Luminescence study of cerium-doped La2Hf2O7: Effects due to trivalent and tetravalent cerium and oxygen vacancies

X-ray excited emission spectra, photoluminescence excitation and emission spectra, optical reflectivity spectra, and pulsed X-ray and optical excited luminescence decay measurements are reported for cerium-doped La₂Hf₂O₇ powders prepared by solid state synthesis. A broad luminescence associated with oxygen vacancies is observed in the region 350–750 nm with a peak around 460 nm. The photoluminescence spectra and the number of oxygen vacancies vary for samples annealed in oxidizing or reducing atmospheres and with the temperature of the synthesis process. Increasing the cerium concentration reduces the oxygen-vacancy-related emission due to the presence of Ce⁴⁺. First principles calculations predict that Ce⁴⁺ can substitute in Hf sites; this is confirmed from the optical reflectivity spectrum of cerium-doped La₂Hf₂O₇. Photoluminescence excitation and emission spectra characteristic of Ce⁴⁺ charge transfer transitions and possibly Ce³⁺ are also observed. Although trivalent cerium may be present, no emission observed from cerium-doped La₂Hf₂O₇ can be attributed to Ce³⁺ in La sites.     

La2O3对Yb:Y2O3透明陶瓷光谱性能的影响

研究了La2O3对Yb:Y2O3透明陶瓷光谱性能的影响,添加适量La2O3以后,Yb:Y2O3透明陶瓷的吸收峰和发射峰的位置不变,但由于La3+的离子半径大于Y3+的离子半径,在Y2O3中引入La3+离子后,导致Y2O3晶格常数变大,晶场强度变弱,同时降低了Y2O3晶体的有序度,致使发射峰强度有所下降,发射截面变小.过量的La2O3(x=0.16)造成Yb3+激活离子发射强度明显下降;其荧光寿命在添加La2O3后总体增大45%-60%.     

Eu3+激活的La2Mo2O9红色荧光粉的制备与性能

利用高温固相法制备了Eu³⁺掺杂的La₂Mo₂O₉红色荧光粉,并对这种荧光粉的结构及发光性质进行了研究。XRD结果表明,实验合成了单一立方相的La₂Mo₂O₉荧光粉体。该荧光粉的激发光谱由一宽带和一系列的锐峰组成;发射光谱由一系列锐峰组成,这些都与Eu³⁺的特征跃迁⁵D_J(J=0,1)和⁷F_J(J=1~4)相对应。结果表明该荧光粉可被395nm的紫外光和470nm的可见光有效激发,并发出峰值位于620nm左右的红光,亮度可达到传统红色荧光粉Y₂O₂S:Eu³⁺的1.5倍以上,这表明它可以作为蓝+黄模式白光LED的红色补光粉,也可以作为UV-LED激发三基色荧光粉体系中的红色荧光粉。研究了Eu³⁺的掺杂浓度以及不同助熔剂对样品发光性质的影响。Eu³⁺的摩尔分数为0.3时,发光强度达到最强。质量分数为3%的NH₄Cl作为助熔剂时效果最好。     

Study of complex impedance spectroscopic properties of the KFeP2O7 compound

The KFeP₂O₇ compound was prepared by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction. The AC electrical conductivity and the dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 200 Hz–5 MHz and 553–699 K, respectively. Both impedance and modulus analysis exhibit the grain and grain boundary contribution to the electrical response of the sample. The temperature dependence of the bulk and grain boundary conductivity were found to obey the Arrhenius law with activation energies Eg?=?0.94 (3)?eV and Egb?=?0.89 (1)?eV. The grain-and-grain boundary conductivities at 573 K are 1.07?×?10^?4 and 1.16?×?10^?5 (Ω^?1 cm^?1). The scaling behavior of the imaginary part of the complex impedance suggests that the relaxation describes the same mechanism at various temperatures. The near value of the activation energies obtained from the equivalent circuit, conductivity data, and analysis of M″ confirms that the transport is through ion hopping mechanism.     

Magnetotransport properties of a layered La1.6Ca1.4Mn2O7 polycrystal

Magnetotransport properties of a two-layered La_1.6Ca_1.4Mn₂O₇ polycrystal have been examined as a function of temperature and applied field. It was found that the magnetic transition temperature (T_c) is about 70 K higher than the insulator–metal transition temperature (T_p). Two peaks were observed on both the temperature dependence of the imaginary part of the ac magnetic susceptibility χ^′′(T ) and that of the magnetoresistance MR(T). One is slightly below T_p∼107 K and the other is near T_c∼170 K. Below 70 K, the MR ratio increases with decreasing temperature. Around and above T_p but below T_c, the magnetization shows some indication of saturation, whereas the MR ratio shows no indication of saturation. The magnetotransport properties can be explained by considering the anisotropy exchange interactions along the a–b plane and the c direction, and the low-temperature MR can be attributed to the effects of the nearly fully spin-polarized carriers’ tunneling through the insulating (La,Ca)₂O₂ layers between the adjacent MnO₂ bilayers. Received: 18 September 2000 / Accepted: 20 February 2001 / Published online: 26 April 2001     

