Preferential Neighboring Substitution-Triggered Full Visible Spectrum

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 33322−33334

Preferential Neighboring Substitution-Triggered Full Visible Spectrum Emission in Single-Phased Ca10.5−xMgx(PO4)7:Eu2+ Phosphors for High Color-Rendering White LEDs Zhihua Leng,† Renfu Li,‡ Liping Li,*,† Dingke Xue,§ Dan Zhang,† Guangshe Li,*,† Xueyuan Chen,‡ and Yu Zhang§

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†

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130026, P. R. China ‡ CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China § State Key Laboratory on Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun, Jilin 130012, P. R. China S Supporting Information *

ABSTRACT: Manipulating the distribution of rare-earth activators in multiple cation lattices can achieve versatile color output for single-phased phosphor-converted white light-emitting diodes (LEDs). However, successful cases are barely reported, owing to the uncertain distribution of rareearth activators and the special combination of three primary colors for white LEDs. Herein, we took whitlockite βCa3(PO4)2 as a multiple cation lattice host to manipulate the redistribution of Eu2+ activators, and the surprising Mg2+guided redistribution of Eu2+ activators among different Ca sites is reported for the first time to regulate the photoluminescence (PL) behavior in series Ca10.5−xMgx(PO4)7:Eu2+ phosphors. The preferential neighboring substitution of smaller Mg2+ cations in Ca(5) and Ca(4) sites triggers a discontinuous evolution of local structure along c axis and induces covalent variable Ca(1), Ca(2), and Ca(3) cation sites for the accommodation of Eu2+ activators. The unique optical feature enables the single-phased Ca9.75Mg0.75(PO4)7:Eu2+ phosphor-converted white LED to exhibit quite high color-rendering index Ra (85) and R9 (91) values. The preferential neighboring-cation substitution reported here can not only manipulate the migration of Eu2+ activators among different cation sites for tunable PL properties, but also carve out a new way for next-generation high-quality solid-state lighting. KEYWORDS: single-phased phosphor, photoluminescence, preferential neighboring substitution, crystal field splitting, centroid shift, white LEDs achieve high color-rendering index in single-phased Eu2+/Ce3+doped phosphors.9−14 However, successful cases are barely reported. It is urgent to explore new strategies for singlephased phosphors with full visible spectrum emission, which may overcome the above disadvantages to further enhance the performance of high color-rendering white LEDs. Generally, host compounds can remarkably influence the luminescence behavior of activators, especially for Eu2+/Ce3+ activators. Recently, the whitlockite β-Ca3(PO4)2-type hosts have attracted growing interest because they can provide versatile local environment and cation sites for the accommodation of Eu2+ activators.15−18 Many isostructural phosphors with whitlockite structure have been synthesized by

1. INTRODUCTION The white light-emitting diodes (LEDs) realized by depositing the Y3Al5O12:Ce3+ (YAG:Ce) yellow phosphor on (In,Ga)Nbased blue LED chip suffer from lower color-rendering index (CRI, Ra ≈ 70−80) and higher correlated color temperature (CCT ≈ 7750 K) due to the serious scarcity of red emission.1−6 Discovering new phosphors with sufficient red emission for high color-rendering white LEDs is still a severe challenge in the field of materials science. Many methods, such as multiphase combination of different phosphors and energy transfer from sensitizer to activator,7,8 have been explored for high color-rendering index. To some extent, these methods could supplement the red color, but inevitable energy reabsorption among different phosphors and energy loss of the sensitizer still remained. Due to the environment-sensitive 4f−5d transitions of Eu2+/Ce3+ activators, deliberately cationic/anionic substitution could be an effective strategy to © 2018 American Chemical Society

Received: July 15, 2018 Accepted: September 10, 2018 Published: September 10, 2018 33322

DOI: 10.1021/acsami.8b11879 ACS Appl. Mater. Interfaces 2018, 10, 33322−33334

Research Article

ACS Applied Materials & Interfaces

diffractometer with graphite monochromatic Cu Kα radiation (λ = 0.15418 nm). The high-quality powder X-ray diffraction (XRD) data for the Rietveld analysis were collected in a step-scanning mode with a scanning speed of 10 s counting time per step and a step width of 0.02° over a 2θ range of 10−120°. Variable-temperature XRD patterns were measured on a Rigaku D/MAX2550 diffractometer with a scanning rate of 2°/min. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken with Tecnai G2 electron microscope. The roomtemperature and low-temperature PL spectra were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with an R928 photomultiplier tube as the detector and a 450 W xenon lamp as the excitation source. The high-resolution emission spectrum was collected with the best instrumental wavelength resolution (0.05 nm; the monochromator slits were set as small as possible to maximize the instrumental resolution). The decay time curves were measured on the same spectrophotometer and detectors equipped with a xenon lamp μF920H or a 100 W pulsed hydrogen lamp nF900 as the excitation source. The quantum efficiencies (QEs) were measured using the integrating sphere on the same spectrofluorimeter. The heat capacity was recorded on a commercial quantum design physical property measurement system. The series of phosphor-converted light-emitting diode (pc-LED) devices were fabricated by depositing the as-obtained Ca10.5−xMgx(PO4)7:Eu2+ phosphors on 365 nm UV LED chips. The electroluminescence (EL) spectral performance of the fabricated LED devices was recorded on an Ocean Optics QE65000 Portable Light Meter.

