Crystal-field and superposition model analysis of :BaY F ( = Er, Dy, Nd)

The experimental values of the energy levels of Er³⁺, Dy³⁺, and Nd³⁺ in BaY₂F₈ were fitted to a single-ion Hamiltonian containing free-ion and crystal-field interactions. The crystal-field parameters so evaluated were then analyzed by using Newman's Superposition Model. The agreement between the two sets of parameters is good, provided a possible distortion of the F^- polyhedron around the rare-earth site is taken into account. The effects of a possible displacement of the rare-earth ion substituting for Y³⁺ are also evaluated. Received 14 May 2002 / Received in final form 12 July 2002 Published online 17 September 2002     

Electronic absorption spectrum of Cr3+ in calcium formate single crystals

Optical absorption spectra of Cr³⁺ ions in calcium formate single crystals with two different dopants have been studied at room and liquid nitrogen temperatures. The observed spectra have been attributed to Cr³⁺ ions in octahedral environment with tetragonal distortion. The values of the crystal field and Racah parameters have been evaluated.     

Thermally stimulated depolarization currents and ionic conductivity of cubic lead fluoride containing small rare earth ions

Abstract

This work is concerned with thermally stimulated depolarization current (TSDC) and ionic conductivity studies in lead fluoride containing the small rare earths Dy, Ho, Er and Yb. The TSDC scans from 80 to 300 K show two peaks. For Pb_1?xEr_xF_2+x one is located at 106 K, and another, which is much stronger, occurs at about 160 K. The former is associated with a dipolar defect containing at least two rare earths and the latter is attributed to the development of F^? space charge during polarization of the sample. The activation energies obtained from both the high temperature TSDC peak and the ionic conductivity are the same, which corroborates the latter assignment. In addition, the ionic conductivity is shown to be independent of concentration. Those results can be understood of rare earth clustering, which is either absent or is unobservable dielectrically for large rare earths such as lanthanum, occurs extensively even at very low concentrations of the small rare earths. The explanation is that the majority of fluorine charge compensators are trapped by clusters.     

Theoretical interpretation of the zero-field splitting parameters for Fe3+ ions in wide-band gap semiconductor TlGaSe2 single crystal

Superposition model (SPM) calculations are carried out to provide theoretical interpretation of the zero-field splitting (ZFS) parameters and investigate the local environment around the Fe³⁺ centers in a TlGaSe₂ single crystal. Experimental electron magnetic resonance (EMR) data are analyzed and compared with the ZFS parameter values predicted by SPM based on the orthorhombic approximation of structural data. The results provide an adequate interpretation of the ZFS parameters obtained by fitting EMR spectra and indicate that Fe³⁺ ions substitute for the Ga³⁺ ions in TlGaSe₂ crystal.     

Site selective spectroscopy of rare earth doped ThO2

The individual sites present in ThO₂ doped with Eu³⁺ and Er³⁺ have been studied by site selective spectroscopy. A cubic site corresponding to a rare earth charge compensated distantly and two sites having local charge compensation have been identified. One of the locally compensated sites is present only at moderate temperature while the other is formed at higher temperatures and is stable to high temperatures. The measurements are related to catalytically excited fluorescence of ThO₂, previous luminescence measurements, and magnetic resonance experiments that observe the presence of local F^_ charge compensation of rare earth dopants.     

Luminescence spectra of doped and undoped lead tungstate crystals

TL spectra of undoped lead tungstate crystals exhibit glow peaks at 30 v K and 85 v K centred at 440 v nm, plus a peak at 50 v K at 530 v nm in an annealed sample. Annealing adds a 170 v K peak. Trivalent dopants of La 3+ and Y 3+ reduce the green luminescence, and Nb 5+ introduces a peak near 100 v K centred at 530 v nm; Sb introduces features between 40 v K and 90 v K and 150 and 180 v K. The luminescence emissions around 50 v K may be attributed to complex intrinsic defect centres, including (WO 4 ) m 3 . Of the four dopants studied in the present research, Sb +5 has the highest luminescence intensity. CL spectra show interesting anomalies near 170 v K which are linked to a phase change of water/ice nanoparticles trapped at dislocations.     

Theoretic analysis on electric conductance of nano-wire transistors

By employing the commercial software nanoMos and Vienna ab Initio Simulation Package (VASP), the performance of nano-wire field-effect transistors is investigated. In this paper, the Density-Gradient Model (DG Model) is used to describe the carrier transport behavior of the nano-wire transistor under quantum effects. The analysis of the drain current with respect to channel length, body dielectric constant and gate contact work function is presented. In addition, Fermi energy and DOS (Density of State) are introduced to explore the relative stability of carrier transport and electrical conductance for the silicon crystal with dopants. Finally, how the roughness of the surface of the silicon-based crystal is affected by dopants and their allocation can be illuminated by a few broken bonds between atoms near the skin of the crystal.     

