Maxim S. Molokeev

Linear structural evolution induced tunable photoluminescence in clinopyroxene solid-solution phosphors.

Clinopyroxenes along the Jervisite (NaScSi2O6) – Diopside (CaMgSi2O6) join have been studied, and a solid-solution of the type (Na1−xCax)(Sc1−xMgx)Si2O6 has been identified in the full range of 0 ≤ x ≤ 1. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show smooth variation of structural parameters with composition. The linear structural evolution of iso-structural (Na1−xCax)(Sc1−xMgx)Si2O6 solid-solutions obeying Vegards rule has also been examined and verified by high resolution transmission electron microscopy (HRTEM). The continuous solid-solutions show the same structural type, therefore the photoluminescence spectra of Eu2+ doped samples possess the superposition of spectral features from blue-emitting component (CaMgSi2O6:Eu2+) and yellow-emitting one (NaScSi2O6:Eu2+). This indicates that the spectroscopic properties of (Na1−xCax)(Sc1−xMgx)Si2O6 clinopyroxene solid-solutions are in direct relations with structural parameters, and it is helpful for designing color-tunable photoluminescence with predetermined parameters.

New Yellow-Emitting Whitlockite-type Structure Sr1.75Ca1.25(PO4)2:Eu2+ Phosphor for Near-UV Pumped White Light-Emitting Devices

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr1.75Ca1.25(PO4)2 newly found by composition design in the Sr3(PO4)2-Ca3(PO4)2 join was reported. Crystal structure and luminescence properties of Sr1.75Ca1.25(PO4)2:Eu(2+) were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu(2+) in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr1.75Ca1.25(PO4)2:Eu(2+) phosphors absorb in the UV-vis spectral region of 250-430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λex = 365 nm which is ascribed to the 5d-4f allowed transition of Eu(2+). The luminescence properties and mechanism are also investigated as a function of Eu(2+) concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties.

Tuning of Photoluminescence by Cation Nanosegregation in the (CaMg)x(NaSc)1-xSi2O6 Solid Solution

Controlled photoluminescence tuning is important for the optimization and modification of phosphor materials. Herein we report an isostructural solid solution of (CaMg)x(NaSc)1-xSi2O6 (0 < x < 1) in which cation nanosegregation leads to the presence of two dilute Eu(2+) centers. The distinct nanodomains of isostructural (CaMg)Si2O6 and (NaSc)Si2O6 contain a proportional number of Eu(2+) ions with unique, independent spectroscopic signatures. Density functional theory calculations provided a theoretical understanding of the nanosegregation and indicated that the homogeneous solid solution is energetically unstable. It is shown that nanosegregation allows predictive control of color rendering and therefore provides a new method of phosphor development.

Pressure-Stimulated Synthesis and Luminescence Properties of Microcrystalline (Lu,Y)3Al5O12:Ce3+ Garnet Phosphors

The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 10(7) Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals ∼19 μm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λ(ex) = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.

Structure, Crystallographic Sites, and Tunable Luminescence Properties of Eu2+ and Ce3+/Li+-Activated Ca1.65Sr0.35SiO4 Phosphors

Eu(2+) and Ce(3+)/Li(+) singly doped and Eu(2+)/Ce(3+)/Li(+)-codoped Ca1.65Sr0.35SiO4 phosphors have been synthesized by a solid-state reaction method. The crystal structure was determined by Rietveld refinement to verify the formation of the αL′-Ca2SiO4 phase with the Sr addition into Ca2SiO4, and the preferred crystallographic positions of the Eu(2+) and Ce(3+)/Li(+) ions in Ca1.65Sr0.35SiO4 were analyzed based on a comparison of the unit-cell volumes and the designed chemical compositions of undoped isostructural compounds Ca(2–x)Sr(x)SiO4 (x = 0.25, 0.35, 0.45, 0.55 and 0.65). Ce(3+)/Li(+) singly activated Ca1.65Sr0.35SiO4 phosphors exhibit strong absorption in the range of 250–450 nm and a blue emission peak centered at about 465 nm. When Eu(2+) ions are codoped, the emission colors of Ca1.65Sr0.35SiO4:Ce(3+)/Li(+),Eu(2+) phosphors under the irradiation of 365 nm can be finely tuned from blue to green through the energy transfer from Ce(3+) to Eu(2+). The involved energy-transfer process between Ce(3+) and Eu(2+) and the corresponding mechanism are discussed in detail. The reported Ca1.65Sr0.35SiO4:Ce(3+)/Li(+),Eu(2+) phosphor might be a candidate for color-tunable blue-green components in the fabrication of near-ultraviolet-pumped white-light-emitting diodes (WLEDs).

Structure evolution and photoluminescence of Lu3(Al,Mg)2(Al,Si)3O12:Ce3+ phosphors: new yellow-color converters for blue LED-driven solid state lighting

This paper reports the development of new phosphors using the chemical unit cosubstituting solid solution design strategy. Starting from Lu3Al5O12, the Al3+–Al3+ couple in respective octahedral and tetrahedral coordination was simultaneously substituted by a Mg2+–Si4+ pair forming the Lu3(Al2−xMgx)(Al3−xSix)O12:Ce3+ (x = 0.5–2.0) series; as a result, the CeO8 polyhedrons were compressed and the emission got red-shifted from green to yellow together with the broadening. The evolution of, the unit cell, the local structural geometry as well as the optical properties of Ce3+ in these garnet creations, in response to the gradual Mg–Si substitution for Al–Al, were studied by combined techniques of structural refinement and luminescence measurements. The new composition Lu2.97Ce0.03Mg0.5Al4Si0.5O12 was comprehensively evaluated regarding its potential application in blue LED-driven solid state white lighting: the maximum emission is at 550 nm under λex = 450 nm; the internal and external quantum efficiencies can reach 85% and 49%, respectively; a 1-phosphor-converted wLED lamp fabricated using the as-prepared phosphor exhibits the luminous efficacy of 105 lm W−1, the correlated color temperature of 6164 K and the color rendering index (Ra) of 75.6. The new solid solution composition series is open for further optimization to enhance the competence for commercial consideration.

