Preparation and characterization of Li0.25Sr0.5(MoO4):Eu 0.253+ red-emitting phosphors for white LEDs by organic gel-thermal decomposition process

Li_0.25Sr_0.5(MoO₄):Eu_0.25³⁺ red-emitting phosphors were prepared by the organic gel-thermal decomposition process with metal salts and citric acid as starting reagents. X-ray diffraction, scanning electron microscopy and photoluminescent spectroscopy were used to characterize the as-prepared phosphors. The Li_0.25Sr_0.5(MoO₄):Eu_0.25³⁺ phase consisting of nanosized crystallites is formed at 400 °C and the nanosized crystallites with a tetragonal-dipyramid morphology increase with the calcination temperature and time. During the early period at 650 °C, the microstructure of the Li_0.25Sr_0.5(MoO₄):Eu_0.25³⁺ crystallites are unstable and the re-crystallization for some particles takes place with a particle morphological modification. The optimized calcination conditions for the Li_0.25Sr_0.5(MoO₄):Eu_0.25³⁺ phosphors are 650 °C for 13 h. The Li_0.25Sr_0.5(MoO₄):Eu_0.25³⁺ phosphors with particle sizes about 0.5 to 2.0 μm obtained under the optimized conditions can be excited by the ultraviolet light 395 nm and blue light 466 nm, which are well met with the requirements for the current commercial near-UV and blue LEDs, and exhibit a high emission performance.     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Syntheses and Characterization of Novel Perovskite-Type LaScO3-Based Lithium Ionic Conductors

Perovskite-type lithium ionic conductors were explored in the (Li_xLa_1−x/3)ScO₃ system following their syntheses via a high-pressure solid-state reaction. Phase identification indicated that a solid solution with a perovskite-type structure was formed in the range 0 ≤ x < 0.6. When x = 0.45, (Li_0.45La_0.85)ScO₃ exhibited the highest ionic conductivity and a low activation energy. Increasing the loading of lithium as an ionic diffusion carrier expanded the unit cell volume and contributed to the higher ionic conductivity and lower activation energy. Cations with higher oxidation numbers were introduced into the A/B sites to improve the ionic conductivity. Ce⁴⁺ and Zr⁴⁺ or Nb⁵⁺ dopants partially substituted the A-site (La/Li) and B-site Sc, respectively. Although B-site doping produced a lower ionic conductivity, A-site Ce⁴⁺ doping improved the conductive properties. A perovskite-type single phase was obtained for (Li_0.45La_0.78Ce_0.05)ScO₃ upon Ce⁴⁺ doping, providing a higher ionic conductivity than (Li_0.45La_0.85)ScO₃. Compositional analysis and crystal-structure refinement of (Li_0.45La_0.85)ScO₃ and (Li_0.45La_0.78Ce_0.05)ScO₃ revealed increased lithium contents and expansion of the unit cell upon Ce⁴⁺ co-doping. The highest ionic conductivity of 1.1 × 10⁻³ S cm⁻¹ at 623 K was confirmed for (Li_0.4Ce_0.15La_0.67)ScO₃, which is more than one order of magnitude higher than that of the (Li_xLa_1−x/3)ScO₃ system.     

Homogeneity regions of double molybdates in the Na2MoO4-MgMoO4 system and structures of triclinic Na1.51Mg2.245(MoO4)3 and Na1.66Mn2.17(MoO4)3

In the samples of the Na₂MoO₄-MgMoO₄ system quenched in the air at above 600°C, by powder X-ray diffraction two double molybdates of variable composition are detected: monoclinic alluaudite-like Na_4?2x Mg_1+x (MoO₄)₃ (0.05 ≤ x ≤ 0.35) and triclinic Na_2?2y Mg_2+y (MoO₄)₃ (0.10 ≤ y ≤ 0.40) isostructural to previously studied Na₂Mg₅(MoO₄)₆. Sodium-magnesium molybdate of the Li₃Fe(MoO₄)₃ structure type is not revealed in this system. By spontaneous flux crystallization, the crystals are obtained and the structures of two triclinic double molybdates of the Na₂Mg₅(MoO₄)₆ structure type (space group $P\bar 1$ , Z = 1) containing magnesium and manganese are determined. The results of the refinement of site occupancies made it possible to determine the composition of the studied crystals: for the compound with magnesium (Na)_0.5(Na_0.255□_0.745)(Na_0.755Mg_0.245)Mg₂(MoO₄)₃ or Na_1.51Mg_2.245(MoO₄)₃ (a = 6.9577(1) Å, b = 8.6330(2) Å, c = 10.2571(2) Å, α = 106.933(1)°, β = 104.864(1)°, γ = 103.453(1)°, R = 0.0188); for the compound with manganese (Na)_0.5(Na_0.33□_0.67)(Na_0.83Mn_0.17)Mn₂(MoO₄)₃ or Na_1.64Mn_2.17(MoO₄)₃ (a = 7.0778(2) Å, b = 8.8115(2) Å, c = 10.4256(2) Å, α = 106.521(1)°, β = 105.639(3)°, Γ = 103.233(1)°, R = 0.0175). The Na₂Mg₅(MoO₄)₆ structure is redetermined and it is shown that actually it corresponds to the composition Na_1.40Mg_2.30(MoO₄)₃.     

