ ions occupy six-fold octahedral sites. Every WO₆ octahedron has only six CaO₆ octahedra as nearest neighbors, and Sr²⁺ ions are located at the 12-coordination sites between them. In double-perovskite A₂B^IIB^VIO₆, the relative arrangement of B^IIO₆ and B^VIO₆ octahedra are governed largely by the tolerance factor, , where r_A and r_O are the ionic radii of A and oxygen, and r_B is the average ionic radius of B^II and B^VI.²⁷ Herein, r_A = 0.144 nm, r_O = 0.140 nm and r_B = (0.1 + 0.06)/2 = 0.08 nm, therefore τ = 0.913. In the ideal cubic structure for which τ = 1, the B^II–O–B^VI bridge is linear. Where τ < 1, it means that the radius of the A cation is relatively small, and thus the double-perovskite will reduce the size of the 12-coordination sites by cooperatively tilting the B^IIO₆ and B^VIO₆ octahedra or by bending the B^II–O–B^VI bridge.²⁸ The chemical bond lengths of Sr–O, Ca–O and W–O in SCWO before and after geometry optimization are given in Table 2. Fig. 3(d) indicates that there are two kinds of Sr²⁺ ions in SCWO, and both of them are surrounded by twelve oxygens. Combined with the information given in Table 2, it can be found that the Sr2–O bond lengths showed significant changes, especially the Sr2–O6 bond. The original lengths of the Sr2–O6 bond were 2.8707 Å and 2.9874 Å, and they became 2.6470 Å and 3.2132 Å, respectively, after geometry optimization, which is more similar to the coordination conditions of Sr1. In addition, it is easy to find that the CaO₆ and WO₆ octahedra have become distorted away from regular geometries.

Fig. 3 (a) The structure of the SCWO 2 × 2 × 2 super cell. (b) Ca–O and W–O bonding. (c) Sr–O bonding. (d) Coordination conditions of Sr1 and Sr2 in the SCWO 2 × 2 × 2 super cell.

Table 2 Comparison of chemical bond lengths (Å) of SCWO before and after geometry optimization

Chemical bond	Number of bonds	Bond length before optimization	Bond length after optimization	Chemical bond	Number of bonds	Bond length before optimization	Bond length after optimization
Sr1–O1	2	3.1324	3.099	W1–O1	2	1.8893	1.8869
Sr1–O2	2	2.8222	2.8148	W1–O2	2	1.8893	1.9384
Sr1–O3	2	2.848	2.8829	W1–O5	2	1.8868	1.913
Sr1–O4	2	2.848	2.842	W2–O3	2	1.8893	1.8869
Sr1–O5	1	2.6435	2.6518	W2–O4	2	1.8893	1.9384
Sr1–O5	1	3.2263	3.2085	W2–O6	2	1.8868	1.913
Sr1–O6	2	2.885	2.8856	Ca1–O1	2	2.2179	2.194
Sr2–O1	2	2.7604	2.8813	Ca1–O2	2	2.2179	2.1976
Sr2–O2	2	3.0835	2.8445	Ca1–O6	2	2.2341	2.2001
Sr2–O3	2	2.8772	3.0962	Ca2–O3	2	2.2179	2.1939
Sr2–O4	2	2.9407	2.8161	Ca2–O4	2	2.2179	2.1974
Sr2–O5	2	2.9097	2.8857	Ca2–O5	2	2.2341	2.1994
Sr2–O6	1	2.8707	2.6470
Sr2–O6	1	2.9874	3.2132

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To understand the electronic origin of the optical transitions and the chemical bonding properties, the band structure and density of states were investigated by the DFT method. The calculated band structure along the high-symmetry points of the first Brillouin zone for SCWO is shown in Fig. 4(a). It can be found that the lowest energy of the conduction bands (CBs) is located at the U point, and the highest energy of the valence band is at point G. The energy gap between the lowest conduction band point and the highest valence band point is 2.887 eV, which also indicates that SCWO is an indirect band gap insulator. Fig. 4(b) presents the total density of states (DOS) of SCWO, and the partial density of states (PDOS) is shown in Fig. 5. From Fig. 5, it can be found that the main contributor into the bottom of the conduction band of SCWO is the W 5d orbital, and that the O 2p orbital is the main contributor into the top of the valence band.

