Lattice Effect on Magnetic and Electrical Properties of Ln2/3Pb1/3MnO3 （Ln=La,Pr, Nd） Films

Lattice effect on magnetic and electrical transport properties of Ln2/3Pb1/3MnO3 （Ln=La, Pr, Nd） films prepared by RF magnetron sputtering technique were investigated. With the decrease of the average ions radius 〈rA〉, the structure of Ln2/3Pb1/3MnO3 （Ln=La, Pr, Nd） targets transit from the rhombohedral phase to the orthorhombic phase, and the Curie temperature reduces rapidly with the decrease of 〈rA〉. The electrical properties show that films are the metallic state which can be fitted to the formula： ρ（T）=ρ0 ＋ ρ1T^2 ＋ ρ2T^4.5 at low temperatures. The temperature range of the ferromagnetic metallic state becomes narrow with the decrease of 〈rA〉. The phenomenon can be explained by the lattice effect.     

Synthesis and luminescence properties of lanthanide (III)-doped YF3 nanoparticles

Synthesis process and luminescence properties of trivalent lanthanide ions (Ln3+) doped YF3 nanoparticles have been investigated. To synthesis Ln(3+)-doped YF3 nanoparticles, the mixture of (YCl3 x nH2O + LnCl3 x nH2O), and NH4F was hydrothermal treated at 180 degrees C in a Teflon-liner auto-clave or heated at higher temperatures (400 degrees C - 600 degrees C) in a stove. The XRD patterns showed that the Ln(3+)-doped orthorhombic YF3 nanoparticles with no second phase have been prepared. The solid solution Y(1-x)Eu(x)F3 (x = 0 - 0.4) nanoparticles have been synthesized. The luminescence concentration quenching resulted from resonance energy transfer between neighboring Eu3+ ions occurred at higher Eu3+ concentrations (30 mol%). The upconversion luminescence of Er(3+)-Yb3+ codoped YF3 nanoparticles under 980 nm excitation has also been observed. With increase of heated temperature, the size of the Er(3+)-Yb3+ codoped YF3 nanoparticles increased gradually, and upconversion luminescence intensity increased significantly.     

Microwave dielectric properties of (A2+(1/3)B5+(2/3))0.5Ti0(0.5)O2 (A2+ = Zn, Mg, B5+ = Nb, Ta) ceramics

Dielectric properties of (A(2+)(1/3)B(5+)(2/3))(0.5)Ti0(0.5)O(2) (A(2+) = Zn, Mg, B(5+) = Nb, Ta) ceramics were investigated at microwave frequencies. A single phase with tetragonal rutile structure was obtained through the entire compositions. Dielectric properties were strongly dependent on the structural characteristics. The specimens with B(5+) = Nb showed a larger dielectric constant than those with B(5+) = Ta due to the decrease of bond valence. Quality factors (Qf) of the specimens with B(5+) = Ta were larger than those with B(5+) = Nb. Temperature coefficient of the resonant frequencies (TCF) of (Zn(1/3)Nb(2/3) )0(0.5)Ti0(0.5)O(2) was larger than that of (Mg(1/3)Ta(2/3))0(0.5)Ti0(0.5)O(2). These results could be attributed to the changes of the temperature coefficient of dielectric constant and the degree of oxygen octahedral distortion.     

Low-frequency and microwave dielectric properties of Ba2LnTaO6 (Ln = La, Pr, Sm, Dy, Ce, Gd, Nd, Tm, Tb) ceramics

Nine Ba₂LnTaO₆ compounds are synthesized, and their low-frequency and microwave dielectric properties are studied. The results are analyzed in relation to earlier structural data and the ionic radius of Ln. The highest permittivity values are offered by the tantalates with the strongest distortion of the perovskite cell and the largest ionic radius of the lanthanide.     

