Studies of the facetting of the polished (100) face of CaF2

The present paper deals with studies of the facetting of the polished (1 0 0) surface of CaF₂ during annealing and growth in UHV using low energy electron diffraction (LEED), atomic force microscopy (AFM), and transmission electron microscopy (TEM). First morphological modifications of the polished surfaces become visible at temperatures of T=874 K. Surfaces annealed at T=974 K exhibit a micro-roughening with pyramidal protrusions and corresponding depressions. LEED studies indicate the evolution of {1 1 1} facets. Reflexes from the (1 0 0) surface are not seen. After growth of about 660 monolayers of CaF₂ at T=1093 K and a saturation ratio S=33 from the vapor phase, larger pyramid-like or hip roof-like crystallites are developed. The results of AFM height profiles as well as of the LEED investigations indicate again the formation of {1 1 1} facets as proved by their angles of 54.7 ^o with the base (1 0 0) surface. This shows that the crystallites are homoepitaxially grown on the underlying CaF₂ substrate.     

氧氩混合气氛下脉冲激光沉积法制备YBa2Cu3O7-δ薄膜的研究

利用脉冲激光沉积法(PLD),在质量流量比为1∶3的氧气和氩气的混合气氛下,在STO(001) 基片上制备了外延的YBa2Cu3O7-δ(YBCO)超导薄膜.尽管薄膜表面分布有亚微米到微米量级颗粒,但与传统PLD制备YBCO的方法相比,大颗粒的密度要小得多.直流电阻和磁化率测量法同时证明了YBCO薄膜的超导转变温度(Tc)大于90 K. 在40 K时,零场临界电流密度(Jc)为63.8 MA/cm2,在5.2 T时达到最大钉扎力密度(Fpmax)387.9 GN/m3;在65 K时,零场Jc为28.3 MA/cm2,在2.6 T时Fpmax达到71.3 GN/m3;在77 K时,零场Jc为8.7 MA/cm2,在0.91 T时Fpmax达到12.1 GN/m3.研究结果为氧气和氩气混合气氛下PLD方法制备YBCO薄膜提供了重要实验数据.     

Synthesis and structural study of 1-(N,N-diethylamino)-2,2-bis(2-nitrophenylthio)ethene

The synthesis, variable temperature NMR spectra, and crystal structures of two crystalline forms, 2a and 2b, of the enamine 1-(N,N-diethylamino)-2,2-bis(2-nitrophenylthio)ethene have been obtained. Both forms crystallize in the monoclinic space group P2₁/a. The two phases have similar molecular structures but possess different intermolecular C–H······O hydrogen bonding interactions. Both forms exhibit disorder within the NEt₂ fragment at 298 K: sufficient disorder persisted with 2a (orange needles) down to 100 K to make the geometric parameters pertaining to the enamine fragment unreliable. The disorder was effectively eliminated on cooling 2b down (red colored blocks) to 150 K. Cell dimensions for the 2a-phase are at 100 K: a = 11.1030(4) Å, b = 15.1325(7) Å, c = 12.4504(7) Å, β = 114.606(3)°, while for the 2b-phase at 150 K, a = 15.5206(4) Å, b = 7.6958(2) Å, c = 15.7137(3) Å, β = 92.580(7)°. The C–N bond length in the β-form at 150 K of 1.335(3) Å indicates considerable double bond character: the rotational barrier of the C–N bond in CDCl₃ was calculated to be 52.4 kJ mol^?1.     

Growth and study of PbFe1/2Ta1/2O3 single crystals

Single crystals of the composition PbFe_1/2Ta_1/2O₃ are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe_1/2Ta_1/2O₃ ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, T _m, increases by more than 70 K with an increase in the frequency from 10² to 10⁶ Hz. It is assumed that the unusual properties of the PbFe_1/2Ta_1/2O₃ crystals are caused by mesoscopically inhomogeneous compositional ordering and comparatively high conductivity providing favorable conditions for the appearance of the volume-charge and thermal electron polarization.     