Fig. 4 Calculated band structure (a) and total densities of states (b) of SCWO near the Fermi energy level (E_F). The Fermi energy is the zero of the energy scale.

Fig. 5 Partial densities of states of SCWO and the atoms constituting SCWO: strontium, calcium, tungsten and oxygen.

Fig. 6 gives the UV-vis diffuse reflectance spectrum of SCWO, and the inset shows the optical band gap of SCWO calculated using the Kubelka–Munk function. The SCWO host lattice shows an energy absorption ability in the UV region, and the absorption edge is near 300 nm. The strong absorption that occurred in the UV region resulted from electron excitation from the valence to the conduction band in SCWO.

Fig. 6 Diffuse reflectance spectrum of SCWO. The inset shows the determination of the optical band gap of SCWO using the Kubelka–Munk function.

The optical band gap, E_g, can be calculated from the UV-vis DR spectrum using the following equation:²⁹

αhν ∝ (hν − E_g)ⁿ, (1)

where α is the absorbance, h is the Planck constant, ν is the photon energy, and n is determined by the transition type (n = 1/2, 2, 3/2 or 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect electronic transitions, respectively). In this case, SCWO has been confirmed to be an indirect band gap material and, therefore, n = 2:

(αhν)^1/2 ∝ hν − E_g. (2)

From a plot of (αhν)^1/2 versus hν, the band gap E_g is evaluated by extrapolating the straight line to (αhν)^1/2 = 0. The value of the optical band gap E_gap is determined to be 3.436 eV for the non-doped SCWO, which is larger than the calculated value by 0.549 eV. The inconsistency between the experimental band gap and the calculated one is generally accepted because DFT calculations usually underestimate the band gap energies of insulators and semiconductors.

Raman spectra

Raman spectra of SCWO:2%Sm³⁺, SCWO:2%Sm³⁺, 2%Li⁺, SCWO:2%Sm³⁺, 2%Na⁺ and SCWO:2%Sm³⁺, 2%K⁺ were measured to identity the cation site substitutions. The spectra in Fig. 7 display Raman shifts at about 100–200, 445 and 820 cm⁻¹. Previous work revealed that the peaks at 100–200, 445 and 820 cm⁻¹ can be assigned to the T_2g(1), T_2g(2) and A_1g modes, respectively.³⁰ According to group theory analysis of SCWO, the A_1g mode is related to the stretch vibration of the oxygen ion of the WO₆ and CaO₆ octahedra. The T_2g(1) mode is an A-site-cation-related vibration, and can be represented as the vibration between Sr and the WO₆ group. In the Raman spectra in Fig. 7, the T_2g(1) mode splits into four peaks, meaning that the symmetry of the Sr site is lowered. A comparison of partial Raman spectra profiles is given in Fig. 8. The four peaks at around 150 cm⁻¹ shift to larger wavenumber upon alkali cation incorporation; however, the peak at around 820 cm⁻¹ is almost unchanged, which implies that the Sm³⁺ ions are mostly incorporated into the Sr²⁺ site of the SCWO.

Fig. 7 Raman spectra of SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K).

Fig. 8 Raman spectra in the ranges of 100–200 cm⁻¹ and 350–1000 cm⁻¹ of (a) SCWO:2%Sm³⁺, (b) SCWO:2%Sm³⁺, 2%Li⁺, (c) SCWO:2%Sm³⁺, 2%Na⁺ and (d) SCWO:2%Sm³⁺, 2%K⁺.

