Crystal-site Engineering Research Articles

Enhancing the cyan light emission of Ba9Lu2Si6O24:Ce phosphors by crystal-site engineering for full spectrum lighting

Enhancing the cyan light emission of Ba9Lu2Si6O24:Ce phosphors by crystal-site engineering for full spectrum lighting

Crystal-Site Engineering of Novel Na3KMg7(PO4)6-x(BO3)x:Eu2+ Phosphors for Full-Spectrum Lighting.

The "cyan gap" is the bottleneck problem in violet-driven full-spectrum white-light-emitting diodes (wLEDs) in healthy lighting. Accordingly, we develop a novel broadband-blue-cyan emission Na3KMg7(PO4)6-x(BO3)x:Eu2+ (NKMPB:Eu2+) phosphor via crystal-site engineering. This phosphor is derived from the Na3KMg7(PO4)6:Eu2+ phosphor, which shows desired abundant cyan emissive components. A comparative study is conducted to reveal the microstructure-property relationship and the key influential factors to its spectrum distribution. It can be found that the introduced (BO3)3- units can manipulate the site-selective occupation of Eu2+ activators, asymmetrically broadening the emission spectrum in NKMPB:Eu2+. Considering detailed luminescence performance analysis and the density functional theory calculations, a new substitution pathway of Eu2+ is created by substituting (PO4)3- with (BO3)3- units, making partial Eu2+ ions enter the Mg2+ (CN = 5, CN = 6) crystallographic sites, and yielding an extra emission band at 600 nm (16667 cm-1) and especially 501 nm (19960 cm-1). Meanwhile, a high-color-quality full-spectrum-emitting wLEDs was fabricated, upon 100 mA forward-bias current driven. Due to the achieved extra cyan emissive components of NKMPB:Eu2+, the constructed NKMPB:Eu2+-based wLEDs show better color rendering ability (∼90.9) than that of Na3KMg7(PO4)6:Eu2+-based wLEDs (∼86.3), and also demonstrate its great potential in full-spectrum healthy lighting.

KYb2F7:Er3+ based nanothermometers: controlled synthesis, enhanced red emission, and improved sensitivities via crystal-site engineering

KYb2F7:Er3+ UC nanocrystal-based nanothermometers have greatly improved the SR with ion substitution of Ti4+, Ge4+, and Y3+ for Yb3+.

Highly efficient and tunable broadband UV excitable Ba9Lu2Si6O24:Eu2+, Mn2+ single-phase white-light-emitting phosphors

Highly efficient and tunable broadband UV excitable Ba9Lu2Si6O24:Eu2+, Mn2+ single-phase white-light-emitting phosphors

Compositional tuning and site engineering of Sr-alloyed Ca3(PO4)2:Mn2+, Eu2+ towards multicenter-activated single-component white light emitter

As a promising alternative to traditional light sources, the phosphor-converted white light-emitting diodes (WLEDs) have rapidly emerged in the fields of lightings and displays. In our work, a novel single-component warm white phosphor Ca 2 Sr(PO 4 ) 2 :Mn 2+ , Eu 2+ was successfully designed and synthesized based on multicenter emission and crystal site engineering. The phosphors were confirmed to have a high phase purity according to X-ray diffraction (XRD) analysis and the scanning electron microscope (SEM). Further XRD Rietveld refinements unraveled the preferential site occupations of both alloying element (Sr) and dopants (Mn and Eu). The feasible site engineering can confine the Mn and Eu dopants in different Ca sites, resulting in regulated optical behaviors. Density functional theory (DFT) calculations were also employed to shed light on the site engineering from the theoretical aspects, which suggest an enlarged formation energy gap for different site occupations after Sr alloying. Finally, a single-component WLED based on a near-ultraviolet (n-UV) chip was successfully fabricated with high color rendering index (Ra = 91.2) and luminous efficacy (65.2 lm/W). The device has also proved its excellent stability via a series of tests, showing the great potential of Ca 2 Sr(PO 4 ) 2 :Mn 2+ , Eu 2+ as phosphors in lighting applications. Confined site occupation of Eu 2+ and Mn 2+ is found in Sr-alloyed Ca 3 (PO 4 ) 2 , leading to efficient single-component white light emission. • Ca3(PO4)2:Sr2+,Mn2+,Eu2+ with single-component warm white light emission was designed and synthesized. • XRD refinements and DFT simulation evidence the site confinements of Mn 2+ and Eu 2+ by Sr alloying. • High-performance and stable WLED was fabricated with Ca2Sr (PO4)2:Mn2+,Eu2+.

