Lattice Constituents Research Articles

Synthesis, structure, and photoluminesence of the chloridoaluminates [BMIm][Sn(AlCl4)3], [BMPyr][Sn(AlCl4)3], and [BMIm][Pb(AlCl4)3

[BMIm][Sn(AlCl4)3] (1) ([BMIm]: 1-butyl-3-methylimidazolium), [BMPyr][Sn(AlCl4)3] (2) ([BMPyr]: 1-butyl-1-methyl-pyrrolidinium), and [BMIm][Pb(AlCl4)3] (3) are obtained by reaction of SnCl2/PbCl2 in [BMIm]Cl/[BMPyr]Cl/AlCl3-based ionic liquids. The colourless crystals of the title compounds contain infinite 1∞[M(AlCl4)3]n- chains (M: Sn, Pb) that are separated by the voluminous [BMIm]+/[BMPyr]+ cations. The central Sn2+/Pb2+ is coordinated by chlorine in the form of distorted squared anti-prismatic polyhedra. Each Cl atom, in turn, is part of an [AlCl4]- tetrahedron that interlinks Sn2+/Pb2+ to the chain-like building unit. In addition to the novel structural arrangement, all title compounds surprisingly show intense white-light emission. Although Sn2+ and Pb2+ are well-known as dopants in conventional phosphors, efficient luminescence via s-p-transitions of compounds containing Sn2+/Pb2+ in molar quantities and as regular lattice constituents is rare. The emission of [BMIm][Sn(AlCl4)3] and [BMPyr][Sn(AlCl4)3] is very efficient with quantum yields of 51 and 76%, which belong to the highest values known for s-p-based luminescence of Sn2+.

The effect of anaerobic processes on the leachability of an arsenopyrite refractory ore

Gold is commonly liberated from sulfide minerals via oxidative destruction techniques. To circumvent the formation of sulfuric acid and to reduce the amount of energy required for these processes two alternative anaerobic processes based on sulfate reducing bacteria are investigated for arsenopyrite in this study. The first alternative, “bio-reduction” is expected to alter the structure of arsenopyrite via reduction of the mineral-sulfur to hydrogen sulfide, yielding a sulfur depleted residue that probably contains the gold. The second alternative “anaerobic oxidation” focuses on the mineral-arsenic which under anaerobic conditions can be oxidized to arsenite and subsequently precipitates as orpiment, which may contain the gold. Both alternatives were investigated with gas lift loop reactor experiments performed at pH 5 and 35 °C. These experiments showed that sulfate reducers were able to reduce sulfate from the reactor fluid, but that they were not able to use arsenopyrite as an electron acceptor (bio-reduction) or donor (anaerobic oxidation) under the selected conditions. As a result the gold leachability of the ore concentrate was not improved. To make the mineral more accessible for the leach solution the solubilization of lattice constituents from arsenopyrite that can be biologically reduced/anaerobically oxidized, should be stimulated. In addition, the concentration of arsenite needs to be limited to preserve the activity of sulfate reducing bacteria.

Crystal structure and DFT calculations of 3,4- seco-lup-20(29)-en-3-oic acid isolated from Wrightia tinctoria: Stacking of supramolecular dimers in the crystal lattice

3,4- Seco-lup-20(29)-en-3-oic acid ( 1), a rare triterpene, was isolated from Wrightia tinctoria R.Br (Family: Apocyanaceae). It appeared promising to study the structural chemistry of this compound because of its unique bio activity. X-ray diffraction analysis demonstrated that this compound consists of three six membered rings and one five membered ring. The first six membered ring (A) is in the twist boat form while the remaining two six membered rings (B, C) are in the chair form. The crystal of this compound belongs – to the monoclinic crystal system and space group P2 1. Lattice constituents are as follows: a = 13.074(6) Å, b = 11.972(5) Å, c = 17.394(7) Å; α = 90.00°, β = 98.20(13)°, γ = 90.00°; V = 2695(2) Å 3, d = 1.094 Mg/m 3, Z = 4. The optimized geometry of the reported molecule has been calculated with the DFT/B3LYP theory using 6-31G(d,p) basis set.