Photoluminescence properties

Fig. 9 displays the photoluminescence excitation (PLE) spectra of the as-prepared Sm³⁺-doped SCWO phosphor and the samples with Li⁺, Na⁺ or K⁺ as charge compensators. The PLE spectra monitored at 646 nm consist of a broad band and some narrow bands; the former is referred to as the charge transfer band and the latter is from the Sm³⁺ ions. According to the band structure and density of states analysis above, the top of the valence band is dominated by the 2p orbital of the O atom, and the W 5d orbital is the main contributor to the bottom of the conduction band. Therefore, the broad excitation band is attributed to the charge transfer process of an oxygen 2p electron going into the empty W 5d orbital. The narrow bands, originating from the f–f transitions of Sm³⁺ ions, can be assigned to the electronic transitions from ⁶H_5/2 to ⁶H_9/2 (345 nm), ⁶H_7/2 (354 nm), ⁴D_3/2 (363 nm), ⁴P_7/2 (377 nm), ⁴L_11/2 (390 nm), ⁴L_13/2 (406 nm), ⁴P_5/2 (419 nm), ⁴G_9/2 (439 nm), ⁴F_5/2 (450 nm), ⁴I_13/2 (462 nm), ⁴I_11/2 (467, 471 nm), ⁴I_11/2 (479 nm) and ⁴I_9/2 (488 nm). It can be found that the PLE intensity of SCWO:Sm³⁺ is obviously enhanced after co-doping with Li⁺, Na⁺ or K⁺ ions. The excitation bands of Sm³⁺ ions in the SCWO:Sm³⁺, Na⁺ phosphor show the strongest intensity. Sm³⁺ ions are expected to occupy Sr²⁺ sites in SCWO:Sm³⁺, and it is difficult to maintain the charge balance. Therefore, the positive-charge defects produced, Sm_Sr⁺, will negatively affect the luminescence. However, when alkali metal ions, M⁺, occupy Sr²⁺ sites, a negative-charge defect, M_Sr⁻, will be obtained, which can counteract the impact of the decrease in luminescence caused by the Sm_Sr⁺ defects. Of the different alkali metal ions acting as the charge transfer compensators (Li⁺, Na⁺ or K⁺), the radius of Na⁺ is most similar to that of Sr²⁺ ions, and therefore the most intense luminescence happens in SCWO:Sm³⁺ phosphors with Na⁺ as a charge compensator.

Fig. 9 Excitation spectra of SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K), monitored at 646 nm.

Another interesting phenomenon observed in Fig. 9 is that the intensity of the excitation band at 406 nm almost doubled after charge compensators were introduced; however, that of the W–O charge transfer band was not enhanced significantly. In order to investigate the influence of charge compensators on intrinsic excitation of the host and f–f transitions from Sm³⁺, the emission spectra of SCWO:Sm³⁺, SCWO:Sm³⁺, Li⁺, SCWO:Sm³⁺, Na⁺ and SCWO:Sm³⁺, K⁺ exposed to 306 nm and 406 nm radiation are given in Fig. 10 and 11.

Fig. 10 Emission spectra of SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) excited at 306 nm.

Fig. 11 Emission spectra of SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) excited at 406 nm.

The emission spectra of the samples in Fig. 10 and 11 exhibited three emission bands, which are assigned to ⁴G_5/2 → ⁶H_5/2 (563, 567, 576 nm), ⁴G_5/2 → ⁶H_7/2 (600, 605, 615 nm) and ⁴G_5/2 → ⁶H_9/2 (646 nm) transitions. The major part of the ⁴G_5/2 → ⁶H_5/2 transition is magnetic-dipole-allowed, and the ⁴G_5/2 → ⁶H_9/2 electronic transition is purely electric dipole dominated. For the ⁴G_5/2 → ⁶H_7/2 transition, the magnetic dipole character is very low.³¹ The intensity ratio I(⁴G_5/2 → ⁶H_9/2)/I(⁴G_5/2 → ⁶H_5/2), denoted as R1, can be used to measure the departure from centrosymmetry of sites occupied by Sm³⁺ ions. The ratio I(⁴G_5/2 → ⁶H_7/2)/I(⁴G_5/2 → ⁶H_5/2), denoted as R2, can also be used to indicate the polarizability of the chemical environment around Sm³⁺. Based on the PL spectra data, R1 and R2 values were calculated and are listed in Table 3. It can be found that for SCWO:2%Sm³⁺, when the excitation wavelength is 406 nm, both R1 and R2 are increased significantly compared with the situation when the radiation wavelength is 306 nm. Because the ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_7/2 transitions are mainly electric-dipole-allowed ones, the bigger the R1 and R2 values of the emission, the more these are related to low-symmetry Sm³⁺ centres. Therefore it can be concluded that low-symmetry Sm³⁺ centres tend to be excited via f–f transitions.³² Both R1 and R2 in SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) increased obviously compared with the values for the SCWO:2%Sm³⁺ sample, when the samples were exposed to 306 nm light. In contrast, when the excitation light was 406 nm, R1 and R2 values were smaller in samples with charge compensators, although they were still larger than those obtained upon excitation by 306 nm light. That is to say, after the charge compensators were added, energy from the W–O charge transfer band preferred to transfer to Sm³⁺ luminescence centres with low symmetry.

