An electron diffraction study of the misfit layer compound (NdS)nNbS2

Misfit layer compounds of the type (NdS)_nNbS₂ have been made by iodine vapour transport. They have been characterised by energy dispersive X-ray analysis, transmission electron microscopy and electron diffraction. An orthorhombic form with Am2m symmetry has been observed. The unit cell contains two layers of NdS and two layers of NbS₂. The layers have rectangular symmetry and are stacked alternately with their planes perpendicular to the c-axis. A curious feature of the structure is that the layers have a common b unit cell parameter but the a values, which depend on the metal element in a given layer, are incommensurate. The lattice parameters for the material are a = 0.5625 nm for the NdS layer, a = 0.331 nm for the NbS₂ layer, b = 0.5733 nm and c = 2.266 nm. The symmetry of the material observed in the present work is Am2m which differs from the Fm2m reported by others. It is argued that the lower symmetry occurs because of a displacement of Nb atoms in one of the layers in the unit cell. A comparison between calculated and observed electron diffraction patterns and energy arguments are used to support the proposed explanation.     

(Tl, Pb, Cu)Sr2(Tl, Pb, Cu)2Cu2O8−δ: a nonsuperconducting Tl-1222 cuprate with both heavy atom single and oxygen depleted double layer motifs

An account is given of the synthesis and structural investigation of a newly discovered tetragonal (Tl, Pb, Cu)Sr₂(Tl, Pb, Cu)₂Cu₂O_8−δ compound (Tl-1222^* phase) with both (Tl, Pb, Cu)O single and oxygen depleted (Tl, Pb, Cu)₂O_x double layer motifs, space group I4/mmm, lattice parameters a=3.8405(2), c=29.2536(5) Å, and D_x=7.46 g cm⁻³. Micaceous and highly reflecting black single crystals of 1 mm×1 mm×3–5 μm size were obtained from a Tl₂O₃- and CuO-rich partial melt. The as-grown compound is nonsuperconducting. The very thin crystals are extremely liable to bend out of shape. Deterioration could not be observed within the period of a year under ambient conditions. The complex crystal structure has been solved using single crystals. We take note of the similarity of the unit cell dimensions to superconducting (Tl, Cu)₂Ba₂(Ca, Tl)Cu₂O₈ (Tl-2212 phase) with TlO double rocksalt layers, and compare crystal-chemical details of similar compounds. Apart from the possibility of obtaining superconducting properties by suitable doping, the mechanical properties of the new material are very attractive from the point of view of its use as highly scattering composite layers of an X-ray monochromator.     

Synthesis and superconductivity of (Pb0.5Cd0.5)(Sr0.9R0.1)2(R′0.7Ce0.3)2Cu2Oy

New Pb based layered superconducting cuprates (Pb_0.5Cd_0.5)(Sr_0.9R_0.1)₂(R′_0.7Ce_0.3)₂Cu₂O_y with (R,R′) = (Eu,Gd), (Eu,Dy) and (Nd,Gd), have been successfully synthesized. The structure of (Pb,Cd)(Sr,Eu)₂(Gd,Ce)₂Cu₂O_y is determined using XRD data. The X-ray powder diffraction pattern can be indexed by the tetragonal structure with lattice parameters a = 3.8250 Å, C = 29.2235 Å and a space group of I4/mmm. The effects of synthesis conditions on (Pb,Cd)(Sr,Eu)₂(Gd,Ce)₂Cu₂O_y are studied and a T_c(onset) of 26 K is obtained by resistance measurements. It is isostructural to the superconductor (Pb,Cu)(Sr,Eu)₂(Eu,Ce)₂Cu₂O_y.     

A new family of Pb based 1222 cuprates (Pb,Mg)(Sr,R)2(R′,Ce)2Cu2Oy (R,R′:rare-earth element)

A series of new Pb based 1222 cuprates, (Pb,Mg)(Sr,R)₂(R′,Ce)₂Cu₂O_y, has been reported in this paper. The structure of the compounds has been determined by the powder X-ray diffraction method. It crystallizes in a tetragonal lattice with space group I4/mmm and lattice parameters a = 3.818 Å and C = 29.085 Å for R,R′=Nd,Dy. XRD results show that the structure is isostructural with (Pb,Cu)(Sr,R)₂(R′,Ce)₂Cu₂O_y with the (Pb,Mg)O layer replacing the (Pb,Cu)O layer. Resistivity measurements show that the compounds are semiconductors under the present experimental conditions.     

