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.     

Acoustic investigations on PbO-Al2O3-B2O3 glasses doped with certain rare earth ions

Elastic moduli (Y, η), Poisson’s ratio (σ), microhardness (H) and some thermodynamical parameters such as Debye temperature (θ_D), diffusion constant (D _i),latent heat of melting (ΔH _m) etc of PbO-Al₂O₃-B₂O₃ glasses doped with rare earth ions viz. Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺ and Yb³⁺, are studied as functions of temperatures (in the temperature range 30–200°C) by ultrasonic techniques. All these parameters are found to increase with increasing atomic numberZ of the rare earth ions and found to decrease with increasing temperature of measurement. From these results (together with IR spectra of these glasses), an attempt is made to throw some light on the mechanical strength of these glasses.     

AC conductivity in rare earth ions doped vanadophosphate glasses

Room temperature density and AC conductivity in the temperature range 300–525 K and frequency range 50 Hz to 5 MHz have been investigated in a set of La₂O doped vanadophosphate glasses. The density decreased and total conductivity increased with increase of La₂O content. The high temperature electrical conductivity has been analyzed using Mott’s small polaron model and polaron activation energies were determined. The polaron activation energy decreased marginally with increase of lanthanum content, at all frequencies of interest. These results have been attributed to the presence of mixed ion–polaron conduction in the present glasses. It is for the first time that La₂O doped vanadophosphate glasses have been investigated for AC conductivity and despite heaviness of lanthanum ions, the mixed ion–polaron conduction has been detected. Frequency dependence of AC conductivity has been considered under the correlated barrier hopping (CBH) model of single electron hopping.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.     

稀土激发的CaMgSi2O6的长余辉发光特性

使用固相反应法在还原气氛中制备了具有长余辉性能的CaMgSi2O6发光材料。研究了不同的共激发离子(Dy^3 和Nd^3 )对于材料发光性能的影响。光谱分析表明了这些磷光体在438nm处有一个宽的发射峰,这个发射峰是由Eu^2 的4f^6 5d^1能级到4f^7能级的跃迁所导致的,而Eu^2 在透辉石中形成六配位的发光中心。获得的3种磷光体都具有长余辉发光性能。其中共掺杂Eu^2 和Dy^3 材料比单掺Eu^2 和共掺Eu^2 扣Nd^3 的材料具有更好的余辉强度扣更长的余辉时间，其原因在于Dy^3 在CaMgSi2O6晶格中形成了更深的和更高密度的陷阱。     

掺稀土离子玻璃的光放大效应

光放大材料是光纤放大器的主要组成部分，它决定光纤放大器的光放大功率。本文介绍了掺稀土离子如Er^3 、Nd^3 、Pr^3 及Dy^3 等玻璃的光放大原理以及影响光放大的各种因素。稀土离子本身存在的多种发射跃迁、无辐射跃迁，稀土离子间相互作用以及玻璃基质都影响稀土离子的光放大跃迁效率。上述分析对掺稀土离子玻璃的光放大材料性能的改进和新材料的设计可提供有益的指导。     

Elastic properties of lanthanum aluminosilicate glasses

The elastic properties of two series of lanthanum aluminosilicate glasses (15La₂O₃-xAl₂O₃-(85−x)SiO₂ and 25La₂O₃-yAl₂O₃-(75−y)SiO₂, where x, y=15, 20, 25, 30, 35 mol%), were obtained by the ultrasonic pulse-echo technique, at room temperature. The correlation of elastic stiffness, the cross-link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio for our glasses are compared with those theoretically calculated values in terms of the Makishima-Mackenzie model.     

Magnetic properties of sulfate glasses doped with transition metal ions

Magnetic susceptibility measurements on ZnSO₄K₂SO₄ glasses containing high concentrations of Mn²⁺, Co²⁺ or Ni²⁺ ions suggest the presence of weak ferromagnetic interaction.     

Physical properties of some rare earth tellurite glasses

Mossbauer and IR spectra as well as the electrical conductivity have been measured to give an idea about the structure and the electrical properties of some rare earth tellurite glasses containing Fe₂O₃. The glasses denoted [1 – (2x + 0.05)] TeO₂ ·xFe₂O₃ · (x + 0.05) Ln₂O₃, wherex = 0.0 and 0.05 and Ln = lanthanum, neodymium, samarium, europium or gadolinium, were prepared by fusing a mixture of their respective reagent grade oxides in a platinum crucible at 800° C for one hour. The Mossbauer parameters such as isomer shift, quadruple splitting and line width were found to be a function of the polarizing power (charge/radius) of the rare earth cations. The Mossbauer parameters were not affected by the heat treatment of the glass samples. Both of the Te-O-Ln and Te-O-Fe stretching vibrations were obtained from the IR results which indicate that the rare earth oxides and iron oxide are partially covalent. The electrical resistivity was measured as a function of temperature from 293 to 520 K. Both the electrical resistivity and the activation energy were found to be a function of the atomic number (Z) of the rare earth cations. The results were interpreted on the basis of the electronic structure of the glass.     

