The magnetocaloric effect in (Gd0.74Tb0.26)5(SixGe1−x)4 alloys

The magnetic phase transition and magnetocaloric effect of the alloys of Gd_0.74Tb_0.26 and (Gd_0.74Tb_0.26)₅(Si_xGe_1−x)₄ (x = 0.43, 0.50, 0.505) have been investigated by magnetization measurement. Experimental results show that partial substitution of the Gd by Tb in Gd₅(Si_xGe_1−x)₄ system keeps the first order magnetic transition. Although the values of transition temperature decrease, the as-cast (Gd_0.74Tb_0.26)₅(Si_0.43Ge_0.57)₄ and annealed (Gd_0.74Tb_0.26)₅(Si_0.50Ge_0.50)₄ alloys display large magnetic entropy change up to 18.89 J kg^− 1 K^− 1 and 13.79 J kg^− 1 K^− 1 near their transition temperatures in the low magnetic field change of 0–2.0 T, respectively.     

Microstructure,luminescence and thermal stability properties of NaSrPO4:Tb3+ phosphors with various doping concentrations prepared using conventional solid-state sintering

This paper reports the microstructure, luminescence and thermal stability properties of the NaSr_1−xPO₄:xTb³⁺ powders (x = 0.008, 0.01, 0.02, 0.04 and 0.06) via the conventional solid-state sintering at 1200 °C for 5 h. The X-ray diffraction result verifies all diffraction peaks are pure phase of NaSrPO₄. The luminescence results show that the NaSrPO₄:xTb³⁺ powders mainly excited at 370 nm have a series of the emission-states, related to the typical 4f → 4f intra-configuration forbidden transitions of Tb³⁺, and a major emission peak of around 546 nm. The concentration quenching of the NaSr_1−xPO₄:xTb³⁺ phosphors is appeared at x = 0.02. The decay time values of the NaSr_1−xPO₄:xTb³⁺ phosphors for the ⁵D₄ state of the Tb³⁺ are around 3.30 ms to 3.60 ms. It is also found the chromaticity coordinate of NaSrPO₄:Tb³⁺ phosphor varies with the increase of the concentration of Tb³⁺ ions from blue to green. Moreover, the thermal stability of the NaSrPO₄:xTb³⁺ phosphors is slightly better than that of conventional YAG phosphors.     

Abnormal blue-shift of Eu2+-activated calcium zirconium phosphates CaZr4(PO4)6

The electronic structure of CaZr₄(PO₄)₆ was calculated using the CASTEP code and the band gap for CaZr₄(PO₄)₆ can reach up to 4.30 eV. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) samples were prepared by a high temperature solid-state reaction method. XRD analysis shows that Eu²⁺ ion can be totally incorporated into CaZr₄(PO₄)₆ forming complete solid solutions with trigonal lattice. Ca_1−xEu_xZr₄(PO₄)₆ (0.01 ⩽ x ⩽ 1) shows typical broad band emission in wavelength range from 400 to 650 nm for both under ultraviolet (UV) light and X-ray excitation, originating from the 4f⁶5d¹ → 4f⁷5d⁰ transition of Eu²⁺ ions. With increasing Eu²⁺ concentration, there is abnormal blue-shift of the emission peaks for Ca_1−xEu_xZr₄(PO₄)₆ due to the decreasing crystal field strength and Stokes shift. With increasing temperature in CaZr₄(PO₄)₆: Eu²⁺, its emission bands show the anomalous blue-shift with decreasing intensity. The overall scintillation efficiency of Ca_0.9Eu_0.1Zr₄(PO₄)₆ is 1.7 times of that of Bi₄Ge₃O₁₂ (BGO) powder under the same conditions. In addition, its predominant decay time is about 50 ns at room temperature. The potential application of Eu²⁺-doped CaZr₄(PO₄)₆ has been pointed out.     

Photoluminescence properties of novel KBaBP2O8:M (M = Pb2+ and Bi3+) phosphors

A series of novel inorganic phosphors KBa_1−xPb_xBP₂O₈ and K_1+xBa_1−2xBi_xBP₂O₈ (0.01 ⩽ x ⩽ 0.08) were synthesized by using a solid-state reaction technique at high-temperature and their photoluminescence properties were investigated. The dependence of the emission intensity on the Pb²⁺ and Bi³⁺ concentration for the KBa_1−xPb_xBP₂O₈ and K_1+xBa_1−2xBi_xBP₂O₈ was studied, in which the optimal concentration as well as the critical transfer distance R_c for Pb²⁺ and Bi³⁺ was obtained and determined. The as-prepared phosphors can be effectively excited with ultraviolet (UV), and exhibit UV − blue emission with large Stokes shift. The above work indicates these phosphors could be potential candidates for application in UV lamps industry.     