First principles study of the electronic and magnetic structures of U2Ni2SnH2

The electronic and magnetic properties and the chemical bonding in recently evidenced U2Ni2SnH2 are self-consistently calculated within the local spin density-functional (LSDF) theory using the scalar-relativistic augmented spherical wave (ASW) method. Trends of the magnetism are discussed in terms of the changes brought by hydrogen within the pure U2Ni2Sn alloy system from both the volume expansion simulating negative pressure and the bonding between H and lattice constituents U, Ni and Sn pointing to a larger Ni–H bonding versus U–H. The ground state is found to be antiferromagnetic in agreement with experiment. Considering the relativistic effects of spin–orbit coupling an ordered magnetic moment, mU=1 μB is calculated for U(5f), close to the experimental magnitude of mU=0.83 μB.

Fast Lithium Ion Conduction in Garnet‐Type Li7La3Zr2O12

Low activation energy and fast lithium ion conduction have been observed for the new compound Li7La3Zr2O12. Relative to previously reported lithium garnets, the solid electrolyte shows a larger cubic lattice constant, higher lithium ion concentration, lower degree of chemical interaction between the Li+ ions and the other lattice constituents, and higher densification.

The prediction of close packed and porous inorganic crystal structures

There are an immense number of possible atomic arrangements that can be generated from the knowledge of the unit cell lattice parameters and constituents. Therefore finding the crystal structure that produces the 'observed' powder diffraction pattern can still be intractable, in some cases, without further information. Hence, techniques for structure prediction have a role to play. Our method (Phys. Chem. Chem. Phys. 1991 1 2535-2542) assumes that the required structure has a lattice energy at either the global minimum or a local minimum of a similar depth. By implementing a genetic algorithm(GA) to generate plausible structures followed by a local optimizer, in order to increase the accuracy of the atomic coordinates, we have efficiently found the structures of a wide range of known close packed oxides, including perovskites, pyrochlores and spinels. Our method, which is capable of reproducing different phases of a compound, is based upon that developed by Bush et al (1995) who successfully predicted the previously unknown structure of Li3RuO4. The method has been implemented within the General Utility Lattice Program (GULP), thus facilitating the calculation of the physical properties (for example, elastic constants) of the phase. Now this method has been extended to enable the prediction of porous materials. To accelerate and guide the GA when trying to generate such materials, several techniques have been investigated, including a simple additive penalty to the cost function when an ion is within a defined 'exclusion zone'(EZ) and the use of a nonlinear grid that has no points within EZ. Further refinements currently being investigated include the use of polyatomic units and symmetry information.

Damage in III/V semiconductors caused by hard- and soft-etching plasmas

Damage in III/V semiconductors caused by processing in high- and low-density plasmas is investigated employing material-specific analytical methods. To distinguish the various regimes of damage, four different ambients out of the palette of plasma constituents were chosen: as borderline cases, argon which is associated with hard etching causing crystal lattice disordering until complete amorphization, and hydrogen which is the prototype of soft etching causing bond breaking between the lattice constituents without changing the crystal site, and the two model ambients Ar/Cl2 and Ar/CH4/Cl2/H2 which are widely used to etch Ga-containing compound semiconductors. Several probes and methods are employed which are selectively sensitive to the different kinds of damage: damage of the crystal lattice with photoluminescence, scanning electron microscopy and scanning transmission electron microscopy, and the concentration gradient of a chemically reactive species (hydrogen) with secondary ion mass spectrometry. Its reaction with lattice atoms (acceptor passivation), is recorded by capacitance/voltage and Hall measurements. For irreversible damage (knock out of atomic lattice sites), there can be detected only one damage zone reaching not deeper than 30 nm. The depth is inversely proportional to the lattice energy and proportional to the kinetic energy of the projectiles. In contrast to this rather shallow damage, the penetration depth of hydrogen amounts up to several microns. Since the subsequent passivation of acceptor atoms is a chemical reaction, it can be reverted at elevated temperatures. It has turned out, with all methods applied, that the gentlest ambient to etch Ga-containing III/V semiconductors is Ar/Cl2. This is not restricted to the smoothness of the surfaces but refers to all quantitative measurements which are applied in this study.