Transport properties of rare earth doped La2−xSrxCuO4

We present measurements of the resistivity, the thermopower, and the thermal conductivity of rare earth (RE) doped La_2–xSr_xCuO₄. Pronounced anomalies occur at the low temperature structural phase transition showing that the electronic properties as well as the lattice dynamics depend strongly on small structural changes. Our results indicate that a suppressed phonon heat transport is a characteristic feature of La₂CuO₄ based superconductors. A possible relationship to the presence of stripe correlations of holes and spins in doped La₂CuO₄ is discussed.This work was supported by the DFG through SFB 341. We thank W. Brenig and A.P. Kampf for many stimulating discussions.     

Composition dependence of electrical properties of ZnF2-MO-TeO2 glasses

Dielectric constant (ε′), loss (tan δ), a.c. conductivity (σ) of ZnF₂-MO-TeO₂ glasses with varying concentrations of MO (P₂O₅, As₂O₃ and Bi₂O₃) were measured as a function of frequency and temperature over moderately wide ranges. From the analysis of these studies along with IR spectra and DTA results of these glasses, the structural changes in the systems with the concentration of metal oxides are discussed.     

Sol–gel derived hybrid materials doped with rare earth metal ions

Sol–gel derived organic–inorganic hybrid materials doped with rare earth metal ions (Pr³⁺) and small amounts of lithium ions (∼0.1–0.2 wt.%) were produced from the tetraethyl orthosilicate (TEOS), AlCl₃·6H₂O (about 10 mol%), ethyl methacrylate, butyl methacrylate and some other organic additions (ca. 35–40 wt.% of organics in the fresh gels) to obtain hybrid organic–inorganic hosts. The gel and hybrid materials obtained were aged at room temperature for three weeks, then heated in an electric drier for 3 h at temperature of 125 °C and investigated for morphology, structure and luminescence properties by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR), ²⁹Si and ²⁷Al MAS nuclear magnetic resonance and fluorescence spectroscopy. An influence of the organic additions and inorganic dopants on microstructure of the materials obtained and their luminescence properties has been examined. Under excitation with UV radiation in a range of ∼210–350 nm, the sharp and relatively intense luminescence emission lines due to ³P₀ → ³H₄ (blue) and ³P₀ → ³F₃ (red) transitions of Pr³⁺ ions were observed in the luminescence spectra of gel and hybrid materials of SA-series.     

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as Eu³⁺ doped Y₂O₃ (red phosphor), Eu²⁺ doped Ba_0·64Al₁₂O_18·64, BaMgAl₁₀O₁₇ and BaMg₂Al₁₆O₂₇ (blue phosphors) and Ce_0·67Tb_0·33MgAl₁₁O₁₉ and Eu²⁺, Mn²⁺ doped BaMgAl₁₀O₁₇ (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400°–500°C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430–450 nm (blue emission) and 515–540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Optical properties of zirconia doped with yttria and some rare earth oxides

Nanometric powders of cubic zirconia stabilized with yttria and rare element oxides (Sm, Nd, Gd) were prepared by crystallization under hydrothermal conditions. The powders were uniaxially compacted, repressed isostatically, pressure-less sintered in oxygen atmosphere and hot isostatically pressed under 300 MPa Ar atmosphere. Fully dense samples were polished from both sides. The optical properties were analyzed based on the spectral dependence of the transmittance (T) and reflectance (R). Spectroscopic measurements have shown that all materials fabricated in the present work are highly transparent, with total transmission above 90% towards the long-wavelength end of the near-IR range of the spectrum. Discussion of these results will be given.     

Excited state absorption in glasses activated with rare earth ions: Experiment and modeling

We present semiempirical approach based on the Judd–Ofelt theory and apply it for modeling the spectral properties of fluoride glasses activated with the rare earth (RE) ions. This method provide a powerful tool for simulating both ground state absorption (GSA) and excited state absorption (ESA) spectra of RE ions, e.g. Nd³⁺, Ho³⁺, Er³⁺ and Tm³⁺ in the ZBLAN glass matrix. The results of theoretical calculations correspond to the experimentally measured data. We also demonstrate that the spectra obtained using the presented approach are applicable in the analysis of up-conversion excitation schemes in these optoelectronically relevant materials.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

Characterization of sol-gel derived scintillating LuBO3 films doped with rare earth ions

Rare earth doped LuBO₃ thin films have been prepared by combining sol-gel process and coating techniques such as spin coating and spray pyrolysis. Annealing treatment results in the crystallization of the film as vaterite phase and incorporation of the doping ions in solid solution. XPS and RBS spectroscopies showed that the composition of the films is close to the nominal one. Adventitious carbon has been observed and attributed to incomplete pyrolysis of metal-organic precursors. XPS concentration profiles show a good homogeneity for the films. RBS demonstrated some inter-diffusion between amorphous carbon substrate and borate films resulting in a gradient of carbon at the interface between the substrate and the film itself. Finally scintillation spectra have been recorded and demonstrate the potentiality of these films to be used as X-ray intensifying screens.     

Thermoluminescence of alkali halides doped with alkaline earth impurities

Doping with alkaline earth impurities converts the alkali halides into very efficient phosphors. LiF doped with Mg is extensively used in radiation dosimetry. However, the phenomenon of thermoluminescence in these materials is far from clear. LiF-tld 100 is artificially separated from the rest of the alkali halide phosphors. The available experimental data are not fully utilised in building a model for thermoluminescence processes. In this review, thermoluminescence properties of LiF-tld 100 and other alkali halide phosphors are discussed. The comparative study which points out several common points could be useful in understanding thetl processes in these phosphors.