Synthesis of high intensity green emitting (Ba,Sr)SiO4:Eu2+ phosphors through cellulose assisted liquid phase precursor process

Green emitting phosphor (Ba_1−x,Sr_1−x)SiO₄:2xEu²⁺, x = 0.03, 0.05, 0.1, and 0.15 were synthesized through a Liquid Phase Precursor Process (LPP). Liquid phase precursor method is reported to result in phosphors with markedly increased emission intensities compared to other synthesis methods. Here microcrystalline cellulose (MCC) and hydroxypropyl cellulose (HPC) templates were studied to achieve high efficiency green phosphors. The phase formation was investigated by XRD analysis which showed the conformation of the Ba₂SiO₄ (JCPDS card number 761631) phase. The obtained samples exhibited broad excitation spectra with maximum at 430 nm and a green emission centered around 520 nm. An optimized dopant concentration was selected and the effect of two different types of cellulose, i.e. MCC and HPC templates on the emission properties was considered. It was found that the samples synthesized using HPC and fired at 1050 °C under a reducing atmosphere, showed a high intensity of almost 2 times that of the MCC sample. Further experiments were conducted by varying viscosity, particle weight, and molecular weight of the HPC template. It was found that the green emission from the (Ba,Sr)SiO₄:Eu²⁺ increased with the increase in viscosity and molecular weight of the template.     

VUV photoluminescence of green-emitting GdPO4:Tb,M (M: Al,Zn) synthesized by ultrasonic spray pyrolysis

We successfully synthesized (Gd_0.94−xTb_0.06M_x)PO₄ (M: Al and Zn; 0 ≤ x ≤ 0.06) green phosphors by ultrasonic spray pyrolysis and investigated their photoluminescence characteristics. The synthesized (Gd_0.94−xTb_0.06M_x)PO₄ phosphor powders showed a spherical morphology and a smooth surface. The phosphors emitted a green light due to the transition from ⁵D₄ to ⁷F_j of Tb³⁺ (j = 3–6) at 489, 544, 585, and 621 nm, respectively. The partial incorporation of Al or Zn for Gd up to x = 0.45 in (Gd_0.94Tb_0.06)PO₄ phosphors yielded a significant rise in the emission intensity. The (Gd_0.895Tb_0.06Al_0.045)PO₄ and (Gd_0.895Tb_0.06Zn_0.045)PO₄ phosphors showed 31 and 13% stronger emission intensity at 544 nm, respectively, compared to (Gd_0.94Tb_0.06)PO₄ phosphor.     

Synthesis and photoluminescence properties of NaLaMgWO6:RE3+ (RE = Eu,Sm, Tb) phosphor for white LED application

Single phase of NaLa_1?xMgWO₆:xRE³⁺ (0 < x ≤1) (RE = Eu, Sm, Tb) phosphors were prepared by solid-state reaction method. X-ray diffraction, scanning electron microscopy, the morphology energy-dispersive X-ray spectroscopy, UV–vis diffuse reflectance spectra and photoluminescence were used to characterize the samples. Under the light excitation, NaLaMgWO₆:Eu³⁺, NaLaMgWO₆:Sm³⁺ and NaLaMgWO₆:Tb³⁺, phosphors showed the characteristic emissions of Eu³⁺ (⁵D₀ → ⁷F_{4, 3, 2, 1}), Sm³⁺ (⁴G_5/2 → ⁶H_{5/2, 7/2, 9/2}), and Tb³⁺ (⁵D₄ → ⁷F_{6, 5, 4, 3}), respectively. The intensity of the red emission for Na(La_0.6Eu_0.4)MgWO₆ is 2.5 times higher than that of (Y_0.95Eu_0.05)₂O₃ under blue light irradiation. The quantum efficiencies of the entitled phosphors excited under 394 nm and 464 nm are also investigated and compared with commercial phosphors Y₂O₃:Eu³⁺, Sr₂Si₅N₈:Eu²⁺ and Y₃A₅G₁₂:Ce³⁺. The results demonstrated NaLaMgWO₆:RE³⁺ phosphors as potential candidates for white light emitting diode pumped by UV or blue chip.     