univalent/bivalent/trivalent cationic substitution, such as Ca 1 0 M(PO 4 ) 7 :Eu 2 + (M = Li, Na, and K), 1 6 , 1 9 , 2 0 Ca3−xMx(PO4)2:Eu2+ (M = Sr and Ba),21,22 Ca10M(PO4)7:Eu3+ (M = Al, Bi, and rare earth),23,24 and Ca8MgLn(PO4)7:Eu3+ (Ln = Y, Gd, and La).25 These whitlockite-type hosts have five different crystallographic cation sites, which are named as M(1), M(2), M(3), M(4), and M(5) sites. Actually, it has been widely accepted that the M(5) site is not suitable for accommodating lager Eu2+ ions due to the shortest average M(5)−O bond length among the five cation sites.15−17,21 However, there is no consensus conclusion whether the lager Eu2+ ions can enter into the M(4) site or not. Some researchers claimed that the larger Eu2+ ions could distribute in M(1), M(2), M(3), and M(4) sites.15−17,26 For the cases of xSr 2 Ca(PO 4 ) 2 −(1−x)Ca 10 Li(PO 4 ) 7 :Eu 2+ 15 and Ca 10 M(PO4)7:Eu2+ (M = Li, Na, and K)16 phosphors, Xia et al. claimed that the emission band at shorter wavelength (around 414 nm) originated from the distribution of Eu2+ ions in M(1), M(2), and M(3) sites and the emission band at longer wavelength is attributed to the accommodation of Eu2+ ions in the M(4) site. Comparatively, Zhou et al.17 have an opposite standpoint, i.e., the Eu2+ ions occupying the M(4) site give rise to the emission band at shorter wavelength (418 nm). However, on the other hand, other researchers declare that the larger Eu2+ ions only enter the M(1), M(2), and M(3) sites.27−29 Fundamentally and systematically investigate is greatly necessary to reveal the relationship between the observed emission peaks and the distribution of Eu2+ activators correctly and widen the applications of these kinds of phosphors. In previous reports, Mg-substituted β-Ca3(PO4)2:Eu2+ phosphor is rarely investigated because it is widely thought that smaller Mg2+ is not suitable for the accommodation of Eu2+.22 Differing from previous reports, for the first time, smaller Mg2+ ions were preferentially substituted in a series of whitlockite-type Ca10.5−xMgx(PO4)7:Eu2+ phosphors to manipulate the redistribution of Eu2+ activators among different Ca sites for the tunable photoluminescence (PL) behaviors. The preferential neighboring-cation substitution strategy reported here can not only carve out a new way for high-quality solidstate lighting via manipulating the migration of Eu2+ activators among different cation sites, but also inevitably exert widespread and far-reaching impact on d band electronic structure-dependent fields.

3. RESULTS AND DISCUSSION 3.1. Preferential Substitution and Local Structures. Previous studies have disclosed the crystal structure of the whitlockite β-Ca3(PO4)2 (R3c space group, Z = 21). In this structure, there are five independent crystallographic Ca sites, which are named as Ca(1), Ca(2), Ca(3), Ca(4), and Ca(5) sites.21,22 Ca(1), Ca(2), Ca(3), and Ca(5) sites are fully occupied by Ca ions, whereas Ca(4) site is randomly halfoccupied by Ca ions and half-occupied by cation vacancies. Considering that the Ca−O bond length is usually less than 2.8 Å, Ca(1), Ca(2), Ca(3), Ca(4), and Ca(5) atoms are strongly coordinated to seven, seven, eight, three, and six O atoms, respectively. The numbers of Ca(1), Ca(2), Ca(3), Ca(4), and Ca(5) atoms are 18, 18, 18, 3, and 6 in a unit cell, respectively.30 Herein, the crystallochemical formula of whitlockite β-Ca3(PO4)2 (Z = 21) can be also written as Ca(1) 3 Ca(2) 3 Ca(3) 3Ca(4) 0.5 Ca(5) 1(PO 4 ) 7 (Z = 6) or Ca10.5(PO4)7 (Z = 6). To concisely express the formula of Ca10.5(PO4)7, Z = 6 is used in this paper. The XRD patterns of Ca10.5−xMgx(PO4)7:Eu2+ (0 ≤ x ≤ 1.5) samples are shown in Figure S1. All of them can be well indexed to the whitlockite βCa3(PO4)2 (JCPDS card No. 09-0169), with no diffraction peaks of any impurity phases. The microstructure analysis of the representative Ca9Mg1.5(PO4)7:Eu2+ sample is further characterized by HRTEM, as displayed in Figure S2. The continuous lattice fringes with interplanar spacing of 0.644 nm agree well with the corresponding (104) planes of whitlockite β-Ca3(PO4)2. However, Mg cannot fully replace Ca to occupy Ca(1)−Ca(5) sites due to the smaller ionic radii of Mg2+. As illustrated in Figure S3, when the Mg concentration exceeds 1.5, a second crystalline phase Ca3Mg3(PO4)4 (JCPDS card No. 11-0234) appears, indicating that the solid solution limit of Mg is about 1.5 in Ca10.5−xMgx(PO4)7:Eu2+. To examine the influence of Mg substitution on the crystal structure, the XRD Rietveld refinements of Ca10.5−xMgx(PO4)7 (0 ≤ x ≤ 1.5) are performed by the GSAS program with the single crystallographic data of β-Ca3(PO4)2 as the initial model

2. EXPERIMENTAL SECTION 2.1. Synthesis. A series of Ca10.5−xMgx(PO4)7:Eu2+ (0 ≤ x ≤ 2.00) samples were synthesized by the conventional solid-state reaction technique. Analytical-purity CaCO3, NH4H2PO4, and MgO and high-purity Eu2O3 (99.99%) were used as the starting materials. The stoichiometric amounts of required raw materials were thoroughly mixed by grinding in an agate mortar for 40 min. The mixtures were pressed into cylindrical disks under a uniaxial pressure of 10 MPa and then presintered at 600 °C for 4 h in air. After being reground for 15 min, the mixtures were pressed into cylindrical disks with 20 mm diameter and approximately 1 mm height under a uniaxial pressure of 10 MPa and annealed again at 1000−1200 °C for 8 h in a tube furnace under a 10% H2−90% Ar reducing atmosphere. The as-obtained samples were subsequently cooled to room temperature in the tube furnace and reground into powders for subsequent measurements. The Ca9Mg1.5(PO4)7:Eu3+ sample was obtained by sintering the parent Ca9Mg1.5(PO4)7:Eu2+ powders at 850 °C for 4 h in the tube furnace under oxygen atmosphere. 2.2. Characterization. The phase structures of the as-obtained samples were determined by a Rigaku D/Max-II B X-ray 33323


