Analytical electron microscopy study of a new phase in an equiatomic Ti50Pd50 alloy

Analytical electron microscopy has been used to study a new phase found in a Ti₅₀ Pd₅₀ alloy. The new phase was formed as ellipsoidal particles along the grain boundaries of the martensitic matrix. The crystal structure was identified as trigonal, with lattice parameters of a=0.83 nm, and c=2.62 nm, and point group ¯3m. The phase contains 68–73 at% Ti (48–54 wt% Ti).     

Preferred orientation of AlN plates prepared by chemical vapour deposition of AlCl3 + NH3 system

Aluminium nitride (AlN) plates about 1 mm thick (maximum) were prepared by chemical vapour deposition (CVD) at the maximum deposition rate of 430 nm s⁻¹ using AlCl₃, NH₃ and H₂ gases at deposition temperatures,T _dep, of 873–1473 K. The effects of deposition conditions on the preferred orientation, morphology and micro-structure were investigated. WhenT _dep was less than 1073 K, the resulting CVD AlN plates contained some impurity chlorine and the aluminium content exceed the nitrogen content. WhenT _dep exceeded 1173 K, no chlorine was detected, and the Al/N atomic ratio matched the stoichiometric value. The lattice parameters (a=0.311 nm,c=0.4979 nm) and density (3.26×10³ kgm⁻³) were in agreement with values reported previously. The crystal planes oriented parallel to the substrates changed from (1 1 ˉ2 0) to (1 0 ˉ1 0) to (0001) with increasing total gas pressure (P _tot) and decreasingT _dep. This tendency is discussed thermodynamically and is explained by the change of supersaturation in the gas phase.     

Structure of Mo2.0 – x Ni1.0 + x P (x = 0.2) and Some Crystal-Chemical Features of Ternary Phases in the Mo–Ni–P System

The crystal structure of Mo_{2.0 – x} Ni_{1.0 + x} P (x= 0.2) is determined by powder x-ray diffraction (monoclinic symmetry, new structure type, sp. gr. Im, a = 1.04036(5) nm, b= 0.84055(4) nm, c= 0.47357(2) nm, = 91.538(3)°; R _int = 0.094, R _prof = 0.197). The 850°C section of the Mo–Ni–P phase diagram is studied in detail, and the crystal-chemical features of molybdenum nickel phosphides are discussed.     

Nitrogen-containing aluminium titanate

In the Ti-Al-O-N system a phase isostructural to aluminium titanate but with expanded unit cell dimensions was observed. It was stable between 1400 and 1700 °C and has unit cell dimensions of a=0.3719 nm, b=0.9703 nm and c=0.9869 nm with a composition of Ti _1.00 ⁴⁺ Al _0.54 ³⁺ Ti _1.46 ³⁺ N _0.28 ^3– O _4.58 ^2– _0.14 Several samples were prepared by reaction sintering mixtures of TiN, Al₂O₃ and AlN powders at 1400 to 1470 °C for 4 h in a nitrogen atmosphere to maximize this phase. One specific advantage of the nitrogen-containing aluminium titanate over aluminium titanate is that the former is unchanged at 1150 °C in a nitrogen atmosphere whereas the latter decomposes. In the Al₂O₃-TiO₂ oxide system Al₂TiO₅ solid solution extends to approximately Al_0.75Ti_2.25O₅ at 1470 °C under the mildly reducing conditions of a graphite furnace. The unit cell volume increases linearly with the increasing replacement of Al³⁺ by Ti³⁺.     

The solid solution structure of BiSr x Ca1−x O2.50

The formation and solid-solution structure of BiSr _x Ca_1–x O_2.50 were studied by X-ray powder diffraction analysis, and the crystallography data of the phase are given. The crystal structure of BiSr _x Ca_1–x O_2.50 belongs to the monoclinic system with space group P1m1 or P12/m1, and the cell parameters for BiCaO_2.50 (x=0.00) are a=1.8363 nm, b=0.5366 nm, c= 1.4670 nm and =100.26°. BiSr _x Ca_1–x O_2.50 is a solid solution of Bi-Ca-O and Bi-Sr-O. When the strontium solid solubility Sr/Sr+Ca=0.50, the strontium dissolved in the phase reaches saturation, while the strontium solid solubility limit of the phase is between 0.67 and 0.75, and beyond this limit the crystal structure is greatly distorted.     