Table 3 Intensity ratios R1 and R2 of SCWO doped with Sm³⁺ and charge compensator Li⁺, Na⁺ or K^+a

λex = 306 nm	R1	R2	λex = 406 nm	R1	R2
a R1 = I(^4G5/2 → ^6H9/2)/I(^4G5/2 → ^6H5/2), R2 = I(^4G5/2 → ^6H7/2)/I(^4G5/2 → ^6H5/2).
SCWO:2%Sm^3+	3.097	2.132	SCWO:2%Sm^3+	3.774	2.314
SCWO:2%Sm^3+, 2%Li^+	3.404	2.176	SCWO:2%Sm^3+, 2%Li^+	3.620	2.248
SCWO:2%Sm^3+, 2%Na^+	3.409	2.170	SCWO:2%Sm^3+, 2%Na^+	3.568	2.252
SCWO:2%Sm^3+, 2%K^+	3.290	2.146	SCWO:2%Sm^3+, 2%K^+	3.579	2.233

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Excited state dynamics of the ⁴G_5/2 level of Sm³⁺

Under excitation into the ⁶H_5/2 → ⁴L_13/2 transition of Sm³⁺ (λ_ex = 406 nm), the excited state dynamics of the ⁴G_5/2 level of Sm³⁺ were measured, ending on the ⁶H_J levels, for the SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) phosphors. The luminescence decay curves are depicted in Fig. 12; they can be well fitted with a first-order exponential decay model using eqn (3):

I(t) = I₀ + A exp(−t/τ), (3)

where I and I₀ are the emission intensities at time t and 0, A is a constant, t is time and τ is the decay time for an exponential component. On the basis of eqn (3) and the decay curves, lifetimes for SCWO:2%Sm³⁺ were determined to be 550.92, 550.81 and 531.44 μs for 563, 600 and 646 nm emissions, respectively (Fig. 12(a)). The interaction of the SCWO:Sm³⁺ phosphor with the excitation wavelength of 406 nm leads to the transition of Sm³⁺ ions from the ground level ⁶H_5/2 to the higher ⁴L_13/2 level. After that, Sm³⁺ ions usually make non-radiative transitions to the ⁴G_5/2 state through multiple channels of cross-relaxation processes, which are due to energy transfer from the excited ⁴G_5/2 state to the ground ⁶H_5/2 state via the ⁶F_11/2, ⁶F_9/2, ⁶F_7/2, and ⁶F_5/2 levels, such as (⁴G_5/2, ⁶F_11/2) → (⁶H_5/2, ⁶F_5/2). Therefore, possible cross-relaxation channels can influence the lifetime of the excited ⁴G_5/2 state. The decay curves for the SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) phosphors are also given in Fig. 12, and the lifetimes corresponding to the ⁴G_5/2 state of Sm³⁺ in SCWO:2%Sm³⁺, 2%Li⁺, SCWO:2%Sm³⁺, 2%Na⁺ and SCWO:2%Sm³⁺, 2%K⁺ are 716.49, 421.71 and 434.59 μs. Compared with SCWO:2%Sm³⁺, SCWO:2%Sm³⁺, 2%Na⁺ and SCWO:2%Sm³⁺, 2%K⁺, the longest lifetime value was observed in SCWO:2%Sm³⁺, 2%Li⁺. In SCWO:2%Sm³⁺, 2%Li⁺, due to the radius of Li⁺ being the smallest, the average Sr–Sr distance is the shortest and, therefore, the compensator Li_Sr⁻ can compensate Sm_Sr⁺ more effectively than can Na_Sr⁻ and K_Sr⁻.

