The role played by hydrogen in the ionic thermocurrents of beryl

Abstract

Ionic thermocurrents (ITC) were measured in pink (morganite) and colourless (goshenite) samples of natural beryl from 77K up to 250K at a constant rate b = 0.1 Ks^?1. The spectra present three distinct bands, at 170K, 200K and 220K. The band at 200K predominates in the untreated crystals. After thermal treatments between 700K and 1300K, the ITC bands at 170K and 220K become prominent, masking the 200K band. The reduction of Fe³⁺ into Fe²⁺ in two distinct positions in the beryl lattice justify this observation. Infrared (IR) spectra show that treatments above 1000K remove water from the channels of the beryls. Ultraviolet (UV) irradiation of these thermally treated samples enhance the two bands at 170K and 220K.     

Dependence of the photoluminescence fatigue on the illumination temperature for a-Si :H

Effects of strong illumination at temperatures between 8 K and room temperature on the photoluminescence intensity in a-Si :H were measured between 8 K and 100 K. The photoluminescence fatigue is more prominent by illumination at 8 K than at temperatures above 50 K. This result appears to contradict the usually accepted mechanism of the photodegradation. Creation of two kinds of defects by illumination is proposed to solve the contradiction.     

The temperature dependence of the electrical resistivity and the negative magnetoresistance of carbon nanoparticles

Temperature dependences of the electrical resistivity of samples of carbon nanoparticles obtained from nanodiamonds by annealing at 1800, 1900, and 2140 K were studied. The magnetoresistance of these samples was measured at 4.5 K. Data on the positive magnetoresistance obtained in fields above 3 T were used to estimate the mean free path l of carriers at liquid-helium temperature, l～12 Å for a sample annealed at 1800 K, l～80 Å for a sample annealed at 1900 K, and l～18 Å for the case of annealing at 2140 K. The samples annealed at 1800 and 2140 K exhibit a negative magnetoresistance in fields below 2 T. The carrier concentrations n in the samples annealed at 1800 and 2140 K were estimated as n～8×10²¹ and 3×10²¹ cm^?3, respectively.     

The influence of potassium and the role of coadsorbed oxygen on the chemisorptive properties of Pt(100)

The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θ_K = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the $\text{2π}{}^1$ orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θ_K = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.     

An infrared study of the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury

The chemistry of dimethyl mercury on a Pt(111) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(111) and readily decomposes on the surface at temperatures of 100 K and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH₃ and CH₃Hg species on the surface, both of which are identified as having C_3v local symmetry. At higher exposures, molecular adsorption dominates with the Hg---C---Hg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH₃Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% of saturated coverage.     

Temperature dependence of the reflectance spectrum of the single crystal of tetrathiafulvalene-chloranil

Near-infrared and visible reflectance spectrum was measured on the tetrathiafulvalene-chloranil (TTF-CA) crystal at temperatures in the range of 40–290 K. An abrupt change of reflectivity was found at about 80 K, and new dispersions associated with the intramolecular transitions of TTF⁺ and CA^? gradually grew up on lowering temperature below 80 K, indicating the increase of the ionized fraction.     

The temperature dependence of the hydroxyl deuterium quadrupole coupling parameter and the rotational correlation time of the OD internuclear vector in neat ethanol-d 1

The temperature dependence of the hydroxyl proton chemical shift and deuterium quadrupolar relaxation time of neat ethanol were measured over the temperature range 190–350 K. The proton isotropic chemical shift varies from 6.2 ppm at 190K to 4.7 ppm at 350 K. The deuterium NMR relaxation time in ethanol-d₁ varies from 6.2 ms to 309 ms over the same range. Ab initio calculations performed on various ethanol clusters ranging in size from monomer to hexamer show a linear correlation (R² = 0.99) between XD, the deuterium quadrupole coupling parameter, and δ_H, the isotropic proton chemical shift in ppm relative to TMS: XD(kHz) = 297.60 ? 15.28δ_H. The temperature dependence of XD ranges from 199.5kHz at 190K to 221.4 kHz at 350 K. Using the values for XD and the relaxation time data, the temperature dependence of the OD rotational correlation time was found to vary from 282 ps at 190 K to 4.5 ps near the boiling point (350 K). Using these correlation times and bulk viscosity data, the Gierer-Wirtz model predicts a supramolecular cluster volume of about 317 ų, the approximate volume of a cyclic pentamer cluter of ethanol molecules. The cluster volume was nearly constant from 340 K to about 290 K.     