A general aqueous sol-gel route to Ln(2)Sn(2)O(7) nanocrystals

This paper describes a general aqueous sol-gel route for the synthesis of a series of rare earth stannates, Ln(2)Sn(2)O(7) (Ln = Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu), with pure pyrochlore phase via the assistance of a cetyltrimethyl ammonium bromide (CTAB) surfactant. The route involves first the formation of CTAB-inorganic lamellar structures and then their thermal decomposition at 800?°C to yield the pyrochlore Ln(2)Sn(2)O(7) nanocrystals. Techniques using a thermo-gravimetric/differential thermal analyzer (TG-DTA), x-ray diffraction (XRD) and transmission electron microscopy (TEM) as well as selected-area electron diffraction (SAED) have been employed to characterize the as-synthesized Ln(2)Sn(2)O(7) nanocrystals. Furthermore, photoluminescence (PL) of the 5% Eu(3+) activated Ln(2)Sn(2)O(7) nanocrystals and carbon monoxide catalytic oxidation over the as-obtained Ln(2)Sn(2)O(7) nanocrystals were investigated. The results indicate that the PL properties as well as the catalytic activity changes significantly with the ionic radii of the rare earth elements.     

Rapid microwave-enhanced hydrothermal synthesis and shape evolution of uniform NaGdF(4):Yb, Er (Tm/Ho) nanocrystals with upconversion and paramagnetic properties

An efficient microwave-enhanced hydrothermal synthesis strategy was developed for the rapid synthesis of β-NaGdF(4):Ln(3+) (Ln?=?Yb, Er/Tm/Ho) nanocrystals (NCs) with multicolour upconversion luminescence and paramagnetic properties. It has been found that the uniform β-NaGdF(4):Ln(3+) NCs could be rapidly formed within a few minutes at 160?°C and the shape of the NCs can be manipulated from uniform rod-like to spherical just by tuning the initial reactants' concentration. In comparison to conventional hydrothermal routes, a burst homogeneous nucleation and higher growth rate as well as enhanced dimensional homogeneity of the NaGdF(4):Ln(3+) was achieved in microwave synthesis. A microwave-heating-based classical crystallization mode and surfactant-assisted anisotropic growth mechanism were proposed for the formation of β-NaGdF(4):Ln(3+) NCs.     

Site-selective spectroscopy in Sm(3+)-doped sol-gel-derived nano-glass-ceramics containing SnO(2) quantum dots

Nano-glass-ceramics of composition 95SiO(2)-5SnO(2) doped with 0.4 Sm(3+) (mol%) were synthesized by the thermal treatment of precursor sol-gel glasses. Structural and luminescence measurements were carried out. The precipitated SnO(2) nanocrystals in the glass matrix constitute a wide bandgap quantum-dot system with size comparable to the bulk exciton Bohr radius. A site-selective excitation, by energy transfer from the semiconductor host, reveals that a fraction of the Sm(3+) ions are incorporated in the SnO(2) nanocrystals, whereas the rest remains in the silica glassy phase. An evolution in the Sm(3+) emission spectra has been observed when the SnO(2) nanocrystals are excited with different UV wavelengths, which has been ascribed to selective excitation of nanocrystal sets with predetermined size.     

Dielectric properties of LiF-B2O3 glasses doped with certain rare earth ions

Dielectric constantɛ, loss tanδ and a.c. conductivityσ of LiF-B₂O₃: Ln³⁺ (where Ln=Ce, Pr, Nd and Tb) glasses are studied as functions of frequency (in the range 10²–10⁶ Hz) and temperature (range 30–200°C). The dielectric breakdown strength of these glasses was also determined at room temperature in an air medium. The rate of increase ofɛ and tanδ with temperature decreases with decrease in the ionic radius of RE³⁺ ion whereas the dielectric breakdown strength, the activation energy for a.c. conduction in the high temperature region decreases with increase in the ionic radius of RE³⁺ ion. An attempt has been made to explain the a.c. conduction in these glasses on the basis of quantum mechanical tunnelling (QMT) model.     