Interdiffusion-induced degradation of 1017 nm ridge waveguide laser diodes

Device degradation of GaAs-based AlGaAs/GaInP/GaInAs ridge waveguide (RW) laser diodes is found to lead to peculiarities in the longitudinal mode spectrum. These features give information on the position of inhomogeneities along the laser stripe. Using monochromatic cathodoluminescence (CL), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) these inhomogeneities are found to be located in the region of the quantum well (QW) and to be caused by interdiffusion of the lattice constituents. This interdiffusion extends over the active region from the interfaces of the GaInP waveguides to the GaAs spacer layers. It is triggered by nonradiative recombination under laser operation.

Solid-phase epitaxial growth of amorphised GaAs in the presence of implantation-induced microscopic non-stoichiometry

Compound semiconductors typically recrystallise in a defective, twinned manner during low-temperature annealing. A variety of factors including non-stoichiometry potentially contribute to the onset of this twinning. For the present report, the influence of non-stoichiometry on the solid-phase epitaxial growth (SPEG) of GaAs has been examined. Two forms of microscopic non-stoichiometry present after the co-implantation of both Ga and As ions were considered: that due to the unequal recoil of the lattice constitutents resulting from differences in mass and that due to the statistical nature of the ion stopping and recoiling processes. (The latter is the chemical disorder on a microscopic scale subsequent to a quenched collision cascade.) Multiple in-situ and ex-situ characterisation techniques have been utilised including transmission electron microscopy, Rutherford backscattering spectrometry, time-resolved reflectivity and double-crystal X-ray diffraction. From such measurements, it was apparent that non-stoichiometry due to the unequal recoil of the lattice constituents was not responsible for the defective SPEG of amorphised GaAs. Furthermore, the onset of twinning and subsequent interfacial non-planarity were independent of the energy deposited in vacancy production, the latter considered a first estimate of relative differences in chemical disorder. This was potentially indicative of the saturation of this form of non-stoichiometry at or below the dose required for amorphisation.

The environment of lattice sites during thermal epitaxial regrowth of ion beam amorphized silicon

To detect changes in the environment of the lattice constituents in ion beam amorphized Si material at various stages of annealing glancing angle X-ray absorption fine structure (XAFS) measurements at the Si K-edge were performed. The recrystallisation of the Si matrix was seen by the emerging of the second and third shell of the diamond structure. The transition took place in a temperature interval of about 100°C centered at 625°C, consistent with former Rutherford-back-scattering/channeling results.

Science of dry etching of III-V materials

A review is given of dry etching of III-V semiconductors. Gas chemistries based on chlorine have traditionally been used for plasma etching of these materials due to the volatilities of both group III and group V chlorides. Recent work on the use of iodine- or bromine-based discharges has shown promising results, although the corrosiveness of the feedstock gases is of concern. For indium-containing III-V semiconductors, CH4/H2 plasmas have produced smooth anisotropic etching at relatively low rates. In applications requiring etch rates near 1 μm min−1, Cl2/CH4/H2 electron cyclotron resonance discharges have proved effective. Examples are given of the structural and chemical changes induced in the semiconductor as a result of dry etching, due to the combined effects of energetic ion bombardment and preferential removal of one of the lattice constituents. Applications for dry etching in modern microelectronic fabrication range from shallow (∼30 nm) mesa formation to creation of through-wafer via holes typically ∼100 μm deep.