Synthesis,Crystal Structure,and Hirshfeld Surface Analysis of Rubidium trans-Bis(N-methyliminodiacetato)chromate(III)

Crystallography Reports - A new rubidium salt, Rb[Cr(C5H7NO4)2] (C5H7NO4 = mida, methyliminodiacetate), was prepared and its molecular structure was determined from synchrotron X-ray data at 173 K....     

Glass formation in the systems (K or Cs)2O(Nb or Ta)2O5Al2O3

The glass-forming ability of melts in the systems K₂O(Nb and/or Ta)₂O₅Al₂O₃ as well as those in which K₂O was replaced with Li₂O, Na₂O, Cs₂O, BaO or PbO was investigated. Some melts in the systems (K or Cs)₂O(Nb and/or Ta)₂O₅Al₂O₃ could be made into glasses by cooling, yielding practically useful amounts. The structures of these glasses were discussed on the basis of their infrared spectroscopic and X-ray emission spectroscopic analyses.     

A Sn-119 and Fe-57 Mossbauer Effect Study of the Crystalline Phase of 7S5

The Mossbauer effect of Sn-119 and Fe-57 has been used to study solutions of trimethyltin-4-methoxybenzylidene-4′-aminocinnamate (4.3% by weight) and Fe-57 enriched 1-1′-diacetylferrocene (0.2% by weight) in the crystalline phase of the liquid crystalline material 4-n-pentylphenyl-4′-n-heptyloxythiobenzoate (7S5). A pronounced deviation from Debye-like behavior was observed at 180 K in the In (recoil-free fraction) vs temperature data for the tin-bearing probe molecule. This deviation may be interpreted as due to a crystal-crystal phase transition which has been observed at 183 K. For the iron-bearing probe molecule, no deviation from Debye behavior was found up to 300 K. The data of this report support the interpretation that the phase transition is due to a rotation of the 7S5 molecule about the C-S bond.     

Temperature Dependence of Static Electrical Conductivity of Li2B4O7 Crystals

Crystallography Reports - The static dc electrical conductivity of one-dimensional ion conductor Li2B4O7 (sp. gr. I41cd) has been investigated in the temperature range of 293–820 K using...     

Magnetic Order in Organic Superconductors

We have undertaken an electron spin resonance study at both low and high magnetic field on the superconducting phase of the ambient pressure superconductor (BEDT-TTF>₂I₃, (ET)₂I₃, over the temperature range 1-300 K and hydrostatic pressure range from 0 to 2 kbar. At ambient pressure the ESR results are consistent with the picture of (ET)₂I₃ as a metal. Superconductivity is observed at 1.6 K via low field ESR. Application of modest pressures strongly suppresses the superconducting transition temperature. At pressures above about 0.3 kbar the superconductivity is suppressed in favor of an as-yet-unidentified magnetic state whose onset is at 7K. Through an analysis of the microwave ESR lineshape we find that the microwave conductivity over the temperature range 5-50 K is in agreement with dc measurements.     

Structural, optical and dielectric properties of glass-nanocomposite

Transparent glass-nanocomposite sample [50Li₂B₄O₇-50(SrO-Bi₂O₃-Nb₂O₅)] (mol %) was prepared by a conventional melt-quenching technique. Fluorite (Bi₃NbO₇) and SrBi₂Nb₂O₉ (SBN) was produced by heat-treating the as-prepared sample at 773 K for 6 h (HT773). The nanocrystallites of fluorite and SBN phases in the Li₂B₄O₇ glass matrix were established by differential thermal analysis (DTA) which is confirmed by X-ray powder diffraction (XRD), infrared spectroscopy and transmission electron microscopic (TEM) studies. The influence of heat-treating the as-prepared sample on the optical properties (transmission, optical band gap and Urbach energy) has been investigated. Properties such as dielectric constant and dielectric loss as a function of frequency (120 Hz -100 k Hz) and temperature (300-829 K) are reported.     

