InCl3-Assisted Surface Defects Restoring to Enhance Lead-Free Cs2ZrCl6 Nanocrystals for X-Ray Imaging and Blue LED Applications.

As one type of recent emerging lead-free perovskites, Cs2ZrCl6 nanocrystals are widely concerned, benefiting from the eminent designability, high X-ray cutoff efficiency, and favorable stability. Improving the luminescence performance of Cs2ZrCl6 nanocrystals has great importance to cater for practical applications. In view of the surface defects frequently formed by the liquid phase method, the particle morphology and surface quality of this material are expected to be regulated if certain intervention is made in the synthesis process. In the work, differing from normal cell lattice modulation based on the ion doping, the grain size and surface morphology of Cs2ZrCl6 nanocrystals are optimized via adding a certain amount of InCl3 to the synthetic solution. The surface defects are restored to inhibit the defect-induced non-radiative transition, resulting in the improvement of the luminescence properties. Moreover, a flexible Cs2ZrCl6@polydimethylsiloxane film with excellent heat, water, and bending resistance and a light-emitting diode (LED) device are fabricated, exhibiting excellent application potential for X-ray imaging and blue LED.

Environment-Dependent Eu2+-Activated Ba3CaK(PO4)3 toward White-Light Emission by Chemical Cosubstitution of the (BO3)3- Anion Group.

Investigating the phosphors doped with single activators in a single component to realize white-light emission is urgently desired for phosphor-converted white-light-emitting diodes. In this work, on the basis of the chemical unit cosubstitution strategy, the new borophosphate phosphors Ba3CaK(PO4)3-x(BO3)x:0.02Eu2+ with a mixed anion group were prepared. Coupling structure refinement and photoluminescence analyses, Ba3CaK(PO4)3-x(BO3)x consists of five different cationic sites with different coordination environments, with Eu2+ occupying the three sites for Ba2+. In the process of partial substituting (BO3)3- for (PO4)3-, because of the greatly distorted coordination field generated from the difference in the geometric configurations between the two anion groups, a red shift and broadening of the emission bands occurs, resulting in a color-adjustable emission from blue to white. A phosphor-converted light-emitting diode has been successfully fabricated with the incorporation of an as-prepared Ba3CaK(PO4)2.6(BO3)0.4:0.02Eu2+ phosphor and a 405 nm near-ultraviolet chip, which exhibits Commission International de I'Eclairage chromaticity coordinates of (0.31, 0.37) and a correlated color temperature of 6295 K. As demonstrated in the present work, an approach adopted from phosphate to borophosphate is conducted to develop high-quality phosphors.

Dependence of Luminous Performance on Eu3+ Site Occupation in SrIn2(P2O7)2: The Effect of the Local Environment.

The photoluminescence behavior of luminescent materials with rare earth (RE) ions as a luminescence center not only depends on the element type and chemical valence of RE ions but also on their concentration and occupation in the matrix, sometimes including the interaction of the matrix and RE ions or between different RE ions. Herein, special SrIn2(P2O7)2 phosphate, assembled by monolayer [SrO10]∞ and bilayer [In2P4O14]∞ consisting of InO6 units and P2O7 groups, was selected as the host material, and different cation positions (Sr and In) were substituted by Eu3+. The structure refinement in combination with Judd-Ofelt theory has shed light on the differences of the Eu3+ coordination environment in SrIn2(P2O7)2. The structural rigidity of the In3+ site is better than that of the Sr2+ site, making SrIn1.92(P2O7)2: Eu0.08 superior in thermal stability. The average distance between adjacent Sr2+ ions is larger than that between adjacent In3+ ions, causing the higher quantum efficiency of Sr0.9In2(P2O7)2: Eu0.1. The present work demonstrates that the site occupation of Eu3+ has an important effect on its luminous performance. Importantly, the newly developed Eu3+-doped SrIn2(P2O7)2 phosphors, exhibiting outstanding luminous efficiency, favorable thermal stability, and excellent color purity, are promising red components of phosphor-based light-emitting diodes.

Daylight-White-Emitting and Abnormal Thermal Antiquenching Phosphors Based on a Layered Host SrIn2(P2O7)2.