HRTEM study of new superconducting lead cuprate (Pb, Cu) (Eu, Ce)2(Eu, Sr)2Cu2Oz

Crystallographic properties of a new lead cuprate superconducting material (Pb, Cu) (Eu, Ce)₂(Eu, Sr)₂Cu₂O_z are examined by electron probe micro-analysis and transmission electron microscopy. It is confirmed that although the sample examined here is not of single-phase, the composition of the superconducting phase is close to the nominal composition. From TEM observations, it follows that the crystal structure of the new phase is similar to that of Tl-1222 which consists of a fluorite type block, a rock salt type layer and a perovskite block.     

(CH3)2和(NH2)2基团修饰的齐聚苯乙炔分子电子输运性质研究

采用密度泛函理论和非平衡格林函数相结合的方法研究了S原子作为单、双端基的(CH₃)₂-OPE (齐聚苯乙炔)和(NH₂)₂-OPE分子在金电极间的电子输运性质. 通过第一性原理优化计算获得分子部分稳定结构, 再置于Au电极之间构成两极系统, 然后再优化整个两极系统获得稳定结构. 另外, 通过非平衡格林函数方法计算了两极系统的电子输运性质. 计算结果表明, 不同的修饰基团和桥接方式可以导致两极系统的开关效应、负微分电阻行为和整流行为等不同的电子输运性质. 通过计算不同偏压下的分子体系投影轨道电子分布、透射谱、态密度, 对这些新异的电输运性质出现的机理进行了解释.     

The crystal structure of (Pb0.5Cd0.5)Sr2 (Y1−xCax)Cu2O7

The crystal structures of (Pb_0.5Cd_0.5)Sr₂YCu₂O₇, (Pb_0.5Cd_0.5)Sr₂(Y_0.6Ca_0.4)Cu₂O₇ and (Pb_0.5Cd _0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O₇ have been refined by the Rietveld method for the X-ray diffraction data. The refinement results indicate that both Pb and Cd atoms in the (Pb, Cd)O layers and O(3) atoms are displaced from their ideal sites, and that there is a complicated occupation in the (Pb, Cd)O layers, i.e., other cations, such as Sr²⁺, Cu²⁺ and Ca²⁺, can occupy the (Pb, Cd) site for these samples. This may be the reason why the lattice constants do not vary monotanically with the calcium content, and why there exist broad superconducting transitions in the system (Pb_0.5Cd_0.5)Sr₂(Y_1−xCa_x)Cu₂O₇. The refinement results also indicate that, as the calcium content increases, the O(2) atoms move close to the CuO₂ planes and far from the (Pb, Cd)O layers. Thus, the increase of calcium content results in charge transfer from the charge reservoir layer (Pb, Cd)O to the CuO₂ planes. This result is consistent with the bond valence sums of the Cu ions in the CuO₂ planes.     

Electron tunneling from a metallic TS2 layer underneath an ultra-thin MS layer with semiconducting properties for misfit-layer compounds

The effects of electron tunneling from the underlying TS₂ (H) layer on the scanning tunneling microscopy (STM) images of the uppermost MS (Q) layer have been studied for the misfit-layer compounds which are represented by the chemical formula {(MS)_1+x}_m{TS₂}_n. Systematic STM observations have been carried out under ultra-high vacuum (UHV) conditions for the 1Q/1H, 1.5Q/1H and 2Q/1H types of misfit-layer compounds. As Q layer thickness increases from about 6 to 12 Å while going from the 1Q/1H type to the 2Q/1H type, pseudo-tetragonal arrays of bright spots as expected from the atomic arrangement of a Q layer are observed more easily and more distinctly. It is found that tunneling electrons from the underlying H layer play an important role on the STM observations of the 1Q/1H and 1.5Q/1H types of compounds. Fast Fourier transform (FFT) analyses give clear evidences for electron tunneling from the underlying H layer and scattering by surface atoms of the uppermost Q layer and a mutual modulation structure peculiar to the compounds.     

