On some low-temperature properties of (La, Ce)B6 alloys

Some recent experimental data on the magnetic susceptibility and the low-temperature magnetization of the (La, Ce)B₆ system are discussed in the light of the theoretical results. A value of the s-f exchange constant J is estimated which is also consistent with the results obtained from both the Kondo effect and the RKKY interaction.     

Magnetoresistivity of the Kondo-system (La,Ce)B6

The electrical resistivity of the Kondo system (La, Ce)B₆ has been measured in longitudinal and transversal magnetic fields up to 6 Tesla in the temperature range 0.04–20K. Corresponding to the strong increase of the resistivity with decreasing temperature the alloys show a very large negative magnetoresistivity with a Kondo temperatureT _K =1.05K and a Kondo magnetic fieldB _K =1.1 Tesla. The observed anisotropy of the resistivity due to the magnetic field direction cannot be explained well by existing theories.     

基于化学计量法对荧光光谱重叠的维生素B1、B2和B6的测定

利用直接正交信号校正( DOSC)-小波包变换(WPT)-偏最小二乘法(PLS)(DOSC-WPT-PLS)新方法对荧光光谱严重重叠的维生素B1、B2和B6三组分混合体系进行了同时测定.设计了 PDOSCWPTPLS程序执行相关计算,并将三种化学计量学方法(DOSC-WPT-PLS、WPT-PLS和PLS)进行了比较....     

Orientation kinetics of the hexavacancies—V6 (B80) in silicon

A phenomenological kinetic equation is proposed to describe a process of a hexavacancy (V₆) orientation in silicon samples. The orientation kinetics of V₆ was investigated under a uniaxial compressional stress along the [111] axis of the crystal. Parameters appeared in the matrix equation of the orientation kinetics of V₆ as functions of pressure and annealing temperature were obtained and their values were estimated. In the case of the inequivalent V₆ centers, the pressure dependence of the difference in the magnitude of their binding energies was determined.     

Spin-glass behaviour of [La,Gd]B6 and [La,Dy]B6 alloys

Electrical resistance and absolute thermoelectric power measurements have been made in the temperature range between 2 and 30 K on a few polycrystalline specimens of [L$\text{a}$,Gd]B₆ and [L$\text{a}$,Dy]B₆ with different concentrations of rare earth ions. The resistance of these alloys varies as $\text{～ T}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ which is characteristic of spin glasses at low temperature. The thermoelectric power of all specimens but one, shows a broad positive peak in the lower part of the temperature range and becomes negative at higher temperatures, a feature that is typical of a spin glass to paramagnetic phase transition. The exceptional specimen has a large Gd concentration and its thermoelectric power remains positive to higher temperatures than would be expected for a spin glass.     

Diborane : High resolution infrared rovibration studies of B2H6 and B2D6

Ground state rotation and quartic distortion constants were obtained for ¹¹B₂D₆ from the analysis of high resolution (0.05 cm⁻¹) Fourier transform infrared spectra. The bands studied comprised the ν₁₇, ν₁₈ type A, and ν₁₄, ν₉ + ν₁₅ type C bands of ¹¹B₂H₆ and the ν₁₆, ν₁₇, ν₁₈ type A, ν₈ type B, and ν₁₄ type C bands of ¹¹B₂D₆. In the case of ¹¹B₂H₆, the authors' ground state data were combined with those of Lafferty et al. obtained from a previous study (J. Mol. Spectrosc. 33, 345–367 (1970)) at comparable resolution of the ν₁₆ type A and ν₈ type B fundamentals. Information on the ground state rotational energy manifold of ¹¹B₂H₆ was accumulated up to J = 23, K_a = 18, and of ¹¹B₂D₆ up to J = 32, K_a = 22. This permitted rather precise determination of the distortion constants Δ_J⁰, Δ_JK⁰, Δ_K⁰, although δ_J⁰ and δ_K⁰ proved to be too small (< 10⁻⁷ cm⁻¹) and were constrained to values calculated from the force field. Sets of upper state parameters were determined for all vibrational levels studied. Although these appear to be essentially unperturbed globally, several localized perturbations were observed and identified.     

Structural, elastic, and electronic properties of icosahedral boron subcarbides (B12C3, B13C2), subnitride B12N2, and suboxide B12O2 from data of SCC-DFTB calculations

The structural, elastic, and electronic properties of a series of icosahedral phases, such as boron subcarbides B₁₂C₃ and B₁₃C₂, subnitride B₁₂N₂, and suboxide B₁₂O₂, have been studied in the framework of the SCC-DFTB method. It has been found that the B₁₂C₂ and B₁₃C₂ phases manifest metal-like properties, while B₁₂C₃ and B₁₂O₂ are semiconductors. The estimates have shown that the insertion of 2p atoms (C, N, or O) into intericosahedral pores of elemental boron can cause both a decrease in its elastic modulus (an increase in the compressibility of B₁₂N₂) and a sharp increase in the modulus B (in subcarbides B₁₂C₃ and B₁₂BCC). On the other hand, the insertion of 2p atoms into α-B₁₂ will favor an increase in its hardness (suboxide B₁₂O₂ will have a maximum hardness).     