Single-phase white-emission phosphors possess a judicious usage potential in phosphor-converted white-light-emitting diodes (WLEDs). Recently, numerous efforts have been made toward the development of new patterns of white-emitting phosphors that achieve excellent quantum yield, superior thermal stability, and applaudable cost effectiveness of WLEDs. Finding suitable single-component white phosphor hosts to provide an ideal local environment for activators remains urgent. Inspired by the original discovery of the promising host MgIn2(P2O7)2 (MIP) and its structural dependence on alkali-metal cations, we synthesized a brand-new phosphor host, SrIn2(P2O7)2 (SIP), via the traditional solid-state reaction. Its crystal structure was determined using an ab initio analysis and the Rietveld method. It belongs to a monoclinic unit cell with the space group C2/c. Besides, SIP exhibits a special layered three-dimensional framework in which the monolayer [SrO10]∞ was surrounded by a bilayer [In2P4O14]∞ made of the InO6 octahedra and P2O7 groups. A series of pure SIP:Tm3+,Dy3+ phosphors with tunable blue-white-yellow emission were prepared by adjusting the dopant concentration and utilizing the Tm3+-Dy3+ energy transfer. The daylight-white-emitting phosphor SIP:0.01Tm3+,0.04Dy3+ (the correlated color temperature is 4448 K) exhibits an abnormal thermal antiquenching property, and the emission intensity of 423 K reaches 103.7% of the initial value at 300 K. On the basis of the temperature-dependent lattice evolution and microenvironment analysis, the reduction of ß and lattice distortion can lead to lower asymmetry of the activators and benefit the compensation of trapped-electron thermal activation. In this work, an integration study was carried out on the crystal structure of the new matrix, the occupation of the luminescent center, the interaction of different activators in the host, and the distortion degree of the local structure for the activators, which is of great practical sense for producing a novel single-matrix white phosphor possessing superior thermal endurance for UV-light-stimulated WLEDs.

Tuning of Emission by Eu3+ Concentration in a Pyrophosphate: the Effect of Local Symmetry.

The local symmetry of a Eu3+ ion has a crucial effect on its luminescence properties. In this work, we show that the red/orange ratio in the emission of Eu3+-doped MgIn2P4O14 phosphors is tunable by adjusting the Eu3+ concentrations, due to the change in the local symmetry of metal ions. The substitution of Eu3+ for In3+ lowers the distortion in the lattice of monoclinic MgIn2P4O14, and an increase in Eu3+ doping concentration causes the metal ion sites to shift closer to an inverse center, leading to a reversal of the relative emission intensity of a magnetic dipole transition to an electric dipole transition. Meanwhile, the higher asymmetry of metal ion sites occupied by Eu3+ in MgIn2P4O14 makes the luminescence less thermally stable than that in Mg3In4P6O24.

The Non-Concentration-Quenching Phosphor Ca3Eu2B4O12 for WLED Application.

Commercial white LED devices usually suffer from a high color temperature and poor color rendering. Developing a new, efficient, and stable red phosphor is the key to solving this problem. In this work, a series of pure Ca3Y2-xB4O12:xEu3+ (0 < x ≤ 2) samples, including the new and fully transitional borate phosphor Ca3Eu2B4O12 (CEBO), have been successfully prepared by solid-state reaction synthesis. CEBO is isostructural with Ca3Y2B4O12 (CYBO), belonging to the orthorhombic system with space group Pnma (No. 62). Under optimal 393 nm excitation, this borate exhibits a strong red emission, peaking at 615 nm, with high color purity. Interestingly, the luminescence of CEBO is relatively higher than that of CYBO:Eu3+ phosphors. The quantum yield of this non-concentration-quenching phosphor reaches 95.6%. Furthermore, a warm pc-WLED device has been fabricated by mixing as-prepared CEBO powders and commercial BaMgAl10O17:Eu2+ and (Sr, Ba)2SiO4:Eu2+ phosphors, which exhibits a high color rendering index (Ra = 83.7) along with a color temperature of around 3883 K. The present work indicates that this new borate, with outstanding quantum efficiency and favorable thermal stability, can be used as a red phosphor for application in WLEDs.

Layered Crystal Structure, Color-Tunable Photoluminescence, and Excellent Thermal Stability of MgIn2P4O14 Phosphate-Based Phosphors.

Single-component white phosphors stand a good chance to serve in the next-generation high-power white light-emitting diodes. Because of low thermal stability and containing lanthanide ions with reduced valence state, most of reported phosphors usually suffer unstable color of lighting for practical packaging and comparably complex synthetic processes. In this work, we present a type of novel color-tunable blue-white-yellow-emitting MgIn2P4O14:Tm3+/Dy3+ phosphor with high thermal stability, which can be easily fabricated in air. Under UV excitation, the MgIn2P4O14:Tm0.02Dy0.03 white phosphor exhibits negligible thermal-quenching behavior, with a 99.5% intensity retention at 150 °C, relative to its initial value at room temperature. The phosphor host MgIn2P4O14 was synthesized and reported for the first time. MgIn2P4O14 crystallizes in the space group of C2/c (No. 15) with a novel layered structure built of alternate anionic and cationic layers. Its disordering structure, with Mg and In atoms co-occupying the same site, is believed to facilitate the energy transfer between rare-earth ions and benefit by sustaining the luminescence with increasing temperature. The measured absolute quantum yields of MgIn2P4O14:Dy0.04, MgIn2P4O14:Tm0.01Dy0.04, and MgIn2P4O14:Tm0.02Dy0.03 phosphors under the excitation of 351 nm ultraviolet radiation are 70.50%, 53.24%, and 52.31%, respectively. Present work indicates that the novel layered MgIn2P4O14 is a promising candidate as a single-component white phosphor host with an excellent thermal stability for near-UV-excited white-light-emitting diodes (wLEDs).