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Figure 1. (a) Crystal structure diagram of whitlockite β-Ca3(PO4)2; variations of Ca/Mg occupancy rates in (b) Ca(4) and (c) Ca(5) sites with Mg doping content (x); and the dependence of lattice parameters (d) a and (e) c and (f) unit cell volume V on Mg doping content (x).

distortion degree of Ca/Mg(5)O6 octahedron using the previously reported formula15,33−35

(Figure S1). On the basis of the refinement results, the structural characteristics and structural evolution with Mg doping content are highlighted in Figure 1. Smaller Mg2+ cannot insert in Ca(1), Ca(2), and Ca(3) sites, but can only substitute for Ca2+ ions at Ca(4) and Ca(5) sites (Figure 1b,c). In whitlockite β-Ca3(PO4)2 structure, Ca(4) site with an unusual coordination to three O atoms offers the possibility to shorten the Ca(4)−O distances, which give the possibility for the incorporation of smaller Mg2+ ions.31 Ca(5) site with an octahedral geometry is especially suitable for doping Mg2+ ions due to the shortest average Ca(5)−O bond length (2.26 Å) among the five Ca sites. As a result, when x ≤ 1, smaller Mg2+ ions (rMg2+ = 0.72 Å for CN = 6, where CN stands for coordination number) preferentially enter Ca(5) site rather than Ca(4) site. Although Mg2+ ions preferentially enter Ca(5) site, the Mg2+ distribution is essentially randomized over Ca(4) and Ca(5) sites at low total Mg2+ content.32 While at x > 1, Ca(5) site is almost filled with Mg2+ and then Mg2+ ions are forced to rapidly occupy Ca(4) site. These results are well consistent with the solid solution limit of Mg (x = 1.5) in Ca10.5−xMgx(PO4)7:Eu2+ (Figure S3). Because the ionic radius of Mg2+ is smaller than that of Ca2+, the lattice shrinkage should take place when smaller Mg2+ is substituted for larger Ca2+. Figure 1d−f shows that lattice parameter a (a = b) and cell volume V decrease continuously with increasing Mg2+ doping content. However, the lattice parameter c gradually decreases when x ≤ 1.25 and displays an abrupt increase at x = 1.5, suggesting a discontinuous evolution of local structure along c axis (Figure 1e). To explain this abnormal structural evolution, the polyhedra distortion index (DI) dependent on bond length is calculated to measure the

DI =

1 n

n

∑ i=1

|li − lav| lav

(1)

where n is the number of anion coordinated with the central cation, li is the distance from the central cation to the ith coordinating anion, and lav is the average bond length. For the regular polyhedron, the DI value is zero. The corresponding bond lengths and DI values for Ca10.5−xMgx(PO4)7 compounds vary with Mg doping content, as illustrated in Figure 2b,c. At higher Mg doping content, the bond lengths and DI values do not exhibit a continuous decrease. The reasons could be explained as follows. When x ≤ 1, the smaller Mg2+ ions preferentially enter Ca(5) site, resulting in a remarkable shrinkage of Ca/Mg(5)O6 octahedron. Consequently, the Ca/ Mg(5)−O(6) bond length decreases rapidly to become nearly equal to the Ca/Mg(5)−O(9) bond length at x = 1.00 (Figure 2b). Meanwhile, the local structure of Ca/Mg(5)O6 octahedron gets more symmetrical (as the decrease of DI values in Figure 2c) and is nearly regular octahedron at x = 1.00 (DI = 0.0077). However, for the Mg content close to solid solution limit (x > 1), an almost complete substitution in Ca(5) site results in an nearly exclusive substitution in Ca(4) site (Figure 1b,c), which could accelerate the decrease of the Ca/Mg(4)− O(3) bond length (Figure 2b). Simultaneously, the rapid reduction of Ca/Mg(4)−O(3) distance would rotate the P(2)O4 tetrahedron, and then the Ca/Mg(5)−O(6) and Ca/ Mg(5)−O(9) distances would be stretched (Figure 2a,b). On the other hand, the symmetry of Ca/Mg(5)O6 octahedron becomes worse (Figure 2c), which agrees well with the 33324


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Figure 2. (a) Local structures of Ca10.5−xMgx(PO4)7 compounds with increasing x; (b) variation of Ca/Mg(4)−O and Ca/Mg(5)−O bond lengths; and (c) distortion indices of Ca/Mg(5)O6 octahedron with Mg doping content (x).

discontinuous evolution of local structure along the c axis (Figure 1e). Namely, the nearly exclusive substitution in the Ca(4) site at x close to 1.5 provokes a noteworthy rearrangement of the Ca10.5−xMgx(PO4)7:Eu2+ unit cell. 3.2. Structure-Dependent Luminescence Property. To probe the structural evolution of Ca10.5−xMgx(PO4)7 compounds with increasing Mg doping content, the luminescence spectra of Ca10.5−xMgx(PO4)7:Eu2+ samples are measured. Under different excitation wavelengths of 340 and 365 nm (Figures 3c and S4), the Mg-free Ca10.5(PO4)7:Eu2+ sample exhibits nearly same spectral profiles, in which a single dominating narrow emission band centered at 418 nm with a weak trailing peak in the range of 480−750 nm. However, with increasing the Mg2+ doping content, the trailing peak at longer wavelength becomes broad. The relative emission intensities of this broad peak display a distinct increase. The co-presence of narrow emission at 418 nm and a broad one at longer wavelength suggests the existence of multiple luminescence centers in the as-obtained Ca10.5−xMgx(PO4)7:Eu2+ samples. Figure 3a displays the excitation spectra of the Ca9Mg1.5(PO4)7:Eu2+ sample under different monitored wavelengths. The different excitation spectra monitored at 418 and 630 nm indicate that the two broad emissions originate from disparate luminescence centers. As displayed in Figure 3d, the site-selective excitation-dependent emission spectra and emission colors further confirm the inference about the multiple luminescence centers in the Ca10.5−xMgx(PO4)7:Eu2+ compounds. Moreover, under 365 nm excitation, the quantum efficiencies of the Ca10.5−xMgx(PO4)7:Eu2+ (x = 0, 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50) samples are calculated to be 19, 16,