用第一性原理研究K空位对 KDP晶体激光损伤的影响

 用基于密度泛函理论及超软赝势的第一性原理研究了KH­­­₂PO₄(KDP)晶体中K空位的电子结构、形成能及驰豫构型。讨论了K空位形成后电荷密度的重新分布、相应的电子态密度和能带结构等性质。计算得到中性K空位的形成能为6.5 eV, 远小于间隙K原子点缺陷形成能13.07 eV。K空位的存在使晶胞体积增大, 分别沿结晶学轴a方向增大近0.8%, b方向增大近0.87%, c方向增大近1.2%,同时使与之配位的8个氧原子发生较大位移,使这8个氧形成的空腔体积增大近3.2%。空腔体积的增大不仅促进了各种点缺陷的扩散迁移,而且有利于其它杂质原子的填隙。K原子迁移率的增大会引起离子电导率的增大,因而会降低KDP的激光损伤阈值,因此从这个方面讲,K空位的存在是不利的。但是如果能从实验上（如热退火）利用K空位所造成的扩散通道排出或改善缺陷结构,则可提高KDP晶体的光学质量。     

Birefringence of PrAlO3 in the vicinity of 118 K

We have measured the birefringence of PrAlO₃ from 300 to 77 K in several different crystalline orientations. The 205 K first order and 151 K second order structural phase transitions are clearly evident as are the effects of twinning below 151 K which take the form of regular oscillations. Near 118 K we have observed anomalies in the amplitude and frequency of the twinning effect which may relate to the Anderson-Blount phase transition reported by Fleury. Direct evidence of the 118 K transition was also obtained when birefringence measurements were taken in a geometry chosen to suppress the twinning effect.     

电场对负电荷激子的能-光谱AB振荡的影响

定量讨论了可控外加均匀电场对量子环上负电荷激子的能-光谱及其AB振荡的影响。文中计算光谱的结果与实验值符合很好。负电荷激子是三个带电粒子的体系,构成本征函数的基矢数以及哈密顿矩阵元都极大,数值计算艰浩。文中提出如何选定基矢组Γ(Kmin,Kmax)以减少基矢数及保证因基矢组的选择所引起的误差极小的方法。为验证基矢组Γ(Kmin,Kmax)的可靠性,还提出了对构成波函数中基矢K的比重D(K)的分析方法。并对哈密顿矩阵元〈H〉中的动能项〈T〉、库仑作用项〈V〉、外加电场作用项〈E〉与环半径R的关系进行了讨论。     

Features of the conductivity of the quasi-one-dimensional compound TiS3

The structure and transport properties of single crystal whiskers of the TiS₃ quasi-one-dimensional semiconductor have been investigated. The anisotropy of the conductivity in the plane of layers (ab) has been measured as a function of the temperature. The anisotropy at 300 K is 5 and increases with a decrease in the temperature. Features on the temperature dependences of the conductivity along and across the chains are observed at 59 and 17 K. Near the same temperatures the form of the current-voltage characteristics measured along the chains is qualitatively changed. The current-voltage characteristics below 60 K exhibit nonlinearity and have a threshold form below 10 K. The results indicate possible phase transitions and the collective conduction mechanism at low temperatures.     

Phase coherence of conduction electrons below the Kondo temperature

We have measured the phase decoherence rate tau_{varphi};{-1} of conduction electrons in disordered Ag wires implanted with 2 and 10 ppm Fe impurities, by means of the weak-localization magnetoresistance. The Kondo temperature of Fe in Ag, T_{K} approximately 4 K, is in the ideal temperature range to study the progressive screening of the Fe spins as the temperature T falls below T_{K}. The contribution to tau_{varphi};{-1} from the Fe impurities is clearly visible over the temperature range 40 mK-10 K. Below T_{K}, tau_{varphi};{-1} falls rapidly until T/T_{K} approximately 0.1, in agreement with recent theoretical calculations. At lower T tau_{varphi};{-1} deviates from theory with a flatter T-dependence. Understanding this anomalous dephasing for T/T_{K}<0.1 may require theoretical models with larger spin and number of channels.     

Measurements of the energy shifts of the K absorption edge and K X-rays of several uranium compounds

The energy shifts in the K absorption edges and the Kα₁ and Kβ_{1, 3} X rays in several uranium compounds were measured with the Caltech 2 m bent crystal spectrometer. The results are compared with free-ion relativistic self-consistent field calculations.     

Temperature dependence of the nuclear quadrupole interaction and the Knight shift in rhenium metal

The temperature dependences of the static nuclear quadrupole interaction and the Knight shift in rhenium metal were investigated for 2K ≦ T ≦150 K by the method of Nuclear Acoustic Resonance (NAR). For the quadrupole interaction ¦e²qQ/h¦ an increase occurs from 255.2(5) MHz at 2 K to 257.2(8) MHz at 150 K in¹⁸⁷Re [269.5(7) to 271.5(10) in¹⁸⁵Re] whereas for the Knight shift a slight decrease from 1.14(4)% at 2K to 1.10(8)% at 150 K [1.16(6) to 1.12(10)] is found.     