正磷酸盐晶体Ba3(PO4)2和Sr3(PO4)2高温拉曼光谱研究

测量了碱土金属正磷酸盐Ba₃(PO₄)₂和Sr₃(PO₄)₂常温及高温拉曼光谱, 对拉曼振动模式进行指认, 并分析了晶体拉曼振动光谱及晶体结构在高温下的变化. 在温度升高的过程中, 拉曼振动频率向低频移动且振动峰宽度展宽, 晶体中的P-O平均键长随温度升高而变长, 但O-P-O的键角并未发生变化. 晶体在900 ℃以下无结构相变发生. 关键词： 3(PO₄)₂和Sr₃(PO₄)₂')" href="#">Ba₃(PO₄)₂和Sr₃(PO₄)₂ 高温拉曼光谱 振动模式 高温结构     

Magnetic susceptibility of (IPA)2FeCl4, (IBuA)2FeCl4 and (IPA)2CuCi4 (IPA = isopropylammonium ion and IBuA = isobutylammonium ion)

The magnetic susceptibility of (IPA)₂FeCl₄, (IBuA)₂FeCl₄ and (IPA)₂CuCl₄, where IPA = (CH₃)₂CHNH⁺₃ and IBuA = (CH₃)₂CHCH₂NH⁺₃, were measured in the temperature range form 80 K up to a temperature near the melting points of the samples. The results obtained were interpreted in termw of a canted two-dimensional antiferromagnet. The variation of the magnetic susceptibility with the change in the composition was also discussed.     

Li2K(IO3)3与Li2NH4(IO3)3的晶体结构

本文用X射线粉末法测定了Li₂K(IO₃)₃与Li₂NH₄(IO₃)₃的晶体结构和原子参数。发现Li₃K(IO₃)₃,Li₂NH₄(IO₃)₃与Li₂Rb(IO₃)₃同晶型,属单斜晶系,空间群为P2₁/α,每个单胞含有四个化合式量。室温的点阵常数分别为α=11.198?,b=11.046?,c=8.254?,β=111.53°,及α=11.327?,b=11.078?,c=8.341?,β=111.87°。讨论了二元化合物的形成与离子半径的关系。 关键词：     

Silver intercalation in PbS1.18(TiS2)n, n=1, 2, misfit layer compounds

The reaction between metallic Ag and PbS_1.18(TiS₂)_n, n=1, 2, misfit layer compounds has been investigated by electrochemical technique, X-ray powder analysis and transmission electron microscopy. It was found that silver intercalation is possible only in the compound with n=2. The thermodynamic behavior and location of phase boundaries were studied in the temperature range 400–650 K. DC-conductivity and magnetic-susceptibility measurements were performed, and the data can be interpreted as an appearance of small polarons during silver insertion.     

Modulation structure in the STM images of the non-metallic misfit-layer compound (PbS)1.12VS2

This paper presents the scanning tunneling microscopy (STM) results of the misfit-layer compound (PbS)_1.12VS₂, which is constructed of alternately stacking of PbS (Q) and VS₂ (H) layers. Temperature dependent resistivity measurements show a semiconducting behavior with small activation energies. Unlike the metallic 1Q/1H type of compounds we have succeeded to take both the STM images of a Q layer and a H layer, because electron tunneling from the underlying H layer is suppressed when intermediate positive bias voltage (V_b) is applied to a tip. At V_b = 0.15 V the image shows pseudo-tetragonal arrays of bright spots, although it is obscure with decreasing bias voltage and disappears at less than 10 mV. A modulation structure is found on the H layer of a stepped surface on which surface atoms are undulated in a period being twice the V-V interatomic distance in the [1 0]_H or the [1 1]_H direction.     