Local structure distortion enhanced the emission intensity of KBaYSi2O7:Eu2+ phosphor for full-visible-spectrum LED lighting

Due to Eu 2+ and Eu 3+ ions often co-exist in the crystal lattices, the luminous intensity of some phosphors is limited greatly. In this work, we propose a crystal-site engineering approach to reduce Eu 3+ to Eu 2+ in the KBaYSi 2 O 7 lattice through substitution of Ba 2+ by Ca 2+ or Sr 2+ . The mechanism between the local environmental regulation of the crystal structure around Eu 2+ as well as the luminous intensity and thermal stability of the phosphor was systematically explored. The results of XRD, Rietveld refinement and EDX confirmed that Ca 2+ and Sr 2+ are dissolved into the crystal lattice effectively without changing the main phase structure. As a result, the luminous intensity of samples doped Ca 2+ and Sr 2+ increased by 39.7% and 41.2%, respectively. The mechanisms for such improvement is ascribed to the reduction of Eu 3+ into Eu 2+ under the lager volume of [EuO 9 ] 2 polyhedron. The treated samples exhibited relatively superior thermal stability with I 150°C > 70%, indicating that doping with Ca 2+ or Sr 2+ have no effect on the thermal stability. A pc-WLED device fabricated by combining (Ba,Sr) 2 SiO 4 :Eu 2+ , (Ca,Sr)AlSiN 3 :Eu 2+ , BaSi 2 O 2 N 2 :Eu 2+ and KBa 0.86 Ca 0.1 YSi 2 O 7 :Eu 2+ with 380 nm chip exhibits CCT (5010 K) and high CRI (96.6). These results indicate that this phosphor has potential for applications in full-spectrum warm white lighting. • The luminous intensity of KBa 0.96 YSi 2 O 7 :Eu 2+ doped Ca 2+ and Sr 2+ increased by 39.7% and 41.2%, respectively. • The effective concentration of Eu 2+ ions was enhanced by structural modulation. • A white-LED device with an excellent CRI (Ra = 96.6) and low CCT (5010 K) was obtained.

Activators lattice migration strategy customized for tunable luminescence of Ce3+ doped β-Ca3(PO4)2

An activators lattice migration strategy was customized for developing tunable- luminescence phosphors in Ce doped β -Ca 3 (PO 4 ) 2 families. By dint of XAFS and DFT calculations, we predicted the local crystal environment of each lattices and migrated Ce 3+ from M(3) to another strengthened site with a lower coordination number, shorter bonding bond length, and greater polyhedral distortion, M(5) . Relevant parameters were identified for dealing with interference caused by the electronegativity change during structural modification and some applications emerged for display and information security. • Activators lattice migration strategy was proposed for tunable-emission phosphors. • The specific occupation and 5 sites local crystal environment of Ce were predicted. • Ce 3+ was migrated to another site with a strengthened crystal field by modification. • The relative intensity of ε CFS and ε NE determined an abnormal emitting mechanism. • Δ D ( C e 3 + ) were identified to deal with the interference of electronegativity difference. The luminescent properties of phosphors are determined by the electronic configuration of their activators and the crystal environment in which they are embedded. By manipulating the site-selective occupancy of activators, we can coordinately control the local crystal field and electron cloud distribution to master the movement of emission wavelength and obtain a color-tunable phosphor. The system of β -Ca 3 (PO 4 ) 2 is an excellent carrier for crystal-site engineering to modify luminescent performance. It has five distinct crystal sites and a strong photoluminescence response. However, due to low doping concentration and structural complexity, the specific occupation of Ce remains unknown. We discussed the specific position of Ce in this study and proposed a novel activators lattice migration strategy for controlling the relative strength of crystal field splitting (CFS) and the nephelauxetic effect (NE) to control luminescence properties. By combining X-ray Absorption Fine Structure (XAFS) with DFT calculation, we predicted the local environment of each crystal lattice and migrated Ce 3+ from M(3) to a strengthened crystal field site with a lower coordination number, shorter bonding bond length, and greater polyhedral distortion, M(5) , resulting in phosphors with tunable luminescence emission. Relevant parameters Δ D ( C e 3 + ) were identified for dealing with interference caused by the electronegativity difference during a structural modification. Some potential applications emerged as a supplement and an intriguing QR-code and response program for epidemic prevention were demonstrated. This structure–activity relationship-based strategy provides new inspiration for designing luminescent materials with tunable emission and a broader range of applications.

