Role of crystallite size on the photoluminescence properties of SrIn2O4:Eu phosphor synthesized by different methods

Photoluminescence (PL) of Eu³⁺ was studied in SrIn₂O₄ host lattice. A complete solid solubility of Eu³⁺ has been found in the series SrIn_2−xEu_xO₄ [x=0-2.0]. The phase formation at a relatively low temperature and in a very short duration was achieved by combustion synthesis (CS). Concentration quenching of luminescence has been observed in SrIn_2−xEu_xO₄ [x=0.1-2.0] and the critical concentration for maximum emission was found to be with x=0.3. In order to find the role of crystallite size on the PL properties of SrIn₂O₄:Eu³⁺, the results obtained with phosphors synthesized by solid state reaction (SSR) and CS methods were compared.     

Eu luminescence—a structural probe in BiCa4(PO4)3O, an apatite related phosphate

Eu³⁺ luminescence is studied in apatite-related phosphate BiCa₄(PO₄)₃O. Compositions of the formula Bi_1−xEu_xCa₄(PO₄)₃O [x=0.05, 0.1, 0.3, 0.5, 0.8 and 1.0] are synthesized and they are isostructural with parent BiCa₄(PO₄)₃O. Room temperature photoluminescence shows the various transitions ⁵D₀→⁷F_J(=0,1,2) of Eu³⁺. The emission results of compositions with different Eu³⁺ content show the difference in site occupancy of Eu³⁺ in Bi_1−xEu_xCa₄(PO₄)₃O. The intense ⁵D₀-⁷F₀ line at 574 nm for higher Eu³⁺ content is attributed to the presence of strongly covalent Eu-O bond that is possible by substituting Bi³⁺ in the Ca(2) site. This shows the preferential occupancy of Bi³⁺ in Ca(2) site and this has been attributed to the 6s² lone pair electrons of Bi³⁺. This is further confirmed by comparing the emission results with La_0.95Eu_0.05Ca₄(PO₄)₃O.     

A novel red emitting phosphor Ca2SnO4: Eu

A novel red emitting phosphor, Eu³⁺-doped Ca₂SnO₄, was prepared by the solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the formation of Ca₂SnO₄: Eu³⁺. Field-emission scanning electron-microscopy (FE-SEM) observation indicated a narrow size-distribution of about 500 nm for the particles with spherical shape. Photoluminescence measurements indicated that the phosphor exhibits bright red emission at about 615 nm under UV excitation. The excellent luminescence properties make it possible as a good candidate for plasma display panels (PDP) application. Splitting of the ⁵D₀-⁷F_J transitions of Ca₂SnO₄: Eu³⁺ suggests that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite. The luminescence lifetime measurement showed a bi-exponential decay, providing other evidence for the existence of two different environments for Eu³⁺ ions.     

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.     

Study of the cation distributions in Eu doped Sr2Y8(SiO4)6O2 by X-ray diffraction and photoluminescent spectra

The crystal structure and photoluminescent properties of europium doped silicate Sr₂Y₈(SiO₄)₆O₂:Eu³⁺ are reported. The Sr₂Y_8−xEu_x(SiO₄)₆O₂ compounds have typical apatite crystal structures with the P6₃/m space group. The distributions of Eu³⁺ between the two crystallographic sites 4f and 6h in the apatite structure are investigated by the powder X-ray diffraction and Rietveld refinement. Results show that Eu³⁺ ions only occupy the 4f sites when the Eu doping concentration is low (x=0-0.5 in Sr₂Y_8−xEu_x(SiO₄)₆O₂). However, in higher concentrations, Eu³⁺ ions begin to enter the 6h sites as well. The distributions of the Eu³⁺ are also reflected in photoluminescent spectra. The CIE coordinates for Sr₂Y₆Eu₂(SiO₄)₆O₂ are (0.63, 0.37), which is close to the pure red color.     