20, 35, 35, 32, and 29%, respectively. To a certain extent, increasing the Mg2+ doping content can remarkably enhance the quantum efficiencies of the Ca10.5−xMgx(PO4)7:Eu2+ phosphors (Figure 3b). As a powerful agent to certify the existence of multiple luminescence centers, the luminescence lifetimes of the Ca10.5−xMgx(PO4)7:Eu2+ samples are measured monitored at 418 and 630 nm (Figure 4). In consideration of the existence of multiple luminescence centers in the Ca10.5−xMgx(PO4)7:Eu2+ samples, the average decay time (τav) is calculated by the following formula36 ∞

τav =

∫0 tI(t ) dt ∞

∫0 I(t ) dt

(2)

where t is the time and I(t) is the luminescent intensity. For the Ca9Mg1.5(PO4)7:Eu2+ sample, the average decay times monitored at 418 and 630 nm are calculated to be 84.5 and 1401.0 ns, respectively. The significant distinction in luminescence lifetime strongly prove that the decay behavior for emissions at 418 and 630 nm should originate from different luminescence centers. Moreover, with increasing Mg doping content, the average decay times monitored at 418 nm continuously reduce from 120.0 ns (x = 0) to 84.5 ns (x = 1.5), but the average decay times monitored at 630 nm continuously increase from 564.1 ns (x = 0) to 1401.0 ns (x = 1.5). This phenomenon suggests that there may exist energy transfer from the luminescence center at 418 nm to the luminescence center at 630 nm, due to the possible spectral overlap among different luminescence centers.37 Noteworthy, the trailing 33325


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Figure 3. (a) Normalized excitation spectra of Ca9Mg1.5(PO4)7:Eu2+ sample under different monitored wavelengths; (b) quantum efficiencies; and (c) normalized emission spectra of Ca10.5−xMgx(PO4)7:Eu2+ samples under 365 nm excitation (inset: from left to right is the corresponding photographs arranged from x = 0 to 1.5 under 365 nm UV lamp); and (d) normalized emission spectra of Ca9Mg1.5(PO4)7:Eu2+ sample under different excitation wavelengths (inset: from left to right is the corresponding photographs under 254, 302, and 365 nm UV lamp).

Figure 4. Decay curves of Eu2+ in the Ca10.5−xMgx(PO4)7:Eu2+ samples monitored at (a) 418 and (b) 630 nm.

Ca10.5−xMgx(PO4)7:Eu2+ samples, but it is really difficult to verify the number of luminescence centers. To determine the number of luminescence centers, the lowtemperature emission spectrum should be helpful. Figure 5a illustrates the emission spectra of the Ca9Mg1.5(PO4)7:Eu2+ sample measured from 11 to 300 K. At low temperature,

emission intensity around 465 nm shows a saltatory variation when x ≥ 1 (Figures 3c and S4). And the emission maximum at longer wavelength displays a slight blue shift when x > 1. These phenomenon will be discussed later. The emission spectra and decay curves recorded at room temperature only indicate the presence of multiple emission centers in the 33326


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Figure 5. (a) Emission spectra of the Ca9Mg1.5(PO4)7:Eu2+ sample recorded at temperatures from 11 to 300 K under 340 nm excitation; (b) temperature dependence of the relative intensity for three emission bands at 418, 465, and 630 nm; (c) temperature-dependent emission intensity of the as-obtained Ca9Mg1.5(PO4)7:Eu2+ sample in the wavelength range of 360−830 nm; and (d) high-resolution emission spectrum of 5D0 → 7F0 transition for Ca9Mg1.5(PO4)7:Eu3+ sample recorded at 11 K.

especially below 130 K, a new and strong emission peak at 465 nm is observed. Figure 5b shows the temperature dependence of emission intensities for three bands at 418, 465, and 630 nm. Different from the intensity variations of emission at 418 and 630 nm, the emission intensity at 465 nm displays a drastic decrease and becomes almost invisible at higher temperature, resulting in an trailing emission peak around 465 nm at room temperature. With increasing the temperature from 11 K, the emission intensities at 418 and 630 nm show a slight increase and then decrease continuously. The slight increase of emission intensity at low temperature may attribute to a small change in the absorption strength at the excitation wavelength, a similar phenomenon reported by Meijerink et al.38 Considering that the Ca(4) and Ca(5) sites are filled with smaller Mg2+ ions, there are only three Ca sites for the accommodation of Eu2+ ions in the Ca9Mg1.5(PO4)7:Eu2+ sample. The observed three emission peaks in the Ca9Mg1.5(PO4)7:Eu2+ sample should be certainly originated from the accommodation of Eu2+ ions in the Ca(1), Ca(2), and Ca(3) sites. On the other hand, the emission maximum at longer wavelength displays a remarkable blue shift (from 650 to 630 nm) with increasing temperature, and this phenomenon will be discussed later. Due to the site-sensitive 5D0 → 7F0 transition, Eu3+ ions have been usually investigated as a spectroscopic probe to certify their site-selective occupations.39,40 To further verify the number of luminescence centers, the Ca9Mg1.5(PO4)7:Eu3+

sample is carefully obtained by oxidizing the parent Ca9Mg1.5(PO4)7:Eu2+ powders at a relatively lower temperature. For our sample, it should be difficult for larger Eu2+ ions to mobilize during secondary sintering process because the parent compound Ca9Mg1.5(PO4)7:Eu2+ is highly crystallized (as confirmed by XRD patterns in Figure S1 and TEM images in Figure S2). As shown in Figures 5d and S6, the highresolution emission spectrum of the Ca9Mg1.5(PO4)7:Eu3+ sample recorded at 11 K shows an asymmetric split peak near 580 nm, which can be attributed to the 5D0 → 7F0 transition of Eu3+. The asymmetric split peak can be well fitted into three Gaussian contributions with peak positions at 578.7, 579.4, and 579.9 nm. This result further confirms that there are only three kinds of emission centers for Eu2+ ions in the Ca9Mg1.5(PO4)7 host. In previous reports about the whitlockite structure, the established consensus is that the M(1), M(2), and M(3) sites can accommodate lager Eu2+ ions, while the M(5) site with the shortest M(5)−O bond length is unsuitable for doping larger Eu2+ ions. However, there still exist a conflicting viewpoint about whether the lager Eu2+ ions can occupy at M(4) site or not. Some researchers claimed that the larger Eu2+ ions could distribute in M(1), M(2), M(3), and M(4) sites.15−17 However, on the other hand, other researchers deny that the larger Eu2+ ions can enter M(4) site.27−29 In this work, because smaller Mg2+ ions preferentially substitute for Ca(4) and Ca(5) sites in the Ca10.5−xMgx(PO4)7:Eu2+ samples, Ca(4) site 33327


