BaAg2S2, ein Thioargentat im CaAl2Si2-Strukturtyp

BaAg₂S₂, a Thioargentate with the CaAl₂Si₂-Type Structure BaAg₂S₂ could be obtained as crystalline powder by the reaction of barium-bis[dicyanoargentate(I)] in a stream of hydrogensulfid at 500°C. Single crystals grew at 480°C in an evacuated glass ampoule filled with a flux of potassium thiocyanate and powdery BaAg₂S₂ as solid. BaAg₂S₂ crystallises in the trigonal CaAl₂Si₂-typ structure, a = 4.386(1) Å, c = 7.194(2) Å, space group P3 m1, Z = 1. The structure was determined from four-circle diffractometer data. The silver-sulphur distances are discussed with respect to the corresponding distances of the hitherto known alkaline earth-transition metal pnictides, also crystallizing in the CaAl₂Si₂-typ structure.     

Untersuchungen zum Existenzgebiet des CaAl2Si2-Strukturtyps bei ternären Siliciden

Investigations about the Stability Range of the CaAl₂Si₂ Type Structure in the Case of Ternary Silicides Five compounds LnAl₂Si₂ (Ln: trivalent rare-earth metal, Y) were synthesized by heating the elements at 800°–1000 °C. They are isotypic and crystallize in the CaAl₂Si₂ type structure (P 3 m1; Z = 1) (lattice constants see “Inhaltsübersicht”). The electronic structures (LMTO band structure calculations) of CaAl₂Si₂ and YAl₂Si₂, the latter one is in accordance to Ln³⁺(Al³⁺)₂(Si^4–)₂ not electrovalent, are discussed with regard to the bondings and the electrical conductivity respectively. Investigations of GdAl_2–xMn_xSi₂ mixed crystals showed, that the structure type already at low Mn content (x ≈ 0,3) changes from CaAl₂Si₂ (GdAl₂Si₂) to ThCr₂Si₂ type structure (GdMn₂Si₂).     

Magnetism and phase relations of the PrAl2CaAl2, GdAl2CaAl2 systems

The magnetic properties and phase relations of the ternary systems Pr_1?xCa_xAl₂ and Gd_1?xCa_xAl₂ are reported. The first system exhibits complete miscibility whereas the second shows a miscibility gap extending from x = 0.5 to 0.8. The ternaries were examined magnetically to ascertain whether antiferromagnetism could be produced by replacing the trivalent rare earth ion with divalent Ca. Results for the (Pr, Ca)Al₂ system give some indication that the exchange changes sign when 80% or more of the Pr is replaced by Ca. The α-phase (Gd-rich) alloys in the (Gd,Ca)Al₂ system are ferromagnetic with Curie temperatures only modestly reduced from that of GdAl₂. The β-phase (Ca-rich) alloys in this system are paramagnetic to 4.2°K. The evidence suggests that PrAl₂ and CaAl₂ are electronically similar in regard to band structure and both differ in band structure from GdAl₂.     

Stapelvarianten aus SrPtSb‐ und CaAl2Si2‐analogen Baublöcken

Stacking Variants of SrPtSb and CaAl₂Si₂ analogous Units Structure determinations on the basis of single crystal X‐ray methods revealed, that the crystal structures of Ca₃Cu₂Zn₂P₄ (P3 m1; Z = 1; a = 4.034(1), c = 14.604(3) Å), the isotypic Eu compound (a = 4.150(9), c = 15.210(7) Å), of Ca₂CuZn₂P₃ (P6₃/mmc; Z = 2; a = 4.048(2), c = 21.466(11) Å) and Ca₄Cu₃Zn₂P₅ (P6₃/mmc; Z = 2; a = 4.041(1), c = 37.060(7) Å) respectively can be described as stacking variants built up by two different segments. The first one corresponds with the hexagonal SrPtSb structure type, the second one with the trigonal CaAl₂Si₂ structure type. The segments along [001] are arranged one another and are represented with different weightiness in the compounds concerned.     

Electrical and optical properties of melting Au/Si eutectics on Si(111)

The Au/Si system exhibits an extremely low eutectic temperature of 363°?C. Thin gold films of 200 nm thickness were deposited on Si(111) single-crystals and the electrical and optical properties of the melting mixtures were investigated. The resistivity measurements were performed in situ in a combined LEED/Auger apparatus. A highly resolving spectroscopic ellipsometer was used for the optical analysis in the wavelength range 400–900 nm. The eutectic temperature was found to be lower than for bulk Au/Si samples. The structure analysis showed that small liquid Au/Si islands embedded in the silicon surface are formed by the melting process. Various heating/ cooling cycles show a characteristic hysteresis behaviour.     