Visible and infrared luminescence properties of Er-doped Y2Ti2O7 nanocrystals

Er³⁺-doped Y₂Ti₂O₇ nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er_xY_2−xTi₂O₇ transfers to well-crystallized nanocrystals, and the average crystal size increases from ∼70 to ∼180 nm under 800-1000 °C/1 h annealing. Er_xY_2−xTi₂O₇ nanocrystals absorbing 980 nm photons can produce the upconversion (526, 547, and 660 nm; ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively) and Stokes (1528 nm; ⁴I_13/2→⁴I_15/2) photoluminescence (PL). The infrared PL decay curve is single-exponential for Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ nanocrystals but slightly nonexponential for Er³⁺ (10 mol%)-doped Y₂Ti₂O₇ nanocrystals. For 5 and 10 mol% doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption. Much stronger intensity of red light relative to green light was observed for the sample with 10 mol% dopant. This phenomenon can be attributed to the reduced distance between Er³⁺-Er³⁺ ions, resulting in the enhancement of the energy-transfer upconversion and cross-relaxation mechanisms.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Optical properties of (Y1−xTmx)3GaO6 and subsolidus phase relation of Y2O3-Ga2O3-Tm2O3

A serial of samples in Y₂O₃-Ga₂O₃-Tm₂O₃ pseudo-ternary system are prepared by solid-state chemical reaction method. The range of solid solution in (Y_1−xTm_x)₃GaO₆ is 0<x<0.384. Powder X-ray diffraction shows that the compounds crystallize in Gd₃GaO₆ (Cmc2₁)-type structure. The solid solubilities of Y_3+xGa_5−xO₁₂ (x=0-0.77) and Tm_3+xGa_5−xO₁₂ (x=0-0.62) are 37.5-47.11 at% Y₂O₃, and 37.5-45.26 at% Tm₂O₃, respectively. PL spectra of Tm-doped Y₃GaO₆ show that there is a sharp blue emission at ∼456 nm from the ¹D₂→³F₄ transition at room temperatures with two lifetimes (∼5 and ∼15 μs) and a narrow saturation range of PL intensity for the Tm³⁺ content from x=0.005 to 0.03. The sharp emission and long lifetime of (Y_1−xTm_x)₃GaO₆ indicate that Y₃GaO₆ is a potential phosphor and laser crystal host material.     

Preparation and photoluminescence property of a loose powder, Ca3Al2O6:Eu by calcination of a layered double hydroxide precursor

A novel red light-emitting material, Ca₃Al₂O₆:Eu³⁺, which is the first example found in the Ca₃Al₂O₆ host, was prepared by calcination of a layered double hydroxide precursor at 1350 °C. The precursor, [Ca_2.9−xAl₂Eu_x(OH)_9.8](NO₃)_2+x·2.5H₂O, was prepared by coprecipitation of metal nitrates with sodium hydroxide. The material is a loose powder composed of irregular particles formed from aggregation of particles of a few nanometers, as shown in scanning electron microscope (SEM) images. It was found that the photoluminescence intensity reached the maximum when the calcination temperature was 1350 °C and the concentration of Eu³⁺ was 1.0%. The material emits bright red emission at 614 nm under a radiation of λ=250 nm.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