Inhomogeneous level splitting in Pr 2-x BixRu2O7

We report that Bi doping drives Pr 2-x BixRu2O7 from an antiferromagnetic insulator (x = 0) to a metallic paramagnet (x approximately 1) with a broad low T maximum in C/T. Neutron scattering reveals local low energy spin excitations (variant Planck's omega approximately 1 meV) with a spectrum that is unaffected by heating to k(B)T > variant Planck's omega. We show that a continuous distribution of splittings of the non-Kramers Pr3+ ground-state doublet such as might result from various types of lattice strain can account for all the data.     

Phonon Spectrum of La2Zr2O7: ab initio Calculation

Optics and Spectroscopy - The crystal structure and the phonon spectrum of a La2Zr2O7 crystal have been investigated in terms of the MO LCAO approach using DFT hybrid functionals that take into...     

Phonon Spectrum of La2Ge2O7: Ab Initio Calculation

Physics of the Solid State - Within the density functional theory, ab initio calculation of the crystal structure, phonon spectrum, and elastic properties of lanthanum germanate La2Ge2O7 with...     

Ga2O3组分对Tm 3+掺杂GeO2-Ga2O3-Li2O-BaO-La2O3玻璃的光谱性能影响

制备了Tm3 (8.0mol%)掺杂(77-x)GeO2-xGa2O3-8Li2O-10BaO-5La2O3(x=4,8,12,16)系列玻璃.系统地研究了Ga2O3从4mol%变化到16mol%时,玻璃的光谱性质与热学性质的变化规律.差热分析表明,随着Ga2O3含量的增加,锗酸盐玻璃的热稳定性增加.运用Judd-Ofelt(J-O)理论计算得到了Tm3 在不同Ga203含量的GeO2-Ga2O3-Li2O-BaO-La2O3玻璃中的J-O强度参数(Ω2,Ω4,Ω6)及Tm3 各激发能级的自发跃迁概率、荧光分支比以及辐射寿命等光谱参量.在808nm激光二极管的激发下,测试并分析了Ga2O3对Tm3 荧光光谱特性的影响.随着Ga2O3从4t001%增加到16mol%,Tm3 在1.8μm处的荧光强度呈现先减弱后增强的特性.当Ga2O3含量大约在12mol%时,Tm3 在1.8μm处的荧光强度最弱,受激发射截面达到最小.还初步讨论了Ga2O3对玻璃结构与光谱参数的影响规律.     

High-pressure Raman and infrared spectroscopic studies of ZrP2O7

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP₂O₇ to 23.2 and 13 GPa respectively. In the pressure range 0.7–4.3 GPa the lattice mode at 248 cm^?1 disappears, new modes appear around 380 and 1111 cm^?1 and the strong symmetric stretching mode at 476 cm^?1 softens, possibly indicating a subtle phase transition. Above 8 GPa all the modes broaden, and all of the Raman modes disappear beyond 18 GPa. On decompression from the highest pressure, 23.2, to 0 GPa all of the modes reappear but with larger full width at half maximum. Lattice dynamics of the high temperature phase of ZrP₂O₇ were studied using first principles method and compared with experimental values.     

Steplike lattice deformation of single crystalline (La0.4Pr0.6)1.2Sr1.8Mn2O7 bilayered manganite

We report a steplike lattice transformation of single crystalline (La0.4Pr0.6)1.2Sr1.8Mn2O7 bilayered manganite accompanied by both magnetization and magnetoresistive jumps, and examine the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase accompanied by a huge decrease in resistance. Our findings support that the abrupt magnetostriction is closely related to an orbital frustration existing in the inhomogeneous paramagnetic insulating phase rather than a martensitic scenario between competing two phases.     

Superconductivity and magnetic properties in Pr0.2Yb0.8−xLaxBa2Cu3O7−δ

The crystal structure, superconductivity and magnetic properties of the title compounds have been investigated. The results revealed that with the concentration of Pr being constant, the transition temperatureT _c of this system decreases with the increasing of La, or rather with the increasing of effective ionic radii of rare earth elements and the superconductivity disappears at aboutx=0.6. The coefficient of electron specific heat and the density statesN (0) which were infered from the magnetic measurements also decrease with the increasing of the effective radii. The relation between the radii of rare earth elements and hybridization is briefly discussed.     

On spectroscopic properties of the KYb(WO4)2:Pr3+ crystal

Spectroscopic properties of Pr³⁺ doped KYb(WO₄)₂ single crystals were investigated. The crystal lattice parameters were determined. Energy levels of Pr³⁺ in KYb(WO₄)₂ were assigned. The absorption, emission, excitation, time-resolved emission and excitation spectra were measured at low (10 K) and at room temperature. Decay times of the praseodymium emissions are non-exponential and unusually short. Site selection spectroscopy evidences several different Pr³⁺ sites. The Judd-Ofelt intensity model was used to analyse the experimental data. The Ω λ parameters, branching ratio and electric dipole transition probabilities were determined.     

