Synthesis,crystal structure,and luminescence properties of CaY2Ge3O10:Ln3+, Ln = Eu,Tb

The crystal structure and luminescence properties of CaY₂Ge₃O₁₀:Ln³⁺ (Ln = Eu, Tb) germanates synthesized via a conventional solid-state reaction and an ethylenediaminetetraacetic acid complexing process are studied. The CaY_{2 ? x} Ln _x Ge₃O₁₀ (Ln = Eu, Tb; x = 0–1.0, 2.0; Δx = 0.1) solid solutions have a monoclinic structure (space group P2₁/c, Z = 4), in which dopant ions occupy three nonequivalent noncentrosymmetric sites with different Ca²⁺/Ln³⁺ ratios. The effect of the synthesis methods, dopant concentrations, and excitation wavelengths on the luminescence properties of the compounds obtained is determined.     

Facile synthesis and luminescence properties of uniform and monodisperse KY3F10:Ln3+ (Ln=Eu,Ce, Tb) nanospheres

Highly uniform and monodisperse KY₃F₁₀:Ln³⁺ (Ln=Eu, Ce, Tb) nanospheres, with an average diameter of 300 nm, have been successfully prepared through a simple template-free and surfactant-free stirring method under ambient conditions. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the samples. The SEM images illustrate that these spheres were actually composed of randomly aggregated nanoparticles. The doped rare earth ions show their characteristic emission in the KY₃F₁₀ samples, i.e., Eu^{3+ 5}D₀–⁷F_J (J=1, 2, 3, 4), Tb^{3+ 5}D₄–⁷F_J (J=6, 5, 4, 3, 2) and Ce³⁺ 5d–4f transition emissions, respectively. An energy transfer phenomenon from Ce³⁺ to Tb³⁺ has been observed in KY₃F₁₀ nanospheres, and the energy transfer efficiency depends on the doping concentration of Tb³⁺ if the concentration of Ce³⁺ is fixed.     

Luminescence properties of Y2O3:Bi3+, Ln3+ (Ln=Sm,Eu, Dy,Er, Ho) and the sensitization of Ln3+ by Bi3+

The spectroscopic characterization of yttria, singly and doubly doped with Ln³⁺ (Ln=Sm, Eu, Dy, Er, Ho) and Bi³⁺ ions, is performed through excitation spectra, emission spectra and decay time measurements. The obtained spectroscopic data clearly indicate that energy transfer takes place from Bi³⁺ to Ln³⁺ ions. The energy transfer efficiency of Bi³⁺→Ln³⁺ and quantum efficiency of Ln³⁺ were calculated. Upon excitation of 370 nm (Bi³⁺ excitation band), the quantum efficiency of Ln³⁺ varies from ～4% to ～44%. The energy transfer efficiency increases continuously with increasing Ln³⁺ concentrations, whereas the variation of the quantum efficiency of Ln³⁺ is complicated. The quantum efficiency of Ln³⁺ is discussed in terms of electron transfer and cross relaxation.     

Photoluminescence properties of red-emitting phosphors Ln3BWO9:Eu3+ (Ln=Y,La, Gd)

Undoped and Eu³⁺ activated Ln₃BWO₉ (Ln=Y, La, Gd) were prepared by the Pechini method and characterized with X-ray diffraction (XRD) and ultraviolet (UV) spectroscopy. All the samples have the hexagonal phase after heat treatment in the range of 850–1000 °C. The Eu³⁺ doped samples emit high-purity red light with peak maximum at about 617 nm under excitation of UV light (～285 nm) at room temperature. When the doping concentration of Eu³⁺ is about 20–30%, luminescence intensity reaches the maximum. Luminescence decay curves indicate that Ln₃BWO₉:Eu³⁺ exhibits a fast decay time of about 0.5 ms. A possible luminescence mechanism has also been proposed. It is worth noting that both the absorption of host lattice and the charge transfer (CT) transition of Eu³⁺ are of great importance to the promising luminescent performance of Ln₃BWO₉:Eu³⁺.     