Cation Substitution Dependent Bimodal Photoluminescence in Whitlockite Structural Ca3–xSrx(PO4)2:Eu2+ (0 ≤ x ≤ 2) Solid Solution Phosphors

Cation substitution dependent tunable bimodal photoluminescence behavior was observed in the Ca3-xSrx(PO4)2:Eu(2+) (0 ≤ x ≤ 2) solid solution phosphors. The Rietveld refinements verified the phase purity and whitlockite type crystal structure of the solid solutions. The tunable photoluminescence evolution was studied as a function of strontium content, over the composition range 0.1 ≤ x ≤ 2. In addition to the emission band peak at 416 nm in Ca3(PO4)2:Eu(2+), the substitution of Ca(2+) by Sr(2+) induced the emerging broad-band peak at 493-532 nm. A dramatic red shift of the emission peak located in the green-yellow region was observed on an increase of x in the samples with 0.75 ≤ x ≤ 2.00. The two emission bands could be related to the EuOn-Ca9 and EuOn-Ca9-xSrx emitting blocks, respectively. The values for the two kinds of emitting blocks in the solid solutions can be fitted well with the observed intensity evolution of the two emission peaks.

Effect of Al/Si substitution on the structure and luminescence properties of CaSrSiO4:Ce3+ phosphors: analysis based on the polyhedra distortion

Blue-emitting CaSrSiO4:Ce3+,Li+ phosphors were prepared by a high temperature solid-state method, and the effect of substituting Al3+ for Si4+ in CaSrSiO4:Ce3+,Li+ has been studied. Crystal structures of the as-prepared Ca1−ySr1−ySi1−xAlxO4:yCe3+,yLi+ phosphors were resolved by the Rietveld method, which suggested that all the samples belonged to the orthorhombic symmetry (Pnma) group of α-CaSrSiO4. The photoluminescence (PL) emission and excitation spectra, the lifetime, and the effect of Al3+ concentration on the PL properties were investigated in detail. The emission peaks of the CaSrSi1−xAlxO4:Ce3+,Li+ (x = 0–0.10) phosphors were red-shifted from 452 to 472 nm with increasing Al/Si ratio. The red-shift of the Ce3+ emission is ascribed to the polyhedra distortion of the cations, originating from the variation in the neighboring [(Si,Al)O4] polyhedra, and the detailed mechanism has been discussed.

New garnet structure phosphors, Lu3−xYxMgAl3SiO12:Ce3+ (x = 0–3), developed by solid solution design

New garnet phosphors, Lu3−xYxMgAl3SiO12:Ce3+ (x = 0–3), which can be efficiently excited by blue light and emit the yellow-orange light, were developed using the solid solution design strategy combining the chemical unit substitution and the cation substitution. Crystal structures of the four compounds were reported for the first time via the Rietveld refinement of their powder XRD patterns. All phosphors show the general cubic garnet structure with the space group Iad. The specific occupancy of Lu/Y, Al/Mg, Al/Si and O atoms in different positions was identified. The evolution of cell parameters and Y/Lu/Ce–O bond lengths were identified. Photoluminescence properties were evaluated on aspects of emission/excitation spectra, internal/external quantum efficiency and thermal emission stability. Under the 450 nm blue light excitation, the phosphors exhibit bright yellow color emission, peaking in the 575–597 nm spectral range. The internal and external quantum efficiency can reach 83% and 58%, respectively. The emission red-shift in response to the Y/Lu ratio variation was discussed in relation to the local structure evolution. The phosphors are relatively promising to act as wavelength converter of blue light in white light emitting diodes.

Structure and luminescence properties of Eu2+ doped LuxSr2−xSiNxO4−x phosphors evolved from chemical unit cosubstitution

The design scheme of the chemical unit cosubstitution of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ has been put into practice to discover the new phosphor systems with tunable luminescence properties, and the structures and photoluminescence tuning of yellow-emitting LuxSr2−xSiNxO4−x:Eu2+ phosphors have been investigated. Crystal structures of LuxSr2−x−ySiNxO4−x:yEu2+ samples were resolved using the Rietveld method, suggesting that the as-prepared Sr2SiO4 belonged to monoclinic symmetry (P21/n) of β-phase Sr2SiO4, while Sr1.97Eu0.03SiO4 and Sr1.965Eu0.03Lu0.005SiO3.995N0.005 belonged to orthorhombic symmetry (Pnma) of α-Sr2SiO4. The emission peaks of LuxSr1.97−xSiNxO4−x:0.03Eu2+ phosphors were red-shifted from 563 to 583 nm upon increasing the [Lu3+–N3−] substitution content from x = 0 to x = 0.005, furthermore, the PL emission peaks of Lu0.005Sr1.965−ySiN0.005O3.995:yEu2+ also showed a red-shift from 583 nm to 595 nm with increasing Eu2+ concentration (y = 0.03, 0.07, 0.10 and 0.15), and their corresponding red-shift mechanism has been discussed. The temperature dependent luminescence results further verified that the introduction of [Lu3+–N3−] for [Sr2+–O2−] in Sr2SiO4:Eu2+ can improve the thermal stability of the photoluminescence, which indicated that the LuxSr2−x−ySiNxO4−x:yEu2+ phosphors have potential applications in white light-emitting diodes (wLEDs).