Radiation-stimulated defects into LaB3O6 and Li6Gd(BO3)3:Ce single crystals

The radiation-induced defect types in LaB₃O₆ and Li₆Gd(BO₃)₃:Ce single crystals have been determined taking into account crystal-lattice features, absorption characteristics, photo- and thermostimulated luminescence data. The possible activator center models in Li₆Gd(BO₃)₃:Ce crystals are proposed.     

Some crystallography,chemistry, physics,and thermodynamics of B12O2, B12P2, B12As2, and related alpha-boron type crystals

This paper shows that several alpha-boron type compounds may be useful as high-temperature semiconductors with decent carrier motilities, high electrical resistivity, good optical transparency, good stability under high radiation bombardment, and possess high neutron capture cross-sections. The most promising are B₁₂O₂, B₁₂P₂, and B₁₂As₂. Their relationship to alpha-boron, B₁₃C₂, and other derivative crystals is explained. A study of their chemical and thermodynamic properties indicates how single crystals useful for electronic devices can be grown.     

B2H6分子的几何构型

采用Gaussian 98程序中B3P86，BP86，B3LYP，BLYP，UHF，HF及LSDA等方法，对B_{2Ｈ6分子可能的D3d，D3h，D2d和D2h平面型状态及立体型状态 的几何构型五种情况的1重态及3重态进行了优化，得到该分子基态为1重态，它的电子状态 是^lＡg,B2H6的几何构型仍然是传统的对 称性为乙烯式的D2h结构 ，其构型如图9所示，其能量最低，是－53．5865H.a.u..两个硼原子与四个H原子所在平面 垂直于两个硼原子及另外二个H原子所在平面，二个硼原子之间的距离是R12=0 ．17542nm，B原子及H原子之间的距离是R17=0．13149nm，R13=0． 1188 4nm,它们之间的夹角分别是，∠314=∠526=121．93°，∠817=∠827=96．3°，B2Ｈ6中硼原子采用不等性的sp³轨道和另一硼原子的不等性的sp< sup>3轨道 及氢原子的1s轨道 交互重叠生成桥式三中心双电子键.三中心双电子键比双中心双电子键更为稳定，计算结果 也证实了这一点. 关键词： 2H6')" href="#">B2H6 密度泛函 几何构型 优化}     

偏硼酸钡低温相的晶体结构

本文报道了新型信频材料——偏硼酸钡[Ba₃(B₃O₆)₂]低温相的晶体结构。晶体属三方晶系,空间群为C₃⁴-R3,取六方坐标系,晶胞参数为a=b=12.532?,c=12.717?,z=6。在PhilipsPW-1100四圆衍射仪上收集到独立衍射693个。采用重原子法通过三维帕特逊和三维傅里叶合成测定结构,并用对角矩阵最小二乘法修正至R=0.046。该晶体是一个由Ba²⁺和(B₃O₆)^3-环交错组成的层状阶梯式结构。其阴离子(B₃O₆)^3-环基本上是平面状的。基团平面垂直于三次轴;Ba²⁺近邻有7个氧原子。将此结构与偏硼酸钡高温相比较表明,由于钡阳离子的非中心对称分布改变了阴离子硼氧环共轭基团的电子云密度,这一结构上的变化对该晶体的倍频效应做了主要贡献。 关键词：     

Synthesis, thermionic emission and magnetic properties of (NdxGd1-x)B6

Polycrystalline rare-earth hexaborides(NdxGd1-x)B6(x = 0,0.2,0.6,0.8,1) were prepared by the reactive spark plasma sintering(SPS) method using mixed powder of GdH2,NdH2 and B.The effects of Nd doping on the crystal structure,the grain orientation,the thermionic emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction,electron backscattered diffraction and magnetic measurements.It is found that all the samples sintered by the SPS method exhibit high densities(> 95%) and high values of Vickers hardness(2319 kg/mm2).The values are much higher than those obtained in the traditional method.With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K.Thus,the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.     

