Ab Initio Calculation of 4f→5d Transition Energy and Electronic Structure of Ce^(3+) Doped in LiYF_4 Crystal

The electronic structures of LiYF4：Ce^3＋ and LiYF4 crystal simulated by an embedded （in a microcrystal containing 1938 ions） cluster CeY4Li8F24, and Y5LisF24 respectively, were computed by the ab initio self-consistent relativistic DV-Xa （discrete variational Xa） method. The ground-state calculation showed that only the lowest 5d level Ed of Ce^3＋ ion lies around the BCB （bottom of the conduction band） while the lowest 4f levels is 2.5 eV lower than BCB. The CB states consist of 4d of Y mixed with 5d of Ce, even for the wavefunctions （WFS） with energy Ed under BCB there are still 24% of Y-4d and 9% of F-2p as components. So, they are not pure crystal-field states at all. Furthermore, transition state （TS） calculation was performed to obtain the 4f→5d transition energies Efd, to improve the previous calculation performed by Andriessen et al, in which a small CeF8 cluster embedded in an array of point charge was used and the results of ground-state calculation were roughly used to compare with the observed 4f→5 d transition energies. The ionic radius of Ce^3＋ is larger than that of y^3＋ , so we had also modeled approximately the lattice relaxation. As results, the CeY4Li8F24 cluster with 4.56 % outward relaxation （of the nearest-neighbor and next nearest-neighbor eight fluorines） has the lowest total energy and gave satisfactory 4f→5d energies Efd, but the ground-state calculated Ed is 0.68 eV higher than BCB. For another cluster with 7.36% outward relaxation the Ed is 0.43 eV lower than BCB, which makes the observation of fine structure （including zero-phonon line） of the lowest 5 d band understandable easier, but the splits between the transition energies Efd were not as good as the former. Therefore, we consider the relaxation is some how around 4. 56% -7.36% outward, not as large as 10% proposed by Andriessen et al.

Research on charge ordering and magnetic properties in La_(0.3)Ca_(0.7)Mn_(1-x)W_xO_3 system

The perovskite manganite sample La0.3Ca0.7Mn1-xWxO3 （x = 0.08, 0.12） was prepared by the solid-state reaction method. The effect of W doping on the Mn site to La0.3Ca0.7MnO3 charge ordering phase and the changing process of magnetic properties were studied through the measurement of the M-T curve, M-H curves, and ESR curves of the sample. The results showed that when x = 0.08, the charge ordering （CO） phase exists in the system, the transition temperature Tco= 275 K, and the system exhibits PM when T 〉 275 K. The system transforms from spin-disordering paramagnetism to spin-ordering antiferromagnetism in the charge ordering state with the temperature decreasing from 275 K to 230 K. The long-range antiferromagnetism forms and AFM/CO states coexist between 230 K and 5 K. There is a little ferromagnetic component in the AFM/CO background in a low temperature range. When x = 0.12, the CO phase in the system has almost melted completely. There is a little remnant of the CO phase below 150 K. The system exhibits paramagnetism when T 〉 150 K and transforms from paramagnetism to ferromagnetism when T〈 150 K.

Influence of Zinc Doping upon Magnetic Structure of LaMnO_3 System

M-T curves, M-H curves and electron spin resonance (ESR) curves of LaMn_(1-x)Zn_xO_3(x=0.05, 0.10, (0.20,) 0.30, 0.40) were studied. Experimental results indicate that: with increasing Zn doping, the T_C value decreases monotonously, and the system undergoes a transition from long-range ferromagnetic order to cluster-spin glass stated. The results are attributed to double exchange interaction, magnetic dilution and lattice effects by Zn doping.

Crystallization and Microstructure Evolution of Amorphous Al_(85)Y_4Nd_4Fe_7 Alloy

A rapidly solidified Al_(85)Y_4Nd_4Fe_7 (%, in nominal atomic fraction) alloy was prepared by melt spinning. As-quenched and as-annealed microstructures were studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Fully amorphous structure could be obtained in the rapidly solidified Al_(85)Y_4Nd_4Fe_7 alloy ribbons. The temperature of first crystallization exceeds 300 ℃. Crystallization of as-annealed Al_(85)Y_4Nd_4Fe_7 alloy is shown to occur in two stages: (1) primary crystallization of α-Al; (2) formation of Al_3Y, Al_(13)Fe_4 and unknown crystalline phases.