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ACS Applied Materials & Interfaces could not accommodate larger Eu2+ ions. If Eu2+ ions prefer to occupy at Ca(4) site, the preferential occupation of Eu2+ ions in Ca(4) site should give rise to a dominant emission peak in the emission spectrum of Mg-free Ca10.5(PO4)7:Eu2+ sample. When the Ca(4) site is gradually filled up with smaller Mg2+ ions, this dominant peak should disappear suddenly in the emission spectrum of Ca9Mg1.5(PO4)7:Eu2+ sample because the Eu2+ ions located at Ca(4) site would migrate from Ca(4) site to Ca(1), Ca(2), or Ca(3) site accompanying the occupation of smaller Mg2+ ions at Ca(4) site. However, this situation was not observed for Ca10.5−xMgx(PO 4) 7:Eu2+ samples, suggesting that Eu2+ ions could hardly occupy at Ca(4) site in the Ca10.5−xMgx(PO4)7:Eu2+ samples. More importantly, our experimental results may shed some new light on the conflicting viewpoint previously reported. To correlate the observed three luminescence centers with the actual Ca sites occupied by Eu2+ ions, an empirical equation reported by Van Uitert expounds the relationship between the emission position of Eu2+ and the local structure in various compounds36,41 ÅÄÅ ÑÉÑ 1/ V ÅÅ ij V yz ÑÑ −(n × Ea × r /80)Ñ Å E = Q ÅÅ1 − jj zz 10 ÑÑÑ ÅÅ ÑÑÖ k4{ (3) ÅÇ where E (cm−1) is the position for the Eu2+ emission peak, Q is the position in energy for the lower d-band edge for the free Eu2+ ion (Q value is 34 000 cm−1 for Eu2+), V is the valence of the Eu2+ (V = 2), n is the number of anions in the immediate shell about the Eu2+ ion, Ea is the electron affinity of the anion atoms (it is a constant for the same host), and r is the radius of the host cation substituted by the Eu2+ ion (rCa2+ = 1.06 Å for CN = 7, rCa2+ = 1.12 Å for CN = 8, and CN stands for coordination number). Namely, the emission position (E) is proportional to the factors n and r. The coordination numbers of Ca(1), Ca(2), and Ca(3) sites are 7, 7, and 8, respectively. Hence, we conclude that the emission peak at 418 nm belongs to the distribution of Eu2+ ions in the Ca(3) site. Under this circumstance, the emission positions of Eu(1) and Eu(2) are unclear. Hereinafter, Eu(1), Eu(2), and Eu(3) stand for Eu2+ located at the Ca(1), Ca(2), and Ca(3) sites. A further analysis is needed to identify the emission position of Eu(1) and Eu(2). Due to the strong interaction between the crystal field and the outermost 5d electrons, the practical 4f−5d level position (Efd(A)) in a certain host is usually lower than that for free lanthanide ion (Efd(free)). The 4f−5d transition energy of rare earths ions can be described as42,43 Efd(A) = Efd(free) − D(A)

Figure 6. Schematic diagram for (a) the degeneration of the free Eu2+ energy states in Ca10.5−xMgx(PO4)7:Eu2+ compounds resulting from the nephelauxetic effect and crystal field interaction and (b) total spectral red shift D(A) of three different luminescence centers.

between the highest and lowest 5d levels. The value of r(A) is usually located between 1.7 and 2.4, and the constant C is the energy difference between centroid position and the lowest 5d level of the free Eu2+ ion. Briefly, the centroid shift εc(A) and crystal field splitting εcfs(A) are two factors that contribute to the total spectroscopic red shift D(A). For the first factor εc(A) in eq 5, the centroid shift εc(A), which is closely related to the nephelauxetic effect, can be expressed as15,44−46 N

(4)

εc(A) =

where D(A), the total downward shift of the 5d level mean energy relative to the free ion value, is the total contribution of the nephelauxetic effect and crystal field splitting. For Eu2+ ions in a host lattice, the emission shifts caused by the nephelauxetic effect and crystal field splitting are mutually independent (Figure 6a). For calculating D(A), a more detailed analysis model had been proposed and modified by Dorenbos44 ε (A ) D(A) = εc(A) + cfs −C r (A )

αi e2 (⟨r 2⟩5d − ⟨r 2⟩4f ) ∑ 1 4πε0 i = 1 R i − ΔR 2

(

N

εc(A) ∝

∑ i=1

χav =

where εc(A) is the centroid shift (closely related to the nephelauxetic effect) relative to the centroid position of free ion. The crystal field splitting εcfs(A) is the energy difference

1 Na

6

(R i − 12 ΔR)

Nc

∑ i

(6)

αi

αspO = 0.33 +

(5)

6

)

4.8 χav2

(7)

(8)

ziχi γ

(9)

In eq 6, e is the elementary charge, ε0 is the permittivity of vacuum, and ⟨r2⟩ represents the expectation value of the radial 33328