Eu2+ Doping Concentration-Induced Site-Selective Occupation and Photoluminescence Tuning in KSrScSi2O7:Eu2+ Phosphor.

Regulation of Eu2+ dopants in different cation sites of solid-state materials is of great significance for designing multicolor phosphors for light-emitting diodes (LEDs). Herein, we report the selective occupation of Eu2+ for multiple cationic sites in KSrScSi2O7, and the tunable photoluminescence from blue to cyan is realized through Eu2+ doping concentration-dependent crystal-site engineering. Eu2+ preferably occupies the K and Sr sites in KSrScSi2O7 at a low doping concentration, resulting in a 440 nm blue emission. As the Eu2+ concentration increases, a new Eu2+ substitution pathway is triggered, that is, Eu2+ enters the Sc site, leading to the red-shifted emission spectra from 440 to 485 nm. The doping mechanism and photoluminescence properties are corroborated by structural analysis, optical spectroscopy study, and density functional theory calculations. The optical properties of the as-fabricated white LEDs are studied, which demonstrates that these phosphors can be applied to full-spectrum phosphor-converted LEDs. This study provides a new design strategy to guide the development of multicolor Eu2+-doped oxide phosphors for lighting applications.

Full-visible-spectrum lighting enabled by site-selective occupation in the high efficient and thermal stable (Rb, K)2CaPO4F: Eu2+ solid-solution phosphors

• The large-scale redshift of emission spectrum was realized in R 2- y K y CPOF: Eu 2+ . • The color-adjustable-emission mechanism in the R 2- y K y CPOF: Eu 2+ was illustrated. • Full visible spectrum lighting was realized by R 2- y K y CPOF: Eu 2+ . Tuning the luminescent properties of phosphors by crystal-site engineering is a good and efficient strategy for developing high-quality white light-emitting diodes (WLEDs). However, it is still challenging to modulate the cyan to red spectral emission in a single matrix and activator phosphor to obtain multicolor output and achieve full-visible-spectrum lighting. Herein, the strategy to manipulate site-selective occupation of Eu 2+ activators by substituting the cation Rb + with K + is reported to obtain a versatile color output. The large-scale redshift regulation of emission spectrum from cyan to red was achieved in fluor-phosphate Rb 2- y K y CaPO 4 F: 0.0085Eu 2+ (R 2- y K y CPOF: 0.0085Eu 2+ ) (0 ≤ y ≤ 1.8) phosphors, which is designed by using Rb 2 CaPO 4 F: 0.0085Eu 2+ (RCPOF: 0.0085Eu 2+ ) cyan-emitting phosphor. The redistribution of Eu 2+ ions in Rb1, Rb2, and Ca sites is achieved by introducing the cation K + to substitute Rb + , and R 2- y K y CPOF: 0.0085Eu 2+ (0 ≤ y ≤ 1.8) is kept in the pure phase. Furthermore, the mixed phosphors of RCPOF: 0.0085Eu 2+ , RKCPOF: 0.0085Eu 2+ , and R 0.2 K 1.8 CPOF: 0.0085Eu 2+ show excellent optical performance and good thermal stability. Full-visible spectrum lighting is successfully achieved with the combination of a 395 nm NUV chip and the blend of phosphor-converted WLEDs. The K + substitution reported in this paper can support a project for warm WLEDs by manipulating the distribution of Eu 2+ activator among different cation sites.