CHARACTERISTICS OF STRUCTURE OF IMPACT COPOLYMERS OF POLYPROPYLENE WITH LOW ETHYLENE CONTENTS

In the present work, the structure and impact properties of copolymers of polypropylenewith low ethylene contents have been investigated. Based on the results of ^(13)C--NMR, FTIR,WAXD, DSC, PLM and SEM, the relationship between impact properties and morphology of thecopolymers has been discussed. The high impact properties of copolymer ICP2 may attribute tothe relatively higher ethylene content and homogeneous ethylene unit distribution. The size andits distribution of spherulite in the copolymers and cycloid cavities dispersed in polypropylenecontinue phase may also be two important factors which affect the impact properties of these materials.

Research of Colossal Magnetoresistance in La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15)

The extraordinary colossal magnetoresistance (CMR) behavior in Mn-site doped system La_(0.67)Ca_(0.33)Mn_(1-x)Cr_xO_3 (0.00x0.15) was reported. It was found that the substitution with Cr on Mn sites introduces an additional bump in zero-field resistivity. With increasing Cr content, this additional bump grows up drastically while the original resistivity peak associated with magnetic order transition diminishes gradually. Under the applied magnetic field, both bumps of resistivity are deeply compressed, which leads to the appearance of two peaks in CMR response. As a result, the temperature range of CMR response is significantly broadened, spanned from the lowest to near room temperature. These results suggest that Mn-site element substitution could be a potent way of tuning CMR response.

C-Axis Current -Voltage Characteristics of Mesa Structures on Bi2Sr2CaCu2O8+δ Single Crystals Fabricated by a Simple Technique Without Photolithography

A simple technique is reported for fabricating the mesa structure on Bi2Sr2CaCu2O8+δ single crystal. In the patterning process, metal masks are used instead of photolithography and argon ion milling is applied to form the small mesa on the Bi2Sr2CaCu2O8+δ crystal surface. Real four-probe transport measurements are made on the mesa structure and typical c-axis current-voltage (I - V) characteristics of the intrinsic Josephson effect have been observed. The superconducting gap parameter can be extracted from the multi-branch structure in the I - V characteristics. Additionally, from the strong hysteresis in the I - V characteristics, the capacitance CJ of the unit intrinsic Josephson junction has been estimated to be 2.3pF, which is in good agreement with that evaluated from the geometric parameters of the unit junction between the two copper oxide layers.

Electrical Properties of ZnO/Si Heterostructure

The electrical Properties of a type of semiconductor heterostructure fabricated by depositing zinc oxide film on a silicon substrate are investigated.The I-V,I-T curves,and deep level transient spectra are measured.From these results,we acquire the information of the characteristics of the junction,and compute some energy levels of the samples.

Structure and Waveguide Properties of Sol-Gel Derived Gd_2O_3 Films

Pure and rare earth doped gadolinium oxide (Gd 2O 3) waveguide films were prepared by a simple sol-gel process and dip-coating method. Structure of Gd 2O 3 films annealed at different temperature was investigated by X-ray diffraction and transmission electron microscopy. Oriented growth of (400) face of Gd 2O 3 has been observed when the films were deposited on amorphous substrate. The refractive index and thickness of films were determined by m-lines spectroscopy. The laser beam (λ=632.8 nm) was coupled into the film by a prism coupler and the propagation length is about 3.5 cm. Luminescence properties of europium ions doped films were measured by waveguide fluorescence spectroscopy, which shows disordered environment for Eu 3+ at 400 ℃.