Sr~(2+(置换对Eu~(2+)掺杂钙长石晶体结构和光谱特性的影响

采用高温固相法在弱还原气氛下制备了Ca0.955-xSrxAl2Si2O8:Eu2+(x=0~0.9)系列荧光粉,研究了Sr2+置换Ca2+对晶体结构和光谱特性的影响。Sr2+进入CaAl2Si2O8晶格与Ca2+发生类质同相替代形成连续固溶体,物相从CaAl2Si2O8相(Triclinic,P1)逐渐转换为SrAl2Si2O8相(Monoclinic,I2/c),晶胞参数a,b,c和晶胞体积都随Sr2+置换量呈线性增加,α,β和γ在置换量为0.1~0.7区间缓慢减小,超过0.7后呈线性急剧减小。位于250~410 nm区间的宽带激发光谱由4个激发峰构成,表观峰值位于356 nm。Eu2+占据两种格位形成两个发光中心,分别产生430和468 nm发射,宽带发射光谱位于390~550 nm区间,呈现近白色发光。控制Sr2+含量可使表观发射峰位置在408~434 nm之间移动,强度随Sr2+含量增加而增强。     

Interfacial properties of thermally oxidized Ta2Si on Si

Many refractory metal silicides have received great attention due to their potential for innovative developments in the silicon‐based microelectronic industry. However, tantalum silicide, Ta₂Si, has remained practically unnoticed since its successful application in silicon carbide technology as a simple route for a high‐k dielectric formation. The thermal oxidation of Ta₂Si produces high‐k dielectric layers, (O? Ta₂Si)‐based on a combination of Ta₂O₅ and SiO₂. In this work, we investigate the interfacial properties of thermally oxidized (850–1050 °C) Ta₂Si on commercial silicon substrates. The implications of diffusion processes in the dielectric properties of an oxidized layer are analyzed. In particular, we observe migration of tantalum pentoxide nanocrystals into the substrate with increasing oxidation temperature. An estimation of the insulator charge and interfacial O? Ta₂Si/Si trap density is also presented. Copyright © 2008 John Wiley & Sons, Ltd.     

钙长石中镓置换骨架铝对结构和Eu~(2+)发光特性的影响

采用高温固相法在弱还原气氛下制备了Ca0.955Al2-xGaxSi2O8∶Eu2+(x=0～1.0)系列荧光粉,研究了Ga3+置换Al3+对晶体结构和光谱特性的影响。Ga3+以类质同相替代CaAl2Si2O8晶格中的Al3+形成连续固溶体,晶胞参数a,b,c和晶胞体积V随Ga3+置换量呈线性增大;晶面夹角α,β和γ随Ga3+置换量的增加呈线性递减。荧光激发谱为宽带,位于230～420 nm,可拟合成4个峰,表观峰值位于352 nm;随着Ga3+置换量的增加,半高宽从112 nm减小到98 nm。发射光谱位于375～560 nm,可由421和457 nm两峰拟合而成,表观峰值位于425 nm,随着Ga3+置换量的增加,两拟合峰均线性红移,且拟合峰强度比呈线性递减。     

Zur kristallstruktur von CaAl2O4

CaAl₂O₄ was re-examined by single crystal X-ray data. The orientation of the tetrahedrals in this stuffed tridymite structure is discussed in comparison with other known compounds.     

High temperature thermal expansion of BaAl2Si2O8, CaAl2Si2O8, and Ca2Al2SiO7 studied by high-temperature X-ray diffraction (HT-XRD)

Sealing of components for high temperature applications with coefficients of linear thermal expansion (CTE) > 10·10⁻⁶ K⁻¹ can be achieved by glasses from which crystalline phases with high CTE are precipitated. Many sealing glasses also contain further components as e.g. aluminium and hence, not only the desired phase is crystallized, but also additional phases. For this purpose, high temperature XRD was performed in order to determine the CTE of BaAl₂Si₂O₈, CaAl₂Si₂O₈, and Ca₂Al₂SiO₇. In the case of BaAl₂Si₂O₈ and Ca₂AlSi₂O₇ the CTEs were 7.8·10⁻⁶/K and 7.9·10⁻⁶/K, respectively. In the case of CaAl₂Si₂O₈ the CTE is 4.4·10⁻⁶/K. Especially the formation of the latter phase should be avoided for a sealing material of high temperature fuel cells. Sintered specimens of the respective compounds were also characterized by dilatometry.     