Hole distribution in La2−xSrxCu1−yZn(Ni)yO4 (x ⩽ 1/8)

Simulation with interacting-potentials has been performed to investigate hole distribution in La_2−xSr_xCu_1−yZn(Ni)_yO₄ (y = 1/32 and x ⩽ 1/8). It is found that the orderly distributed hole configurations are more energetically favorable than the randomly distributed hole configurations. Moreover, the doped Zn²⁺ and Ni²⁺ ions can both act as stabilizing centers to localize the holes in their vicinity leading to an atomic-scale inhomogeneous distribution of holes. This atomic-scale inhomogeneities of holes obtained from our simulation is similar to that predicted by the “Swiss Cheese” model.     

Microstructure and photoluminescent properties of BaY2ZnO5:Tb3+ phosphors with addition of lithium carbonate

In this paper, Tb³⁺ activated BaY₂ZnO₅ phosphors with addition of lithium carbonates (0, 0.0125, 0.025, 0.05, 0.1, and 0.2 molar ratio) were synthesized using the solid-state reaction, and the morphology and photoluminescent properties are investigated. When BaY₂ZnO₅:Tb³⁺ phosphors to which lithium carbonates were added were sintered at 1250 °C for 12 h, X-ray powder diffraction analysis showed that the un-reacted Y₂O₃ raw material decreased and scanning electron microscopy showed that the grain growth is improved. From PL studies, the photoluminescent properties were decreased when Li⁺ ion was added, which might be resulted from the increase of the oxygen vacancy in BaY₂ZnO₅:Tb³⁺ phosphors. The green emission of the BaY₂ZnO₅:Tb³⁺, Li⁺ phosphors showed the CIE chromaticity coordinates in the range of x = 0.3361–0.3510 and y = 0.5202–0.5492.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Microwave-induced combustion synthesis and electrical conductivity of Ce1−xGdxO2−1/2x ceramics

Ce_1−xGd_xO_2−1/2x nanopowder were successfully synthesized by microwave-induced combustion process. For the preparation, cerium nitrate, gadolinium nitrate hexahydrate, and urea were used for the microwave-induced combustion process. The process took only 30 min to obtain Ce_1−xGd_xO_2−1/2x powders. The exo-endo temperature, phase identification, and morphology of resultant powders were investigated by TG/DTA, XRD, and SEM. The as-received Ce_1−xGd_xO_2−1/2x powders showed that the average particle size ranged from 18 to 50 nm, crystallite dimension varied from 11 to 20 nm, and the specific surface area was distribution from 16 to 46 m²/g. As for Ce_1−xGd_xO_2−1/2x ceramics sintered at 1450 °C for 3 h, the bulk density of Ce_1−xGd_xO_2−1/2x ceramics were over 91% of the theoretical density, the maximum electrical conductivity, σ_700 °C = 0.017 S/cm with minimum activation energy, E_a = 0.869 eV was found at Ce_0.80Gd_0.20O_1.90 ceramic.     

Interplay between random laser performance and self-frequency conversions in NdxY1.00−xAl3(BO3)4 nanocrystals powders

Random Laser emission at 1.06 μm, self-second-harmonic generation at 0.53 μm and self-sum-frequency generation at 0.46 μm were investigated in Nd_xY_1.00−xAl₃(BO₃)₄ nanocrystalline powders, for 0.05 ⩽ x ⩽ 1.00, excited by a pulsed laser operating at 808 nm, focusing on the interplay between the RL performance and the second-order nonlinear processes. The RL performance, characterized by a figure-of-merit relating the laser slope efficiency and the excitation pulse energy threshold, improved as x increased up to 1.00 while the efficiency of the self-frequency conversion processes reduced for increasing x because of distortions introduced in the crystalline structure of the grains. The RL wavelength was also dependent on the Nd³⁺ concentration and presented a redshift from 1061.9 nm to 1063.5 nm for increasing values of x.     