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ACS Applied Materials & Interfaces position of the electron in 5d or 4f orbital of Eu2+. Namely, the 2

factor

e (⟨r 2⟩5d 4πε0

Ca9Mg1.5(PO4)7:Eu2+ sample originated from the accommodation of the Eu2+ activator in Ca(3), Ca(1), and Ca(2) sites, respectively (as illustrated in Figure 6b). In addition, according to order II, the Eu(2) ions have the largest crystal field splitting, which is well consistent with the broad emission band around 630 nm. The large crystal field splitting of the 4f65d1 level gives rise to multiple 4f65d1 excited states (Figure 6a), and the broad emission band around 630 nm is attributed to the overlap of multiple 4f65d1 → 4f7 transitions. The relatively large energy difference between Eu(1) emission (at 465 nm) and Eu(2) emission (at 630 nm) may be attributed to the different distortion degrees of Ca(1)O7 (DI = 0.027) and Ca(2)O7 (DI = 0.042) polyhedrons. It is worth mentioning that the Eu2+ doping contents were kept at a low level (about 0.6 mol %). At such low doping concentration, the migration of a small amount Eu2+ activators could not give rise to obvious change of the Ca(1)O7, Ca(2)O7, and Ca(3)O8 polyhedrons. In addition, it is difficult to accurately determine the migration of Eu2+ ions by XRD at such low doping level. Due to its structure-dependent PL feature, Eu2+ ions were used as a luminescence probe to investigate the structural evolution of Ca10.5−xMgx(PO4)7:Eu2+ phosphors and the migration of Eu2+ activators among different cation sites. The assignment of three emission centers in the Ca9Mg1.5(PO4)7:Eu2+ sample could help us to explain the intensity variation and blue shift of the broad peak at longer wavelength in the emission spectra of Ca10.5−xMgx(PO4)7:Eu2+ (Figures 3c and S4). The increasing relative emission intensities of the broad peak at longer wavelength in Figures 3c and S4, which originated from the Eu2+ activator located at Ca(1) and Ca(2) sites, indicate that the deliberate introduction of the smaller Mg2+ ions in Ca(4) and Ca(5) sites gives rise to a distinct redistribution of Eu2+ activators among Ca(1), Ca(2), and Ca(3) sites. In the Mgfree Ca10.5(PO4)7:Eu2+ sample, the Ca(3) site, having the highest coordination number, provides a weaker covalent local environment for the accommodation of Eu2+ activators, compared to Ca(1) and Ca(2) sites. In addition, the longest average bond length of Ca(3)−O also offers a looser Ca(3)O8 dodecahedra for accommodating the larger Eu2+ ions to minimize the lattice strain. As a consequence, in the emission spectra of Mg-free Ca10.5(PO4)7:Eu2+ sample, there is a single dominant emission band centered at 418 nm, which originated from Eu2+ ions predominantly entry into the Ca(3) site (Figure 7a). The weak trailing peak at longer wavelength can attribute to the distribution of a very small amount of Eu2+ in Ca(1) and Ca(2) sites. To further verify this hypothesis, the emission spectra of Mg-free Ca10.5(PO4)7:Eu2+ sample under different excitation wavelengths were recorded and are displayed in Figure 7a. Similar to the site-selective excitation-dependent emission phenomenon in Figures 3c and S4, the weak trailing emission of Mg-free Ca10.5(PO4)7:Eu2+ sample increases continuously, suggesting that this weak trailing emission at longer wavelength also originated from different luminescence centers. On the other hand, the very low emission intensity of the trailing peak at longer wavelength implies that it is more difficult for Eu2+ ions to enter the Ca(1) and Ca(2) sites than Ca(3) site in the Mg-free Ca10.5(PO4)7:Eu2+ sample. When the smaller Mg2+ ions are deliberately introduced in Ca(4) and Ca(5) sites, the average Ca/Mg(5)−O bond length decreases drastically, while those of Ca(1)−O, Ca(2)−O, and Ca(3)−O change slightly (Figure S8). Therefore, a covalence competition between the

− ⟨r 2⟩4f ) is a constant. Therefore, eq 6 can

briefly rewrite as eq 7. In eq 7, Ri is the distance between the center metal ion and the ligand anion i, ΔR is the ionic radius difference between Eu2+ ion and its substituted Ca cation, and 1 ΔR expresses an approximate correction for lattice relaxation 2 around Eu2+ (rEu2+ = 1.20 Å for CN = 7, rEu2+ = 1.25 Å for CN = 8; ΔR = 0.14 Å for CN = 7, ΔR = 0.13 Å for CN = 8). αi (Å−3) is the spectroscopic polarizability of the ligand anion i. As a phenomenological parameter in the Dorenbos model, αOsp is linearly dependent on the inverse square of the average cation electronegativity (χav), as described in eq 8.45 And χav can be calculated using eq 9, where Na and Nc are the sum of the number of anions and cations in the formula of the compound, respectively, Z and γ are the formal charge of cations and anions, respectively, and χ is the cation electronegativity. The Pauling-type electronegativity values of relevant cations (χCa =1.00, χMg =1.31, and χP = 2.19) are obtained from previous Allred’s report.47 Taking Ca9.5Mg1.5(PO4)7 compound as an example, χav = (18 χCa + 3 χMg + 35χP)/56 = 1.7604 in this case. The calculated O anion spectroscopic polarizability is αOsp = 1.879 Å−3. In the Ca9Mg1.5(PO4)7:Eu2+ sample, according to eq 7, the centroid shift for Eu(1), Eu(2), and Eu(3) decreases in the sequence of (order I) εc(Eu2) > εc(Eu3) > εc(Eu1). Next, for the second factor εcfs(A) in eq 5, the crystal field splitting εcfs(A), which is mainly determined by the anion polyhedral shape and size around Eu2+, can be expressed as46,48 −2 1 i y εcfs(A) = βploy R−2 = βploy jjjR av − ΔR zzz 2 k {

(10) 2+

where R is the average distance from the center metal Eu ion to its ligand anions and Rav is the average bond length between the center metal Eu2+ ion to its ligand anions. Similar to eq 7, 1 ΔR expresses an approximate correction for lattice relaxation 2 around Eu2+, and βploy is a constant solely determined by the shape of the coordination polyhedron. The shapes of the Ca(1)O7, Ca(2)O7, and Ca(3)O8 polyhedrons are decanedron, decanedron, and dodecahedron, respectively. Previous investigation showed that the βploy values are arranged in the order of βocta > βdeca > βddh > β3ctp = βcubo for octahedron, decanedron, dodecahedron, tricapped trigonal prism, and cuboctahedron, respectively.46,48 In other words, εcfs(A) usually decreases with increasing coordination number (CN) and the average distance (Rav). For Ca9Mg1.5(PO4)7 host, the 1