Solution precursor synthesis and magnetic properties of Eu1−xCaxTiO3

Europium titanate, EuTiO₃, is a paraelectric/antiferromagnetic cubic perovskite with T_N=5.5 K. It is predicted that compressive strain could induce simultaneous ferroelectricity and ferromagnetism in this material, leading to multiferroic behavior. As an alternative to epitaxial strain, we explored lattice contraction via chemical substitution of Eu²⁺ with the smaller Ca²⁺ cation as a mechanism to tune the magnetic properties of EuTiO₃. A modified sol-gel process was used to form homogeneously mixed precursors containing Eu³⁺, Ca²⁺, and Ti⁴⁺, and reductive annealing was used to transform these precursors into crystalline powders of Eu_1−xCa_xTiO₃ with x=0.00, 0.05, 0.10, 0.15, 0.25, 0.35, 0.50, 0.55, 0.60, 0.65, 0.80, and 1.00. Powder XRD data indicated that a continuous Eu_1−xCa_xTiO₃ solid solution was readily accessible, and the lattice constants agreed well with those predicted by Vegard's law. SEM imaging and EDS element mapping indicated a homogeneous distribution of Eu, Ca, and Ti throughout the polycrystalline sample, and the actual Eu:Ca ratio agreed well with the nominal stoichiometry. Measurements of magnetic susceptibility vs. temperature indicated antiferromagnetic ordering in samples with x≤0.60, with T_N decreasing from 5.4 K in EuTiO₃ to 2.6 K in Eu_0.40Ca_0.60TiO₃. No antiferromagnetic ordering above 1.8 K was detected in samples with x>0.60.     

Quantitative determination of site occupancy of multi-rare-earth elements doped into Ca2SnO4 phosphor by electron channeling microanalysis

X-ray fluorescence analysis based on electron channeling effects in transmission electron microscopy (TEM) was performed on Ca₂SnO₄ phosphor materials doped with Eu³⁺/Y³⁺ at various concentrations, which showed red photoluminescence associated with the ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ ions. The method provided direct information on which host element site dopant elements occupy, the results of which were compared with those of X-ray diffraction (XRD)-Rietveld analysis. The obtained results indicated that while it is not favorable for a part of Eu³⁺ to occupy the smaller Sn⁴⁺ site, this is still energetically better than creating Ca vacancies or any other of the possible charge balance mechanisms. The local lattice distortions associated with dopant impurities with different ionic radii were also examined by TEM-electron energy-loss spectroscopy (TEM-EELS). The change in PL intensity as a function of dopant concentration is discussed based on the experimental results, although the general concept of concentration quenching applies.     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.     

New transparent conductors with the M7O12 ordered oxygen-deficient fluorite structure: from In4Sn3O12 to In5.5Sb1.5O12

A new transparent conductor, containing pentavalent antimony, In_4+xSn_3−2xSb_xO₁₂, has been synthesized for 0?x?1.5. The latter exhibits an ordered oxygen-deficient fluorite structure with an ordered distribution of Sb⁵⁺ and In³⁺/Sn⁴⁺ species in the octahedral and seven-fold coordinated sites, respectively. More importantly, it is shown that the electronic conductivity of this transparent conducting oxide (TCO) at room temperature, is one order of magnitude larger for x=1 (In₅SnSbO₁₂) than for x=0 (In₄Sn₃O₁₂) and it turns to a semi-metallic behavior in contrast to In₄Sn₃O₁₂ which is a semi-conductor. The potential of this new material, as TCO, is also shown by its reflectance spectra, similar to In₄Sn₃O₁₂, involving only a small increase of the optical bandgap, by 0.15 eV.     

Vacuum Ultraviolet Excited Photoluminescence Properties of Y2O2S：Eu^3＋,Bi^3＋ Phosphor

As an Hg-free lamp using phosphor, the Bi^3＋ and EH^3＋ co-doped Y2O2S phosphors were prepared and their luminescence properties under vacuum ultraviolet（VUV） excitation were investigated. The VUV photoluminescent intensity of Y2O2S：Eu^3＋ was weak, however, considerably stronger red emission at 626 nm with good color purity was observed in Y2O2S：Eu^3＋,Bi^3＋ systems. Investigation on the photoluminescence reveals that the strong VUV luminescence of Y2O2S：Eu^3＋,Bi^3＋ at 147 nm is mainly because the Bi^3＋ acts as a medium and effectively performs the energy transfer process： Y^3＋-O^2-→Bi^3＋→Eu^3＋, while the intense emission band at 172 nm is attributed to the absorption of the characteristic ^1So-^1P1 transition of Bi^3＋ and the direct energy transfer from Bi^3＋ to Eu^3＋. The Y2O2S：Eu^3＋,Bi^3＋ shows excellent VUV optical properties compared with the commercial （Y,Gd）BO3：Eu^3＋. Thus, the Y2O2S：Eu^3＋,Bi^3＋ can be a potential red VUV-excited candidate applied in Hg-free lamps for backlight of liquid crystal display.     