Low temperature specific heats and magnetic properties of LaRh2Si2 and HoRh2Si2

Specific heats, static magnetizations and a.c. susceptibilities have been measured on polycrystalline LaRh₂Si₂ and HoRh₂Si₂. Both compounds have been confirmed to have the ThCr₂Si₂-type crystal structure down to 4 K.LaRh₂Si₂ is not superconducting and shows no ordered magnetism above 1.3 K. It has a Debye temperature θ_D = 330 K, electronic specific heat coefficient γ = 6.4mJ mol⁻¹ K⁻², temperature-independent magnetic susceptibility χ_m = 1.6 × 10⁻⁴ e.m.u. mol⁻¹, and is considered to be an ordinary metal.HoRh₂Si₂ has two peaks in the specific heat vs. temperature curve, a broad peak at 11 K and a very sharp one at 27 K. The magnetization vs. temperature curve also shows two peaks at 10 K and 29 K. The magnetic part of the specific heat obtained by subtraction of the specific heat of LaRh₂Si₂ from that of HoRh₂Si₂, provides the total entropy ΔS = 24 J mol⁻¹ K⁻¹, which is close to Rln(2J + 1) for J = 8. This fact suggests that all the excess entropy is attributed to the ground state multiplet of Ho³⁺ ion (⁵I₈) and that the lower temperature peak observed in the specific heat (and also in the magnetization) does not originate in itinerant electrons nor in an impurity phase.     

Metal-particle-induced, highly localized site-specific etching of Si and formation of single-crystalline Si nanowires in aqueous fluoride solution

A straightforward metal-particle-induced, highly localized site-specific corrosion-like mechanism was proposed for the formation of aligned silicon-nanowire arrays on silicon in aqueous HF/AgNO3 solution on the basis of convincing experimental results. The etching process features weak dependence on the doping of the silicon wafers and, thus, provides an efficient method to prepare silicon nanowires with desirable doping characteristics. The novel electrochemical properties between silicon and active noble metals should be useful for preparing novel silicon nanostructures and also new optoelectronic devices.     

Proton conduction in hydrous glasses of the join CaAl2Si2O8-CaMgSi2O6: an impedance and infrared spectroscopic study

Hydrous silicate glasses with compositions along the join diopside-anorthite (An, CaAl(2)Si(2)O(8))-(Di, CaMgSi(2)O(6)) containing up to 3 wt. % H(2)O were synthesized at temperatures 1523-1723 K and pressures of 200 MPa in an internally heated gas pressure vessel. The water content of the glasses was analyzed by Karl-Fischer titration. Infrared microspectroscopy was used to test the homogeneity of the water distribution and to measure the concentrations of OH groups and H(2)O molecules before and after conductivity measurements. The electrical conductivity was measured by impedance spectroscopy at temperature up to 685 K. A positive correlation between water content and conductivity was observed for An(100) from 0 to 1.8 wt.% H(2)O, for An(50)Di(50) (in mol.%) from 1.5 to 2.8 wt.% H(2)O, and for Di(100) from 0 to 1.2 wt.% H(2)O. At same water content of ～1.2 wt.%, the conductivity was three orders of magnitude higher in Di(100) than in the other two glasses, emphasizing the importance of non-bridging oxygens on the transport of hydrous charge carriers. Consistent with findings in literature, we conclude that protons are the predominant mobile charge carriers in alkali-free hydrous silicate glasses. Conductivity data were evaluated in terms of proton diffusivity by the Nernst-Einstein equation. The obtained diffusion coefficients range from 10(-17) m(2)/s for An(50)Di(50) with 1.50 wt.% of H(2)O at 596 K to 10(-12) m(2)/s for An(50)Di(50) with 2.77 wt.% of H(2)O at 685 K.     