Ca0.8-2x(YbxTb0.1Na0.1+x)2xWO4近红外下转换荧光粉水热合成及发光性质

一种在近红外光谱(NIR)区域高效的量子剪裁现象已在Ca_0.8-2x(Yb_xTb_0.1Na_0.1+x)_2xWO₄(x=0~0.2)荧光粉中得到证实,该量子剪裁通过吸收紫外线光子发射近红外光子,能量传递包括两个协同过程,分别是WO₄^2-基团到Yb³⁺离子和WO₄^2-基团到Tb³⁺离子再到Yb³⁺离子,Yb³⁺离子的掺杂浓度对荧光粉在可见光和近红外光谱的发光,荧光寿命和量子效率的影响已进行了详细得研究。经计算,量子效率最大达到135.7%。铽与镱共掺钨酸钙的近红外量子剪裁,通过吸收太阳光谱的1个紫外光到2个1000nm光子(2倍光子数增加)的下转化机制实现高效率硅太阳能电池的途径。     

The Crystal Chemistry of Lithium in the Alluaudite Structure: A Study of the (Na1−xLix)CdIn2(PO4)3 Solid Solution (x=0 to 1)

Several compounds of the (Na_1−xLi_x)CdIn₂(PO₄)₃ solid solution were synthesized by a solid-state reaction in air, and pure alluaudite-like compounds were obtained for x=0.00, 0.25, and 0.50. X-ray Rietveld refinements indicate the occurrence of Cd²⁺ in the M(1) site, and of In³⁺ in the M(2) site of the alluaudite structure. This non-disordered cationic distribution is confirmed by the sharpness of the infrared absorption bands. The distribution of Na⁺ and Li⁺ on the A(1) and A(2)′ crystallographic sites cannot be accurately assessed by the Rietvled method, probably because the electronic densities involved in the Na⁺→Li⁺ substitution are very small. A comparison with the synthetic alluaudite-like compounds, (Na_1−xLi_x)MnFe₂(PO₄)₃, indicates the influence of the cations occupying the M(1) and M(2) sites on the coordination polyhedra morphologies of the A(1) and A(2)′ crystallographic sites.     

A study of phase formation in the subsolidus region of the system Na<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-MnMoO<Subscript>4</Subscript>-Cr<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Phase relationships in the subsolidus region of the system Na₂MoO₄-MnMoO₄-Cr₂(MoO₄)₃ were studied by means of X-ray diffraction and differential-thermal analyses. The possibility of obtaining a variablecomposition phase Na_1?x Mn_1?x Cr_1+x (MoO₄)₃ (0 ≤ x ≤ 0.5) and ternary molybdate NaMn₃Cr(MoO₄)₅ was examined. The temperature dependence of the conductivity of the phase Na_1?x Mn_1?x Cr_1+x (MoO₄)₃ was analyzed.     

Luminescent properties and energy transfer process of Sm3+-Eu3+ co-doped MY2(MoO4)4 (M=Ca,Sr and Ba) red-emitting phosphors