Fig. 12 PL decay curves of SCWO:2%Sm³⁺ (a), SCWO:2%Sm³⁺, 2%Li⁺ (b), SCWO:2%Sm³⁺, 2%Na⁺ (c) and SCWO:2%Sm³⁺, 2%K⁺ (d) under 406 nm radiation.

CIE chromaticity coordinates and quantum efficiency

The CIE chromaticity diagram for the SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) samples is displayed in Fig. 13. As indicated in Fig. 13, the CIE coordinates for SCWO:2%Sm³⁺, SCWO:2%Sm³⁺, 2%Li⁺, SCWO:2%Sm³⁺, 2%Na⁺ and SCWO:2%Sm³⁺, 2%K⁺ are (0.5952, 0.4022), (0.6001, 0.3984), (0.5998, 0.3987) and (0.5992, 0.3993), respectively, which are close to those of commercial red Sr₂Si₅N₈:Eu²⁺ phosphors (0.62, 0.37). The quantum efficiency of the Sr₂CaWO₆:2%Sm³⁺, 2%Na⁺ was measured, because the emission intensity of Sr₂CaWO₆:2%Sm³⁺, 2%Na⁺ under 406 nm excitation was the most intense. Its external quantum efficiency is 5.017% and internal quantum efficiency is 21.066%. The quantum efficiency of our sample is lower compared with those of the state-of-the-art red phosphors, such as Y₂O₃:Eu³⁺. This is partly because the method that we used to synthesize the phosphors is high-temperature solid-state reaction, which usually leads to particle agglomeration and non-uniform size distribution. It is believed that the quantum efficiency can be further enhanced by controlling the particle size and morphology through optimizing the synthetic route.

Fig. 13 CIE chromaticity coordinates of (a) SCWO:2%Sm³⁺, (b) SCWO:2%Sm³⁺, 2%Li⁺, (c) SCWO:2%Sm³⁺, 2%Na⁺ and (d) SCWO:2%Sm³⁺, 2%K⁺.

Conclusions

In conclusion, SCWO:2%Sm³⁺ and SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K) phosphors were prepared via a solid-state reaction method. XRD patterns show that all the samples can be indexed to orthorhombic SCWO, according to the JCPDS file no. 76-1983. The results of crystal structure refinement by GSAS program confirmed the reduced lattice parameters of SCWO unit cell after incorporation of Sm³⁺. More reasonable chemical bond lengths were given after geometry optimization using the program Materials Studio. The calculation results from the CASTEP mode show that the main contributor into the bottom of the conduction band of SCWO is the W 5d orbital, and that the O 2p orbital is the main contributor into the top of the valence band. The band gap is calculated to be 2.887 eV, which is smaller than the optical band gap E_gap for the non-doped SCWO determined from UV-vis diffuse reflectance spectroscopy. The inconsistency between the calculated band gap and the experimental one is generally accepted because DFT calculations usually underestimate the band gaps of insulators and semiconductors. Sr²⁺ 12-coordination sites are confirmed to be substituted by Sm³⁺ ions through Raman spectra. Charge compensators can influence the local chemical environment of Sm³⁺, and near-UV excitation at about 406 nm is more intense than the charge transfer band in SCWO:2%Sm³⁺, 2%M⁺ (M = Li, Na and K). The emission color of SCWO:2%Sm³⁺, 2%Li⁺ (0.6001, 0.3984) is closest to that of the commercial red phosphor Sr₂Si₅N₈:Eu²⁺ (0.62, 0.37).

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2013012655). This work also supported by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. The Sm³⁺-doped SCWO with Li⁺, Na⁺ or K⁺ was supplied by the Display and Lighting Phosphor Bank at Pukyong National University.

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