−2

βploy and R av − 2 R values are in the sequence of βdeca (Ca(2)) = βdeca (Ca(1)) > βddh (Ca(3)) and (RCa(2))−2 > (RCa(1))−2 > (RCa(3))−2, respectively. Thus, in Ca9Mg1.5(PO4)7:Eu2+, the εcfs(A) values of Eu(1), Eu(2), and Eu(3) vary in the sequence of (order II) εcfs(Eu2) > εcfs(Eu1) > εcfs(Eu3). Combining the order I and order II, we determined the sequence of the total spectroscopic red shift using eq 5 as D(Eu2) > D(Eu1), D(Eu3). These results can confirm that the peak at 630 nm belongs to the distribution of Eu2+ ions in the Ca(2) site. Considering the foregoing result that the emission peak at 418 nm originated from Eu2+ located at the Ca(3) site, we can deduce that the emission peak at 465 nm is attributed to the distribution of Eu2+ ions in the Ca(1) site. Therefore, the three emission peaks (located at 418, 465, and 630 nm) observed in the low-temperature emission spectrum of the

(

)

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Differing from Ca(5) site that shares the edge of the Ca(1)O7 and Ca(2)O7 decanedrons, Ca(4) site shares a corner with the Ca(1)O7 and Ca(3)O8 units. Furthermore, because the Ca(4) site is only half-occupied by Ca/Mg cations, we conclude that the guiding effect of the substituted Ca(4) site is weaker than that of the substituted Ca(5) site. When x > 1, after Ca(5) site is almost filled, Mg2+ ions preferentially enter Ca(4) site. Guided by the preferentially substituted Mg2+ ions in Ca(4) site, some Eu2+ activators may migrate from Ca(1) site to Ca(2) and Ca(3) sites, resulting in the saltatory variation of emission intensity around 465 nm (Figures 3c and S4). In addition, the migration of Eu2+ activators among different cation sites may be responsible for the slight blue shift in Figures 3c and S4. Certainly, thermal expansion of lattice might have some influence on the emission spectra of Ca10.5−xMgx(PO4)7:Eu2+ phosphors. As shown in Figure 8c, the heat capacities of Ca10.5(PO4)7:Eu2+ and Ca9Mg1.5(PO4)7:Eu2+ samples increase continuously with temperature and no thermal anomalies are observed ranging from 2 to 300 K, suggesting that no phase transitions occurs in low temperature. Therefore, we can roughly estimate the lattice expansion rate using the XRD data recorded in the temperature range of 20−600 °C and further explain the blue shift phenomenon with increasing temperature (as arrow indication in Figure 5a). The XRD patterns of the Ca9Mg1.5(PO4)7:Eu2+ sample measured in the temperature range of 20−600 °C are shown in Figure S10. With increasing temperature, the diffraction peaks shift slightly toward the lower angle side (Figure S11), indicating a continuous lattice expansion. The lattice parameters and unit cell volume obtained from the XRD Rietveld refinements results exhibit a linear increase ranging from 20 to 600 °C (Figure 8a). We can speculate that the lattice thermal expansion shows a linear increase ranging from 2 to 300 K. The thermal lattice expansion would decrease the centroid shift and crystal field splitting, giving rise to the blue shift in Figure 5a. On the other hand, the thermal lattice expansion for Ca9Mg1.5(PO4)7:Eu2+ sample is anisotropic (Figure 8b) because the relative thermal expansion rate along the c axis (slope = 2.07 × 10−5) is almost twice larger than that along the a axis (slope = 1.10 × 10−5). Explicitly, the polyhedral shape around Eu2+ ions would expand anisotropically with increasing temperature. Considering the crystal field splitting is highly dependent on the cation−anion local geometry, the decrease of crystal field splitting caused by the anisotropic lattice expansion may be responsible for the blue shift of Eu2+ emission in Figure 5a. 3.3. EL Properties of the Fabricated pc-LED Devices. The Mg2+-oriented strong red emission for our designed Ca 10.5−x Mg x (PO 4 ) 7 :Eu 2+ phosphors may make up the deficiency of red emission in traditional white LEDs. To evaluate the potential applications of as-obtained Ca10.5−xMgx(PO4)7:Eu2+ phosphors in white LEDs, series LEDs were fabricated by depositing the representative Ca10.5−xMgx(PO4)7:Eu2+ phosphors on 365 nm UV LED chips. The EL spectra, photographs, and the CIE chromaticity coordinates of typical LEDs are displayed in Figure 9. And important optical parameters, such as warm CIE coordinate, CCT, color-rendering index (CRI), and luminous efficacy, are given in Table 1. Noteworthily, a warm white output is achieved in LED-1. The broad emission of the as-obtained Ca10.5−xMgx(PO4)7:Eu2+ phosphors almost cover the whole visible light range. Differing from multiphased phosphorconverted LEDs, our fabricated LEDs no longer need

Figure 7. (a) Normalized emission spectra of the Mg-free Ca10.5(PO4)7:Eu2+ sample under different excitation wavelengths and (b) schematic diagram for the redistribution of Eu2+ activators in selective Ca2+ sites guided by the preferential Mg2+ substitution.

substituted Mg2+ and the neighboring Ca2+ for the oxygen electron cloud occurs. Because the electronegativity of Mg2+ ions (χMg = 1.31) is higher than that of Ca2+ ions (χCa = 1.00), the introduced Mg2+ ions show a stronger attraction to the oxygen electron cloud. According to the previous reports,44,49 the stronger binding of the oxygen anion to neighboring Mg2+ will reduce the covalency of Ca(1)−O(6) and Ca(2)−O(9) bonds. In other words, the neighboring Mg2+ forms relatively covalent bonds to oxygen anion, resulting in a more ionic surrounding around Ca(1) and Ca(2) sites (Figure 7b). The decrease of electron density between Ca(1) and O(6) as well as between Ca(2) and O(9) supplies two weaker covalent Ca(1) and Ca(2) sites for the accommodation of Eu2+ activators. To compensate the covalence change, more Eu2+ activators would occupy the Ca(1) and Ca(2) sites because the electronegativity of Eu (χEu = 1.20) is higher than that of Ca (χCa = 1.00). Consequently, guided by the preferential substitution of Mg2+ ions at Ca(4) and Ca(5) sites, more Eu2+ activators would occupy the Ca(1) and Ca(2) sites, giving rise to a continuous increase for the relative emission intensities at longer wavelengths. Additionally, the local structures around Ca(1) and Ca(2) sites, directly connected to Ca/Mg(5) site, varied with increasing Mg doping content in series Ca10.5−xMgx(PO4)7:Eu2+ samples. Therefore, the emission maximum at the long wavelength region of Mg-doped samples should be varied with the Mg doping content. 33330


