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.     

Photoluminescence properties of a novel phosphor, Na3La9O3(BO3)8:RE(RE=Eu,Tb)

The new oxyborate phosphors, Na₃La₉O₃(BO₃)₈:Eu³⁺ (NLBO:Eu) and Na₃La₉O₃(BO₃)₈:Tb³⁺ (NLBO:Tb) were prepared by solid-state reactions. The photoluminescence characteristics under UV excitation were investigated. The dominated emission of Eu³⁺ corresponding to the electric dipole transition ⁵D₀→⁷F₂ is located at 613 nm and bright green luminescence of NLBO:Tb attributed to the transition ⁵D₄→⁷F₅ is centered at 544 nm. The concentration dependence of the emission intensity showed that the optimum doping concentration of Eu and Tb is 30% and 10%, respectively.     

Thermochemistry of Li1+xMn2−xO4 (0?x?1/3) spinel

Lithium substituted Li_1+xMn_2−xO₄ spinel samples in the entire solid solution range (0?x?1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x?0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na₂O·4MoO₃ at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x?0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn⁴⁺ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn³⁺ to Mn⁴⁺ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.     

Solid-state synthesis, characterization and luminescent properties of Eu-doped gadolinium tungstate and molybdate phosphors: Gd(2−x)MO6:Eux (M=W, Mo)

Red phosphors gadolinium tungstate and molybdate with the formula Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) were successfully synthesized by the solid-state reaction at 900 and 1300 °C for 4 h, respectively. The products were characterized by an X-ray powder diffractometer (XRD), TG-DSC, FT-IR, PL, UV-vis and SEM. Room-temperature photoluminescence indicated that the as-prepared Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) had a strong red emission, which is due to the characteristic transitions of Eu³⁺ (⁵D₀→⁷F_J, J=0, 1, 2, 3, 4) for these phosphors. Meanwhile, the ⁵D₀→⁷F₂ is in the dominant position. The emission quantum efficiency of Eu³⁺ in the Gd_(2−x)MO₆:Eu_x³⁺ (M=Mo, W) system has been investigated. The XRD results indicate that both Gd₂WO₆ and Gd₂MoO₆ belong to the monoclinic system with space group C2/c [A. Bril, G. Blasse, J. Chem. Phys. 45 (1966) 2350-2356] and I2/a [A. Bril, G. Blasse, J. Chem. Phys. 45 (1966) 2350-2356], respectively. SEM images indicate that the shape of Gd_1.96WO₆:Eu_0.04³⁺ is aggregated small particles with a mean diameter of about 300 nm, and the shape of Gd_1.96MoO₆:Eu_0.04³⁺ is block-like structures.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

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.     

Synthesis and structural study of a new NASICON-type solid solution: Li1−xLax/3Zr2(PO4)3

A new complete solid solution of NASICON-type compounds between LiZr₂(PO₄)₃ and La_1/3Zr₂(PO₄)₃ was evidenced with the general formula Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1). These phases were synthesized by a complex polymerizable method and structurally characterized from Rietveld treatment of their X-ray and neutron powder diffraction data. This solid solution results from the substitution mechanism Li⁺→1/3La³⁺+2/3□ leading to an increase of the vacancies number correlated to an increase of the La content. According to this substitution mechanism, the general formula can then be written Li_1−xLa_x/3□_2x/3Zr₂(PO₄)₃ (0?x?1) in order to underline the correlation between the La content and the vacancies rate. For all the compounds, the structure is clearly related to that of the NASICON family with three crystallographic domains evidenced. For 0?x?0.5, all the members adopt at high temperature the typical NASICON-type structure (s.g. R3¯c), while at lower temperature, their structure distorts to a triclinic form (s.g. C 1¯), as observed for LiZr₂(PO₄)₃ prepared above 1100 °C. Moreover, in this domain, the reversible transition is clearly soft and the transition temperature strongly depends of the x value. For 0.6?x?0.9, the compounds crystallize in a rhombohedral cell (s.g. R3¯), while for x=1, the phase La_1/3Zr₂(PO₄)₃ is obtained (s.g. P3¯, Z=6, a=8.7378(2) Å, c=23.2156(7) Å).This paper is devoted to the structure analysis of the series Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1), from X-ray and neutron powder thermo diffraction and transmission electron microscopy (TEM) studies.     