Luminescence of Ln3+ and WO 2 2+ ions in wolframylphosphate glasses

We investigate the luminescent properties of potassium wolframylphosphate glasses doped with Eu³⁺, Tb³⁺, and Dy³⁺ ions whose luminescence is excited by donor-acceptor interaction between the active WO ₂ ²⁺ and Ln³⁺ ions, as well as the migration of energy in the subsystems of each type of the active ions. Comparison of the obtained data with the results of investigation of the spectroscopic properties of Ln³⁺ in uranylphosphate materials shows that a sufficiently high degree of the ionicity of bonds of Ln³⁺ with the atoms of its first coordination sphere is preserved in wolframylphosphate matrices. We show that three stages of the decomposition of electron excitations are typical of the WO ₂ ²⁺ ions in wolframylphosphate glasses doped with Ln³⁺ and two stages in nonactivated glasses. The electron excitation energy transfer in the WO ₂ ²⁺ −Ln³⁺ system occurs due to induction-resonance interaction. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 620–625, September–October, 1997.     

Submillimeter electron-nuclear excitation spectra in CsCdBr3:Ln3+ (Ln=Tm, Ho) crystals

The ESR spectra of single and pair impurity centers of thulium and holmium ions in CsCdBr₃:Tm³⁺ and CsCdBr₃:Ho³⁺ crystals are measured in the frequency range 160-400 GHz. Analysis of the characteristic features of the hyperfine structure of the ESR lines and analysis of the variations in the spectra as a function of the temperature and external magnetic field shows that the Ln³⁺ ions substitute for Cd²⁺ ions and predominantly form symmetric pair centers of the type Ln³⁺-(vacancy at a neighboring Cd²⁺ site)-Ln³⁺. The ESR spectra of CsCdBr₃: Ln³⁺ crystals are used to make a positive identification of the optical spectra of selective laser excitation. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 535–540 (10 April 1997)     

Influence of co‐dopant in the europium reduction in SrAl2O4 host

Xerogels of strontium chlorate and aluminium chlorate doped with europium (un‐co‐doped) and co‐doped with rare earth ions (Ln = Gd, Dy, Er and Y) were prepared using the proteic sol–gel route. Synchrotron radiation was used to investigate the effect of different co‐dopants on the Eu³⁺→ Eu²⁺ reduction process during the synthesis of the samples. Samples were excited at the Eu L_III‐edge and the XANES regions were analyzed. The results suggest that some of the Eu ions can be stabilized in the divalent state and that it is difficult to completely reduce Eu³⁺ to Eu²⁺ during thermal treatment. The mechanisms of the Eu reduction processes are explained by a proposed model based on the incorporation of charge‐compensation defects.     

Co-reduction synthesis of new LnxSb2−xS3 (Ln: Nd, Lu, Ho) nanomaterials and investigation of their physical properties

New Ln_xSb_2−xS₃ (Ln: Lu³⁺, Ho³⁺, Nd³⁺)-based nanomaterials were synthesized by a co-reduction method. Powder XRD patterns indicate that the Ln_xSb_2−xS₃ crystals (Ln=Lu³⁺, Ho³⁺, x=0.00−0.1 and Ln=Nd³⁺, x=0.00−0.08) are isostructural with Sb₂S₃. SEM images show that doping of Lu³⁺ and Ho³⁺ ions in the lattice of Sb₂S₃ results in nanorods while that in Nd³⁺ leads to nanoflowers. UV-vis absorption and emission spectroscopy reveal mainly electronic transitions of the Ln³⁺ ions in case of Ho³⁺ and Nd³⁺ doped nanomaterials. Emission spectra show intense transitions from excited to ground state of Ln³⁺. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂S₃, show other emission bands originating from f-f transitions of the Ho³⁺ ions. TGA curves indicated that Sb₂S₃ has the highest thermal stability. The electrical conductance of Ln-doped Sb₂S₃ is higher than undoped Sb₂S₃, and increase with temperature.     