CO-DEPOSITION OF FULLERENES AND SULFUR FROM DILUTED SOLUTION

ABSTRACT

A new method to synthesize fullerene and sulfur compounds has been developed. Using this method, C₆₀S₁₆ and C₇₀S₁₆ compounds were grown from dilute fullerene and sulfur toluene solution. Their atomic structures were analyzed by x-ray diffraction with the single crystal. The C₆₀S₁₆ crystal is C-centered monoclinic structure of a=2.0874?nm, b=2.1139?nm, c=1.05690?nm and β=111.93°, and the C₇₀S₁₆ has a primitive monoclinic, P2₁/c, with lattice parameters of a=1.5271?nm, b=1.49971?nm, c=2.18024?nm and β=109.791°. In this compound, the structure of fullerenes is maintained and sulfur atoms form S₈ rings placed around the fullerenes.     

Metastable phases in vapour-deposited Al-Ru alloys

Vapour deposition of two Al-Ru alloys containing 10 and 15wt% Ru resulted in the formation of supersaturated solid solutions. Electron microscopy and electron diffraction techniques have been used to characterize this as-quenched structure and also those formed on subsequent decomposition. Elevated-temperature (650–800 K) annealing treatments resulted in the formation of a metastable intermediate phase. Electron diffraction evidence suggested that this phase can be assigned a simple cubic structure either of the Mo₃N₂-type witha=0.42 nm or of the Mg₂Zn₁₁-type witha=0.84 nm. Further annealing led to the formation of the equilibrium AI and AI₆ Ru phases. The morphology and structural features of transformation products and the crystal structure details of the metastable intermediate phase have been discussed.     

Tilted and crossing vortex chains in layered superconductors

In presence of the Josephson vortex lattice in layered superconductors, small c-axis magnetic field penetrates in the form of vortex chains. In general, structure of a single chain is determined by the ratio of the London [λ] and Josephson [λ _J ] lengths, α=λ/λ _J . The chain is composed of tilted vortices at large α's (tilted chain) and at small α's it consists of crossing array of Josephson vortices and pancake-vortex stacks (crossing chain). We study chain structures at the intermediate α's and found two types of phase transitions. For α≲0.6 the groud state is given by the crossing chain in a wide range of pancake separationsa≳[2−3]λ _J . However, due to attractive coupling between deformed pancake stacks, the equilibrium separation can not exceed some maximum value depending on the in-plane field and α. The first phase transition takes place with decreasing pancake-stack separation a ata≳[1−2]λ _J , and rather wide range of the ratio α, 0.4≲α≲0.65. With decreasing a, the crossing chain goes through intermediate strongly-deformed configurations and smoothly transforms into the tilted chain via the second-order phase transition. Another phase transition occurs at very small densities of pancake vortices,a ~ [20−30]λ _J , and only when α exceeds a certain critical value ∼0.5. In this case small c-axis field penetrates in the form of kinks. However, at very small concentration of kinks, the kinked chains are replaced with strongly deformed crossing chains via the first-order phase transition. This transition is accompanied by a very large jump in the pancake density.     

Cathodoluminescence emission spectra of trivalent europium-doped yttrium oxysulphide

Europium-doped yttrium oxysulphide phosphor (Y₂O₂S:Eu³⁺) was prepared. Y₂O₂S:Eu³⁺ was found to have a hexagonal structure with lattice parametersa=0.3776 nm andc=0.6538 nm. The Cathodoluminescence emission spectra of Y₂O₂S:Eu³⁺ were recorded between 400 and 635 nm at both 77 and 293 K. Most of the numerous emission lines were assigned on the basis of known energy levels of Eu³⁺ ions, crystal field splittings, and selection rules. More attention was paid to the high-intensity emission lines in the spectral red region, because this phosphor can be used as an efficient red-emitting phosphor in colour televisions. The highly intense lines at 611.3, 616.9 and 626.5 nm are attributed to the transitions from⁵ D ₀ to one of the Stark components of the⁷ F ₂ level. The other transitions from⁵ D ₁ ⁵ D ₂ and⁵ D ₃ to⁷ F _J (J=0, 1–5) were found and assigned.     

Growth and characterization of SnSe2

Tin diselenide single crystals were grown by direct synthesis using the Bridgman technique and were subsequently purified by successive recrystallizations. An X-ray powder diffraction study verified that the crystals obtained belong to the D _3d ³ — P¯3m1 space group with lattice parameters a=0.3811 nm and c=0.6137 nm. A stoichiometric analysis gave a maximum deviation of their nominal SnSe₂ composition of 4%. Study of their morphology by scanning electron microscopy reveals the layered-type structure of the obtained material. Finally, the microhardness parallel and perpendicular to the layer planes was found to be strongly anisotropic, as expected from the layered structure of SnSe₂.     