Spontaneous-reduction and photoluminescence tuning in singly-doped Ba5-Ca (PO4)3Cl:Eu2+/Eu3+ phosphors

Developing a feasible scheme for solid-state lighting with high-quality white light remains a significant challenge. Particularly, tunable luminescence with single-component white emission have been widely studied to improve the luminescence performance of phosphor-converted white light-emitting diodes (pc-WLEDs) phosphors. In this work, a novel spontaneous reduction phenomenon was first found in Eu-activated apatite-type Ba5(PO4)3Cl phosphors prepared by a solid-phase reaction in air. Under 362 nm excitation, the luminescence results showed that a broad blue Eu2+ emission band with a peak at 437 nm appears unexpectedly in addition to the usual sharp orange-red emission of Eu3+. The mixed-valence fact of Eu2+/Eu3+ was further confirmed by X-ray photoelectron spectra and time-resolved spectroscopy techniques. The underlying mechanism could be explained by a charge compensation model. Accordingly, the local crystal-site engineering control of the luminescence in Ba5-yCay(PO4)3Cl:Eu2+/Eu3+ had been studied in detail by the introduction of Ca2+. It is found that the Ca2+ substitution of Ba2+ will lead to new splitting peaks of Eu3+ emission and a red-shift followed by a blue-shift of Eu2+ emission respectively, which could eventually adjust the luminescence of phosphors to the proper white light region. These phenomena are mainly ascribed to the introduction of Ca2+ to the structure modification of activator coordination environment. Finally, based on the thermal quenching results indicate that the potential application of the single-doped mixed-valence phosphor system in pc-WLEDs.

Photoluminescence tuning in Ba3ScB3O9:Eu2+ phosphor by crystal-site engineering

Controlled photoluminescence tuning is essential for optimizing and accelerating the application of phosphor materials. Here, we adopt the crystal-site engineering method to tailor the photoluminescence properties in Ba3ScB3O9:Eu2+. Ba3ScB3O9 host contains multiple cationic sites which help to finely regulate Eu2+ site occupancies and thus the emission color. The relationship between Eu2+ emission and the local environments has been analyzed, and we demonstrate that the emission colors of the phosphors can be widely tuned from NIR to yellow by chemically driving the Eu2+ from octahedrally-coordinated Sc3+ sites to nine-fold coordinated Ba2+ sites. Our study will initiate and guide more explorations on discovering new phosphors through crystal-site engineering.

Crystal structure, dielectric and optical properties of β-Ca3(PO4)2-type phosphates Ca9-xZnxLa(PO4)7:Ho3+

A series of new phosphates Ca 9- x Zn x La(PO 4 ) 7 :Ho 3+ with the β -Ca 3 (PO 4 ) 2 -type structure was synthesized by the solid state route. An intense near infra-red (NIR) emission according to intraconfigural 4 f -4 f transitions of Ho 3+ ions 5 I 7 – 5 I 8 (~2 μm) and 5 I 6 – 5 I 8 (~1.156 μm) was observed. The obtained phases were studied by a combination of methods including synchrotron powder X-ray diffraction, dielectric spectroscopy, second harmonic generation, differential scanning calorimetry, luminescence spectroscopy. The structure of Ca 8 ZnLa(PO 4 ) 7 was refined by the Rietveld method in centrosymmetric space group R 3 ‾ c . The Ca 2+ → Zn 2+ substitution in the M5 site leads to a transformation from polar R 3 c space group ( x = 0 – 0.5) to centrosymmetric R 3 ‾ c space group ( x = 0.6–1) and to the increased integral intensity of luminescence with maxima at x = 1. It was concluded that the crystal site engineering in the Ho 3+ -containing β -Ca 3 (PO 4 ) 2 -type hosts offers a promising way to obtain new NIR phosphors for use in the creation of biocompatible bone tissue fillers. • A series of new phosphates Ca 9- x Zn x La(PO 4 ) 7 :Ho 3+ with β -Ca 3 (PO 4 ) 2 -type structure were synthesized by solid state route. An intense near infra-red emission according to 4 f -4 f transition of Ho 3+ ions 5 I 7 – 5 I 8 (~2 μm) and 5 I 6 – 5 I 8 (~1.156 μm) was observed. Structure of Ca 8 ZnLa(PO 4 ) 7 were refined by the Rietveld method in centrosymmetric space group R 3 ‾ c. Under Ca 2+ → Zn 2+ substitution a transformation from polar to centrosymmetric ( R 3 c → R 3 ‾ c ) space group was occurred, which leads to increasing of integral intensity of luminescence with maxima at x = 1.