Emission properties of hydrothermal Yb(3+), Er(3+) and Yb(3+), Tm(3+)-codoped Lu2O3 nanorods: upconversion, cathodoluminescence and assessment of waveguide behavior

Yb(3+) and Ln(3+) (Ln(3+) = Er(3+) or Tm(3+)) codoped Lu(2)O(3) nanorods with cubic Ia3 symmetry have been prepared by low temperature hydrothermal procedures, and their luminescence properties and waveguide behavior analyzed by means of scanning near-field optical microscopy (SNOM). Room temperature upconversion (UC) under excitation at 980 nm and cathodoluminescence (CL) spectra were studied as a function of the Yb(+) concentration in the prepared nanorods. UC spectra revealed the strong development of Er(3+) (4)F(9/2) → (4)I(15/2) (red) and Tm(3+) (1)G(4) → (3)H(6) (blue) bands, which became the pre-eminent and even unique emissions for corresponding nanorods with the higher Yb(3+) concentration. Favored by the presence of large phonons in current nanorods, UC mechanisms that privilege the population of (4)F(9/2) and (1)G(4) emitting levels through phonon-assisted energy transfer and non-radiative relaxations account for these observed UC luminescence features. CL spectra show much more moderate development of the intensity ratio between the Er(3+) (4)F(9/2) → (4)I(15/2) (red) and (2)H(11/2), (4)S(3/2) → (4)I(15/2) (green) emissions with the increase in the Yb(3+) content, while for Yb(3+), Tm(3+)-codoped Lu(2)O(3) nanorods the dominant CL emission is Tm(3+) (1)D(2) → (3)F(4) (deep-blue). Uniform light emission along Yb(3+), Er(3+)-codoped Lu(2)O(3) rods has been observed by using SNOM photoluminescence images; however, the rods seem to be too thin for propagation of light.     

Effect of structure, particle size and relative concentration of Eu(3+) and Tb(3+) ions on the luminescence properties of Eu(3+) co-doped Y(2)O(3):Tb nanoparticles

Eu(3+) co-doped Y(2)O(3):Tb nanoparticles were prepared by the combustion method and characterized for their structural and luminescence properties as a function of annealing temperatures and relative concentration of Eu(3+) and Tb(3+) ions. For Y(2)O(3):Eu,Tb nanoparticles annealed at 600 and 1200?°C, variation in the relative intensity of excitation transitions between the (7)F(6) ground state and low spin and high spin 4f(7)5d(1) excited states of Tb(3+) is explained due to the combined effect of distortion around Y(3+)/Tb(3+) in YO(6)/TbO(6) polyhedra and the size of the nanoparticles. Increase in relative intensity of the 285?nm peak (spin-allowed transition denoted as peak B) with respect to the 310?nm peak (spin-forbidden transition denoted as peak A) with decrease of Tb(3+) concentration in the Y(2)O(3):Eu,Tb nanoparticles heated at 1200?°C is explained based on two competing effects, namely energy transfer from Tb(3+) to Eu(3+) ions and quenching among the Tb(3+) ions. Back energy transfer from Tb(3+) to Eu(3+) in these nanoparticles is found to be very poor.     

Frequency-temperature compensation in Ti(3+) and Ti(4+ ) doped sapphire whispering gallery mode resonators

A new method of compensating the frequency-temperature dependence of high-and monolithic sapphire dielectric resonators near liquid nitrogen temperature is presented. This is achieved by doping monocrystalline sapphire with Ti(3+) ions. This technique offers significant advantages over other methods.     

Mechanisms of combustion synthesis and magnetic response of high-surface-area hexaboride compounds

We present an analysis of the combustion synthesis mechanisms for the preparation of hexaboride materials using three compounds as model systems: EuB(6), YbB(6), and YB(6). These three hexaborides were chosen because of the differences in ionic radii between Eu(3+), Yb(3+), and Y(3+), which is a factor in their stability. The powders were prepared using metal nitrates, carbohydrazide, and two different boron precursor powders. The resulting materials were analyzed by X-ray diffraction, which showed that combustion synthesis is effective for the synthesis of EuB(6), since the Eu(3+) ion has an ionic radius greater than ～1 ?. The synthesis of YbB(6) and YB(6) is not as effective because of the small size of the Yb(3+) and Y(3+) ions, making the hexaborides of these metals less stable and resulting in the synthesis of borates due to the presence of oxygen during the combustion process. Scanning electron microscopy and dynamic light scattering of the EuB(6) powders shows that the particle size of the hexaboride product is dependent on the particle size of the boron precursor. The magnetic susceptibility of our EuB(6) powders manifests irreversible behavior at low applied fields, which disappears at higher fields. This behavior can be attributed to the increase in size and number of magnetic polarons with increasing magnetic field.     