Influence of the Type of a Ligand on the Intensity of Luminescence of Ln3+ Ions in Aqueous Solutions

The luminescence characteristics of hydrated Ln³⁺ ions and their complexes with some acidic ligands have been investigated. The possibility of determining the stability of the complexes of lanthanides in solutions from the intensity of luminescence bands is shown. The influence of the characteristic features of the f-electron shell of Ln³⁺ on the formation of the spectrochemical series is discussed.     

Rare-earth ion distribution in sol–gel glasses co-doped with Al3+

Terbium spectroscopy is used to probe clustering of rare earth (RE) ions in sol–gel glasses. We use pulsed laser experiments to study energy transfer among Tb³⁺ dopants, and analyze the fluorescence decay curves using the Inokuti–Hirayama model. The analysis yields new insights about the role of Al³⁺ co-doping. Assuming a multipolar interaction, we derive an effective Tb³⁺ concentration for a series of samples with varying amounts of RE. When actual doping concentration is varied over 2 orders of magnitude, the effective Tb³⁺ concentration of centers that emit light changes by only a factor of 10. The results indicate that Al³⁺ co-doping is only effective at dispersing RE ions when the ratio of Al:RE is 10:1 or greater. At higher RE concentrations, most ions reside in clusters; ⁵D₃ fluorescence is observed from a minority of isolated ions. This interpretation contradicts earlier work that used fluorescence line narrowing to demonstrate that Al³⁺ co-doping caused RE dispersal at high doping levels.     

Synthesis and characterization of new LnxSb2−xSe3 (Ln: Yb, Er) nanoflowers and their physical properties

New Ln_xSb_2−xSe₃ (Ln: Yb³⁺, Er³) based nanomaterials were synthesized by a co-reduction method. Powder XRD patterns indicate that the Ln_xSb_2−xSe₃crystals (Ln=Yb³⁺, Er³⁺, x=0.00-0.12) are isostructural with Sb₂Se₃. The cell parameters b and c decrease for Ln=Er³⁺ and Yb³⁺ upon increasing the dopant content (x), while a increases. SEM images show that doping of the lanthanide ions in the lattice of Sb₂Se₃ generally results in nanoflowers. UV-vis absorption and emission spectroscopy reveals mainly electronic transitions of the Ln³⁺ ions in case of Yb³⁺ doped nanomaterials. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂Se₃, show additional emission bands centered at 955 nm, originating from the ²F_7/2→²F_5/2 transition (f-f transitions) of the Yb³⁺ ions. DSC curves indicate that Sb₂Se₃ has the highest thermal stability. The temperature dependence of the electrical resistivity of doped-Sb₂Se₃ with Yb³⁺ and Er³⁺ was studied.     

Novel red phosphors for solid state lighting; the system BixLn1−xVO4; Eu/Sm (Ln=Y, Gd)

Solid solutions of vanadates of formula Bi_xLn_1−xVO₄ (Ln=Y, Gd) doped with Eu³⁺ or Sm³⁺ ions have been synthesized by solid-state reactions. Intense red/orange-red luminescence is obtained in these samples on excitation in the broad charge-transfer band in the near UV. The excitation in the Eu³⁺ levels leads to much less intense red emission. These materials could find applications as red phosphors for solid-state white lighting devices utilizing GaN-based excitation in the near UV.     