MY₂(MoO₄)₄:Sm³⁺ and MY₂(MoO₄)₄:xSm³⁺,yEu³⁺ (M=Ca, Sr and Ba) phosphors were successfully prepared using solid-state reaction route, and their luminescent properties and energy transfer process from Sm³⁺ to Eu³⁺ were systematically investigated. The results indicate that MY₂(MoO₄)₄:Sm³⁺ phosphors can be effectively excited by 407 nm near UV light originating from the ⁶H_5/2 → ⁴F_7/2 transition of Sm³⁺, and exhibit a satisfactory red emission at 646 nm attributed to the ⁴G_5/2 → ⁶H_9/2 transition of Sm³⁺, in which the emission intensity of SrY₂(MoO₄)₄:Sm³⁺ is the strongest among the MY₂(MoO₄)₄:Sm³⁺ (M=Ca, Sr and Ba) phosphors. For Eu³⁺ co-doped MY₂(MoO₄)₄:Sm³⁺ samples, with increasing Eu³⁺ doping content, the main emission peaks of Sm³⁺ (approximately 646 nm) are decreased, but the emission peaks and intensity of Eu³⁺ are increased while the maximum intensity of luminescence at the Eu³⁺ concentration 0.9. The introduction of Eu³⁺ in the MY₂(MoO₄)₄:Sm³⁺ phosphors can remarkably generate a strong emission line at 616 nm, originating from the ⁵D₀→⁷F₂ transition of Eu³⁺ and Sm³⁺ (⁴G_5/2) → Eu³⁺ (⁵D₀) effective energy transfer process. The energy transfer mechanism from Sm³⁺ to Eu³⁺ was discussed in detail.     

Hydrothermal Synthesis and Up‐conversion Luminescence of Ho3+/Yb3+ Co‐doped PbTiO3

Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals with different content of dopant were successfully prepared via a facile hydrothermal method. The purity, morphology, element distribution, chemical state and up‐conversion (UC) photoluminescence (PL) of PbTiO₃ nanocrystals affected by Ho³⁺ dopant are investigated systematically. X‐ray diffraction (XRD) results illustrate that PbTiO₃ samples with the doping Ho³⁺ concentration ranging from 0 to 5 mol‐% are perovskite structure. The doping Ho³⁺ ions have no change on the crystal structure of perovskite PbTiO₃. Owing to the non‐equivalent substitution of Ho³⁺ to Ti⁴⁺ in PbTiO₃, the particle size of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples is decreased as well as the particle agglomeration is detected. Moreover, Ho and Yb ions have uniform distributions in the PbTiO₃ nanoparticles as the presence of Ho³⁺ and Yb³⁺ cations. The up‐conversion spectra demonstrate that Ho/Yb co‐doped PbTiO₃ samples have up‐conversion emissions centered at 550 nm, 660 nm and 755 nm, corresponding to the transitions of ⁵F₄(⁵S₂)→⁵I₈, ⁵F₅→⁵I₈ and ⁵S₂(⁵F₄)→⁵I₇ of Ho³⁺ ions. Additionally, the effect of temperature on the UC PL property of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ system is further investigated. The sensitivity and the trend of Ho³⁺/Yb³⁺ co‐doped PbTiO₃ samples in temperature from 298 k to 493K are calculated on the basis of fluorescence intensity ratio (FIR) method. Ho³⁺/Yb³⁺ co‐doped PbTiO₃ nanocrystals are verified the high potential in the optical temperature sensing.     

Vibrational spectra and electrophysical properties of a NASICON phase of variable composition Na1 − x Co1 − x Sc1 + x (MoO4)3 (0 ≤ x ≤ 0.5)

Subsolidus phase ratios of the Na₂MoO₄-CoMoO₄-Sc₂(MoO₄)₃ system have been studied by X-ray diffraction, differential thermal analysis, and vibrational spectroscopy. A phase of variable composition Na_{1 ? x} Co_{1 ? x} Sc_{1 + x} (MoO₄)₃, 0 ≤ x ≤ 0.5 having NASICON structure (space group \(R\bar 3c\) ) and triple molybdate NaCo₃Sc(MoO₄)₅ crystallizing in triclinic system (space group \(P\bar 1\) ) have been obtained. The high conductivity of Na_{1 ? x} Co_{1 ? x} Sc_{1 + x} (MoO₄)₃ allows the phase of variable composition to be regarded as a promising sodium-ion conducting solid electrolyte.     

Revised phase diagram of Li2MoO4-ZnMoO4 system, crystal structure and crystal growth of lithium zinc molybdate

Orthorhombic lithium zinc molybdate was first chosen and explored as a candidate for double beta decay experiments with ¹⁰⁰Mo. The phase equilibria in the system Li₂MoO₄-ZnMoO₄ were reinvestigated, the intermediate compound Li₂Zn₂(MoO₄)₃ of the α-Cu₃Fe₄(VO₄)₆ (lyonsite) type was found to be nonstoichiometric: Li_2−2xZn_2+x(MoO₄)₃ (0≤x≤0.28) at 600 °C. The eutectic point corresponds to 650 °C and 23 mol% ZnMoO₄, the peritectic point is at 885 °C and 67 mol% ZnMoO₄. Single crystals of the compound were prepared by spontaneous crystallization from the melts and fluxes. In the structures of four Li_2−2xZn_2+x(MoO₄)₃ crystals (x=0; 0.03; 0.21; 0.23), the cationic sites in the face-shared octahedral columns were found to be partially filled and responsible for the compound nonstoichiometry. It was first showed that with increasing the x value and the number of vacancies in M3 site, the average M3-O distance grows and the lithium content in this site decreases almost linearly. Using the low-thermal-gradient Czochralski technique, optically homogeneous large crystals of lithium zinc molybdate were grown and their optical, luminescent and scintillating properties were explored.     