Effects of a site substitution of bivalent ions on Ag(Nb0.8Ta0.2)O3 ceramics

Bivalent ions equivalent substitution and equimolar substitution for Ag⁺ in silver niobate tantalate ceramic were studied, which will result in Ag_0.98A_0.01(Nb_0.8Ta_0.2)O₃ and Ag_0.98A_0.02 (Nb_0.8Ta_0.2)O₃ respectively, where A²⁺ = Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺, Pb²⁺. The samples were synthesized by traditional solid method and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of a little Ag⁺ (in mol ratio 2 %) on the dielectric properties in Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺, Pb²⁺ substituted samples, and the influences of radius and ionic electronegativity on the dielectric properties were discussed. The results showed that the substitution of most bivalent ions except large Ba²⁺ ions for a little Ag⁺ (in mol ratio 2 %) showed no obvious change on the perovskite structure and microstructure of Ag(Nb_0.8Ta_0.2)O₃ ceramics. Basically, the dielectric properties of equivalent substituted samples were better than that of equimolar equimolar substituted samples, Ca²⁺ equivalent substituted samples showed lower dielectric loss, and Cu²⁺ equivalent substituted samples showed higher dielectric constant.     

White luminescence by up-conversion from thin film made with Ln(3+)-doped NaYF4 nanoparticles

White light-emitting thin films containing Ln(3+)-doped NaYF4 nanoparticles were prepared by a simple spin-coating method. White light was generated by two different lanthanide ions, Er3+ (red and green) and Tm3+ (blue), by upconversion process under the excitation of a 980-nm laser diode. The ratio of the intensity of the three main emissions was tuned by controlling the concentration of the nanoparticles in the thin film and the concentration of the lanthanide ions in the nanoparticles. The color coordinates corresponding to emissions of different nanoparticle concentrations and with the different pump powers were investigated. When the pump power was fixed at 900 mW, the thin film with a concentration ratio of 2.5:1 emitted pure white light with coordinates of (0.333, 0.339).     

Luminescence enhancement and quenching by codopant ions in lanthanide doped fluoride nanocrystals

Luminescence enhancement (LE) and quenching for lanthanide (Ln) doped nanocrystals is obtained by a second Ln(3+) ion doping method. Singly or doubly doped LaOF, LaF(3) and NaYF(4) nanocrystals are studied in detail under selective or two-color excitations. The underlying reason for LE by codoping is explored, and a mechanism of the enhancement based on the low local point symmetry effect of the matrix is proposed. It is found that the modification of the local environment induced by dopant ions can result in LE if the non-radiative relaxation probability can be ignored. The observations reported here should be useful for improving the quality of Ln(3+) doped nanomaterials.     

Phase transition characteristics and dielectric properties of rare-earth (La,Pr, Nd,Gd) doped Ba(Zr0.09Ti0.91)O3 ceramics

A-site deficient rare-earth doped barium zirconate titanate (BZT) ceramics (Ba_1−yLn_2y/3)Zr_0.09Ti_0.91O₃ (Ln = La, Pr, Nd, Gd) are obtained by a modified solid-state reaction method. Perovskite-like single-phase compounds were confirmed from X-ray diffraction data. Morphological analysis on sintered samples shows that the addition of rare-earth ions inhibits the growth of the grain and remarkably changes the grain morphology. The effect of rare-earth addition to BZT on phase transition and dielectric properties is analyzed. A dramatic fall in the transition temperature occurs when BZT ceramic is doped with rare-earths. Moreover, diffusivity degree of the phase transition increases and a relaxor-type behaviour is induced due to both the increment of the lanthanide content and the increase of the ionic radius of the dopant element. High values of dielectric tunability are obtained for lanthanum doped BZT. A direct relation between transition temperature and tunability is discussed. Conclusively, low permittivity and high tunability materials can be obtained by the adequate substitution of rare-earths into BZT ceramics.     