First-principles study of the binary Ni60Ta40 metallic glass: The atomic structure and elastic properties

Ni–Ta bulk metallic glass (BMG) with compositions around Ni₆₀Ta₄₀ is a newly found binary BMG with high glass forming ability and extraordinary mechanical strength. Using ab initio molecular dynamics, the local atomic structure, elastic properties and electronic structures of Ni₆₀Ta₄₀ glass have been explored. The pair-correlation functions, coordination numbers, and chemical compositions of the most abundant local clusters have been analyzed. We demonstrated the existence of icosahedral Ni₇Ta₆ clusters as the major Ni-centered clusters, while the most popular Ta-centered cluster is Ta₇Ni₈. These findings agree with our previous cluster model of Ni–Ta binary BMG. The elastic moduli of Ni₆₀Ta₄₀ glass were also computed and the experimental Young's modulus is well reproduced. Analysis of electronic structures further revealed that the interaction between d electrons of Ni and Ta atoms is responsible for the experimentally observed ultrahigh mechanical strength for the Ni–Ta BMGs.     

Cracking assisted nucleation in chemical vapor deposition of silicon nanoparticles on silicon dioxide

Deposition of sub-monolayer silicon on SiO₂/Si(1 0 0) greatly facilitates nucleation in subsequent thermal chemical vapor deposition (CVD) of silicon nanoparticles. Sub-monolayer seeding is accomplished using silicon atoms generated via disilane decomposition over a hot tungsten filament. The hot-wire process is nonselective towards deposition on silicon and SiO₂, is insensitive to surface temperature below 825 K, and gives controlled coverages well below 1 ML. Thermal CVD of nanoparticles at 1×10⁻⁴ Torr disilane and temperatures ranging from 825 to 925 K was studied over SiO₂/Si(1 0 0) surfaces that had been subjected to predeposition of Si or were bare. Seeding of the SiO₂ surface with as little as 0.01 ML is shown to double the nanoparticle density at 825 K, and densities are increased twenty fold at 875 K after seeding the surface with 30% of a monolayer.     

Study of the Fracture Toughness of K2NixCo(1 – x)(SO4)2 · 6H2O Crystals in Dependence of the Growth Direction and Rate

Crystallography Reports - K2Ni(SO4)2 · 6H2O (KNSH), K2Co(SO4)2 · 6H2O (KCSH), and K2NixCo(1 – x)(SO4)2 · 6H2O (KCNSH) crystals have been grown by traditional...     

Structural Analysis of the Phase Transitions of (NH<Subscript>4</Subscript>)<Subscript>4</Subscript>HgBr<Subscript>6</Subscript>

Abstract Single crystal diffraction and differential scanning calorimetry DSC techniques have been used to investigate the different phases of (NH₄)₄HgBr₆, tetrammonium mercury hexabromide, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 190 and 268 K, by DSC experiment. X-ray diffraction measurements confirm these anomalies. At room temperature the structure is tetragonal P4/mnc (No. 128) with lattice parameters a = b = 9.25560(8) ?; c = 8.8657(11) ?; V = 759.49(9) ?³ and Z = 2. At T = 250 K the structure is orthorhombic Pnnm with lattice parameters a = 8.8436(8) ?; b = 9.2191(8) ?; c = 9.2232(7) ?; V = 751.97(11) ?³ and Z = 2. Below approximately 200 K the structure is monoclinic P2₁/n (No. 14) with: a = 8.8080(9) ?; b = 9.1608(8) ?; c = 9.1498(8) ?; β = 90.230(7)°; V = 738.28(12) ?³ and Z = 2 (T = 120 K). The structure of (NH₄)₄HgBr₆ consists of isolated HgBr₆-octahedra in the whole temperature range which are slightly compressed in c-direction. The ammonium groups are located between the octahedra ensuring the stability of the structure by hydrogen bonding contacts: N–H···Br. The structural phase transformations are described by a rotation of the [HgBr₆]²⁻ octahedra around the c-axis, and this behaviour is attributed to an orientational disorder of ammonium groups. Index abstract Structural analysis of the phase transitions of (NH₄)₄HgBr₆; M. Loukil, A. Kabadou, I. Svoboda, A. Ben Salah and H. Fuess; The phase transformations in (NH₄)₄HgBr₆ are explained by large rotation of [HgBr₆]²⁻ octahedra around the c-axis.      