Growth,thermal and optical properties of Yb:GdYAl3(BO3)4 crystal

Single crystal of Yb:GdYAl₃(BO₃)₄(Yb:GdYAB) has been grown by the flux method. The structure of Yb:GdYAB crystal has been determined by X-ray diffraction analysis. The experiment show that the crystal has the same structure as that of YAl₃(BO₃)₄ crystal and its unit cell constants have been measured to be a = 9.30146 Å, c = 7.24164 Å, Vol = 542.59 Å³. The absorption and fluorescence spectrum of Yb:GdYAl₃(BO₃)₄ crystal have also been measured at room temperature. In the absorption spectra, there are two absorption bands at 938 nm and 974 nm, respectively, which is suitable for InGaAs diode laser pumping. In the fluorescence spectra, there are two fluorescence peaks at 992 and 1040 nm. The thermal properties of Yb:GdYAl₃(BO₃)₄ crystal have been studied for the first time. The thermal expansion coefficient along c-axis is almost 5.4 times larger than that along a-axis. The specific heat of the crystal has been measured to be 0.77 J/g °C at room temperature. The calculated thermal conductivity is 5.26 Wm⁻¹ K⁻¹ along a-direction.     

Growth and characterization of 2-(meta-methoxyphenyl) thiazolidine

2-(meta-methoxyphenyl) thiazolidine (hereafter called 2mmpT) has medicinal importance, as the five-membered thiazolidine ring happens to be the active part of penicillin. This material has been synthesized and crystallized. The crystals are needle-shaped and of dimensions 0.3 × 0.5 × 1.5mm³ using X-ray diffraction technique, oscillation and Weissenberg patterns of the sample are taken. It is shown that the crystal is monoclinic and the unit cell parameters are found to be a = 1.007(5) nm, b=0.545(3) nm, c=1.720(8) nm and =71.3(3)°. The space group is P2₁/c.From IR absorption spectrum of the sample, the characteristic peak pertaining to NH at 3250cm^–1 has been identified. The proton magnetic resonance (PMR) spectrum of the sample unambiguously supports the proposed structure of the compound. It has been confirmed that thiazolidine complex in the present case is a cyclic and not a linear one. The implications are discussed.     

1070-K section of the Yb—Co—B phase diagram

Phase relations in the Yb—Co—B system are studied at 1070 K and Yb contents in the range 0–33 at %. The existence of the three known phases in the Yb—Co—B system is confirmed. The structure of YbCoB₄ is refined by single-crystal x-ray diffraction: sp. gr. Pbam , YCrB₄ type, a = 0.5857(2) nm, b = 1.1343(3) nm, c = 0.3360(2) nm; R_F = 0.0398 and R_w = 0.0434 for 894 independent hkl reflections.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1483–1488.Original Russian Text Copyright © 2004 by Veremchuk, Chaban, Davydov, Kuzma.     

Crystallization characteristics of an amorphous Nb81Si19 alloy under high pressure and formation of the A15 phase

Amorphous Nb-19 at% Si alloy, prepared by rapid quenching from the molten state, was annealed while being subjected to a pressure of 10 GPA. X-ray diffraction investigations on the alloy specimens quenched to ambient conditions have shown that pressure greatly alters the crystallization characteristics and the cubic A15 (Nb₃Si)-phase forms in preference to the tetragonal Nb₃Si-phase at temperatures in the range from 710° C to 800° C. Up to 680° C, the component atoms do not show any tendency towards ordering upon crystallization and the body-centred tetragonal solid solution forms; while, at 830° C, niobium atoms diffuse to form the body-centred cubic Nb precipitates. Superconducting properties have been measured for the single-phase A15 structure with the lattice parametera=0.5155 nm with the results that the transition temperature,T _C, is 3.4 K and the temperature coefficient of the upper critical field,H _C2, is 1.2 MA m^–1 K^–1 (15 kOe K^–1).     

A new organically templated monodimensional mixed valence (Fe/Fe) phosphite: (C4H12N2)[FeFe(HPO3)2F3]: Solvothermal synthesis, crystal structure, spectroscopic and magnetic properties

The organically templated (C₄H₁₂N₂)[Fe^IIFe^III(HPO₃)₂F₃] compound has been synthesized under mild solvothermal conditions. The crystal structure has been determined from X-ray single-crystal diffraction data. The compound crystallizes in the P2₁/n monoclinic space group, with the unit-cell parameters a = 12.935(1), b = 6.4476(7), c = 15.693(2) Å, β = 105.630(9)° and Z = 4. The crystal structure consists of [Fe^IIFe^III(HPO₃)₂F₃]²⁻ chains formed by a central chain built of [Fe(2)O₄F₂] edge-sharing octahedra, and two side chains formed by alternating [Fe(1)O₃F₃] octahedra and [HP(1)O₃] tetrahedra. The piperazinium cations are placed between the chains linked by ionic and hydrogen interactions. The IR and Raman spectra show the existence of two phosphite crystallographically independent. From the diffuse reflectance spectrum the D_q parameter for the iron(II) cations has been calculated (D_q = 820 cm⁻¹). The Mössbauer spectrum in the paramagnetic state shows the simultaneous presence of Fe²⁺ and Fe³⁺. The magnetic measurements indicate the existence of antiferromagnetic interactions.     