Composition-induced structural phase transitions in the (Ba1−xLax)2In2O5+x (0?x?0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba_1−xLa_x)₂In₂O_5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

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, optical properties and electronic structures of polyoxometalates K3P(Mo1−xWx)12O40 (0?x?1)

Various compositions of solid solutions K₃P(Mo_1−xW_x)₁₂O₄₀ (0?x?1) were prepared using two solid state synthetic routes. The crystallite size was determined by linewidth refinements of X-ray diffraction patterns using the Warren-Averbach method, and the grain size distribution by laser scattering experiments. Optical properties were determined by diffuse reflectance measurements in the UV-visible range. The optical gap E_g was found to increase exponentially from ∼2.5 to ∼3.30 eV with increasing x, and is systematically shifted to a higher energy when the grain size decreases. The relation between E_g and x was analyzed by calculating the HOMO-LUMO gaps of the [P(Mo_1−xW_x)₁₂O₄₀]³⁻ anions on the basis of tight-binding electronic structure calculations.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.     

A new promising phosphor, Na3La2(BO3)3:Ln (Ln=Eu, Tb)

We present an efficient way to search a host for ultraviolet (UV) phosphor from UV nonlinear optical (NLO) materials. With the guidance, Na₃La₂(BO₃)₃ (NLBO), as a promising NLO material with a broad transparency range and high damage threshold, was adopted as a host material for the first time. The lanthanide ions (Tb³⁺ and Eu³⁺)-doped NLBO phosphors have been synthesized by solid-state reaction. Luminescent properties of the Ln-doped (Ln=Tb³⁺, Eu³⁺) sodium lanthanum borate were investigated under UV ray excitation. The emission spectrum was employed to probe the local environments of Eu³⁺ ions in NLBO crystal. For red phosphor, NLBO:Eu, the measured dominating emission peak was at 613 nm, which is attributed to ⁵D₀-⁷F₂ transition of Eu³⁺. The luminescence indicates that the local symmetry of Eu³⁺ in NLBO crystal lattice has no inversion center. Optimum Eu³⁺ concentration of NLBO:Eu³⁺ under UV excitation with 395 nm wavelength is about 30 mol%. The green phosphor, NLBO:Tb, showed bright green emission at 543 with 252 nm excited light. The measured concentration quenching curve demonstrated that the maximum concentration of Tb³⁺ in NLBO was about 20%. The luminescence mechanism of Ln-doped NLBO (Tb³⁺ and Eu³⁺) was analyzed. The relative high quenching concentration was also discussed.     

Evolution of structure and magnetic properties in electron-doped double perovskites, Sr2−xLaxMnWO6 (0?x?1)

Powder neutron and X-ray diffraction studies show that the double perovskites in the region 0?x?1 exhibit two crystallographic modifications at room temperature: monoclinic P2₁/n and tetragonal I4/m, with a boundary at 0.75<x<0.9. Magnetic susceptibility measurements indicate that for x=0 and 0.5 Sr_2−xLa_xMnWO₆ orders antiferromagnetically (AFM) at 15 and 25 K, respectively, for 0.75?x<1.0, a contribution of weak ferromagnetism (FM), probably due to canted-AFM order, increases with increasing x. The end point compound SrLaMnWO₆ shows the strongest FM cluster effect; however, no clear evidence of magnetic order is discernable down to 4.2 K. X-ray absorption spectroscopy (XAS) confirms Mn²⁺ and mixed-valent W^6+/5+ formal oxidation states in Sr_2−xLa_xMnWO₆.     