Effect of thermal diffusion on the excitonic reflectivity spectra of InGaAs/GaAs quantum wells

We have investigated the effect of intermixing on the excitonic reflectivity spectra of a 100 A thick InGaAs/GAas quantum well (QW). We could resolve both the E 1 -H 1 and E 2 -H 2 excitonic features, up, to LD=80 A. We show that both energy shifts agree well with a simple model which assumes a Fick's law diffusion, with a diffusion coefficient that is independent of the concentration of the cationic species. Next, considering the change (improvement) in signal to noise ratio versus thermal annealing, we suggest that, in our samples, we not only get a broadening of the quantum well as the diffusion proceeds but, also, a diffusion of the lattice constituents in the plane of the well. This in-plane diffusion smoothes the interfaces and reduces the monolayer well width fluctuations

Dry-etching techniques and chemistries for III–V semiconductors

Abstract Dry etching of III–V materials using both chlorine-based (CCl2F2, SiCl4, BCl3 and Cl2) and CH4-H2 discharges is briefly reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4-H2, but the latter gives smoother surface morphologies for indium-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional r.f. (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both indium- and gallium-based III–V semiconductors using CCl2F2-O2 and CH4-H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage in the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Measurement of electron densities in electron cyclotron resonance plasmas for etching of III-V semiconductors

The average electron densities in CH4/H2/Ar and CCl2F2/O2 electron cyclotron resonance discharges typical of those used for etching InP, GaAs, and related materials have been measured approximately 4 cm downstream from the multipolar microwave source as a function of microwave power (50–300 W), additional radio-frequency power (10–50 W), pressure (1–20 mTorr), flow rate (30–90 standard cubic centimeters per minute) and gas composition. At 1 mTorr pressure and 10 W rf, the electron densities (and semiconductor etch rates) increase rapidly with microwave power, from 1.3×1011 cm−3 for 5CH4/17H2/8Ar and 6×1010 cm−3 for 28CCl2F2/2O2 discharges at 50-W microwave power, to 9×1011 cm−3 and 3×1011 cm−3, respectively at 300-W microwave power. At the highest microwave power levels (≥200 W) the InP and GaAs etched surface morphologies are rough due to preferential removal of one of the lattice constituents from each material. The electron densities in both types of discharge show moderate increases with increasing rf power level, pressure or higher Ar or O concentrations.

Dry Etching Techniques and Chemistries for III-V Semiconductors

ABSTRACTDry etching of HI-V materials using both Cl-based (CCl2F2, SiCl4, BCl3, Cl2) and CH4/H2 discharges will be reviewed. The etch rates using chlorine-based mixtures are generally faster than those utilizing CH4/H2, but the latter gives smoother surface morphologies for In-containing compounds. The use of microwave (2.45 GHz) electron cyclotron resonance (ECR) discharges minimizes the depth of lattice disorder resulting from dry etching, relative to conventional RF (13.56 MHz) discharges. Recent results on the systematics of ECR plasma etching of both In- and Ga-based III-V semiconductors using CCl2F2/O2 and CH4/H2 mixtures will be discussed, including the determination of the maximum self-biases allowable which do not induce near-surface damage to the semiconductor. A further key issue is the prevention of changes in the surface stoichiometry of materials such as InP, where the lattice constituents may have considerably different volatilities in the particular discharge.

Implant-induced high-resistivity regions in InP and InGaAs

We have investigated the effects of ion bombardment on the electrical properties of intentionally doped InP and InGaAs grown by metalorganic molecular-beam epitaxy. The sheet resistivity and mobility of n+InP (Sn) and n+InGaAs (Sn) or p+InGaAs (Be) epilayers grown on semi-insulating InP substrates were measured as a function of ion species (O, B, H, or Fe), ion dose (1012–1015 cm−2), and post-implant annealing temperature (100–600 °C). In n+InP, the resistivity after bombardment goes through a maximum with annealing temperature, reaching a value of ∼106 Ω/⧠ for 0.5-μm-thick films after implantation with H or O and annealing at 200–300 °C. The as-grown resistivity is restored by annealing above 500 °C. Ion doses below 1012 cm−2 actually lead to a decrease in resistivity through the creation of shallow donor levels. By contrast, the implantation of Fe above a critical dose where the Fe density exceeds the dopant concentration leads to the formation of thermally stable, high-resistivity (>106 Ω/⧠) material. The temperature dependence of the resistivity shows an activation energy of 0.67 eV, which corresponds to the acceptor level of substitutional Fe in InP. Both n+InGaAs and p+InGaAs show somewhat similar behavior after implantation with maximum resistivities of ∼105 Ω/⧠ regardless of implant species. Once again for relatively low doses of O or H (below ∼1013 cm−2 in this case) there is creation of shallow defect levels that lower the resistivity of the material. The formation of these levels in InP has been investigated in more detail by measuring the depth-dependent carrier profile in implanted high-resistivity InP. The profile of the damage-induced centers is in close correlation with the nuclear energy deposition profile of the implanted ion in some cases, and with the profiles of stoichiometric excess due to unequal recoil of the lattice constituents in other cases.