硼(氮、氟)掺杂对TiO2纳米颗粒光学性能的影响

利用X射线衍射谱、拉曼光谱和紫外-可见光吸收光谱研究了硼(氮、氟)掺杂对TiO₂纳米颗粒光学性能的影响.X射线衍射谱和拉曼光谱结果表明,掺硼(氮、氟)对TiO₂纳米颗粒的锐钛矿相晶体结构无明显影响,而其锐钛矿晶格出现畸变(c/a值增大),这被归因于掺杂原子对TiO₂纳米颗粒表面氧原子缺位沿晶格c轴方向的占据.另外,掺硼(氮、氟)TiO₂纳米颗粒吸收带红移与TiO相似文献   

Thermoluminescence mechanism in rare-earth-doped magnesium tetra borate phosphors

Magnesium tetra borate (MTB) doped with rare earths (REs) was prepared by the solid state sintering technique. Among the different RE dopants studied in this phosphor, gadolinium-doped phosphors resulted in a dosimetric peak at a relatively higher temperature. The thermoluminescence (TL) emission spectra of RE-doped MTB showed characteristic RE ³⁺ emissions. Electron paramagnetic resonance measurements were carried out in these phosphors to identify the defect centers formed during gamma irradiation and to establish a mechanism for the TL process. Signals corresponding to (BO ₃)^2?, O _v^? were seen upon irradiation which vanished on annealing at 250 °C, showing the role of these centers in the TL process. The thermal activation energies calculated based on the decay of these signals matched well with those calculated on the basis of the usual conventional method showing the validity of the mechanism of TL.     

EPR theoretical study of local lattice structure in ZnS:Cr system

By analyzing the EPR parameters a, D and F of Cr²⁺ ion located at tetrahedral site in ZnS, the local structure around Cr²⁺ in the crystal has been investigated on the basis of the complete energy matrix for a d⁴ configuration in a tetragonal ligand-field within a strong-field-representation. It is shown that there exists an expansion distortion in the local lattice structure. From EPR calculation, the distortion parameters ΔR=0.13 Å and Δθ=1.417° are determined.     

Magnetic interactions between Copper and RE in RE Ba2Cu3O7-δ

The antiferromagnetic ordering temperatures of the rare earth (RE) moments in RE Ba₂ Cu₃O_7-gd, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the ordering mechanism can be mainly seen as an indirect spin-spin exchange. However, the oxygen dependence of T _N is found to be reversed for the light rare earths compared to the heavy rare earths. As origin for this systematic observation an indirect interaction between the 3d-moments of copper and the 4f-moments of rare earths is discussed as a second order effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1: 2: 3 cuprates by inelastic magnetic neutron scattering. Here, the usual thermally driven increase of the magnetic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat- Mignod in YBa₂Cu₃O₇ and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO₂ planes. Furthermore, the quasielastic magnetic response has a Gaussian contribution at temperatures below 100 K, i.e., much above the long ranged ordering temperatures T _N. Magnon-like excitations appear already at slightly larger temperatures than T _N. In addition the paramagnetic inelastic spectra show only little dependence of the crystal field scheme on the oxygen concentration.     

XMCD at LII,III edges of rare-earths: electric quadrupolar and dipolar effects

X-ray Magnetic Circular Dichroism (XMCD) at L_II,III edges of rare earths in rare earth–3d transition metal compounds is still a subject of study. We present experimental evidence of electric quadrupolar transitions at L_III and L_II edges of Er in the ErFe₂ Laves phase. These experimental results are compared with atomic calculation taking into account crystal electric field effects on the Er site. Received: 30 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Investigation of local lattice structure around Cr3+ ions at the trigonal and tetragonal sites in RbCdF3: Cr3+ crystal

The local lattice structure and EPR, optical spectra for Cr³⁺ doped in RbCdF₃ crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr³⁺ ions in RbCdF₃ exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr³⁺ ion is smaller than that of Cd²⁺ ion, and therefore Cr³⁺ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr³⁺ doped in RbCdF₃ crystal R =?1.9491 Å ～?1.9814 Å, θ?= 55.234° ～?55.286° at the trigonal site and R ₁ =?1.8617 Å ～?1.8928 Å, R ₂ =?1.9527 Å ～?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results.     