Energy transfer phenomena in ordered perovskites of the type Sr2Na0.5Ln3+0.5X6+O6

We have investigated perovskites with composition Sr₂Na_0.5Ln³⁺_0.5WO₆ and Sr₂Na_0.5Ln³⁺_0.5 UO₆ (Ln = La, Gd, Eu). Their luminescence gives information on crystallographic details of the crystal structure and on a number of different energy transfer phenomena in these compounds. For Ln = La the Na⁺ and La³⁺ ions are disordered; for Ln = Gd(Eu) they are ordered. Single-step energy transfer is observed for the couples U⁶⁺ -Eu³⁺ and W⁶⁺ - Eu³⁺; energy migration occurs within the uranium and the europium sublattices.     

Destruction of the short‐range disorder due to erbium doping in Pb0.8La0.2TiO3films

We show the destruction of a displacement of Ti in the short‐range structure by observing the disappearance of emission and Raman signals when the Er³⁺ concentration exceeds 7 mol% in sol–gel‐derived Pb_0.8La_0.2TiO₃ polycrystalline films. It is believed that there always exists disorder due to displacement of B ions in the skeleton of BO₆ in perovskite ABO₃ materials. This disorder due to the displacement of Ti ions breaks the center of symmetry to activate emission of rare‐earth ions such as Er³⁺ and Raman modes of perovskites. We found that the breaking of symmetry can be diminished by introducing more Er³⁺ ions. Copyright © 2009 John Wiley & Sons, Ltd.     

Multiphonon relaxation of optical excitations in CsCdBr<Subscript>3</Subscript>:Pr<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> crystals

The technique of calculation of the n-phonon transition rates between electronic sublevels of impurity rare earth ions in dielectric crystals is developed in the case when n>2. The n-phonon transition probabilities are calculated according to the 1st and 2nd orders of perturbation theory. The Hamiltonian of the electron-phonon interaction is constructed in the framework of the exchange charge model and developed as series in relative displacements of the rare earth ion and ligands. The contribution of the lattice anharmonicity on the probabilities of n-phonon transitions is taken into account. On the basis of the developed technique, the nonradiative relaxation rates of ⁴ G _7/2 multiplet of Nd³⁺ ions in LiYF₄:Nd³⁺crystal and ³P₁ multiplet of Pr³⁺ ions in CsCdBr₃:Pr³⁺ crystal were computed. The results of our calculations show that the 2nd order terms in the expressions for the probabilities studied here are comparable with, and in some cases prevail over the 1st order terms. An account of lattice anharmonicity in case of LiYF₄:Nd³⁺ crystal substantially modifies the corresponding multiphonon relaxation rates. The calculated nonradiative relaxation rates for both crystals agree well with the experimental data.     

On the magnetism of Ln<Subscript>2/3</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (Ln = lanthanide)

The magnetic and thermodynamic properties of the complete Ln_2/3Cu₃Ti₄O₁₂ series were investigated. Here Ln stands for the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. All the samples investigated crystallize in the space group Im[`3]Im\bar{3} with lattice constants that follow the lanthanide contraction. The lattice constant of the Ce compound reveals the presence of Ce⁴⁺ leading to the composition Ce_1/2Cu₃Ti₄O₁₂. From magnetic susceptibility and electron-spin resonance experiments it can be concluded that the copper ions always carry a spin S = 1/2 and order antiferromagnetically close to 25 K. The Curie-Weiss temperatures can approximately be calculated assuming a two-sublattice model corresponding to the copper and lanthanide ions, respectively. It seems that the magnetic moments of the heavy rare earths are weakly coupled to the copper spins, while for the light lanthanides no such coupling was found. The 4f moments remain paramagnetic down to the lowest temperatures, with the exception of the Tm compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic singlet ground state of the crystal-field split 4f manifold. From specific-heat measurements we accurately determined the antiferromagnetic ordering temperature and obtained information on the crystal-field states of the rare-earth ions.     