Ab initio study of structural, electronic and optical properties of MnHg(SCN)4 and FeHg(SCN)4

 The structural, electronic and optical properties of MnHg(SCN)₄ and FeHg(SCN)₄ were studied by means of quantum-mechanical calculations based on the density-functional theory and pseudopotential method. The lattice constants can be compared with the experimental values when the effects of temperature are considered. The peaks of partial density of states of S, C, N and Hg of FeHg(SCN)₄ have a tendency of shifting to the higher energy levels relative to those of MnHg(SCN)₄. The distributions of the 3d electronic states in the transition metal atoms show quite large difference and decide different optical properties. We found that absorptional peaks of FeHg(SCN)₄ lag behind those of MnHg(SCN)₄ and the peak in the infrared range has a higher absorptional intensity, which are in accord with the experimental results. By analyzing the distributions and transitions of the 3d electronic states, we explained the different absorption phenomena.     

Electronic density of states in pseudobinary compounds Y(Fe-Co)2 and Y(Mn-Co)2

Densities of states in pseudobinary compounds are calculated by making use of the coherent potential approximation (CPA) [3] and the recursion method. Applications are made for Y(Fe-Co)₂, Y(Mn-Co)₂ and Zr(Fe-Co)₂. The total and local magnetic moments of Y(Fe-Co)₂ and Zr(Fe-Co)₂ are calculated and compared with the experimental results.     

Dense-Kondo antiferromagnetism in (CePd3)8Ge

We report the existence of (CePd₃)₈Ge and (LaPd₃)₈Ge, and find them to have the same cubic superstructure as reported for (CePd₃)₈T (T=Ga, In, Sn, Sb, Pb and Bi) by Gordon and co-workers [1 and 2]. The cerium ions in (CePd₃)₈Ge are trivalent and undergo an antiferromagnetic transition below 5 K with a dense-Kondo-lattice behaviour.     

Interpretation of scanning tunneling microscopy and atomic force microscopy images of 1T-TiS2

The surface of 1T-TiS₂ was examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The STM and AFM images of this compound were interpreted on the basis of the partial electron density ρ(r,E_F) and total electron density ρ(r) of a slab which consists of six (001) 1T-TiS₂ layers. Electronic structure calculations were performed using the ab-initio Hartree–Fock program crystal. It was found that the bright spots in experimental STM images correspond to sulfur atoms at both positive and negative bias voltages. The AFM image showed a periodicity which can be explained by the atomic corrugation at the surface. Structural defects on the surface were also investigated, and their interpretation constitutes experimental proof that only sulfur atoms were detected by scanning probe microscopies.     

Luminescence and OSL study of the inorganic compounds Tl-doped (NH4)2BeF4 and (NH4)2SiF6

The search of materials with efficient optically stimulated luminescence (OSL) properties and a high hydrogen content for fast neutron dosimetry has led to the development of Tl⁺-doped ammonium salts (NH₄)₂SiF₆ and (NH₄)₂BeF₄. Preliminary OSL studies have shown that the OSL in (NH₄)₂SiF₆:Tl⁺ and (NH₄)₂BeF₄:Tl⁺is efficient. In order to optimise the OSL emission and its detection, the identification of the luminescent centres in the OSL process is necessary. This study focuses on the identification of the luminescent centres associated with Tl⁺ based on optical studies in the vacuum UV (VUV) at different temperatures.     

EPR evidence of local lattice mode in K3H(SO4)2 and Rb3H(SO4)2 fast-proton conductors

The observation of an anomalous temperature dependence of Mn²⁺ EPR spectra linewidth and nonaxial crystal-field parameter in K₃H(SO₄)₂ and Rb₃H(SO₄)₂ allows one to suggest the presence of “local mode” predicted by Yamada (Ferroelectrics 170 (1995) 23). The activation energy for this kind of excitation was found and equals 11.3 (0.5) and 7.4 (0.3) meV for Mn²⁺ doped K₃H(SO₄)₂ and Rb₃H(SO₄)₂, respectively.     

IR-spectra of (CO2)2 dimers and collision-induced absorption of carbon dioxide in the region of the fermi doublet (ν1, 2ν2)

The positions and intensities of all vibrational transitions in the (ν₁, 2ν₂) Fermi doublet region of (CO₂)₂ dimers are obtained by means of variational method. The collision-induced absorption in compressed carbon dioxide is shown to be mainly due to dimeric absorption. Assignment of ν₁ and 2ν₂ vibrational bands in isolated CO₂ molecule is performed by consideration of the symmetries and intensities of dimeric vibrations.