Geometric properties of covalently bonded DNA on single-crystalline diamond

Diamond is a promising candidate for bioapplications. Properties of hybridized DNA arrays on single-crystalline diamond are studied on a microscopic level by atomic force microscopy (AFM) in buffer solutions. Compact DNA layers in a thickness of 76 A are resolved by optimizing phase and height contrast in AFM. The height shows some long-range (30 nm) undulations of +/-5 A due to tip and DNA interactions. The axis of double helix DNA is oriented at about 36 degrees with respect to the diamond surface. DNA molecules can be removed by contact-mode AFM with forces >45 nN, indicating stronger DNA bonding than on gold substrates.     

One-step preparation of single-crystalline beta-MnO2 nanotubes

Single-crystal beta-MnO(2) nanotubes with diameters in the range 200-500 nm and lengths up to several micrometers were successfully prepared by a simple hydrothermal method through oxidizing MnSO(4) with NaClO(3) in the presence of poly(vinyl pyrrolidone) (PVP). It was found that the formation process of beta-MnO(2) nanotubes included two primary evolution stages over time: (1) the MnOOH nanoparticles initially formed in the hydrothermal system and anisotropic growth to nanorods and nanorod aggregates, and (2) the MnOOH nanorods transformed into beta-MnO(2) tubular structure and grown into beta-MnO(2) nanotubes due to continuous growth through a dissolution-recrystallization process eventually. Based on a series of experimental analysis, the formation mechanism of these nanostructures was discussed briefly. The present study has enlarged the family of nanotubes available and offers a possible new, general route to one-dimensional single-crystalline nanotubes of other materials.     

Sonochemical synthesis and luminescence properties of single-crystalline BaF2:Eu nanospheres

BaF₂ nanocrystals doped with 5.0 mol% Eu³⁺ has been successfully synthesized via a facile, quick and efficient ultrasonic solution route employing the reactions between Ba(NO₃)₂, Eu(NO₃)₃ and KBF₄ under ambient conditions. The product was characterized via X-ray powder diffraction (XRD), scanning electron micrographs (SEM), transmission electron microscopy (TEM), high-resolution transmission electron micrographs (HRTEM), selected area electron diffraction (SAED) and photoluminescence (PL) spectra. The ultrasonic irradiation has a strong effect on the morphology of the BaF₂:Eu³⁺ particles. The caddice-sphere-like particles with an average diameter of 250 nm could be obtained with ultrasonic irradiation, whereas only olive-like particles were produced without ultrasonic irradiation. The results of XRD indicate that the obtained BaF₂:Eu³⁺ nanospheres crystallized well with a cubic structure. The PL spectrum shows that the BaF₂:Eu³⁺ nanospheres has the characteristic emission of Eu^{3+ 5}D₀-⁷F_J (J=1-4) transitions, with the magnetic dipole ⁵D₀-⁷F₁ allowed transition (590 nm) being the most prominent emission line.     

Laves-phase structural changes in the system CaAl2-xMgx

Compounds CaAl(2)(-)(x)Mg(x) (0 < or = x < or = 2) were synthesized and structurally characterized by X-ray diffraction experiments. With increasing Mg content x the sequence of Laves phase structures MgCu(2) --> MgNi(2) --> MgZn(2) is revealed. The homogeneity ranges of the underlying phases were determined to be 0 < or = x < 0.24(1) (MgCu(2) type), 0.66(2) < x < 1.07(3) (MgNi(2) type), and 1.51(5) < x < or = 2.0 (MgZn(2) type). Mg/Al site occupancies in CaAl(1.34)Mg(0.66) and in CaAl(0.44)Mg(1.56) were refined from neutron powder diffraction experiments and exposed a pronounced segregation of Al and Mg in MgNi(2)-type CaAl(1.34)Mg(0.66) where Al atoms preferentially occupy the positions corresponding to trigonal bipyramids. In MgZn(2)-type CaAl(0.44)Mg(1.56), however, the Mg/Al distribution was found to be nearly uniform. Structural stability in the quasi-binary system CaAl(2)(-)(x)Mg(x) was investigated by first-principles calculations in which random occupational disorder of Mg and Al was modeled with the virtual crystal approximation. The theoretical calculations reproduced the experimental compositional stability ranges of the three different Laves phase structures very well. Structural changes in the quasi-binary system CaAl(2)(-)(x)Mg(x) are induced by the electron concentration, which decreases with increasing x. The stability of the different Laves phase structures as a function of electron concentration was analyzed by the method of moments.