Electrical transport and magnetic properties of perovskite-type electron-doped La–Pr–Mn–O epitaxial films

La_1−xPr_xMnO₃ (LPrMO) thin films have been epitaxially grown on (1 0 0)SrTiO₃ single-crystal substrates by pulsed-laser deposition. The films have a perovskite structure and give rise to the colossal magnetoresistance effect with the maximum magnetoresistance ratio of 10³% (at 240 K and 5 T). The electrical transport and magnetic properties have been investigated for the La_0.8Pr_0.2MnO₃ film with thickness 3000 Å. The results indicate that the films have quite a distinctive magnetotransport behavior compared to the bulk. The analysis of X-ray photoemission spectroscopy suggests that the valence state of Pr is 4+ in LPrMO film. Therefore, the epitaxial film is most likely an electron-doped colossal magnetoresistance system.     

Pr3+ luminescence center in Lu2Si2O7 host

Pr³⁺‐doped Lu₂Si₂O₇ (LPS:Pr) microcrystalline phosphor was prepared by the sol–gel method. We study the LPS:Pr luminescence properties under UV and X‐ray excitation within 80–500 K. The emission spectrum is dominated by fast 5d–4f band peaking at 261 nm having 16 ns decay time. By purely optical contactless methods we determine the energy barrier of 300 meV for thermal ionization of the Pr³⁺ 5d₁ relaxed excited state in LPS host. The barrier is high enough to keep the room temperature quantum efficiency of the Pr³⁺ luminescence center close to unity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SrB4O7：Pr3+,Mn2+中的Pr→Mn能量传递

从能量传递的角度出发,利用同步辐射光源(德国HASYLAB实验室的SUPERLUMI实验站)对Pr³⁺和Mn²⁺掺杂的SrB₄O₇粉末样品进行了光谱研究.206nm激发下,在SrB₄O₇:Pr³⁺(0.1%,摩尔分数)样品中观察到了来自Pr³⁺离子1S0能级的光子级联发射.SrB₄O₇:Pr³⁺样品的发射谱与SrB₄O₇:Mn²⁺样品监测Mn²⁺离子640nm发射的激发谱在330~430nm的波长范围里存在显著的光谱重叠.这个光谱重叠有利于Pr³⁺→Mn²⁺的能量传递发生,从而将Pr³⁺离子级联发射中第一步不实用的紫外或近紫外光子转换为Mn²⁺的红光发射.双掺杂样品SrB₄O₇:Pr³⁺,Mn²⁺与单掺杂样品SrB₄O₇:Pr³⁺的发射谱比较揭示出Pr³⁺→Mn²⁺的能量传递的确存在,并且提供了一种传递效率的估算方法,表明通过“Pr³⁺-Mn²⁺”组合有可能获得量子效率大于1的高效真空紫外激发发光材料.     

Oxygen concentration-related impedance spectroscopic studies of La2Mo2O9 oxide ion conductors

Ionics - The frequency dispersion of impedance, relaxation, and modulus formalism under oxygen, air, and nitrogen atmospheres are performed on La2Mo2−x W x O9 (x = 0, 0.3, 0.5,...     

Fabrication of Cu2O nanocrystalline thin films photosensor prepared by RF sputtering technique

Cuprous oxide (Cu₂O) nanocrystalline thin films were prepared on two types of substrates known as crystalline silicon and amorphous glass, by radio frequency reactive magnetron sputtering method. Scanning electron microscopy images confirmed that Cu₂O particles covered the entire surface of both substrates with smoothing distribution. The root mean square surface roughness for the prepared Cu₂O thin films on glass and Si (111) substrates is 4.16, and 3.36 nm, respectively. Meanwhile, X-ray diffraction results demonstrated that the two phases of Cu₂O and CuO were produced on Si (111) and glass substrates. The optical bandgap of Cu₂O thin films synthesised on glass substrate is 2.42 eV. Furthermore, the prepared Cu₂O nanocrystalline thin films have showed low reflectance value in the visible spectrum. Metal-Semiconductor-Metal photodetector based Cu₂O nanocrystalline thin films deposited onto Si (111) was fabricated using aluminium and platinum, with the current-voltage and photoresponse characteristic investigated under various applied bias voltages. The fabricated Metal-Semiconductor-Metal (M-S-M) photodetector had shown 126% sensitivity in the presence of 10 mW/cm² of 490 nm light with 1.0 V bias, displaying 90 and 100 ms response and recovery times, respectively. These findings have demonstrated the suitability of M-S-M Cu₂O photodetector as an affordable photosensor in the future.