真空紫外光激发下Tb~(3+)激活的稀土正硼酸盐的发光

报导了Tb3+、Ce3+激活的稀土正硼酸盐的真空紫外光谱。分析了Ce3+的4f75d能级随基质结构、基质阳离子的变化,讨论了温度、Tb3＋离子的浓度对发光的影响以及Ce3＋－Tb3＋间的能量传递.     

The two-photon excitation of SiO(2)-coated Y(2)O(3):Eu(3+) nanoparticles by a near-infrared femtosecond laser

In order to improve the photoluminescence property of Eu(3+)-doped nanoparticles, Y(2)O(3):Eu(3+) nanoparticles were synthesized using the Pechini-type sol-gel method, then coated with SiO(2) shells by using the St?ber method for different coating times. The SiO(2)-coated nanoparticles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and Raman spectroscopy, and their photoluminescence spectra were recorded under 800?nm femtosecond laser excitation. The results indicate that a two-photon simultaneous absorption upconversion luminescence is obtained, and their upconversion luminescence intensities are further enhanced after the surfaces of the nanoparticles are coated with different thickness SiO(2) shells. Compared to the upconversion luminescence intensity of non-coated nanoparticles at 611?nm, the upconversion luminescence intensities of SiO(2)-coated Y(2)O(3):Eu(3+) nanoparticles with coating times of 60, 90 and 120?min were enhanced by 3.30, 3.96 and 4.13 times, respectively. This can be attributed to the contributions of the increased amounts of Eu(3+) ions populated at the (5)D(0) level on the surfaces of the nanoparticles because the cooperative ligand fields between the Y(2)O(3) core and non-crystalline SiO(2) shell interfaces activate the 'dormant' Eu(3+) ions near or on the surfaces of the nanoparticles. From a Judd-Ofelt (J-O) theory analysis, the coated shell structures can improve the radiative quantum efficiencies of Eu(3+)-doped nanoparticles. It is therefore concluded that more intense red upconversion luminescence with high radiative quantum efficiencies can enable the SiO(2)-coated Y(2)O(3):Eu(3+) nanoparticles to have the great potential to be used as a fine resolution phosphor.     

White Light Emission Characteristics of Tb~(3+)/Sm~(3+) Co-Doped Glass Ceramics Containing YPO_4 Nanocrystals

The Tb~(3+)/Sm~(3+) single-doped and co-doped glasses and glass ceramics containing YPO_4 nanocrystals have been synthesized by melt quenching method. The structural and luminescent properties of these glass specimens were investigated. Under 375 nm wavelength excitation, the emission spectra combined with blue, green and red bands were observed, which achieved the white light emission. Moreover, the energy transfer between Tb~(3+) and Sm~(3+)ions was validated by decay lifetime measurement and energy level diagram.The color coordinates(x = 0.333, y = 0.333), correlated color temperature(5595 K) and the color rendering index(Ra = 80.5) indicated that the glass ceramics were considered to be good lighting source. Hence,the YPO_(4-) based Tb~(3+)/Sm~(3+) co-doped glass ceramics can act as potential matrix materials for white lightemitting diodes under ultraviolet excitation.     

Ln(Ln=Gd3+,Cu+,Sm3+,Dy3+)助激活的Ca8Mg（SiO4）4Cl2∶Eu2+荧光粉的光谱特性和Eu2+的格位计算

采用高温固相法合成了掺杂Ln(Ln=Gd3+,Cu+,Sm3+,Dy3+)作助激活离子的氯硅酸镁钙荧光粉。通过X射线衍射(XRD)对Ca8Mg(SiO4)4Cl2∶Eu2+,Ln进行了表征,结果表明Ln的共掺杂并没有影响基质晶体的面心立方结构。所合成的荧光粉发射峰值位于507nm的绿光区,激发光谱在330~430nm之间均有较强吸收,与紫光InGaN芯片(395nm)相匹配。掺杂Ln作助激活剂增强了荧光粉的发光强度。借助Uitert经验公式计算出Eu2+在Ca8Mg(SiO4)4Cl2基质中占据八配位Ca(Ⅱ)格位。