A qualitative study of the instability of fractal aggregates

The instability of fractal aggregates is discussed qualitatively using the self-consistent harmonic approximation. The density of elastic vibrational states originating from phonon and elastic fracton modes is used to calculate the mean square displacement of lattice constituents, and the transition temperature is estimated for fractal aggregates with different dimensionalities of elastic fractons.

Thermodynamic excess functions for quasiternary solid solutions of type (MkN(1‐k)) (XlY(1‐l))

AbstractThe excess enthalpy for a quasiternary system of type MkN(1‐k)XlY(1‐l) is derived by use of a model, which considers the interaction of clusters composed of four particles. The distribution of the lattice constituents is assumed to be random. Based on this model the excess enthalpy can be described by a cubic function of the mole fractions k and l with the standard reaction enthalpy and 8 interaction parameters as constants. It is shown, that the ternary interaction parameters can be calculated from the interaction parameters of the subregular quasibinary subsystems, whereas the reaction enthalpy for the double conversion between the binary components can be derived from their standard enthalpies. Thus the phase diagrams of quasiternary systems with two mixed sublattices can be derived from the quasibinary data without introducing any special ternary interaction parameter. ‐ The influence of the interaction parameters on the shape of the phase diagram and especially the influence of the standard reaction enthalpy on the tie line field is demonstrated by some calculated phase diagrams.

Range phenomena of low energy ions in solids

While at intermediate reduced energies of about 10 −2 ≲ ϵ ≲ 1 a nearly universal relation of reduced ranges, ρ= f( ϵ), holds, the situation is different at low energies of about ϵ ≲ 10 −2. Here both projectile and target individualities may show up depending on their combinations. In homopolar materials, such as semiconductors, an oscillatory dependence of ρ on Z 1 has been reported previously; in metallic targets, however, this Z 1-effect is quenched. In ionic solids, an additional stopping mechanism, due to the Coulomb interaction of projectile ions with ionic lattice constituents, can appear as theoretically predicted. A review of these effects will be given with particular emphasis on the latter phenomenon.

ChemInform Abstract: CATALYSIS BY TRANSITION METAL OXIDES

In the catalytic reaction of organic molecules with gas phase oxidants (e.g. oxygen, sulfur, chlorine), either the oxidant is activated and performs an electrophilic attack, or the organic molecules are activated and the reaction proceeds in consecutive steps. These steps involve: a) activation of the organic molecule, which is then oxidized by lattice cations, b) nucleophilic insertion of the appropriate anionic lattice constituent into the organic species at a position which depends on the mode of activation, and c) reoxidation of the lattice cations by the gas phase. The development of a theory of catalysis requires a determination of the mechanism of intermediate complex formation and the nature of the interaction between a reacting molecule and the active site as it affects selectivity by influencing reactivity along different reaction paths. The structure of such a complex depends on: a) the type of cation at the active site, i.e., the composition of surface vs. bulk, b) its coordination by oxygen ions, i.e. the geometry of the catalyst surface, and c) its orbital structure and occupancy, which can be modified either by introduction of additives, shifting of the Fermi level, or by supporting on an appropriate carrier. Geometry is of particular importancemore » in the case of Group V to VII transition metal oxides and oxysalts, which form layer structures with strongly pronounced anisotropy of chemical properties such as mobility of lattice constituents. The latter is a function of defect concentration and the facility of the mode of rearrangement in which coordination polyhedra are linked together.« less