Ab Initio Calculation of 4f→5d Transition Energy and Electronic Structure of Ce^(3+) Doped in LiYF_(4) Crystal

The electronic structures of LiYF4：Ce^3＋ and LiYF4 crystal simulated by an embedded （in a microcrystal containing 1938 ions） cluster CeY4Li8F24, and Y5LisF24 respectively, were computed by the ab initio self-consistent relativistic DV-Xa （discrete variational Xa） method. The ground-state calculation showed that only the lowest 5d level Ed of Ce^3＋ ion lies around the BCB （bottom of the conduction band） while the lowest 4f levels is 2.5 eV lower than BCB. The CB states consist of 4d of Y mixed with 5d of Ce, even for the wavefunctions （WFS） with energy Ed under BCB there are still 24% of Y-4d and 9% of F-2p as components. So, they are not pure crystal-field states at all. Furthermore, transition state （TS） calculation was performed to obtain the 4f→5d transition energies Efd, to improve the previous calculation performed by Andriessen et al, in which a small CeF8 cluster embedded in an array of point charge was used and the results of ground-state calculation were roughly used to compare with the observed 4f→5 d transition energies. The ionic radius of Ce^3＋ is larger than that of y^3＋ , so we had also modeled approximately the lattice relaxation. As results, the CeY4Li8F24 cluster with 4.56 % outward relaxation （of the nearest-neighbor and next nearest-neighbor eight fluorines） has the lowest total energy and gave satisfactory 4f→5d energies Efd, but the ground-state calculated Ed is 0.68 eV higher than BCB. For another cluster with 7.36% outward relaxation the Ed is 0.43 eV lower than BCB, which makes the observation of fine structure （including zero-phonon line） of the lowest 5 d band understandable easier, but the splits between the transition energies Efd were not as good as the former. Therefore, we consider the relaxation is some how around 4. 56% -7.36% outward, not as large as 10% proposed by Andriessen et al.

Structure Property and Visible Upconversion of Er^(3+) Doped Gd_2O_3 Nanocrystals

Gd_2O_3∶Er nanoparticles were prepared by a simple sol-gel method. The structure properties of Gd_2O_3∶Er were studied by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy. The visible up-converted luminescence spectra of Er^(3+) were investigated under excitation to (()~4I_(9/2)) level by 785 nm laser. Laser power, Er^(3+) ion concentration and temperature dependences of the upconverted emissions were investigated to understand the upconversion mechanisms. Excited state absorption and energy transfer process are discussed as the possible mechanisms for the upconversion.

Structure of melt-spun Al-2.5Ti-2.5Fe-2.5V alloy and structural changes during annealing

Rapidly solidified Al 2.5Ti 2.5Fe 2.5V alloy was prepared by melt spinning. As spun and as annealed microstructures were studied by TEM, HREM, XRD and EDS analysis. The microhardness of the alloy at different annealing temperature was measured. The results show that the as spun microstructure of the alloy includes four kinds of primary phases: Al 11 (V,Ti), AlTi 3, Al 80 (V,Ti) 20 and Al 82 (Fe,Ti,V) 18 phase. After annealing at 300 ℃ for 10 h, metastable phase Al 82 (Fe,Ti,V) 18 transforms to the stable phase Al 13 Fe 4. After annealing at 400 ℃ for 10 h, the primary phases Al 11 (V,Ti) and AlTi 3 has completely transformed to Al 80 (V,Ti) 20 and Al 23 Ti 9 phase, respectively. The addition of element V increases the microhardness of melt spun Al 2.5Ti 2.5Fe alloy at elevated temperatures.

OPTICAL EFFECTS AND MICROSTRUCTURE OF BURIED INSULATOR LAYER FORMED BY O^+ AND N^+ CO-IMPLANTATION

The microstructure and optical properties of a buried layer formed by O+(200keV,1.8×1018/cm2)and N+(180 keV,4×1017/cm2)co-implantation and annealed at 1200℃for 2 h have been investigated by Auger electron,IR absorption and reflection spectroscopicmeasurements.The results show that the buried layer consists of silicon dioxide and SiOx（x【 2）and the nitrogen segregates to the wings of the buried layer where it forms an oxynitride.Bydetail theoretical analysis and computer simulation of the IR reflection interference spectrum,therefractive index profiles of the buried layer were obtained.

Structure and Luminescent Enhancement Effect of Na_5Eu(MoO_4)_4 by Doping(WO_4)^(2-)

The luminescent enhancement effect of Na_5Eu(MoO_4)_4 by doping(WO_4)^(2-) has been studied. When the value x in Na_5Eu(Mo_(1-x)W_xO_4)_4 is in 0<x<0.21,its photoluminescent spectrum is near to that of Na_5Eu(MoO_4)_4,but its luminescent intensity increases.When x=0.033,the intensity is increased by about 30%.According to the X-ray diffraction analysis,W ions get into the lattice and occupy the sites of the Mo ions.The change of crystal environment of Eu^(3+) may result in the luminescent enhancement.