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Figure 8. (a) Temperature dependence of lattice parameters a and c and unit cell volume V; (b) relative thermal expansion for Ca9Mg1.5(PO4)7:Eu2+ sample; and (c) heat capacity of Ca10.5(PO4)7:Eu2+ and Ca9Mg1.5(PO4)7:Eu2+ samples.

Figure 9. (a) EL spectra and corresponding photographs of pc-LED devices fabricated with a 365 nm UV chip and (b) CIE chromaticity coordinates of corresponding LEDs.

85) and a lower correlated color temperature (CCT = 3788 K) are obtained in LED-1. As an agent to quantify the rendering of deep red, the special CRI R9 is widely used to appraise the

additional blue, green, or red phosphors, which will remarkably decrease the energy reabsorption effect among different phosphors. Importantly, a higher color-rendering index (Ra = 33331


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ACS Applied Materials & Interfaces Table 1. Optical Parameters of the Fabricated pc-LEDs under 365 nm UV LED Chips devices

chip (nm)

phosphor

LED-1 LED-2 LED-3 LED-4 refs 7 and 50

365 365 365 365 460

Ca9.75Mg0.75(PO4)7:Eu2+ Ca9.50Mg1.00(PO4)7:Eu2+ Ca9.25Mg1.25(PO4)7:Eu2+ Ca9Mg1.5(PO4)7:Eu2+ Y3Al5O12:Ce3+

CIE coordinate (0.385, (0.435, (0.450, (0.447, (0.292,

performance of white LEDs. Compared to the lower R9 value (14.3) of the YAG:Ce-based white LED,7 the quite high R9 value (91) of LED-1 is particularly noteworthy. It is worth mentioning that the luminous efficiencies of the fabricated LED are not better than those of commercial white LEDs (460 nm blue chip + YAG:Ce phosphor).7,50 However, it is sufficiently high for high color-rendering white LED (Ra = 85, R9 = 91), and can be further improved by optimizing the processing conditions of phosphors and packaging technology, including enhancing the performance of chips.6,51 Considering that white LEDs with high CRI Ra and R9 values are in urgent need in high-quality solid-state lighting, the higher CRI Ra (85) and R9 (91) values of LED-1 reported here allow the assynthesized Ca9.75Mg0.75(PO4)7:Eu2+ phosphor to be an excellent candidate for next-generation UV-excited white LEDs.

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CCT (K)

Ra

R9

efficiency (lm/W)

3788 2663 2434 2567 7756

85 62 55 60 75

91 0 −25 11 14.3

23.0 31.1 18.4 19.7 38.9

ples; TEM image, HRTEM image, low-temperature spectra, variable-temperature XRD patterns, and refinement results of the Ca9Mg1.5(PO4)7:Eu2+ sample; and emission spectra of Ca10.5−xMgx(PO4)7:Eu2+ samples under 290 and 340 nm excitation (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.L.). *E-mail: [email protected] (G.L.). ORCID

Liping Li: 0000-0002-6732-4902 Guangshe Li: 0000-0002-3278-1804 Xueyuan Chen: 0000-0003-0493-839X Yu Zhang: 0000-0003-2100-621X Notes

4. CONCLUSIONS In summary, structural analysis demonstrated that the solid solution limit of Mg2+ ions in Ca10.5−xMgx(PO4)7:Eu2+ is 1.5. Initially, Mg2+ ions preferentially occupy at Ca(5) site and then Mg2+ ions preferentially occupy at Ca(4) sites. The preferential occupation of Mg2+ ions not only triggers a discontinuous evolution of local structure along c axis, but also induces a covalence competition between the substituted Mg2+ and the neighboring Ca2+ for the oxygen electron cloud. Guided by the preferentially substituted Mg2+ ions, Eu2+ activators migrate from Ca(3) site to Ca(1) and Ca(2) sites, giving rise to a broad emission at longer wavelength. Low-temperature emission spectra prove that there are three luminescence centers in the Ca9Mg1.5(PO4)7:Eu2+ sample. The evaluation of centroid shift, crystal field splitting, and the total downward shift verifies that the three observed emission bands at 418, 465, and 630 nm originated from the accommodation of Eu2+ activators in Ca(3), Ca(1), and Ca(2) sites, respectively. The anisotropic thermal expansion of the Ca9Mg1.5(PO4)7:Eu2+ lattice produces a decrease of crystal field splitting, which may be responsible for the blue shift of Eu2+ emission with increasing temperature. The preferential neighboring substitution-triggered redistribution of Eu2+ activators in multiple cation lattice reported here could be a new and general way to tune photoluminescence. To achieve these objects, future works should focus on (i) Eu2+/Ce3+-activated phosphors with multiple cation sites, (ii) possible preferential substitution at specific sites in multiple cation/anion lattice hosts, and (iii) size/electronegativity mismatch between native and foreign cations/anions.

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0.365) 0.362) 0.362) 0.372) 0.325)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (Grants 21671077, 21771171, 21571176, 21611530688, and 21025104). The authors thank Dr. Renfu Li and Prof. Xueyuan Chen for the low-temperature PL investigations, and M.S. Dingke Xue and Prof. Yu Zhang for the EL investigations. The CAS/SAFEA International Partnership Program for Creative Research Teams is also acknowledged.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b11879. XRD patterns, refinement results, and average cation electronegativity of the Ca10.5−xMgx(PO4)7:Eu2+ sam33332


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Preferential Neighboring Substitution-Triggered Full Visible Spectrum

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