Penrose structures described by different kite-clusters

The family of kite-clusters, i.e. K4, K7 and K17, obtained by applying inflation rules to the smallest, initial cluster (K4), were successfully used to cover the rhombic Penrose atomic structure. These clusters, especially the first two, namely four-atom K4 and seven-atom K7 clusters were thoroughly studied and compared with the previously described kite-like cluster K17 and the standard 33-atom Gummelt’s cluster (G33). The first in series – monatomic K4 cluster – is probably too small for any real application. The next – K7 – cluster is the simplest one that can be decorated by three different types of atoms. Their concentration is comparable to the concentration of atoms of the stable, decagonal quasicrystals Al–Ni–Co. This leads to the conclusion that the K7 could be also considered as a starting model for the structure refinement.     

Study of the Radial Heterogeneity and Mosaic Microheterogeneity in KCNSH Mixed Crystals

Crystallography Reports - Mixed crystals of potassium?cobalt sulfate hexahydrate/potassium?nickel sulfate hexahydrate (K2Co(SO4)2 · 6H2O/K2Ni(SO4)2 · 6H2O (KCNSH)) have been...     

The growth and physics of ultra-high-mobility two-dimensional hole gas on (311)A GaAs surface

We report on the molecular beam epitaxy growth of modulation-doped GaAs-(Ga,Al)As heterostructures on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases (2DHGs) with low-temperature hole mobility exceeding 1.2×10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8×10¹¹ cm⁻² have been obtained. This hole mobility is the highest ever observed at such low densities by any growth technique. We also report the first observation of persistent photoconductivity in a 2DHG. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.     

The effect of hydrostatic pressure on the dielectric constants, and their temperature dependences, of phosphate and tellurite glasses

Measurements of the pressure dependence of the static dielectric constant of tellurite (pure TeO₂ and 67%TeO₂ + 33%WO₃) and samarium phosphate (5%Sm₂O₃ + 95%P₂O₅ and 15%Sm₂O₃ + 85%P₂O₅) glasses at elevated pressures (0–70 kbar) for a range of temperatures (77–380 K) are reported. The electrical properties under pressure have been determined from the low-frequency complex plane analysis of glass discs contained within a Bridgman opposed anvil cell. The most notable observation is that the pressure dependence of the static dielectric constant, of all glasses studied, is positive, for example for vitreous TeO₂ ln ε/P is equal to 4.41 × 10⁻¹¹ (Pa⁻¹) at 293 K. Behaviour of this type is common to a number of materials (plastics and chalcogenide glasses) for which it is not possible to define any long-range order. It is in direct contrast with the behaviour of crystalline insulators, for which ε/P is usually negative. The effect of pressure on the dielectric constant has been analysed using two different approaches based on the macroscopic Clausius-Mossotti equation. The effects of high pressure on the dielectric constant have been correlated with the temperature dependence of the dielectric constant at atmospheric pressure.     

Mechanochemical–hydrothermal synthesis of hydroxyapatite from nonionic surfactant emulsion precursors

Nanocrystalline hydroxyapatite [HA, Ca₁₀(PO₄)₆(OH)₂] powders were synthesized by the mechanochemical–hydrothermal method using emulsion systems consisting of aqueous phase, petroleum ether (PE) as the oil phase and biodegradable Tomadol 23–6.5 as the nonionic surfactant. (NH₄)₂HPO₄ and Ca(NO₃)₂ or Ca(OH)₂ were used as the phosphorus and calcium sources, respectively. The calcium source and emulsion composition had significant effects on the stoichiometry, crystallinity, thermal stability, particle size and morphology of final products. Disperse HA crystals with a 160 nm length and aspect ratio of ca. 6 were formed in an emulsion system containing 10 wt% PE, 60 wt% water and 30 wt% surfactant. The HA particles had needle morphology with a specific surface area of . With this technique, HA nanopowders with specific surface areas in the range of 72– were produced.     

Morphology of {100} and {201} faces and the optical quality of KTiOPO4 crystals

Crystallography Reports - Potassium titanyl phosphate crystals up to 350 g in weight have been grown from a high-temperature solution in K4P2O7 by the flux method on a [100]-oriented seed. The...