Two-phase flow regimes in round, square and rectangular tubes during condensation of refrigerant R134a

An experimental investigation of two-phase flow mechanisms during condensation of refrigerant R134a in six small diameter round (4.91 mm), square (D_h=4 mm, α=1), and rectangular (4×6 and 6×4 mm: D_h=4.8 mm, α=0.67 and 1.5; 2×4 and 4×2 mm: D_h=2.67 mm, α =0.5 and 2) was conducted. Unique experimental techniques and test sections were developed to enable the documentation of the flow mechanisms during phase change. For each tube under consideration, flow mechanisms were recorded over the entire range of qualities for five different refrigerant mass fluxes between 150 and 750 kg m⁻² s⁻¹. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. In addition, the large amount of data enabled the delineation of several different flow patterns within each flow regime, which provides a clearer understanding of the different modes of two-phase flow. Transition lines between the respective flow patterns and regimes on these maps were established based on the experimental data. It was found that for similar hydraulic diameters, flow regime transitions are not very strongly dependent on tube shape or aspect ratio. These maps and the transition lines can be used to predict the particular flow pattern or regime that will be established for a given mass flux, quality and tube geometry.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Crystal growth and investigation of efficient non-linear optical materials in the mixed (2,-4-dinitrophenyl)-L-alanine (DPA) and 2-methyl-4-nitroaniline (MNA) system

When a 11 weight mixture of (2,-4-dinitrophenyl)-L-alanine (DPA) and 2-methyl-4-nitroaniline (MNA) is dissolved in a 5050 volume mixture of ethyl acetate and hexane, two types of crystals were observed on slow evaporation. In the first stage of crystallization, light-yellow plates of MNA measuring up to 25 mm×5 mm×1 mm were formed, which belong to the triclinic space group P1 with lattice parameters a=0.7621 (3) nm, b=0.795 2(5) nm, c=0.8200 (2) nm, =111.84(2)°, = 93.76(2)°, =116.84(3)°, Z=2. The volume of the unit cell containing two molecules is 361.9(1)×10^–3 nm³. The powder SHG efficiency of the triclinic polymorph (MNA(T)) is about the same as that of the monoclinic MNA(M). The Raman spectrum of the new crystal is similar to that of MNA(M) crystallized from the vapour phase or solution, except for the change in the intensity of a few bands. The DSC data show minor differences in the melting points and specific heats for the two phases. The second stage of crystallization resulted in the formation of orange-yellow coloured crystals of the DPA:MNA complex, in the monoclinic space group P2₁ with lattice parameters a =0.6876(1) nm, b=0.7621(3) nm, c=1.7626(4) nm, =96.62(2)°, V=917.5 ×10^–3 nm³, Z=2 for C₁₆H₁₇N₅O₈. The Raman spectrum of DPA:MNA consists of bands from both DPA and MNA. The complex melts at 138(1)°C with a specific heat of 25.3(4) cal g^–1 according to the DSC measurements. The powder SHG efficiency is nearly the same as that of methyl-(2,4-dinitriphenyl)-aminoproanoate (MAP).     

Synthesis and structural characterization of calcia-stabilized tetragonal zirconia

Synthesis of calcia-stabilized tetragonal zirconia by the reaction of precipitated zirconium hydroxide with calcium oxide at ∼1400 K is described. The unit cell parameters of the oxidea ₀=b ₀=0.3644 nm,c ₀=0.5092 nm andZ=2 are in good agreement with the high-temperature tetragonal crystallographic modification of pure zirconia.     

Morphological studies on single crystals and nanofibers of poly(heptamethylene terephthalate)

Poly(heptamethylene terephthalate) (poly(7GT)) was synthesized, and its lamellar single-crystals were grown isothermally at 70 °C from a dilute solution in 1-octanol. Poly(7GT) nanofibers were prepared via electrospinning of its solution in 1,1,1,3,3,3-hexafluoro-2-propanol. Morphology of the single crystals and that of as-spun and annealed nanofibers were investigated by transmission electron microscopy. Selected-area electron diffraction (SAED) of the crystals gave a well-defined N-pattern consisting of spot-like hk0 reflections, and that of bundles of the annealed nanofibers indicated a highly oriented fiber pattern. From the analysis of SAED diagrams for single crystals and nanofibers, it can be assumed that poly(7GT) takes an orthorhombic crystal system and its unit cell parameters estimated are as follows: a = 1.409 nm, b = 1.480 nm, c (chain axis) = 3.392 nm, and α = β = γ = 90°.