Synthesis and properties of Ce1−xGdxO2−x/2 solid solution prepared by flame spray pyrolysis

Flame spray pyrolysis, which produces ultrafine particles, was applied to the synthesis of Ce_1−xGd_xO_2−x/2 solid solutions by substituting Gd from a mole fraction of 0–0.40. The solubility limit of Gd in the Ce_1−xGd_xO_2−x/2 solid solution produced by flame spray pyrolysis was between 0.25 and 0.30, which is consistent with the reported value. The as-prepared Ce_1−xGd_xO_2−x/2 particles had a square morphology and a nanometer range in the equivalent diameter. The small particle size made it possible to reduce the sintering temperature of the Ce_1−xGd_xO_2−x/2 solid solution from 1650 °C to 1400 °C for the ceria-based solid electrolytes produced by the solid state preparation. The maximum ionic conductivity was achieved when the mole fraction of Gd was 0.25. The mole fraction for the highest ionic conductivity was the same as the particles produced by hydrothermal synthesis. However, the ionic conductivity of the Ce_1−xGd_xO_2−x/2 prepared by the flame spray pyrolysis (1.01 × 10⁻² S/cm at 600 °C) was higher than that prepared by the hydrothermal synthesis (7.53 × 10⁻³ S/cm at 600 °C).     

Nanosized pure and Cr doped Al2−xScx(WO4)3 solid solutions

Nanosized solid solutions of the formula Al_2?x?ySc_xCr_y(WO₄)₃, where x varies from 0 to 2 and y from 0.02 to 0.1 are synthesized for the first time by the co-precipitation method. X-ray powder diffraction, DTA/TG and TEM analyses demonstrate that the powders are pure solid solution compounds with orthorhombic structure, space group Pnca. Particle sizes between 10 and 70 nm are obtained after thermal treatment of the precipitates at 550 °C for 1 h for all compositions except in the case of Sc_1.9Cr_0.1(WO₄)₃. For the last one mean particle size of 64 nm was obtained after thermal treatment at 500 °C. The influence of the concentrations of Sc and Cr as well as of the temperature and duration of the thermal treatment on the particle size and size distribution are established and discussed.     

Visible quantum cutting through downconversion in Eu3+-doped K2GdZr(PO4)3 phosphor

K₂Gd_1?xZr(PO₄)₃:Eu_x³⁺ (0.02 ≤ x ≤ 0.1, x is in mol.%) were prepared by solid-state reaction method and their photoluminescence properties were investigated in ultra-violet (UV) and vacuum ultra-violet (VUV) region. The phenomenon of visible quantum cutting through downconversion was observed for the Gd³⁺–Eu³⁺ couple in this Eu³⁺-doped K₂GdZr(PO₄)₃ system. Visible quantum cutting, the emission of two visible light photons per absorbed VUV photon, occurred upon the 186 nm excitation of Gd³⁺ at the ⁶G_J level via two-step energy transfer from Gd³⁺ to Eu³⁺ by cross-relaxation and sequential transfer of the remaining excitation energy. The results revealed that the efficiency of the energy transfer process from Gd³⁺ to Eu³⁺ in the Eu³⁺-doped K₂GdZr(PO₄)₃ system could reach to 155% and K₂GdZr(PO₄)₃:Eu³⁺ was effective quantum cutting material.     

Crystal growth and characterization of Tm doped mixed rare-earth aluminum perovskite

In this work, we present results of structural characterization and optical properties including radio luminescence of (Lu_xGd_yY_0.99?x?yTm_0.01)AP single crystal scintillators for (x, y) = (0.30, 0.19), (0, 0.19) and (0, 0) grown by the micro-pulling-down (μ-PD) method. The grown crystals were single phase materials with perovskite structure (Pbnm) as confirmed by XRD and had a good crystallinity. The distribution of the crystal constituents in growth direction was evaluated, and significant segregation of Lu and Gd was detected in (Lu_0.30Gd_0.19Y_0.50Tm_0.01)AP sample. The crystals demonstrated 70% transmittance in visible wavelength range and some absorption bands due to Tm³⁺, Gd³⁺ and color centers were exhibited in 190–900 nm. The radioluminescence measurement under X-ray irradiation demonstrated several emission peaks ascribed to 4f–4f transitions of Tm³⁺ and Gd³⁺. The ratio of emission intensity in longer wavelength range was increased when Y was replaced by Lu or Gd.     