Anti-thermal quenching phosphors based on the new phosphate host Ca3.6In3.6(PO4)6.

To enhance the working quality of WLEDs, considerable efforts have been made to upgrade the thermal quenching resistance of existing phosphors or design new anti-thermal quenching (ATQ) phosphors. Developing a new phosphate matrix material with special structural features has great importance for the fabrication of ATQ phosphors. By phase relationship and composition analysis, we have prepared a novel compound Ca3.6In3.6(PO4)6 (CIP). Coupling ab initio and Rietveld refinement techniques, the novel structure of CIP with partly vacant cationic positions was solved. Taking this unique compound as the host and using the inequivalent substitution of Dy3+ for Ca2+, a series of C1-xIP:Dy3+ rice-white emitting phosphors were successfully developed. When the temperature was raised to 423 K, the emission intensity of C1-xIP:xDy3+ (x = 0.01, 0.03, and 0.05) increased to 103.8%, 108.2%, and 104.5% of the original intensity at 298 K, respectively. Except for the strong bonding network and inherent cationic vacancy in the lattice, the ATQ property of the C1-xIP:Dy3+ phosphors is mainly attributed to the generation of interstitial oxygen from the substitution of unequal ions, which releases electrons with the thermal stimulus, causing anomalous emission. Finally, we have explored the quantum efficiency of C1-xIP:0.03Dy3+ phosphor and the working performance of PC-WLED prepared with C1-xIP:0.03Dy3+ phosphor and 365 nm chip. The research work sheds light on the relationship between lattice defects and thermal stability, and meanwhile offers a new strategy for the development of ATQ phosphors.

Large optical polarizability causing positive effects on the birefringence of planar-triangular BO3 groups in ternary borates.

The structure-property relationship of photoelectric functional materials has been recognized as a hot topic. The study of the inner link between the band gaps and birefringence of optical materials is extremely crucial for the design and creation of novel optical devices, but still remains rather unexplored. In this work, taking a series of borates with only planar-triangular BO3 groups, α-/ß-TM3(BO3)2 (TM = Zn, Cd), Cd2B2O5, and M3(BO3)2 (M = Hg, Mg, Ca, Sr) as the research subject, the comprehensive relationship between their electronic structures and linear optical properties has been systematically investigated. Through combining experimental measurements and theoretical calculations, the effect of optical polarizability on the birefringence of these borates was clarified. Based on the present discussion, the relationship between the O (2p) bandwidth of the highest valence band and the HSE06 band gaps is opposite. Meanwhile, a method involving the determination of so-called optical permittivity Δε to evaluate the magnitude of birefringence Δn is found to be feasible. A large Δε makes a positive contribution to Δn. In addition, the experimentally measured band gaps and IR vibrations are in good agreement with theoretical results for the compounds α-Cd3(BO3)2 and Cd2B2O5.

Efficient and stable Sr3Eu2B4O12 red phosphor benefiting from low symmetry and distorted local environment.

The development of suitable red phosphors to obtain improved white color stands a good chance to serve in the new generation of white light-emitting diodes. Owing to multi-elements via doping and oxidation of reduced valence state of lanthanide or transition metal ions, most of the reported phosphors usually suffer from complex synthetic processes and unstable color of the lighting industry cycle. In this work, we present a new red emitting and stable Sr3Eu2B4O12 phosphor with regard to its special structure. It crystallizes as an orthorhombic cell, with Sr and Eu atoms co-occupying three different lattice sites in the space group of Pnma (no. 62). It is proposed that the long bond distance between activators minimizes the content quenching, while the high disorder of location restricts the thermal quenching. This phosphor emits bright red light with good color purity under UV excitation, with the luminescence intensity and quantum yield tunable via the fabrication temperature. Through a preliminary optimization of the synthesis process, the Sr3Eu2B4O12 phosphor prepared at 1250 °C has high quantum yields of about 94.7% and excellent thermal stability of 85.6% intensity retention at 150 °C relative to the initial value at room temperature. The calculated Judd-Ofelt intensity parameters (Ω2, Ω4) further clarified that the Eu3+ site in Sr3Eu2B4O12 had lower symmetry without an inversion center, and more distorted local environment and structural rigidity of the host, predicting excellent thermal stability. Finally, a warm pc-WLED device has been produced by mixing as-prepared Sr3Eu2B4O12 powders and commercial BaMgAl10O17:Eu2+ and (Sr, Ba)2SiO4:Eu2+ phosphors, which exhibits a high color rendering index (Ra = 83.4) along with a color temperature at around 4102 K. The present work indicates that the Sr3Eu2B4O12 phosphor is an efficient red component with excellent thermal stability for white-light production of near-UV-excited w-LEDs.