Synthesis and characterization of a NASICON phase of variable composition Na1 − x Ni1 − x Sc1 + x (MoO4)3 (0 ≤ x ≤ 0.5)

Subsolidus phase ratios in the Na₂MoO₄-NiMoO₄-Sc₂(MoO₄)₃ system have been studied using X-ray diffraction, differential thermal analysis, and vibrational spectroscopy. A phase of variable composition Na_{1 ? x} Ni_{1 ? x} Sc_{1 + x} (MoO₄)₃ (0 ≤ x ≤ 0.5) having NASICON structure (space group \(R\bar 3c\) ) and a triple molybdate crystallizing in triclinic system (space group \(P\bar 1\) ) have been obtained. The high conductivity of Na_{1 ? x} Ni_{1 ? x} Sc_{1 + x} (MoO₄)₃ allows the phase of variable composition to be regarded as a promising sodiumion-conductive solid electrolyte.     

Synthesis,characterization, and x-ray structural study of binary lithium managanese(II) molybdate

The binary molybdate of variable composition Li_2?2nMn_2+x(MoO₄)₃ (O2Fe₂(MoO₄)₃, was discovered in the Li₂MoO₄-MnMoO₄ system. We have grown single crystals of Li_1.60Mn_2.20(MoO₄)₃) and determined its crystal structure (space group Pnma, a=5.145, b=10.681, c=17.985 Å, Z=4). Along with statistical arrangement of Li and Mn in three different atomic positions, cation vacancies in one of these were found. Based on the data obtained, we propose to revise the compositions of some lithium-containing phases with the Li₂Fe₂(MoO₄)₃-type structure.     

Vacuum ultraviolet and near-infrared excited luminescence properties of Ca3(PO4)2:RE, Na (RE=Tb, Yb, Er, Tm, and Ho)

Tb³⁺, Yb³⁺, Tm³⁺, Er³⁺, and Ho³⁺ doped Ca₃(PO₄)₂ were synthesized by solid-state reaction, and their luminescence properties were studied by spectra techniques. Tb³⁺-doped samples can exhibit intense green emission under VUV excitation, and the brightness for the optimal Tb³⁺ content is comparable with that of the commercial Zn₂SiO₄:Mn²⁺ green phosphor. Under near-infrared laser excitation, the upconversion luminescence spectra of Yb³⁺, Tm³⁺, Er³⁺, and Ho³⁺ doped samples demonstrate that the red, green, and blue tricolored fluorescence could be obtained by codoping Yb³⁺-Ho³⁺, Yb³⁺-Er³⁺, and Yb³⁺-Tm³⁺ in Ca₃(PO₄)₂, respectively. Good white upconversion emission with CIE chromaticity coordinates (0.358, 0.362) is achieved by quadri-doping Yb³⁺-Tm³⁺-Er³⁺-Ho³⁺ in Ca₃(PO₄)₂, in which the cross-relaxation process between Er³⁺ and Tm³⁺, producing the ¹D₂-³F₄ transition of Tm³⁺, is found. The upconversion mechanisms are elucidated through the laser power dependence of the upconverted emissions and the energy level diagrams.     

A general approach for selective enhancement of green upconversion emissions in Er3+ doped oxides by Yb3+–MoO4 2− dimer sensitizing

In this paper, we report a general approach to enhance the upconversion (UC) luminescence of Er³⁺ doped oxides phosphors by Yb³⁺–MoO₄ ^2? dimer sensitizing, which induced strong green UC emissions under the 976 nm laser diode excitation. By codoping of Yb³⁺ and Mo⁶⁺ in the Er³⁺ doped TiO₂ and ZnO, the green UC emissions intensity can be selectively increased about 10 and 500 times than those of Er³⁺–Yb³⁺ codoped TiO₂ and ZnO, respectively. The high excited state energy transfer between |²F_7/2, ³T₂> state of Yb³⁺–MoO₄ ^2? dimer and ⁴F_7/2 level of Er³⁺ significantly avoids the nonradiative decay processes happened at lower energy levels of Er³⁺, and then increases the green UC emissions efficiently. The proposed Yb³⁺–MoO₄ ^2? dimer sensitizing has been realized as an efficient way to enhance the green UC emissions in other Er³⁺ doped oxides phosphors. It is expected that the selective enhanced green UC emissions sensitized by Yb³⁺–MoO₄ ^2? dimer in Er³⁺ doped oxides phosphors can greatly extend their scope of applications.     