中子探测晶体Li6Gd(BO3)3:Ce的光谱特性研究

文章用提拉法生长出Li₆Gd(BO₃)₃:Ce晶体,并对其光谱性能与发光过程进行了探索. 借助于真空紫外-紫外透过光谱测试,发现晶体的透过光谱中存在Ce³⁺离子和Gd³⁺的特征吸收峰,同时还存在与Ce⁴⁺离子相关的电荷迁移带. 对晶体的真空紫外-紫外激发发射光谱进行研究发现,在晶体存在着Ce³⁺离子的5d→4f辐射跃迁发光与Gd³⁺离子的4f→4f辐射跃迁发光,而且存在着Gd³⁺→Ce³⁺之间的能量传递. 对Li₆Gd(BO₃)₃:Ce晶体的X射线与γ射线激发发射光谱研究可知,晶体在高能射线激发下的闪烁光主要是Ce³⁺离子的发光. 关键词： 6Gd(BO₃)₃:Ce晶体')" href="#">Li₆Gd(BO₃)₃:Ce晶体 真空紫外-紫外透过光谱 真空紫外-紫外激发发射光谱 能量传递     

Evidence for a three Tc behavior of the Kondo superconductor (La,Y)Ce

The dependence of the superconducting transition temperature T_c on Ce impurity concentration is reported for the (La_0.80Y_0.20)Ce system. Susceptibility and resistivity measurements on samples with about 0.85 at. % Ce show unique behavior which must be interpreted as a superposition of three transition curves: a first transition at 0.55 K from the normal to the superconducting state, a second one at 0.27 K back to the normal state, and finally a third one at about 0.05 K again to the superconducting state. The experimental evidence of this third transition (T_c3) is of special importance for the decision about the relevant physical mechanism of pair-breaking at low temperature (T ? T_K).     

Electronic structure of (Nd, Ce)2(Ba, Nd)2Cu3Oy

The dependence of T_c on oxygen content was determined for (Nd_0.67Ce_0.33)₂(Ba_0.67Nd_0.33)₂Cu_3.01O_y. It was found that superconduction appears in a range of y=9.10−9.16 and that T_c is enhanced remarkably by the increase of oxygen content. In an oxygen content range where superconduction appeared, the Ce valence was nearly +4 and the Cu valence changed from 2.31 to 2.33. The conductivity was measured in a wide temperature range between T_c and 1200 K as a function of oxygen partial pressure. The high-temperature conductivity increased with increasing oxygen content. It was found that both carrier concentration and mobility are increased with oxygen content. The conduction at low temperatures was semiconductor-like, and increased with increasing oxygen content, where the energy gap E_A-E_V between acceptor level and top of filled band approached zero.     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureT _f() is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f() is found to be weak, for (Y, Gd)Al₂ T _f() is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.     

Luminescence properties of novel NaBa4(BO3)3:Ce, Eu phosphors

Novel Eu³⁺, Ce³⁺ activated NaBa₄(BO₃)₃ phosphors were synthesized by solid-state reactions. The excitation spectrum of NaBa₄(BO₃)₃:Ce³⁺ consists of an intense band peaking at 350 nm and a weak band in the higher energy side, and the emission spectrum exhibits a blue band with a maximum at about 420 nm. The Eu³⁺ emission in NaBa₄(BO₃)₃ consists of the transitions from ⁵D₀ to ⁷F_J, and the excitation spectrum consists of broad excitation band peaking at 270 nm and some intense narrow lines. The optimum doped concentration, the critical distance of the concentration quenching, and the fluorescence lifetime have also been investigated.     

Thermoelectric power of the vitreous electrolyte (AgI)0.79(Ag2O.B2O3)0.21

We have determined the thermoelectric power ? of the high ionic conductivity glass (AgI)_0.79(Ag₂O.B₂O₃)_0.21; ? is negative throughout the investigated T range, 320–500 K. The heat of transport of the mobile Ag⁺, Q_Ag, taken as the slope of the straight line fitting ? versus 1/T, is quite lower than the activation energy obtained from conductivity data, viz. Q_Ag = 2.81 kcal/mole^-1 < E_act = 4.34 kcalmole^-1. To circumvent this discrepancy, the analysis of the experimental data is carried out as follows: (i) it is supposed that Q_Ag = E_act in agreement with the free ion theory for solid electrolytes; (ii) the vibrational part of the silver ion entropy, S(Ag⁺, vib), is assumed to be equal to the entropy of silver, S(Ag); (iii) on the ground of a structural model for this kind of glasses, the ideal configurational entropy of the mobile Ag⁺, S(Ag⁺, conf)_id, is evaluated through a statistical approach. The ideal ionic entropy is defined as S(Ag⁺)_id = {S(Ag⁺, vib) + S(Ag⁺, conf)_id}; (iv) the difference {S(Ag⁺)_exp - S(Ag⁺)_id} is viewed as an excess entropy and is described according to the classical model of the regular solutions.     

Near stabilization of the Ce moment upon dilution in (Ce,La)Al3

Susceptibility measurements taken for 1.5 ≤ T ≤ 17 K on the system Ce_xLa_1?xAl₃ indicate that the Ce moment is nearly stabilized on dilution. Evidence for this is the fact that the fractional occupancy of the 4f¹ configuration and the spin correlation time both increase as x → 0. Crystalline electric fields influence the susceptibility of these valence fluctuation systems.