Synthesis, vacuum ultraviolet and near ultraviolet-excited luminescent properties of GdCaAl3O7: RE (RE=Eu, Tb)

Vacuum ultraviolet (VUV) excitation and photoluminescent (PL) properties of Eu³⁺ and Tb³⁺ ion-doped aluminate phosphors, GdCaAl₃O₇:Eu³⁺ and GdCaAl₃O₇:Tb³⁺ have been investigated. X-ray diffraction (XRD) patterns indicate that the phosphor GdCaAl₃O₇ forms without impurity phase at 900 °C. Field emission scanning electron microscopy (FE-SEM) images show that the particle size of the phosphor is less than 3 μm. Upon excitation with VUV irradiation, the phosphors show a strong emission at around 619 nm corresponding to the forced electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺, and at around 545 nm corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺. The results reveal that both GdCaAl₃O₇:RE³⁺ (RE=Eu, Tb) are potential candidates as red and green phosphors, respectively, for use in plasma display panel (PDP).     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.     

Synthesis, crystal structure, infrared and Raman spectra of Sr4Cu3(AsO4)2(AsO3OH)4·3H2O and Ba2Cu4(AsO4)2(AsO3OH)3

The two new compounds, Sr₄Cu₃(AsO₄)₂(AsO₃OH)₄·3H₂O (1) and Ba₂Cu₄(AsO₄)₂(AsO₃OH)₃(2), were synthesized under hydrothermal conditions. They represent previously unknown structure types and are the first compounds synthesized in the systems SrO/BaO-CuO-As₂O₅-H₂O. Their crystal structures were determined by single-crystal X-ray diffraction [space group C2/c, a=18.536(4) Å, b=5.179(1) Å, c=24.898(5) Å, β=93.67(3)°, V=2344.0(8) Å³, Z=4 for 1; space group P4₂/n, a=7.775(1) Å, c=13.698(3) Å, V=828.1(2) Å³, Z=2 for 2]. The crystal structure of 1 is related to a group of compounds formed by Cu²⁺-(XO₄)³⁻ layers (X=P⁵⁺, As⁵⁺) linked by M cations (M=alkali, alkaline earth, Pb²⁺, or Ag⁺) and partly by hydrogen bonds. In 1, worth mentioning is the very short hydrogen bond length, D···A=2.477(3) Å. It is one of the examples of extremely short hydrogen bonds, where the donor and acceptor are crystallographically different. Compound 2 represents a layered structure consisting of Cu₂O₈ centrosymmetric dimers crosslinked by As1φ₄ tetrahedra, where φ is O or OH, which are interconnected by Ba, As2 and hydrogen bonds to form a three-dimensional network. The layers are formed by Cu₂O₈ centrosymmetric dimers of CuO₅ edge-sharing polyhedra, crosslinked by As1O₄ tetrahedra. Vibrational spectra (FTIR and Raman) of both compounds are described. The spectroscopic manifestation of the very short hydrogen bond in 1, and ABC-like spectra in 2 were discussed.     

A new structure type of phosphate: Crystal structure of Na2Zn5(PO4)4

A new compound, Na₂Zn₅(PO₄)₄, was identified in the system ZnONa₂OP₂O₅ and high-quality crystal was obtained by the melt method. The crystal structure of this compound was solved by direct method from single crystal X-ray diffraction data. The structure was then refined anisotropically using a full-matrix least square refinement on F² and the refinement converged to R₁=0.0233 and wR₂=0.0544. This compound crystallizes in the orthorhombic system with space group Pbcn, lattice parameters a=10.381(2) Å, b=8.507(1) Å, c=16.568(3) Å and Z=4. The structure is made up of 3D [Zn₅P₄O₁₆]_n²ⁿ⁻ covalent framework consisting of [Zn₄P₄O₁₆]_n⁴ⁿ⁻ layers. The powder diffraction pattern of Na₉Zn₂₁(PO₄)₁₇ is explained by simulating a theoretical pattern with NaZnPO₄ and Na₂Zn₅(PO₄)₄ in the molar ratio of 1:4 and then by Rietveld refinement of experimental pattern. Na₂Zn₅(PO₄)₄ melts congruently at 855 °C and its conductivity is 5.63×10⁻⁹ S/cm.