Luminescent properties of Er3+ in centrosymmetric and acentric phosphates Ca8MEr(PO4)7 (M = Ca, Mg, Zn) and Ca9-xZnxLa(PO4)7:Er3+

A series of centrosymmetric and acentric whitlockite-type stoichiometric phosphates Ca8MEr(PO4)7, M = Ca2+, Mg2+, Zn2+ and Ca9-xZnxLa0.9(PO4)7:0.1Er3+ were synthesized through the high-temperature solid state route. The obtained phases were studied by a combination of methods including Synchrotron powder X-ray diffraction, dielectric spectroscopy, second harmonic generation, luminescence spectroscopy. There are two single-phase regions of polar (0 ≤ x < 0.5) and nonpolar (0.6 ≤ x ≤ 1) crystal structures were distinguished in Ca9-xZnxLa0.9(PO4)7:0.1Er3+. These areas are separated by a two-phase region (0.5 ≤ x < 0.6). It was shown, that under Ca2+ → Zn2+ substitution a transformation from ferroelectric to antiferroelectric properties was occurred. The luminescence properties of the Ca8MEr(PO4)7 were characterized by NIR spectroscopy at 300 K and at 5 K. It was concluded that the crystal site engineering in Er3+-containing β-Ca3(PO4)2-type hosts offering a promising way to obtain efficient NIR phosphors.

Modern aspects of strategies for developing single-phase broadly tunable white light-emitting phosphors

The latest developments of single-phase white-light-emitting Ba9Lu2Si6O24(BLSO) silicate phosphors are presented. The emerging approaches to crystal-site engineering and the energy transfer mechanisms are discussed at great length.

Zero-thermal-quenching and improved chemical stability of a UCr4C4-type phosphor via crystal site engineering

• CsNa 2 K(Li 3 SiO 4 ) 4 :Ce 3+ phosphor was synthesized with 80% quantum efficiency. • First zero-thermal-quenching phosphor among other Ce 3+ -doped phosphors. • Upon Ce 3+ doping in host shows excellent chemical stability than Eu 2+ doping. • CsNa 2 K(Li 3 SiO 4 ) 4 :Ce 3+ shows high color rendering index value of 95 at 1000 mA. Europium-doped UCr 4 C 4 cuboid phosphors with narrow and tunable emission show promise for application in designing light-emitting diodes with high color purity and thermal stability. Nevertheless, their poor chemical stability bottlenecks their use in lighting applications. Herein, a dual narrow-band blue-emitting Ce 3+ -doped CsNa 2 K(Li 3 SiO 4 ) 4 (CNKLSO) cuboid phosphor with a quantum yield of 80% is synthesized. The dual-band emission is attributed to the occupation of Ce 3+ at two distinct sites of CNKLSO, which is consistent with the density functional theory outcomes. Ce 3+ doping produces stable emission characteristics, in contrast to Eu 2+ incorporation, because of the improved site stability, as realized from bond valence sum calculations. Remarkably, no drop in the emission intensity is observed even at 200 °C, making CNKLSO:Ce 3+ the first Ce 3+ -based zero-thermal-quenching phosphor. A CNKLSO:Ce 3+ -based white light-emitting diode displays an excellent color rendering index (~95) at a high flux current of 1000 mA. The proposed study indicates that Ce 3+ doping in other UCr 4 C 4 - type oxide phosphors may improve the site stability, which in turn the chemical stability of the phosphor materials can be boosted.