Crystal structure and the magnetic properties of CeTiGe3

The crystal structure of the RTiGe₃ compounds (R=La, Ce and Pr) has been studied by X-ray powder diffraction methods; Rietveld refinement has been carried out on the La homologue. These compounds crystallise in the BaNiO₃ prototype structure, hP10-P6₃/mmc, also called the hexagonal perovskite (a=6.300(1), c=5.915(1) Å for LaTiGe₃). This seems to be the first example in which an intermetallic phase adopts such a structure type which can be considered as derived from the Ni₃Sn type (anti) by a distortion of the lattice and the occupation of the quasi-octahedral 2a site at the origin of the cell (0, 0, 0) by Ti. The composition, also confirmed by microprobe analyses, was found to be strictly 1:1:3 indicating that these are line compounds, forming very likely by a peritectoid reaction. The existence of homologous compounds has been established for the lighter rare earths La, Ce and Pr. Heat capacity and magnetisation data show that CeTiGe₃ orders ferromagnetically with a Curie temperature of nearly 14 K. On the other hand a−ln T variation of the magnetic part of the resistivity below 300 K is consistent with that expected for single impurity Kondo behaviour. CeTiGe₃ is thus an uncommon example of a ferromagnetic dense Kondo lattice.     

Excitation spectra of ferromagnets with crystal fields

We have investigated the ferromagnetic phase transition and excitation spectrum of systems, such as the rare earths, where the crystalline fields are significant in determining the magnetic properties. Using the full level scheme appropriate to a system ofJ-4 ions in a crystal field of cubic symmetry and assuming a Heisenberg interaction, we have calculated the full temperature- andq-dependence of the dynamic susceptibility within the RPA, both in the paramagnetic and ordered regimes. Variations in behavior, both with changes in crystal field parameter and with changes in magnetic interaction strength are discussed, and emphasis is placed on the limiting case which corresponds to current experiments in the light rare earths.     

Magnetic Moment of Fe and Magnetocrystalline Anisotropy of Fe-rich Intermetallic Compounds Under Pressure

Magnetization and magnetocrystalline anisotropy of Fe-rich R -Fe intermetallics ( R =Y and rare earths) have been studied under pressure up to 1 GPa in temperatures from 5 K to 350 K using a miniature CuBe pressure cell and a SQUID magnetometer. A decrease of Fe-moment under pressure depends strongly on local environment parameters such as a local atomic volume, local coordination of Fe-atoms and hybridization. The pressure-induced changes of magnetocrystalline anisotropy energy are caused by both, the pressure decrease of magnetization M S and the pressure effect on the crystal electric field interactions. The pressure experiments allow to distinguish both the effects.     

Luminescence properties of Nd3+, Sm3+ and Eu3+ intercalated CdPS3

Nd³⁺, Sm³⁺ and Eu³⁺ have been intercalated by cation exchange into CdPS₃. The photoluminescence and IR spectra show the creation of cadmium cation vacancies. Crystal field analysis indicates that the rare earths have entered the intralamellar vacancies and formed a complex defect center with C₂ or lower symmetry.     

掺钇钨酸铅晶体缺陷的理论研究

采用基于密度泛函理论的相对论性离散变分和嵌入团簇方法，计算了掺钇PbWO₄ 晶体中多 种相关缺陷的电荷分布和不同团簇缺陷结合能，由能量最低原理发现［2(Y³⁺Pb)-V″_Pb］缺陷在各相关缺陷形式中最为稳定.并运用过渡 态方法计算了轨 道跃迁的激发能，算出掺Y后晶体中O2p→Y4d的跃迁能量为3.9eV，表明掺Y不会引起晶体中3 50nm和420nm吸收.从掺Y对PbWO₄晶体电子结构的影响来看，其作用机理与掺La 的情况也有较大差异. 关键词： 4晶体')" href="#">PbWO₄晶体 密度泛函 掺Y 态密度分布     

NaCl（OH－）中K~＋对F＋2型心的形成及其光谱性质的影响

研究了Ｋ＋对ＮａＣｌ（ＯＨ－）中（Ｆ＋２）Ｈ心的形成及其光谱性质的影响。结果表明，Ｋ＋的存在抑制了Ｆ＋２型心的形成，改变了（Ｆ＋２）Ｈ心的光谱性质，拓宽了发射谱的半功率谱宽。从而修正了Ｐｏｌｏｃｋ的实验结果。同时指出，研制具有更宽波长发射范围的Ｎａ１－ｘＫｘＣｌ（ＯＨ－）复合基质Ｆ＋２型色心激光材料是可能的     

CaS:Pr,Na的激光选择激发

本文用激光选择激发研究了CaS:Pr,Na中Pr³⁺的³P₀→³F₂发光,分辨出5种不同的Pr中心。用三个参数描述立方晶场中受电荷补偿结构影响的稀土离子的能级劈裂,其中两个参数是立方晶场参数B₄和B₆,另一个反映引入电荷补偿结构后立方晶场的畸变。对³F₂能级劈裂的拟合结果表明,发光来自受不同距离上电荷补偿结构影响的Pr中心.