Eu luminescence in La5Si2BO13 with apatite related structure and magnetic studies in Ln5Si2BO13 (Ln=Gd, Dy)

Eu³⁺ photoluminescence is studied in La₅Si₂BO₁₃ with apatite related structure. La_5−xEu_xSi₂BO₁₃ [x=0.05, 0.1, 0.3, 0.5, 0.7, 1.0, 2.0] compositions are synthesized. The emission results shows that Eu³⁺ ions occupy two different cationic sites viz., La(1) and La(2). The increase in the intensity of ⁵D₀-⁷F₀ line with increasing Eu³⁺ content shows the preferential occupancy of Eu³⁺ in La(2) site due to the existence of short La(2)-O(4) (free oxide ion) bond. The observation of antiferromagnetic interactions in Gd and Dy analogues supports the structural features elucidates from photoluminescence studies.     

Luminescence of Tb-doped Ca3Y2(Si3O9)2 oxide upon UV and VUV synchrotron radiation excitation

Powders of calcium yttrium silicate, Ca₃Y₂(Si₃O₉)₂, containing 0.1-3% Tb³⁺ were prepared using a sol-gel method and characterized with XRD, IR, UV-vis and UV-VUV spectroscopies at room temperature and 10 K. Structural analysis revealed pure monoclinic phase of Ca₃Y₂(Si₃O₉)₂ after heat-treatment at 1000 °C. Infrared spectroscopy showed that between 800 and 900 °C a short-range structural organization of the components proceeded, yet without crystallization. A strong emission of Tb³⁺ had been observed both in the green part of the spectrum due to the ⁵D₄→⁷F_J transitions and in the blue-violet region owing to the ⁵D₃→⁷F_J radiative relaxation. The color of the light could be tuned from yellowish-green to bluish-white both by means of the dopant content and the temperature of synthesis. Efficient luminescence of Tb³⁺-doped Ca₃Y₂(Si₃O₉)₂ phosphors could also be obtained upon stimulation with vacuum ultraviolet synchrotron radiation demonstrating that an energy transfer from the host to the Tb³⁺ ions takes place.     

Influence of the structure on electric and magnetic properties of La0.8Na0.2Mn1−xCoxO3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La_0.8Na_0.2Mn_1−xCo_xO₃ (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn³⁺→Mn⁴⁺ leading to the formation of stable couples Mn⁴⁺-Co²⁺. Therefore the double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ partly vanish and they are replaced by positive superexchange interactions Mn⁴⁺-O²⁻-Co²⁺, but of a semiconducting character.     

Aurivillius phases of PbBi4Ti4O15 doped with Mn synthesized by molten salt technique: Structure, dielectric, and magnetic properties

Doping of manganese (Mn³⁺/Mn⁴⁺) into the Aurivillius phase Pb_1−xBi_4+xTi_4−xMn_xO₁₅ was carried out using the molten salt technique for various Mn concentrations (x=0, 0.2, 0.4, 0.6, 0.8, and 1). Single phase samples could be obtained in the composition range with x up to 0.6 as confirmed by X-ray and neutron diffraction analysis. Dielectric measurements show a peak at 801, 803, 813 and 850 K for samples with x=0, 0.2, 0.4, and 0.6, respectively, related to the ferroelectric transition temperature (T_c). The main contribution of the in-plane polarization for x≤0.2 which was calculated from the atomic positions obtained by the structure analysis is the dipole moment in the Ti(1)O₆ layer; however, for x≥0.4 the polarization originates from the dipole moment in the Ti(2)O₆ layer. Mn doping in the Pb_1−xBi_4+xTi_4−xMn_xO₁₅ does not show any long range magnetic ordering.