Hydrothermal synthesis and photoluminescence of novel green-emitting phosphor Y1−xBO3:xTb3+

Y_1−xBO₃:xTb³⁺ phosphors were first synthesized by hydrothermal reaction, and the samples were characterized by X-ray powder diffractometry, infrared absorption, nuclear magnetic resonance, scanning electron microscopy and photoluminescence. The results show that single phase is obtained with Tb concentration up to 0.28 and all the samples exhibit flake-like morphology. The sample was determined to be vaterite-type orthoborate and the boron is both four-coordinated (chief) and three-coordinated (few). The Y_1−xBO₃:xTb³⁺ phosphors showed intense green emission at 550 nm and the intensity of the emission increases with Tb³⁺ substitution up to 0.22 and then decreases for higher Tb³⁺ content. In the phosphors prepared by the hydrothermal method the concentration quenching is higher than in the phosphors prepared by solid-state reaction; the intensity of emission is stronger in the former than that of the latter. Y_1−xBO₃:xTb3⁺ is a promising phosphor for plasma display panels and hydrargyrum-free lamps.     

Crystal structure and magnetic properties of Pr- and Ti-substituted La2RuO5

Polycrystalline samples of Pr- and Ti-substituted La₂RuO₅ were prepared applying a soft-chemistry route based on the thermal decomposition of citrate-stabilized precursors. The simultaneous substitution on the La-sites by Pr and on the Ru-sites by Ti results in samples of the composition La_2−xPr_xRu_1−yTi_yO₅ with 0 ≤ x ≤ 0.75 and 0 ≤ y ≤ 0.4. The crystal structures of these compounds were analyzed by Rietveld refinement of powder X-ray diffraction patterns. For pure La₂RuO₅ a structural transition from a monoclinic room-temperature modification to a triclinic low-temperature structure was found at 161 K. This structural change is linked to a low-temperature long-range ordered spin-singlet ground state formed by Ru⁴⁺ spin-moments. Both the structural transition and the formation of the singlet ground state become progressively suppressed with higher Ti contents, while the Pr substitution has only a minor influence on the dimerization. The behavior of the Curie–Weiss temperatures can be explained assuming two almost independent magnetic sublattices corresponding to the ruthenium and the rare-earth ions, respectively. For all investigated properties, i.e. crystal structure, magnetic susceptibilities, and dimerization temperature T_d, a completely additive behavior of the effects of Pr-substitution and Ti-substitution is observed.     

Luminescence properties of Sn2P2Se6 crystals

A large family of Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) semiconductor-ferroelectric crystals were obtained by the Bridgman technique. The photoluminescence properties of the Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) family crystals strongly depend on their chemical composition, excitation energy and temperature. The influence of the Pb → Sn and S → Se isovalent substitutions on the luminescence properties of a crystal with the Sn₂P₂Se₆ basic composition was investigated. A broad emission band observed in the Sn₂P₂Se₆ crystal with a maximum roughly at 600 nm (at T = 8.6 K) was assigned to a band-to-band electron-hole recombination, whereas broad emission bands, peaked near 785 nm (at T = 8.6 K) and 1025 nm (at T = 44 K) were assigned to an electron-hole recombination from defect levels localised within the bandgap. Possible types of recombination defect centres and specific mechanisms of luminescence in the Sn₂P₂Se₆ semiconductor-ferroelectric crystals were considered and discussed on the basis of the obtained results and the referenced data.     

Electrical conductivity of LaxSr2−xFe1−yRuyO4±δ

Materials of the K₂NiF₄ structure type have been prepared and the electrical conductivity in air determined for a number of compositions in the La_xSr_2−xFe_1−yRu_yO_4±δ solid solution series including three with Ru substituted for Fe on the B site: La_0.2Sr_1.8Fe_0.6Ru_0.4O_4±δ, La_0.4Sr_1.6Fe_0.7Ru_0.3O_4±δ, and La_0.6Sr_1.4Fe_0.8Ru_0.2O_4±δ. Overall the total conductivity values measured were lower than expected for the Ru-doped materials, with a peak conductivity of ≈2 S cm⁻¹ at 700 °C. In the undoped La_xSr_2−xFeO_4±δ materials, a significant jump in conductivity was observed between the x = 0.7 and 0.8 compositions and was related to the bonding in the materials and the Fe³⁺/Fe⁴⁺ redox behaviour. In all materials, the conduction behaviour was found to follow a semi-conducting trend.