Structure, tunable luminescence and energy transfer in Tb3+ and Eu3+ codoped Ba3InB9O18 phosphors.

The borate Ba3InB9O18 (BIBO) is a promising host material for phosphors. A series of Tb3+ and Eu3+ codoped Ba3InB9O18 phosphors were synthesized. Based on the Rietveld method, structure refinement of the codoped BIBO phosphor was carried out. Then, the luminescence properties of BIBO:Tb3+, Eu3+ phosphors were extensively investigated under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation. The measured PL spectra and decay times evidenced that energy transfer occurs between the Tb3+ and Eu3+ ions. The energy-transfer mechanism from Tb3+ to Eu3+ in Ba3InB9O18 is dominated by electric multipolar interactions, with the critical distance calculated to be 10.97 Å. The temperature sensitivity of the Tb3+ and Eu3+ codoped sample under VUV was also investigated at the low temperature range from 25 K to 298 K. The emission color could be tuned from green to the red region by adjusting the concentration of codoped ions. The results indicate that the BIBO-based phosphors are valuable candidates for applications in the display and lighting fields.

Enhanced thermoelectric properties in N-type Mg2Si0.4-x Sn0.6Sb x synthesized by alkaline earth metal reduction.

Mg2Si1-x Sn x -based compounds have been recognized as promising thermoelectric materials owing to their high figure-of-merit ZTs, abundance of raw constituent elements and nontoxicity. However, further improvement in the thermoelectric performance in this type of material is still constrained by the high thermal conductivity. In this work, we prepared a series of representative Mg2Si0.4-x Sn0.6Sb x (x = 0, 0.0075, 0.008, 0.009, 0.01, 0.011) samples via the alkaline earth metal reduction method through a combination of ball milling and spark plasma sintering (SPS) processes. The samples featured many dislocations at the grain boundaries and plenty of nanoscale-coherent Mg2Si-Mg2Sn spinodal phases; both of which can effectively scatter heat-carrying phonons and have nearly no impact on the carrier transport. Meanwhile, Sb-doping can efficiently optimize the carrier concentration and significantly suppress the bipolar effects. As a result, a maximal ZT of 1.42 at 723 K and engineering (ZT)eng of 0.7 are achieved at the optimal Sb-doping level of x = 0.01. This result indicates that the alkaline earth metal reduction method could be an effective route to engineer phonon transport and improve the thermoelectric performance in Mg2Si1-x Sn x -based materials.

Increased effective mass and carrier concentration responsible for the improved thermoelectric performance of the nominal compound Cu2Ga4Te7 with Sb substitution for Cu.

Although the ternary chalcopyrite compound Cu2Ga4Te7 has relatively high thermal conductivity and electrical resistivity, it has a high carrier concentration, thus making it a good thermoelectric candidate. In this work we substitute Sb for Cu in this compound, aiming at engineering both the electrical and thermal properties. Rietveld refinement revealed that the nominal compounds Cu2-x Sb x Ga4Te7 (x = 0-0.1) crystallize with the crystal structure of CuGaTe2 with the real compositions deviating from those of their nominal ones. Besides, Sb resides in Cu sites, which increases both the effective mass and the Hall carrier concentration. Therefore, the Seebeck coefficient increases at high temperatures, and the lattice thermal conductivity reduces due to increased phonon scattering from point defects and electron-phonon interactions. As a consequence, the thermoelectric (TE) performance improves with the highest TE figure of merit (ZT) of 0.58 at 803 K. This value is about 0.21 higher than that of the pristine Cu2Ga4Te7.

Improvement of thermoelectric performance of copper-deficient compounds Cu2.5+δ In4.5Te8 (δ = 0-0.15) due to a degenerate impurity band and ultralow lattice thermal conductivity.

Cu-In-Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration (n H) and ultralow lattice thermal conductivity (κ L) when Cu was added to ternary Cu2.5+δ In4.5Te8 (δ = 0-0.15) compounds. The enhancement in n H is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low κ L results from the increased lattice disorder. We thus obtained the minimum κ L value of only 0.23 W K-1 m-1 in the sample at δ = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at δ = 0.1, which is about 0.38 higher than that of the pristine Cu2.5In4.5Te8.