Manipulating the redistribution of single dopant via crystal-site engineering approach in Ba4Gd3(K1-xNax)3(PO4)6F2:Eu2+ phosphors for warm white light emitting diodes

Manipulating the redistribution of single dopant among multiple crystallographic sites can optimize and design luminescence materials for single-doped warm white phosphor with high color rendering. However, successful cases are barely reported, because the substitution of all sites and the warm white emitting of multiemission bands cannot be guaranteed. Herein, we synthesized a series of Ba 4 Gd 3 (K 1-x Na x ) 3 (PO 4 ) 6 F 2 :Eu 2+ (0 ≤ x ≤ 1.0) solid solutions via crystal-site engineering approach to manipulate the redistribution of Eu 2+ dopant for tunable photoluminescence property. Na + ions preferentially substitution in the A and B columns along the c -axis direction gives rise to the anisotropic lattice shrinkage. The multiemission bands of Ba 4 Gd 3 (K 1-x Na x ) 3 (PO 4 ) 6 F 2 :Eu 2+ phosphors almost cover the whole visible light range because Eu 2+ dopant can occupy the M(1), Gd(2) and A(3) sites. The Na-substitution results in a redistribution of Eu 2+ dopants among different cation sites, and induces a continuous increase of the relative emission intensities at longer wavelength with increasing Na + doping content. Moreover, the unique optical feature enables the single-phased Ba 4 Gd 3 Na 3 (PO 4 ) 6 F 2 :Eu 2+ phosphor converted WLED to exhibit quite high color rendering index (R a = 90.2) and moderate correlated color temperature (CC T = 4125 K). The manipulating the redistribution of single dopant via crystal-site engineering approach reported here can not only carve out a new avenue for warm WLEDs, but also be applied to develop novel potential phosphors for practical optical application. • Ba 4 Gd 3 (K 1-x Na x ) 3 (PO 4 ) 6 F 2 :Eu 2+ (0 ≤ x ≤ 1.0) solid solutions are successfully prepared. • Na-substitution triggers an anisotropic lattice shrinkage along the c -axis. • Na-substitution triggers migration of Eu 2+ from M(1) and Gd(2) sites to A(3) sites. • The relative emission intensity can be regulated by increasing Na doping content. • The Ba 4 Gd 3 Na 3 (PO 4 ) 6 F 2 :Eu 2+ phosphor converted WLED exhibits quite CRI (R a = 90.2).

Cation-Deficiency-Induced Crystal-Site Engineering for ZnGa2O4:Mn2+ Thin Film.

Zn-deficient spinel-type ZnGa2O4:Mn2+ phosphor thin films were prepared using pulsed laser deposition. With an increase (decrease) in the Zn deficiency (concentration) of the films, changes in lattice constant, optical band gap, and photoluminescence spectra were observed. All films without γ-Ga2O3:Mn showed green luminescence attributable to the transition from the 4T1 state to the 6A1 state. In addition, the spectral shape changed depending on the temperature. The luminescence spectra have two peaks resulting from the Mn2+ ions located in the tetrahedral and octahedral sites. These peaks had different thermal quenching temperatures, which were around 320 and 260 K, respectively. Therefore, the spectral shape changed with increasing temperature. The spectral shape also depended on the Zn concentration. With an increase (decrease) in the Zn concentration (deficiency) of the films, the intensity of emission from Td increased in comparison with that from Oh. Therefore, the position of Mn2+ was controlled by Zn deficiency similarly to the effect of crystal-site engineering.