Synthesis and electrochemical properties of Li9V3 − x Ti x (P2O7)3(PO4)2/C compounds via wet method for lithium-ion batteries

A series of Ti⁴⁺-doped Li₉V_3???x Ti _x (P₂O₇)₃(PO₄)₂/C compounds have been prepared by using wet method. X-ray diffraction measurement shows that single phase region can be expressed as x?≤?0.10. The effects of substitution of Ti for V on the electrochemical properties of Li₉V_3???x Ti _x (P₂O₇)₃(PO₄)₂ compounds have been studied. Our investigations show that Ti doping can improve the electrochemical performance. The Li₉V_2.95Ti_0.05(P₂O₇)₃(PO₄)₂/C exhibits the best cycle performance and the highest first discharge capacity of 120.7 mAh g^?1 at 0.2 C. The electrochemical impedance spectroscopy indicates that the charge transfer resistance initially decreases with x and then for x?>?0.05 increases monotonically with Ti⁴⁺ content.     

Synthesis and characterization of the variable-composition phase Na1−x Ni1−x Cr1+x (MoO4)3 (0 ≤ x ≤ 0.4) with a NASICON structure

Synthetic conditions are determined for the variable-composition phase Na_1?x Ni_1?x Cr_1+x (MoO₄)₃ (0 ≤ x ≤ 0.4) with a NASICON structure. The unit cell parameters of this phase are derived from X-ray powder diffraction data, and the phase is characterized by IR and Raman spectroscopy.     

A new high voltage alluaudite sodium battery insertion material

Large-scale stationary storage forms a key sector that can be economically served by sodium-ion batteries. In realizing practical sodium-ion batteries, discovery and development of novel cathodes is essential. In this spirit, alluaudite-type Na₂Fe₂(SO₄)₃ was reported in 2014 to have the highest Fe³⁺/Fe²⁺ redox potential (～3.8 V vs. Na). This finding led to reports on various PO₄^3? and SO₄^2? based alluaudite compounds exhibiting high energy densities. In 2017, MoO₄^2? based alluaudite, Na_2.67Mn_1.67(MoO₄)₃, was found as a 3.45 V cathode material. Exploring molybdenum chemistry further, this work reports alluaudite type Na_3.36Co_1.32(MoO₄)₃ (NCMo) as a novel versatile electroactive cathode for Li-ion and Na-ion batteries. It was synthesized by a wet solution-combustion route with a restricted annealing duration of 1 min at 600 °C. Calorimetric study revealed the formation enthalpy from component oxides (ΔH°_f,ox = ?575.49 ± 7.75 kJ/mol) to be highly exothermic. Unlike the sulfate class of alluaudites, this material is highly stable in air and moisture (ΔH_ds = 537.42 ± 0.78 kJ/mol). Having an ionic conductivity of 6.065 × 10^?8 S/cm (at 50 °C), it offers a pseudo two-dimensional Na⁺ migration pathway. Without any material optimization, NCMo was found to work as a high-voltage insertion cathode (ca. 4.0 V vs. Na/Na⁺ and 4.1 V vs. Li/Li⁺) in sync with theoretically predicted potential of 3.98 V (vs. Na/Na⁺). Ex-situ X-ray diffraction and photoelectron spectroscopy studies revealed the occurrence of solid-solution redox mechanism solely involving Co³⁺/Co²⁺ redox centre. It benchmarks Na_3.36Co_1.32(MoO₄)₃ as a novel electrochemically active Mo-based alluaudite-type polyanionic cathode insertion material.     

Electroconductance of single-crystal Li2 + x Fe 2 − 2x 2+ Fe x 3+ (MoO4)3 (x = 0.22)

Electrophysical properties of single-crystal Li_{2 + x} Fe _{2 ? 2x} ²⁺ Fe _x ³⁺ (MoO₄)₃ (x = 0.22) are studied at 25–400°C. It is found that the conduction is of electronic nature and the conductivity equals 5 × 10^-2 S/cm at 300°C. The activation energy for the electron transport is 0.23 eV. The conductance in molybdate Li_2.22Fe _1.56 ²⁺ Fe _0.22 ³⁺ (MoO₄)₃ is markedly anisotropic.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.