Self-assembled 3D sphere-like SrMoO4 and SrMoO4:Ln3+ (Ln = Eu,Sm, Tb,Dy) microarchitectures: Facile sonochemical synthesis and optical properties

Three-dimensional (3D) well-defined SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures of obvious sphere-like shape have been successfully synthesized using a large-scale and facile sonochemical route without using any catalysts or templates. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and photoluminescence (PL) spectra were used to characterize the samples. The intrinsic structural feature of SrMoO₄ and external factor, namely the ultrasonic time and the pH value, are responsible for the ultimate shape evolutions of the product. The possible formation mechanism for the product is presented. Additionally, the PL properties of SrMoO₄ and SrMoO₄:Ln³⁺ (Ln = Eu, Sm, Tb, Dy) hierarchical structures were investigated in detail. The Ln³⁺ ions doped SrMoO₄ samples exhibit respective bright red–orange, yellow, green and white light of Eu³⁺, Sm³⁺, Tb³⁺ and Dy³⁺ under ultraviolet excitation, and have potential application in the field of color display. Simultaneously, this novel and efficient pathway could open new opportunities for further investigating about the properties of molybdate materials.     

Synthesis and luminescent properties of spindle-like YVO4:Ln3+ (Ln=Eu,Dy) self-assembled of nanoparticles

Large-scale spindle-like YVO₄ particles with an euatorial diameter of 100–150 nm and a length of 300–350 nm were synthesized by utilizing the Y(OH)CO₃ colloid spheres as the precursor and NH₄VO₃ as the vanadium source through a simple solution-based hydrothermal process, for the first time. In the first stage of the reaction, hierarchical flower-like YVO₄ spheres were formed. Then, petals of spindle-like YVO₄ particles were obtained via a following self-abscission process from these flower spheres. The possible formation mechanism has been discussed in detail. Moreover, the photoluminescent properties of spindle-like YVO₄:Ln³⁺ (Ln=Eu, Dy) nanoparticles were investigated. They might have potential application in advanced flat panel display, minioptoelectronic devices, and biological labeling.     

Weak ferromagnetism in BiFeO<Subscript>3</Subscript>-based multiferroics

The magnetic properties of the Bi_{1 ? x} Ln _x FeO₃ (Ln is a rare-earth ion), Bi_{1 ? x} A _x FeO_{3 ? x/2} (A is an alkali earth ion), and BiFe_{1 ? x} Ti _x O_{3 + δ} solid solutions in magnetic fields up to 14 T have been studied. The concentration ranges of the existence of the ferroelectric phase described by the space group R3c have been determined. It is shown that the substitution of the rare-earth ions for the Bi³⁺ ions leads to a sharp decrease in the critical fields inducing the metamagnetic transition from a modulated antiferromagnetic state to a weakly ferromagnetic one; however, the modulated structure in the concentration range of the R3c phase is mainly retained. The substitution of the alkali earth ions (x ～ 0.1) for the bismuth ions leads to the total destruction of the modulated structure and to the implementation of the weakly ferromagnetic state within the R3c phase. A homogeneous weakly ferromagnetic state has been revealed when the Ti⁴⁺ ions (x = 0.1) are substituted for the Fe³⁺ ions in the ferroelectric R3c phase.     

Heavy ion irradiation-induced phase transformation in polycrystalline Dy2O3

Radiation damage effects in polycrystalline pellets of the rare earth sesquioxide Dy₂O₃ irradiated with 300?keV Kr²⁺ ions were studied by combining grazing incidence X-ray diffraction (GIXRD) with transmission electron microscopy (TEM). Radiation damage was introduced using 300?keV Kr²⁺ ions to fluences up to 1?×?10²⁰?Kr?m^?2 at cryogenic temperature. GIXRD and cross-sectional TEM observations revealed that the crystal structure of the irradiated Dy₂O₃ transformed from a cubic, so-called C-type rare earth sesquioxide structure to a monoclinic, B-type rare earth sesquioxide structure upon ion irradiation. In addition, TEM and high-resolution electron microscopy (HREM) indicated that the transformed surface Dy₂O₃ layer adopts an epitaxial orientation relationship with the substrate Dy₂O₃.