温度对金刚石涂层膜基界面力学性能的影响

利用分子动力学方法建立了硬质合金基底金刚石涂层膜基界面模型, 并采用Morse势函数和Tersoff势函数相互耦合的方法来表征模型内原子间的相互作用关系, 在此基础上对不同温度(0–800 K)条件下硬质合金基底金刚石涂层膜基界面的力学性能进行了分子动力学仿真计算. 结果表明: 当温度由0 K上升到800 K的过程中, 金刚石涂层膜基界面拉伸强度呈下降趋势, 并且在0–300 K范围内下降趋势明显, 在300–800 K范围内下降趋势缓和; 体系能量随温度的变化具有相同的下降趋势.     

Temperature dependence of the quadrupole interaction in potassium heptafluohafnate

The quadrupole interaction between the tantalum nucleus and its environment in potassium heptafluohafnate incorporating radioactive¹⁸¹Hf has been studied over the temperature range from 77 K to 318 K. A discontinuity in the field gradient tensor is observed at 250 K together with a more gradual alteration with midpoint temperature of approximately 210 K. Below 250 K, the data indicate two inequivalent sites for the tantalum nuclei. The values obtained for the electric field gradient tensor suggest that motional averaging may be significant in this compound.     

粉防己碱与牛血清白蛋白相互作用的研究

利用荧光光培和紫外-可见吸收光谱,研究了粉防己碱与BSA相瓦作用的光谱学行为.研究结果表明,粉防己碱埘BSA有较强的荧光猝灭作用,静态猝灭和非辐射能量转移是导致粉防己碱猝火BSA内源荧光的主要原因.利用Stern-Volmer方程处理实验数据,获得了猝火常数Ksv,不同温度下的Ksv分别为1.26×104 L·mol-1(300 K),1.17×104 L·mol-1(310 K),1.12×104 L·mol-1(320 K).根据Forster非辐射能量转移理论计算出了粉防己碱与BSA间的结合距离r(300 K:3.24 nm;310 K:3.31 nm;320 K:3.50nm).此外,还求得了粉防己碱与BSA的结合常数KA(300 K:1.52×105 L·mol-1;310 K:2.03×105 L·mol-1;320 K:2.89×105 L·mol-1)及相应温度下的热力学参数,热力学数据表明二者主要靠疏水作用力结合.粉防己碱与BSA相互作用的同步荧光光谱表明,二者的结合对BSA构象产生了影响.     

TSDC studies of the effects of plasticizer and water on the sub-T g relaxations of an epoxy resin system

The sub-glass-transition-temperature (sub-T _g relaxations of an epoxy resin system based on diglycidyl ether of bisphenol-A (DGEBA) and cured with triethylenetetramine (TETA) were investigated by means of the thermally stimulated depolarization current (TSDC) technique in the temperature range 77 K to 300 K. Three relaxation regions were observed: the γ relaxation at about 150 K, the β relaxation at 160 K to 200 K and the ω relaxation at 240 K to 270 K. The plasticization effect on the aforementioned relaxations of a plasticizer chemically connected to the epoxy resin network was investigated. The amount of plasticizer (commercial name THIOCOL LP3) was varied between 0% and 60% by weight of the epoxy. The broad range of the β relaxation reflects the heterogeneous structure of the material. The activation energies of the γ and β relaxations were calculated using the thermal sampling technique. Physical aging strongly influenced the TSDC thermogram of the ω relaxation, indicating phase separation during the aging procedure. Water effects were systematically investigated from dry samples to water-saturated samples (water content, 2.5%). In all specimens, water induced plasticization, which is expressed by the shifting of the β relaxation toward lower temperatures. The mode of water absorption (immersion in water or exposure to humid environment) also influenced the evolution of the ω relaxation.     

On the crystallography of the TTF-TCNQ salt at reduced temperatures

A structure determination of the one-dimensionally conducting salt tetrathiofulvalinium-tetracyanoquinodimethanide (TTF-TCNQ) at 100°K, and additional measurements at lower temperatures, show that no major change in crystal structure occurs between room temperature and 40°K. At 100°K the interplanar spacings are reduced from 3.47 to 3.43 and from 3.17 to 3.11 Å for the TTF and TCNQ stacks, respectively. The bond lengths at 100°K agree better with the values listed for salts of TCNQ^-0.5, than with those for TCNQ^-1. On cooling a broadening of the b1 axis reflections is observed, which would be compatible with one-dimensional distortions of the crystal lattice.