The crystal site engineering and turning of cross-relaxation in green-emitting β-Ca3(PO4)2-related phosphors

Abstract Series of Tb3+-doped phosphates Ca9–xZnxTb(PO4)7 and Ca9–xZnxLa0.99(PO4)7:0.01 Tb3+ with β-Ca3(PO4)2-type structure were synthesized using a standard high temperature route. The luminescence properties of these compounds were determined by the photoluminescence emission (PL), the excitation (PLE) spectra and lifetime measurements. A green-emitting LED device was fabricated using the most effective one. No concentration quenching was detected in the whole region of solid solutions. Upon Ca2+ → Zn2+ substitution the crystal structure transformation from the polar to centrosymmetric. The ways of control the PL spectra and cross-relaxation process were determined. In both solid solutions a “green” magneto-dipole 5D4 → 7F5 transition is predominant. The integral intensity increases in 3 times in the most effective Ca8ZnTb(PO4)7 phosphor. The measured quantum yields (QY) reached 31%. Thus, a target substitution of Ca2+ by Zn2+ is a new path for crystal site engineering and allows to obtain an efficient green luminescence of Tb3+ ions in the β-Ca3(PO4)2-type hosts.

Full visible spectra emission introduced by crystal-site engineering in β-Ca3(PO4)2-type solid solution phosphors for high quality white light emitting diodes application

Abstract Exploring the phosphors with full visible spectra emission in a single system to realize versatile color output is highly desired but still challenging for high quality phosphor converted white light emitting diodes (WLEDs). Here, inspired by the multiple crystallographic sites in β -Ca 3 (PO 4 ) 2 structures and facile crystal-site engineering method, full visible spectra emission was obtained via the design of two kinds Ca 9 Y(PO 4 ) 7 -Ca 10 Li(PO 4 ) 7 :Eu 2+ , Mn 2+ and Ca 9 Y(PO 4 ) 7 -Ca 10 Na(PO 4 ) 7 :Eu 2+ , Mn 2+ solid solution phosphors. The structure characteristics and photoluminescence properties were investigated by the Rietveld refinement on the basis of X-ray diffraction data, density functional theory, and low/room/high temperature spectra. The nonlinear phase evolutions introduced by the disappearances of vacancy, decreases of polyhedron number and changes of occupying atom in the different five cationic sites aroused the redistributions of Eu 2+ among the cationic sites, which achieved full visible spectra emission and then enabled the fabricated WLEDs to exhibit high color rendering index (CRI = 81.5) and low correlated color temperature (CCT = 4087 K) under 365 nm chip excitation. Moreover, the R9 value was further enhanced from 24.6 to 93.1 with the help of construction of Eu 2+ -Mn 2+ energy transfer in selected phosphors, which improved the qualities of device with higher CRI and lower CCT (CRI = 95.1 and CCT = 3213 K). The new kinds of solid solution phosphors reported here could not only manipulate the redistributions of Eu 2+ among different cationic sites for full visible spectra emission, but also pave a way to develop phosphors for high quality WLEDs.

Yellow Emission from Low Coordination Site of Sr2 SiO4 :Eu2+ , Ce3+ : Influence of Lanthanide Dopants on the Electron Density and Crystallinity in Crystal Site Engineering Approach.

Lanthanide doping through a crystal site engineering approach tunes the emission wavelength suitable for LED applications, but weak emission from low coordination sites remains a huge challenge. Herein the use of a sensitizer is reported to enhance the emission strength and unravel the crystallinity and phase, as this approach demands a large amount of dopants. Doping of Eu2+ ions at SrO10 and SrO9 sites of Sr2 SiO4 (S2 S), respectively, tunes the emission from green to yellow and controlled doping of a Ce3+ sensitizer quadruples the quantum efficiency of yellow emission. Remarkably, doping of Eu2+ at the SrO9 site produces polycrystals, whereas co-doping of Ce3+ and Eu2+ at the same site produces single crystals. DFT calculations further delineate the underlying changes wherein strong interaction of dopant with its neighbours determines the electron density, and thus the crystallinity and phase, rather than usual explanation of aliovalent conditions, which is further substantiated by TEM results. Irrespective of dopant valence, use of large amounts of dopants and their interaction with the host is responsible for the crystallinity and phase change (α'-S2 S to β-S2 S). The XPS valence band spectra experimentally evidences the changes in bonding nature of O 2p and O 2s orbitals of silicate and its electron density, due to doping at the two sites. In short, the outcomes resulting from this work could be extended for the development of other two-coordination site lanthanide-doped materials and crystallization of inorganic materials.