Microhardness measurements on single crystals of flux-grown rare earth perovskites (orthoferrites,orthochromites and aluminates)

The results of indentation-induced microhardness testing studies of flux-grown single crystals of rare earth orthoferrites, RFeO₃ (R=Gd to Er and Yb), rare earth orthochromites RCrO₃ (R=La, Eu and Dy), and rare earth aluminates RAlO₃ (R=La, Sm, Gd, Eu and Ho) are presented. The variation in the value of microhardness with load is observed to be non-linear in the case of all these materials. It is found that the results are not in accordance with Kick's law. The results have been analysed and the applicability of the idea of materials resistance pressure in the modified law as proposed by Hays and Kendall [Metallography 6 (1973) 275] is discussed.     

Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium

Thermal behaviour of gel-grown pure and mixed rare earth tartrates of yttrium and samarium is investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal behaviour suggests that the materials are unstable at lower energies and pass through various stages of decomposition, decomposing to respective rare earth oxides which remain stable on further heating. It is estimated that both pure yttrium and pure samarium tartrate crystals carry eight waters of hydration, while mixed yttrium samarium tartrate crystals carry six waters of hydration. Critical examination of TG and DSC curves shows that the initial decompositions are endothermic and the latter are exothermic. Thermal kinetics of these materials has been worked out using Horowitz-Metzger, Piloyan-Novikova and Coats-Redfern equations. Application of these equations to these materials yields values of activation energy, order of reaction and frequency factor which are in reasonably good agreement.     

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。     

Resonance modes in rare earth doped microcrystals

Alkaline earth sulfide phosphors doped with Eu²⁺ or Ce³⁺ are good candidates for wavelength conversion in LEDs for general lighting applications. A very broad emission spectrum can be obtained by preparing SrS:Eu-SrS:Ce core-shell particles. These microcrystals are rectangular cuboids and can also be efficiently excited with electrons in a scanning electron microscope. The broad emission spectrum, ranging from 450 nm to 650 nm, consists of the combined emission of Ce³⁺ and Eu²⁺ centers. Using cathodoluminescence (CL) in a scanning electron microscope, evaluation of the emission characteristics of individual cuboids was possible. Sharp peaks in the CL-spectra of individual particles could be observed and described as being resonance modes. The spectral positions and the line widths of resonance modes in these core-shell microcrystals are predicted by an extension of the plane wave model, taking into account the dispersion of the refractive index and the existence of open orbits.     

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     

The flux growth of rare earth germanomolybdates and germanotungstates

The growth of R₂GeMoO₈ and R₂GeWO₈ crystals from the flux systems R₂O₃-GeO₂-MO₃-PbO, where M = Mo or W, is described. A preliminary investigation of their magnetic properties is reported. The X-ray powder pattern data for R₂GeWO₈ with R = Gd, Tb, Dy, Ho and Er are given.     

Reactions of rare earth acetates with aluminum isopropoxide in ethylene glycol: Synthesis of the garnet and monoclinic phases of rare earth aluminates

The reactions of mixtures of rare earth (RE) acetates and aluminum isopropoxide (Al/RE = 3/5) in ethylene glycol at 300°C afforded amorphous products. The IR spectra of these products were essentially identical with each other and indicated the presence of an acetate group and ethylene glycol moiety. When the content of RE (La–Gd) acetate in the starting mixture was increased, a crystalline glycol complex of RE oxide acetate was also formed together with the amorphous phase, but the reactions of acetates of the smaller RE ions (Tb–Lu) yielded amorphous products with wider compositions (Al/RE = 5/3–1/6). Phase-pure RE3Al5O12 (garnet) and RE4Al2O9 (monoclinic) were obtained by calcination of the amorphous products with the corresponding stoichiometric compositions at low temperatures.     

Thermoluminescence dosimetry of rare earth doped calcium aluminate phosphors

The thermoluminescence (TL) properties of calcium aluminate (CaAl²O⁴) doped with different rare earth ions have been studied and their suitability for radiation dosimetry applications is discussed. It is observed that monocalcium aluminate doped with cerium is a good dosimeter having linear response up to about 4 kGy of radiation doses. Dopant concentration of 0.25 mol% cerium gives maximum TL emission. The well-defined single peak observed at 295°C can be advantageously used for high temperature dosimetry applications.     

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.     

Phase transition of nanostructure zirconias doped with rare earth elements

Nanostructure zirconias doped with rare earth elements were obtained by coprecipitation. The morphology, structure, and phase transition of the as-prepared samples were studied. According to transmission electron microscopy, the particles appear in near spherical shape and have a mean particle sizes of about 150 nm without change with the rare earth elements. From laser Raman and X-ray diffraction results, it is known that the tetragonal phase of zirconia is stabilized by a certain concentration of the rare earth dopants, and the cubic phase is obtained if enough dopants are added. The crystal lattice increases with the increase of dopant concentration. Moreover, the heavy rare earths are better than the light ones in stabilizing effect. According to our study, laser Raman is more sensitive than X-ray diffraction in monitoring the phase transition.     

Growth and characterization of gel grown pure and mixed iron-manganese levo-tartrate crystals

Several applications of iron tartrate and manganese tartrate compounds are reported in the literature. In the present investigation, we have grown pure and mixed iron (II)-manganese levo-tartrate crystals by single diffusion gel growth technique. Crystals with spherulitic morphology were harvested. The colouration of the crystals changed from black to pinkish brown upon increasing the content of manganese in the crystals. The crystals were characterized by FTIR spectroscopy, powder XRD, TGA, VSM and dielectric study. Crystal structures of different mixed crystals were studied. From TGA it was observed that on heating the hydrated crystals became anhydrous and then converted into oxides. Paramagnetic nature of the crystals was revealed from VSM study. The variation of the dielectric constant with frequency was studied. The results are discussed.     

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.     

稀土铈掺杂纳米ZnO抗菌复合材料研究

采用纳米ZnO为载体,按照不同掺杂比例制备了复合无机抗菌剂 Ce4 /ZnO,采用抑菌圈实验和最小抑菌浓度实验来检测其抗菌性能。结果表明,掺杂稀土Ce4 明显提高了纳米 ZnO的抗菌性。通过对ZnO白样和Ce4 /ZnO试样的XRD测试, 结果表明 Ce4 已进入 ZnO晶格,同时对复合抗菌机理进行了探索研究。     

Magnetic moments for mixed substituted rare earth iron garnets

The molecular field coefficients for the individual rare earth iron garnets have been determined by a trial and error procedure using the phenomenological molecular field theory of Neél and the experimental curves for each rare-earth iron garnet. These results were then used to calculate magnetic moment-temperature curves for various film compositions subject to the conditions that the lattice parameter of the film and λ     

Thermoluminescent spectra of rare earth doped MgB4O7 dosemeters

This paper presents X ray excited TL spectra of magnesium borate doped with either single rare earth ions Dy or Tm, or co-doped with Dy/Tm, Tm/Mn or Dy/Tb. Intrinsic emission from the host material is in the UV/blue region at approximately 375 nm, with a tail extending to 200 nm. The main dosimetric peak is detected at approximately 180 degrees C but slight differences are noted between the glow peak maxima from the different rare earth ions and there were changes following thermal treatments. The results are discussed according to the interaction between trapping and rare earth sites.     

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.