Photovoltaic Effect of ZnO/Si Heterostructure

A type of semiconductor heterostructure fabricated by zinc-oxide films deposited on silicon substrates is investigated.The current-voltage characteristics under dark or illumination were determined.Tt is indicated that the sample is likely a semiconductorjunction,and this structure generates an obvious photovoltaic effect.Spectral responses of photovoltage,cathodotoluminescence and excitation spectra at room temperature were employed to study the structural properties and the mechanism generating photovoltaic effect of the samples.The energy level and the process of electron transition in the ZnO film have also been deduced.

Electronic Structure of Si-C Alloys

We have systematically studied the electronic structure of the ordered Si-C alloys,fourteen models were created according to the different carbon contents.The linear muffin-tin orbitals method was used to calculate their energy band informations.The functional relation of band gap upon carbon contents conflicts with the point of view of Soref[J.Appl.Phys.Lett.56(1990)734],but agrees well with that of Alexander's[Phys.Rev.B 48(1993)2207].

Microstructure Investigation of (Y,Ce)-TZP-Al_(2)O_(3) Composite Materials

The microstructure of selected specimens in the(Y,Ce)-TZP-Al_(2)O_(3) composite systems was studied.The investigation techniques used in this work included X-ray diffractometry(XRD),Scanning Electroscope(SEM),Transmission Electroscope(TEM),X-ray Energy Dispersion Analysis(EDAX)and High Resolution Electron Microscopy(HREM).Based on the experimental results,the relationship between microstructure and mechanical properties of the specimens was discussed.

聚电解质电流变效应中的E_c及其产生机理的研究

为了深入研究电流变体中电压阀值 ( Ec)产生的机理 ,我们设计了一种全新的测试方法——饱和电压法 ,并用此方法明确地证实了电压阀值 ( Ec1和 Ec2 )的存在 ,同时对此作出了解释 ;而且 ,我们还对两种典型的电流变模型在不同电流变体中的适用性作出了定义。计算机模拟的结果分别给出了两个对应于电压阀值 Ec1和 Ec2 的序参量 φ1=1.5和 φ2 =4 .5。

Enhancement of room-temperature magnetoresistance in La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/(Ag_2O)_(x/2)

La0.5Sm0.2Sr0.3MnO3/（Ag2O）x/2 （x = 0.00, 0.04, 0.08, 0.25, 0.30） samples were prepared by the solid-state reaction method, and their transport behaviors, transport mechanism, and magnetoresistance effect were studied through the measurement and fitting of p-T curves. The results show that the element Ag takes part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the host material and is in metallic state when the doping amount is relatively large; then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by the spin-dependent scattering. The resistivity of the sample doped with 30 mol% Ag is one order of magnitude smaller than that of low-doped samples, and its magnetoresistance in the magnetic field of 0.5 T and at 300 K is strengthened apparently reaching 9.4%, which is connected not only with the improvement of the grain-boundary structure of the host material but also with the decrease of material resistivity.

Influence of the substitution of Sm, Gd, and Dy for La in La_(0.7)Sr_(0.3)MnO_3 on its magnetic and electric properties and strengthening effect on room-temperature CMR

A series of La0.7-xSmxSr0.3MnO3, La0.7-xGdxSr0.3MnO3, and La0.7-xDyxSr0.3MnO3 （x=0.00, 0.10, 0.20, 0.30） samples were prepared by the solid-state reaction method. The influence of the substitution of Sm, Gd, and Dy for La on the magnetic and electric properties and on the magnetoresistance （MR） was studied through measurements of M-T curves and p-T curves. The results showed that： lattice distortion induced by substitution of Sm, Gd, and Dy for La and extra magnetism of substitution had great influence on the magnetic and electric properties of pcrovskite manganites; substitution of magnetic rare earth element for La was an effective way to change Curie temperature and to strengthen MR in perovskite manganites; and appropriate substitution proportion would generate large MR near room temperature.